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Phytophagous insects commonly interact through shared host plants. These interactions, however, do not occur in accordance with traditional paradigms of competition, and competition in phytophagous insects is still being defined. It remains unclear, for example, if particular guilds of insects are superior competitors or important players in structuring insect communities. Gall-forming insects are likely candidates for such superior competitors because of their ability to manipulate host plants, but their role as competitors is understudied. We investigate the effect of invasive populations of an oak gall wasp, Neuroterus saltatorius, on a native specialist butterfly, Erynnis propertius, as mediated by their shared host plant, Quercus garryana. This gall wasp occurs at high densities in its introduced range, where we stocked enclosures with caterpillars on trees that varied in gall wasp density. Biomass production of butterflies was lower in enclosures on high-density than on low-density trees because overwintering caterpillars were smaller, and fewer of them eclosed into adults the following spring. To see if the gall wasp induced changes in foliar quality, we measured host plant quality before and after gall induction on 30 trees each at two sites. We found a positive relationship between gall wasp density and the percentage change in foliar C:N, a negative relationship between gall wasp density and the percentage change in foliar water at one site, and no relationship between the percentage change in protein-binding capacity (i.e., phenolics) and gall-wasp density. Additionally, there was a negative relationship between foliar quality and butterfly performance. Our results provide evidence for a plant-mediated impact of an invasive oak gall wasp on a native butterfly and suggest that gall wasps could act as superior competitors, especially when they occur at high densities. Key words: cynipids; Erynnis propertius; gall-forming insect; induced defense; interspecific competition; invasive species; Neuroterus saltatorius; plant-mediated interactions; plant-insect interactions; Quercus garryana; range expansion.

Variation in invasion success may result from the divergent evolutionary histories of introduced species compared to those of native taxa. The vulnerability of native biotas to ecological disruption may be especially great on oceanic islands invaded by continental species with unique ecological traits. In part because Hawaii lacks native eusocial insects, social invaders may threaten endemic taxa that are ecologically similar but solitary. Using a combination of field manipulations, molecular analyses, physiological data, and behavioral assays, we identify the mechanisms underlying the displacement of two genera of native solitary Hymenoptera in Hawaii by a social continental invader, the western yellowjacket (Vespula pensylvanica). Experimental removal of V. pensylvanica colonies resulted in increased densities of native Hymenoptera. Endemic Hylaeus bees directly suffer through predation by yellowjackets, and perhaps as a consequence, avoid floral resources occupied by V. pensylvanica. Native Nesodynerus wasps also avoid V. pensylvanica but are negatively affected by yellowjackets not through predation, but through exploitative competition for caterpillar prey. Displacement of native solitary Hymenoptera may be heightened by the ability of V. pensylvanica to prey upon and scavenge honey bees and to rob their honey stores, resources unavailable to endemic bees and wasps because of their specialized niches. Our study provides a unique example of an ecologically generalized social invader that restructures native assemblages of solitary Hymenoptera by interacting with endemic taxa on multiple trophic levels. Key words: exploitative competition, Hawaii; Hylaeus; invasions; Nesodynerus; predation; Vespula pensylvanica; yellowjacket.

Adjacent food webs may be linked by cross-boundary subsidies: more-productive donor systems can subsidize consumers in less-productive neighboring recipient systems. Introduced species are known to have direct effects on organisms within invaded communities. However, few studies have addressed the indirect effects of normative species in donor systems on organisms in recipient systems. We studied the direct role of introduced trout in altering a lake-derived resource subsidy and their indirect effects in altering a passerine bird's response to that subsidy. We compared the abundance of aquatic insects and foraging Gray-crowned Rosy-Finches (Leucosticte tephrocotis dawsoni, 'Rosy-Finch') at fish-containing vs. fishless lakes in the Sierra Nevada Mountains of California (USA). Introduced trout outcompeted Rosy-Finches for emerging aquatic insects (i.e., mayflies). Fish-containing lakes had 98% fewer mayflies than did fishless lakes. In lakes without fish, Rosy-Finches showed an aggregative response to emerging aquatic insects with 5.9 times more Rosy-Finches at fishless lakes than at fish-containing lakes. Therefore, the introduction of nonnative fish into the donor system reduced both the magnitude of the resource subsidy and the strength of cross-boundary trophic interactions. Importantly, the timing of the subsidy occurs when Rosy-Finches feed their young. If Rosy-Finches rely on aquatic-insect subsidies to fledge their young, reductions in the subsidy by introduced trout may have decreased Rosy-Finch abundances from historic levels. We recommend that terrestrial recipients of aquatic subsidies be included in conservation and restoration plans for ecosystems with alpine lakes. Key words: aquatic-insect emergence; aquatic-terrestrial linkages: Callibaetis ferrugineus; density-mediated indirect effect; ecosystem boundary; ecosystem subsidy; fish stocking; food web; land--water interface; Oncorhynchus mykiss; Passeriformes: Salvelinus fontinalis.

Invasive plants may compete with native plants by increasing the pressure of native consumers, a mechanism known as 'apparent competition.' Apparent competition can be as strong as or stronger than direct competition, but the role of apparent competition has rarely been examined in biological invasions. We used four years of demographic data and seed-removal experiments to determine if introduced grasses caused elevated levels of seed consumption on native plant species in a coastal dune system in California, USA. We show that the endangered, coastal dune plant Lupinus tidestromii experiences high levels of predispersal seed consumption by the native rodent Peromyscus maniculatus due to its proximity to the invasive grass, Ammophila arenaria. We use stage-structured, stochastic population models to project that two of three study populations will decline toward extinction under ambient levels of consumption. For one of these declining populations, a relatively small decrease in consumption pressure should allow for persistence. We show that apparent competition with an invasive species significantly decreases the population growth rate and persistence of a native species. We expect that apparent competition is an important mechanism in other ecosystems because invasive plants often change habitat structure and plant-consumer interactions. Possible implications of the apparent-competition mechanism include selective extinction of species preferred by seed consumers in the presence of an invasive species and biological homogenization of communities toward non-preferred native plant species. Key words: Ammophila arenaria; apparent competition; biotic homogenization; coastal dune ecosystems; life-table response experiment; Lupinus tidestromii; matrix population model; Point Reyes National Seashore, northern California, USA; Peromyscus maniculatus; population viability analysis; predispersal seed consumption; stochastic population model.

Plants interact with numerous enemies and mutualists simultaneously and sequentially. Such multispecies interactions can give rise to trait-mediated indirect effects that are likely to be common in nature but which are also inherently difficult to predict. Understanding multispecies interactions is also important in the use of biological control agents to control invasive plants because modern approaches to biocontrol rely on releasing multiple agents for each target weed. Centaurea solstitialis is one of the most problematic invasive weeds in California, USA, and the weevil Eustenopus villosus is its dominant biological control agent. We conducted a field experiment to quantitatively assess the direct effect of the recently approved biocontrol pathogen Puccinia jaceae f.s. solstitialis on plant performance and any indirect effects that might arise by altering the plant's interactions with Eustenopus or its pollinators (principally the nonnative Apis mellifera). We documented both synergy and interference between the two biocontrol agents depending on the life stage of the weevil. Puccinia infection increased the impact of bud-feeding by the adult weevils but reduced the impact of seed-feeding by larval weevils. Neither infection nor Eustenopus attack had an effect on pollinator visitation. The net effect was that attack by both the pathogen and the weevil did not reduce plant fitness relative to plants attacked only by the weevil. Because the consequence to the plant of interacting with one species may depend on the presence or absence of other interacting species, a careful consideration of multispecies interactions may be necessary for the selection of biocontrol agents that act in a complementary manner to reduce plant fitness. Likewise, relatively tractable weed biocontrol systems allow us to examine multispecies interactions that can be difficult to study experimentally in native systems that are composed of numerous species with well-established populations. Key words: biological control; California, USA; Centaurea solstitialis; cumulative stress; Eustenopus villosus; indirect interactions; interference; Puccinia jaceae f.s. solstitialis; synergy: yellow starthistle.

We addressed the potential for climate change to facilitate invasions and precipitate shifts in community composition by testing effects of ocean warming on species in a marine fouling community in Bodega Harbor, Bodega Bay, California, USA. First, we determined that introduced species tolerated significantly higher temperatures than natives, suggesting that climate change will have a disproportionately negative impact on native species. Second, we assessed the temperature dependence of survival and growth by exposing juveniles to an ambient control temperature and increased temperatures predicted by ocean warming scenarios (+3[degrees]C and +4.5[degrees]C) in laboratory mesocosms. We found that responses differed between species, species origins, and demographic processes. Based on the temperature tolerance, survival, and growth results, we predict that, as ocean temperatures increase, native species will decrease in abundance, whereas introduced species are likely to increase in this system. Facilitation of invasions by climate change may already be underway; locally, invasive dominance has increased concurrent with ocean warming over the past ~40 years. We suggest that the effects of climate change on communities can occur via both direct impacts on the diversity and abundance of native species and indirect effects due to increased dominance of introduced species. Key words: Bodega Bay, California USA; bryozoan; climate change; community ecology; fouling community; global warming; introduced species; invasion; nonindigenous species; survival; temperature; tunicate.

This study addresses characteristics of source and island populations of nonraptorial land birds that have colonized the Lesser Antilles to determine whether colonization by individual source populations is a continuous process or occurs in transient phases. Species were classed as non-colonists, recent colonists, old endemic residents in the Lesser Antilles, and old endemic taxa that had recently spread within the archipelago. If colonization were transient, source populations of recent colonists would be more widely distributed than source populations of older, endemic island species. I compared the ecological and geographic distributions of source populations and island populations for each of the colonization groups based on a variety of literature and field data. Ages of colonization events and spread within the islands were determined by sequence divergence in mitochondrial genes, where available; relative ages were otherwise inferred from taxonomic differentiation and gaps in island distribution. Within the Lesser Antilles, old colonists, whether endemic or recently spread, tended to inhabit forest rather than open environments. Recently spread old colonists, like young colonists, had relatively greater abundance and broader habitat distribution within islands. These patterns were paralleled in northern South American source populations, but the trends were relatively weak. The strongest pattern was the propensity for source populations of young colonists and older, re-expanded endemics to occur independently on islands off the Caribbean coast of Venezuela. In comparisons with non-colonists in Trinidad, species that have recently colonized the Lesser Antilles tended to occur in more open habitats and to be more abundant locally, as well as more widespread through continental zoogeographical zones. Nested ANOVA based on a taxonomic hierarchy demonstrated that relative abundance and ecological and geographic distributions are labile, most of the variation being among species within genera. This is consistent with the idea that periods of high population productivity leading to colonization of offshore islands are transient. The existence of such phases can be inferred from correlations, albeit weak, between the status of populations in the Lesser Antilles and the ecological and geographic distribution of their putative source populations in the mainland. Key words: colonization; ecological distribution; island biogeography; land birds; Lesser Antilles; mainland source populations; mitochondrial DNA sequence divergence.

Habitat-modifying invasive species can influence rates of predation on native prey either directly by providing protective structure or indirectly by modifying traits of prey species responding to the habitat. The alga Caulerpa taxifolia is one of the most successful invasive species of shallow-water marine systems globally, often provisioning habitat in areas previously lacking in vegetated structure. We experimentally evaluated the direct effect of Caulerpa to provide refuge for the native clam Anadara trapezia and how this balances with its influence on two trait-mediated indirect interactions that may increase Anadara's susceptibility to predators. Specifically, Caulerpa's alteration of physical and chemical properties of the surrounding water and sediment deteriorate Anadara's condition and predator resistance properties and also cause Anadara, though normally buried, to project from beneath the sediment, exposing it to predators. Our results show that Anadara are somewhat (but not consistently) protected from predators by living among Caulerpa. Shallow burial depth did not counteract this protective effect. However at times of year when predator activity diminishes and conducive environmental conditions develop, negative effects of Caulerpa habitat such as hypoxia and lowered flow may dominate. Under such situations, poor clam condition accentuates Anadara's susceptibility to mortality. Ultimately, a slight and inconsistent positive effect of Caulerpa to protect Anadara from predators is exceeded by the strong negative effect of Caulerpa on clam mortality, which is heightened by clams' weakened condition produced by chronic exposure to Caulerpa. Our results show that invasive habitat-modifying species can affect mortality of native species not simply through obvious positive direct effects of their protective structure, but indirectly through contrasting negative modification of the traits of prey species responding to the habitat. Key words.' Anadara trapezia; behavioral modification; benthic infauna; Caulerpa taxifolia; ecosystem engineering; foundation species; New South Wales, Australia; nonnative species; prey refuges; soft sediment communities; sublethal effects; trait-mediated indirect interactions.

A central question in ecology concerns how some exotic plants that occur at low densities in their native range are able to attain much higher densities where they are introduced. This question has remained unresolved in part due to a lack of experiments that assess factors that affect the population growth or abundance of plants in both ranges. We tested two hypotheses for exotic plant success: escape from specialist insect herbivores and a greater response to disturbance in the introduced range. Within three introduced populations in Montana, USA, and three native populations in Germany, we experimentally manipulated insect herbivore pressure and created small-scale disturbances to determine how these factors affect the performance of houndstongue (Cynoglossum officinale), a widespread exotic in western North America. Herbivores reduced plant size and fecundity in the native range but had little effect on plant performance in the introduced range. Small-scale experimental disturbances enhanced seedling recruitment in both ranges, but subsequent seedling survival was more positively affected by disturbance in the introduced range. We combined these experimental results with demographic data from each population to parameterize integral projection population models to assess how enemy escape and disturbance might differentially influence C. officinale in each range. Model results suggest that escape from specialist insects would lead to only slight increases in the growth rate ([lambda]) of introduced populations. In contrast, the larger response to disturbance in the introduced vs. native range had much greater positive effects on L. These results together suggest that, at least in the regions where the experiments were performed, the differences in response to small disturbances by C. officinale contribute more to higher abundance in the introduced range compared to at home. Despite the challenges of conducting experiments on a wide biogeographic scale and the logistical constraints of adequately sampling populations within a range, this approach is a critical step forward to understanding the success of exotic plants. Key words: Cynoglossum officinale; disturbance; escape from enemies; herbivory; integral projection model (IPM); Life Table Response Experiment (LTRE); Montana, USA; native and introduced ranges; plant demography; Saxony-Anhalt, Germany.

The pervasive impact of invasive species has motivated considerable research to understand how characteristics of invaded communities, such as native species diversity, affect the establishment of invasive species. Efforts to identify general mechanisms that limit invasion success, however, have been frustrated by disagreement between landscape-scale observations that generally find a positive relationship between native diversity and invasibility and smaller-scale experiments that consistently reveal competitive interactions that generate the opposite relationship. Here we experimentally elucidate the mechanism explaining the large-scale positive associations between invasion success and native intertidal diversity revealed in our landscape-scale surveys of New England shorelines. Experimental manipulations revealed this large-scale pattern is driven by a facilitation cascade where ecosystem-engineering species interact nonlinearly to enhance native diversity and invasion success by alleviating thermal stress and substrate instability. Our findings reveal that large-scale diversity--invasion relationships can be explained by small-scale positive interactions that commonly occur across multiple trophic levels and functional groups. We argue that facilitation has played an important but unrecognized role in the invasion of other well studied systems, and will be of increasing importance with anticipated climate change. Key words: biodiversity; ecosystem engineer; facilitation cascade; foundation species; invasion paradox; invasive species; marine conservation; nonlinear ecological interactions.

Geographic variation in species interactions can have major effects on distributions. Effects of such variation can be particularly evident for invasive species, in which variation in competitive ability can influence invasive success and impacts. We tested the hypothesis that coexistence or exclusion of the resident mosquito Aedes aegypti results from variation among local populations of the invasive Aedes albopictus in competitive interactions with A. aegypti. We also examined the role of variation in fecundity-size relationships in these competitive interactions. We compared competitive abilities of nine North American populations of A. albopictus, three populations from each of three site types: extinction of A. aegypti following A. albopictus invasion, coexistence following A. albopictus invasion, and A. albopictus allopatric to A. aegypti. Competition among larvae from each A. albopictus population and a single A. aegypti population was tested in laboratory microcosms in a response surface design. We found interpopulation differences in competitive ability of A. albopictus, but no strong patterns among site types. Extinction sites had steeper average fecundity-size relationships than coexistence sites and allopatric sites, but this did not translate into superior population performance. Certain individual A. albopictus populations had exceptionally large competitive effects on A. aegypti or poor competitive responses to competition from A. aegypti, but competitive effect and response were not correlated. These results suggest that interpopulation variation in the competitive ability of A. albopictus may only partly explain the geographic pattern of coexistence with or extinction of A. aegypti. Environmental differences among regions may affect the competitive ability of A. albopictus and influence its invasion success and impact. Key words: Aedes aegypti; Aedes albopictus; competitive asymmetry; competitive hierarchies; fecundity--size relationship; interpopulation divergence; invasive species; mosquitoes; reproductive tactics.

Selection and adaptation are important processes in the coevolution between parasites and their hosts. The slave-making ant Protomognathus americanus, an obligate ant social parasite, has previously been shown to evolve morphological, behavioral, and chemical adaptations in the coevolutionary arras race with its Temnothorax hosts. Yet empirical studies have given variable results on the strength of the selection pressure this parasite exerts on its host populations. In this study, we directly investigated the pressure exerted by P. americanus and the reactions of the main host species, T. longispinosus, in two ant communities by manipulating parasite density in the field over several years, in addition, a cross-fostering design with the exchange of parasites between host populations allowed us to investigate local adaptation of parasite of host. We demonstrate a severe impact of the social parasite on the two host populations in West Virginia and New York, but also variation in host reactions between sites, as expected by the geographic mosaic theory of coevolution. Host density decreased at the West Virginia site with the presence of local slave-makers, whereas at the ecologically favorable New York site, density was unaffected. Nevertheless, social organization, colony size, and investment patterns of these host colonies at this site changed in response to our parasite manipulation. The release of P. americanus colonies led to a reduction in the number of resident queens and workers, an increase in intranest relatedness, and lower productivity, but also a higher investment in reproductives. In West Virginia, colony demography did not change, but raiding activity by New York slave-makers caused different investment patterns of host colonies. In addition, the cross-fostering element revealed local adaptation of the parasite P. americanus: slave-making colonies fared better in their sympatric host population, as they contained more slave-making ant workers and slaves at the end of our 27-month experiment. Key words: coevolution; coevolutionary arms race, geographic mosaic; host--parasite interactions; Protomognathus; selection; slave-making ants; social insects; Temnothorax.

Species abundances in natural systems are usually close to some equilibrium, making mechanisms that maintain or prevent species coexistence difficult to discern. Biological control projects provide an opportunity to observe systems transition between equilibriums as a result of the influence of the newly introduced species. In the southeastern United States and Texas, species of phorid fly parasitoids are being sequentially introduced as control agents for imported fire ants. The first two species introduced, Pseudacteon tricuspis and P. curvatus, partition the host niche based upon body size and co-exist broadly in their native range in Argentina, indicating they would forma co-existing and complementary suite of parasitoids in North America. This study examines the interaction between these parasitoids at multiple temporal and spatial scales. Surprisingly, data at all scales reveal that as P. curvatus establishes at a site it competitively displaces P. tricuspis. However, the speed of this reduction appears to differ between ecoregions, suggesting that the rate of displacement depends on environment. At the site where P. curvatus has been established the longest, this population interaction approaches complete displacement. Tests of potential mechanisms causing this displacement reveal that direct competition for host workers alters the operational sex ratio of the P. tricuspis population, but the strength of this effect is insufficient to explain the displacement. Experiments reveal the operation of a strong, indirect effect whereby locally common species preempt reproductive opportunities from rarer species by inducing host behavioral defenses. Finally, a re-examination of published data from their native range reveals that a previously overlooked negative relationship between the densities of these two species also exists there, suggesting that the same processes as those reported here also operate in South America. Key words: hiological control: central Texas. USA: competition: competitive displacement; fire ant control; phorid fly parasitoid; Pseudacteon curvatus; Pseudacteon tricuspis; rare species; Solenopsis invicta; trait-mediated indirect effect.

Belowground interactions between herbaceous native species and nonnative species is a poorly understood but emerging area of interest to invasive-species researchers. Positive feedback dynamics are commonly observed in many invaded systems and have been suspected in California grasslands, where native plants associate strongly with soil mutualists such as arbuscular mycorrhizal fungi. In response to disturbance, invading nonnative plants proliferate, and to the degree these species associate weakly with soil mutualists, we would expect mutualist efficacy to degrade over time. Degraded mutualist efficacy would negatively impact mutualist-dependent native species or their recruitment following a disturbance. We investigated the feedback dynamics of soil conditioned both with native and normative herbaceous communities of southern California grasslands to test this degraded mutualist hypothesis. Using a mesocosm approach, we inoculated each community with live soil originating from a remnant native grassland and varied the plant communities (i.e., native or nonnative) along a plant-species-richness gradient. After one year, we then used this conditioned soil for reciprocal feedback tests on a native and nonnative indicator species. We show that a native herbaceous forb (Gnaphalium californicum) grows best in soil conditioned by a diverse mix of other native species that includes G. californicum but is inhibited by soil conditioned by a diverse mix of normative species. We also show that an invasive, nonnative herbaceous forb (Carduus pycnocephalus) exhibits strong growth in soil lacking arbuscular mycorrhizal fungi and in soil conditioned by a diverse mix of nonnative species that include C. pycnocephalus, and that it is inhibited by the same soil that best promotes the native, G. californicum. Separate bioassays for mycorrhizal density show a reduction of arbuscular mycorrhizal fungi in the nonnative-conditioned soil relative to the native-conditioned soil, which suggests that nonnative species do not promote the growth of mycorrhizal fungi in the same way that native species do. The growth patterns resulting from the vegetative history of these distinct soil communities provide evidence of a biotic feedback mechanism that may account for the maintenance of persistent communities of nonnative (and often invasive) plants ubiquitous throughout California grasslands. Key words: AMF; arbuscular mycorrhizal fungi; Carduus pycnocephalus; native; normative; invasive; California grasslands; feedback; Gnaphalium californicum; mutualist degradation.

Although much of the theory on the success of invasive species has been geared at escape from specialist enemies, the impact of introduced generalist invertebrate herbivores on both native and introduced plant species has been underappreciated. The role of nocturnal invertebrate herbivores in structuring plant communities has been examined extensively in Europe, but less so in North America. Many nocturnal generalists (slugs, snails, and earwigs) have been introduced to North America, and 96% of herbivores found during a night census at our California Central Valley site were introduced generalists. We explored the role of these herbivores in the distribution, survivorship, and growth of 12 native and introduced plant species from six families. We predicted that introduced species sharing an evolutionary history with these generalists might be less vulnerable than native plant species. We quantified plant and herbivore abundances within our heterogeneous site and also established herbivore removal experiments in 160 plots spanning the gamut of microhabitats. As 18 collaborators, we checked 2000 seedling sites every day for three weeks to assess nocturnal seedling predation. Laboratory feeding trials allowed us to quantify the palatability of plant species to the two dominant nocturnal herbivores at the site (slugs and earwigs) and allowed us to account for herbivore microhabitat preferences when analyzing attack rates on seedlings. The relationship between local slug abundance and percent cover of five common plant taxa at the field site was significantly negatively associated with the mean palatability of these taxa to slugs in laboratory trials. Moreover, seedling mortality of 12 species in open-field plots was positively correlated with mean palatability of these taxa to both slugs and earwigs in laboratory trials. Counter to expectations, seedlings of native species were neither more vulnerable nor more palatable to nocturnal generalists than those of introduced species. Growth comparison of plants within and outside herbivore exclosures also revealed no differences between native and introduced plant species, despite large impacts of herbivores on growth. Cryptic nocturnal predation on seedlings was common and had large effects on plant establishment at our site. Without intensive monitoring, such predation could easily be misconstrued as poor seedling emergence. Key words: cryptic seedling herbivory; Deroceros reticulatum; earwig; Forficula auricularia; generalist herbivores; Helix aspersa; introduced species; large-scale laboratory and field trials; native distribution; nocturnal herbivores; slug; snail.

The dominance of nonnative plants coupled with declines of native plants suggests that competitive displacement drives extinctions, yet empirical examples are rare. Herbivores, however, can alter vegetation structure and reduce diversity when abundant. Herbivores may act on mature, reproductive life stages whereas some of the strongest competitive effects might occur at early life stages that are difficult to observe. For example, competition by perennial nonnative grasses can interfere with the establishment of native seeds. We contrasted the effects of ungulate herbivory and competition by neighboring plants on the performance of native plant species at early and established life stages in invaded oak meadows. We recorded growth, survival, and flowering in two native species transplanted as established plants, six native species grown from seed, and five extant lily species as part of two 2 x 2 factorial experiments that manipulated herbivory and competition. Herbivory reduced the performance of nearly all focal native species at early and established life stages, whereas competition had few measurable effects. Our results suggest that herbivory has a greater local influence on native plant species than competition and that reducing herbivore impacts will be required to successfully restore endangered oak meadows where ungulates are now abundant. Key words: black-tailed deer, British Columbia; competition; conservation; exotic; grass; invasive; oak savanna; Odocoileus hemionus; sheep.

Although biological invasions are of considerable concern to ecologists, relatively little attention has been paid to the potential for and consequences of indirect interactions between invasive species. Such interactions are generally thought to enhance invasives' spread and impact (i.e., the 'invasional meltdown' hypothesis); however, exotic species might also act indirectly to slow the spread or blunt the impact of other invasives. On the east coast of the United States, the invasive hemlock woolly adelgid (Adelges tsugae, HWA) and elongate hemlock scale (Fiorinia externa, EHS) both feed on eastern hemlock (Tsuga canadensis). Of the two insects, HWA is considered far more damaging and disproportionately responsible for hemlock mortality. We describe research assessing the interaction between HWA and EHS, and the consequences of this interaction for eastern hemlock. We conducted an experiment in which uninfested hemlock branches were experimentally infested with herbivores in a 2 x 2 factorial design (either, both, or neither herbivore species). Over the 2.5-year course of the experiment, each herbivore's density was ~30% lower in mixed- vs. single-species treatments. Intriguingly, however, interspecific competition weakened rather than enhanced plant damage: growth was lower in the HWA-only treatment than in the HWA + EHS, EHS-only, or control treatments. Our results suggest that, for HWA-infested hemlocks, the benefit of co-occurring EHS infestations (reduced HWA density) may outweigh the cost (increased resource depletion). Key words: Adelges tsugae; biological invasions; competition; Fiorinia externa; hemlock woolly adelgid; Tsuga canadensis.

While an extensive literature exists on the negative effects of invasive species, little is known about their facilitative effects on native species, particularly the role of invasives as trophic subsidies to native predators. The invasive gypsy moth (Lymantria dispar) undergoes periodic outbreaks during which it represents a super-abundant food source for predators capable of consuming it, particularly native cuckoos (Coccyzus erythropthalmus and C. americanus). We examined how gypsy moth outbreaks affect the abundance and distribution of cuckoos using the North American Breeding Bird Survey and 29 years of U.S. Forest Service gypsy moth defoliation records. Abundances of both Black-billed and Yellow-billed Cuckoos were significantly above average during outbreaks, but populations were average or below average in preceding and subsequent years, suggesting that cuckoos are immigrating to defoliations during outbreak years. Spatial analyses showed that cuckoo abundances ~40-150 km outside of defoliation areas were significantly below average, and these under-occupied breeding areas extended in all four compass directions around outbreaks. This result supports the idea that cuckoos locate gypsy moth outbreaks during a post-migratory nomadic phase. By shifting the annual distribution of cuckoos, gypsy moths may be shifting the trophic impact of cuckoos across large distances, which could affect native insect herbivores and plants. Key words: Coccyzus; cuckoo; gypsy moth; invasive species; Lymantria dispar; outbreak; predation; spatial distribution.

Territorial defense by breeders influences access to resources near defended nest sites by intruder species and may have indirect effects on other species within the territory, leading to local patchiness in distribution patterns. The present study demonstrates that adult males of a damselfish, Pomacentrus amboinensis, indirectly facilitate the increased survival of conspecific juveniles through the territorial defense of their nesting site from potential egg predators. Moreover, male territoriality results in a shift in the selectivity of predation on newly settled juveniles. We monitored the fate of pairs of predator-naive, newly settled P. amboinensis placed inside and outside nesting territories. Individuals within a pair differed in size by ~1 mm and were tagged for individual identification. Away from male territories larger juveniles had greater survival, while within territories, larger juveniles suffered higher mortality. Behavioral observations indicated that the moonwrasse Thalassoma lunare, a predator of benthic eggs and small fishes, had reduced access to juveniles within male territories, while another predator on small fishes, the dottyback Pseudochromis fuscus, had unobstructed access to male territories. Experimental removal of P. fuscus indicated that the shift in the direction of phenotypic selection on newly settled juveniles was the indirect effect of aggression by nest-guarding male damselfish, which resulted in differential access to male territories by these two predators of small fishes. Evidence suggests that behavioral interactions between the resident community and intruders will influence patchiness in selective pressures imposed on benthic prey by influencing both the composition of predator types that can access the prey resource and their relative abundance. How this spatial and temporal patchiness in predator pressure interacts with spatial patchiness of recruiting prey will have a major influence on the resulting distribution of juveniles and their phenotypic traits. Key words: behavior; coral reef fish; density-mediated indirect effects; phenotypic selection; predator interference; predator-prey; selective mortality; settlement; size-selection; territoriality.

The differences in phenotypic plasticity between invasive (North American) and native (German) provenances of the invasive plant Lythrum salicaria (purple loosestrife) were examined using a multivariate reaction norm approach testing two important attributes of reaction norms described by multivariate vectors of phenotypic change: the magnitude and direction of mean trait differences between environments. Data were collected for six life history traits from native and invasive plants using a split-plot design with experimentally manipulated water and nutrient levels. We found significant differences between native and invasive plants in multivariate phenotypic plasticity for comparisons between low and high water treatments within low nutrient levels, between low and high nutrient levels within high water treatments, and for comparisons that included both a water and nutrient level change. The significant genotype X environment (G X E) effects support the argument that invasiveness of purple loosestrife is closely associated with the interaction of high levels of soil nutrient and flooding water regime. Our results indicate that native and invasive plants take different strategies for growth and reproduction; native plants flowered earlier and allocated more to flower production, while invasive plants exhibited an extended period of vegetative growth before flowering to increase height and allocation to clonal reproduction, which may contribute to increased fitness and invasiveness in subsequent years. Key words: common garden; genotype X environment (G X E) interactions; invasive species; Lythrum salicaria; multivariate analysis; native vs. invasive; phenotypic plasticity; purple loosestrife; reaction norm.

Theory predicts that damage by a shared herbivore to a secondary host plant species may either be higher or lower in the vicinity of a preferred host plant species. To evaluate the importance of ecological factors, such as host plant proximity and density, in determining the direction and strength of such herbivore-mediated indirect effects, we quantified oviposition by the exotic weevil Rhinocyllus conicus on the native wavyleaf thistle Cirsium undulatum in midgrass prairie on loam soils in the upper Great Plains, USA. Over three years (2001-2003), the number of eggs laid by R. conicus on C. undulatum always decreased significantly with distance (0-220 m) from a musk thistle (Carduus nutans L.) patch. Neither the level of R. conicus oviposition on C. undulatum nor the strength of the distance effect was predicted by local musk thistle patch density or by local C. undulatum density ([less than or equal to]5 m). The results suggest that high R. conicus egg loads on C. undulatum near musk thistle resulted from the native thistle's co-occurrence with the coevolved preferred exotic host plant and not from the weevil's response to local host plant density. Mean egg loads on C. undulatum also were greater at sites with higher R. conicus densities. We conclude that both preferred-plant proximity and shared herbivore density strongly affected the herbivore-mediated indirect interaction, suggesting that such interactions ate important pathways by which invasive exotic weeds can indirectly impact native plants. Key words: apparent competition; associational defense; biological control, Carduus nutans L.; Cirsium undulatum Spreng.; insect herbivory; invasive plants; non-target effects; Rhinocyllus conicus; thistle; weeds; weevil.

The invasion paradox describes the co-occurrence of independent lines of support for both a negative and a positive relationship between native biodiversity and the invasions of exotic species. The paradox leaves the implications of native--exotic species richness relationships open to debate: Are rich native communities more or less susceptible to invasion by exotic species? We reviewed the considerable observational, experimental, and theoretical evidence describing the paradox and sought generalizations concerning where and why the paradox occurs, its implications for community ecology and assembly processes, and its relevance for restoration, management, and policy associated with species invasions. The crux of the paradox concerns positive associations between native and exotic species richness at broad spatial scales, and negative associations at fine scales, especially in experiments in which diversity was directly manipulated. We identified eight processes that can generate either negative or positive native--exotic richness relationships, but none can generate both. As all eight processes have been shown to be important in some systems, a simple general theory of the paradox, and thus of the relationship between diversity and invasibility, is probably unrealistic. Nonetheless, we outline several key issues that help resolve the paradox, discuss the difficult juxtaposition of experimental and observational data (which often ask subtly different questions), and identify important themes for additional study. We conclude that natively rich ecosystems are likely to be hotspots for exotic species, but that reduction of local species richness can further accelerate the invasion of these and other vulnerable habitats. Key words: biodiversity; exotic; invasibility; native; nonindigenous; scale; species diversity.

Although invasive alien species (IAS) are a major threat to biodiversity, human health, and economy, our understanding of the factors controlling their distribution and abundance is limited. Here, we determine how environmental factors, land use, life-history traits of the invaders, residence time, origin, and human usage interact to shape the spatial pattern of invasive alien plant species in South Africa. Relationships between the environmental factors and the extrinsic and intrinsic attributes of species were investigated using RLQ analysis, a multivariate method for relating a species-attribute table to an environmental table by way of a species presence/absence table. We then clustered species according to their position on the RLQ axes, and tested these groups for phylogenetic independence. The first three axes of the RLQ explained 99% of the variation and were strongly related to the species attributes. The clustering showed that, after accounting for environmental factors, the spatial pattern of IAS in South Africa was driven by human uses, life forms, and reproductive traits. The seven clusters of species strongly reflected geographical distribution, but also intrinsic species attributes and patterns of human use. Two of the clusters, centered on the genera Acacia and Opuntia, were phylogenetically non-independent. The remaining clusters comprised species of diverse taxonomic affinities, but sharing traits facilitating invasion in particular habitats. This information is useful for assessing the extent to which the potential spread of recent introductions can be predicted by considering the interaction of their biological attributes, region of origin, and human use. Key words: biological invasions; co-inertia analysis; environment; exotic species; human uses; invasive alien plants; life-history traits; plant invasions; RLQ analysis; South Africa.

It is hard to defend the view that biotic communities represent a simple and predictable response to the abiotic environment. Biota and the abiotic environment interact, and the environment of an individual certainly includes its neighbors and visitors in the community. The complexity of community assembly calls forth a quest for general principles, yet current results and theories on assembly rules differ widely. Using a grassland microcosm as a model system, we manipulated fertility, disturbance by defoliation, soil/microclimate, and arrival order of species belonging to two groups differing in functional attributes. We analyzed the outcome of community assembly dynamics in terms of species richness, invasibility, and species composition. The analyses revealed strong environmental control over species richness and invasibility. Species composition was mainly determined by the arrival order of species, indicating that historical contingency may change the outcome of community assembly. The probability for multiple equilibria appeared to increase with productivity and environmental stability. The importance of arrival order offers an explanation of the difficulties in predicting local occurrences of species in the field. In our experiment, variation in fertility and disturbance was controlling colonization with predictable effects on emergent community properties such as species richness. The key mechanism is suggested to be asymmetric competition, and our results show that this mechanism is relatively insensitive to the species through which it works. While our analyses indicate a positive and significant correlation between richness and invasibility, the significance disappears after accounting for the effect of the environment. The importance of arrival order (historical contingency) and environmental control supports the assumption of the unified neutral theory that different species within a trophic level can be considered functionally equivalent when it comes to community assembly. However, our results indicate that variation in asymmetric competition is the key factor determining the richness of the resulting communities, and this is far from neutral. Key words: diversity: invasibility; multiple equilibria; niche differentiation; neutral theory; richness; species pool; vegetation.

The introduction of chronic, infectious diseases by colonizing populations (invasive or reintroduced) is a serious hazard in conservation biology, threatening the original host and other spillover species. Most research on spatial invasion of diseases has pertained to established host populations, either at steady state or fluctuating through time. Within a colonizing population, however, the spread of disease may be influenced by the expansion process of the population itself. Here we explore the simultaneous expansion of a colonizing population and a chronic, nonlethal disease introduced with it, describing basic patterns in homogeneous and structured landscapes and discussing implications for disease management. We describe expected outcomes of such introductions for three qualitatively distinct cases, depending on the relative velocities at which the population and epidemic expand. (1) If transmissibility is low the disease cannot be sustained, although it may first expand its range somewhat around the point of introduction. (2) If transmissibility is moderate but the wave-front velocity for the population, [v.sub.p], is higher than that for the disease, [v.sub.d], the disease wave front lags behind that of the population. (3) A highly transmissible disease, with [v.sub.d] > [v.sub.p], will invade sufficiently rapidly to track the spread of the host. To test these elementary theoretical predictions, we simulated disease outbreaks in a spatially structured host population occupying a real landscape. We used a spatially explicit, individual-based model of Persian fallow deer (Dama mesopotamica) reintroduced in northern Israel, considering a hypothetical introduction of bovine tuberculosis. Basic patterns of disease expansion in this realistic setting were similar to our conceptual predictions for homogeneous landscapes. Landscape heterogeneity, however, induced the establishment of population activity centers and disease foci within them, leading to jagged wave fronts and causing local variation in the relative velocities at which the population and epidemic expanded. Based on predictions from simple theory and simulations of managed outbreaks, we suggest that the relative velocities at which the population and epidemic expand have important implications for the impact of different management strategies. Recognizing which of our three general cases best describes a particular outbreak will aid in planning an efficient strategy to contain the disease. Key words: bovine tuberculosis; chronic disease; colonizing populations; Dama mesopotamica; infectious disease; invasive species; Israel; managed disease outbreak; reintroduction; Persian fallow deer; population expansion; spatially explicit model.

Human activity is central in promoting invasion of arthropod generalist predators (AGP) and they have widespread and varied ecological impacts, many of which stem from displacement of similar native predators. Invasive AGP weakens or strengthens top-down control and they contribute to either invasional meltdown or to prevention of further damage from other invaders.

Hypothesized factors determining alien dominance were tested with structural equation modeling. Fire history is an important indirect factor as it affects both prefire stand structure and postfire alien seed banks.

Condition-specific competition, wherein competitive superiority varies with the abiotic environment, can determine species' distributions in a spatially heterogeneous environment. We investigated this phenomenon with two competing container-dwelling mosquitoes. We tested the hypothesis that habitat drying alters the outcome of interspecific competition, predicting that the competitive impact of Aedes albopictus on Aedes aegypti would be severe in wetter environments, but greatly reduced in drier environments. We tested these predictions in a laboratory experiment within cages, with aquatic larvae residing in water-filled cups within the cage, and adults emerging within the cage and ovipositing on the cups. We raised each species alone or with the competitor. Environmental treatments were fluctuating (water in cups evaporated to 50% of the original volume and then cups were refilled), and drying (water in cups evaporated completely and cups remained dry for two weeks before refilling). There was a significant interaction between treatment and species combination for adult populations of both species. Interspecific competition was highly asymmetrical. In the wetter fluctuating environment, interspecific competition had a large negative effect on A. aegypti, but in the drying environment, interspecific competition had a large negative effect on A. albopictus, and relatively little impact on A. aegypti. The main cause of the shift in competitive advantage appeared to be a greater increase in egg mortality for A. albopictus under dry conditions, compared to A. aegypti. Thus, mortality impinging on noncompeting life cycle stages can alter the population level impact of interspecific competition. The hypothesis that dry conditions shift competitive advantage away from A. albopictus is supported in this experiment. Key words: abiotic factors; Aedes aegypti; Aedes albopictus; environmental tolerances; invasive species.

Despite recent advances in the study of invasive, nonnative species, we still understand very little about the variability of their population dynamics over space and time, and the processes that contribute to their demographic success. The goal of this study was to quantify the population dynamics of the invasive, subtropical shrub, Ardisia elliptica (Myrsinaceae), growing in a disturbed area of Everglades National Park, Florida, USA. We used bootstrap sampling and randomization tests to test hypotheses about spatial and temporal variation in population dynamics. Five populations spanning a range of habitats were studied for three years. Significant spatial and temporal variation in fruit production, relative growth, fates of vegetative individuals, and population growth rates were detected; however, spatial variation among populations had an overall stronger effect than temporal variation. Population growth rates were greater than 1 or not different from 1, implying that populations are on average increasing in size. Similar to most other shrubs, prospective analysis showed that stasis of large stage classes had the highest elasticities. However, a retrospective analysis indicated that it was variation in growth to larger stage classes that contributed the most to observed variation in population growth rates. Simulations suggest that historically higher population growth rates of A. elliptica were necessary to produce current population sizes in Everglades National Park. Key words: Ardisia elliptica; biological invasions; elasticity structure; invasive nonnative species; life table response experiment; matrix models; population dynamics; randomization tests.

Invasion biologists typically regard susceptibility of an invasive species to native predators as a fortuitous condition that increases biotic resistance to the invasion. The line of reasoning is that predation weakens the net impact of the invader and reduces its ability to displace native competitors. However, predation on invasives is a coupled interaction; every invader consumed also enhances the predator population. If these predators also consume native species, then the invader's indirect effect via predators (i.e., apparent competition) could be more harmful to natives than the effect of resource competition from the exotic. We apply general community ecology theory to determine the conditions under which the net effect of predation on the exotic species is to extirpate the native competitor. An approximation to these conditions provides a simple metric to estimate the threat of native species extinction due to apparent competition posed by an invader. Key words: apparent competition; biocontrol agent; biological control; biological invasions; biological resistance; competitive exclusion; exotic species; exploitative competition; nonindigenous species.

The relative importance of plasticity vs. adaptation for the spread of invasive species has rarely been studied. We examined this question in a clonal population of invasive freshwater snails (Potamopyrgus antipodarum) from the western United States by testing whether observed plasticity in life history traits conferred higher fitness across a range of temperatures. We raised isofemale lines from three populations from different climate regimes (high- and low-elevation rivers and an estuary) in a split-brood, common-garden design in three temperatures. We measured life history and growth traits and calculated population growth rate (as a measure of fitness) using an age-structured projection matrix model. We found a strong effect of temperature on all traits, but no evidence for divergence in the average level of traits among populations. Levels of genetic variation and significant reaction norm divergence for life history traits suggested some role for adaptation. Plasticity varied among traits and was lowest for size and reproductive traits compared to age-related traits and fitness. Plasticity in fitness was intermediate, suggesting that invasive populations are not general-purpose genotypes with respect to the range of temperatures studied. Thus, by considering plasticity in fitness and its component traits, we have shown that trait plasticity alone does not yield the same fitness across a relevant set of temperature conditions. Key words: adaptation; clonal reproduction; general-purpose genotypes; invasiveness; phenotypic plasticity; Potamopyrgus antipodarum; reaction norm.

The invasion of natural and anthropogenic habitats by alien plants is a global problem. To understand the factors that regulate the success of alien plant species we sampled alien plant biomass and species composition in 66 sites representing a wide variety of habitat types within Bruce Peninsula National Park, Ontario, Canada. We quantified the response of existing alien biomass to a suite of habitat variables including habitat productivity, standing biomass, species richness, soil nutrients and disturbance factors. Standing biomass in the sites varied from 0 to nearly 60 000 g/[m.sup.2] and annual productivity ranged from 0 to nearly 800 g*[m.sup.-2]-[yr.sup.-1]. The response of both absolute and relative alien biomass (dominance) to the independent variables was assessed using univariate plots and multivariate canonical correspondence analysis. The results show that total standing alien biomass and site disturbance history can predict alien dominance, but other factors including primary productivity, nutrient levels, and site species richness cannot. In the multivariate analysis, species composition of both aliens and natives was best explained by nutrient levels, disturbance type, biomass, and, to a lesser extent, productivity. The study illustrates the value of examining both absolute abundance and dominance of aliens across a broad array of habitat types, and in the context of a large suite of possible driving functions. Key words: alien plants; Bruce Peninsula National Park, Ontario, Canada; dominance; invasion biology; invasive species, success determinants; productivity; standing biomass.

Invasive species have the potential to alter trade-offs leading to selection in the populations they invade. Here we quantify the demographic and selective effects of herbivory by native insects and the introduced floral feeder Rhinocyllus conicus on Platte thistle (Cirsium canescens), a sparse monocarpic thistle endemic to North America. Rhinocyllus first invaded the Platte thistle population in 1993. Since then, its numbers have increased exponentially, while the Platte thistle population size has decreased. Data from 11 years were analyzed to determine how demographic rates varied with plant size and damage by native insects and Rhinocyllus. Individual growth, survival, flowering probability, and seed set were all size dependent. Damage to vegetative structures did not influence demographic rates; damage to flower heads did because Platte thistle is seed limited. These analyses were used to parameterize a series of integral projection models (IPMs) that investigated the effects of floral herbivory on the population growth rate [lambda], equilibrium population size, and the evolutionary stable (ES) flowering strategy. The IPMs showed that native insects have significant impact on the equilibrium population size and X, but not the ES flowering strategy, because they use the flowers of different-sized plants indiscriminately. In contrast, Rhinocyllus has the potential to drive Platte thistle extinct. Rhinocyllus preferentially fed and oviposited on the flowers of larger plants and therefore selected for a reduction in flowering size. However, as the thistle population went into decline, this pattern reversed. Thus, selection imposed by an invader may be complex and will reflect behavioral interactions between herbivore and host, as well as demographic changes in the host population. Key words: biological control; Cirsium canescens; evolutionary stable strategy; insect herbivore; integral projection model; invasive species; life history evolution; monocarpic perennial; Platte thistle; Rhinocyllus conicus; selection.

Humans are altering the composition of biological communities through a variety of activities that increase rates of species invasions and species extinctions, at all scales, from local to global. These changes in components of the Earth's biodiversity cause concern for ethical and aesthetic reasons, but they also have a strong potential to alter ecosystem properties and the goods and services they provide to humanity. Ecological experiments, observations, and theoretical developments show that ecosystem properties depend greatly on biodiversity in terms of the functional characteristics of organisms present in the ecosystem and the distribution and abundance of those organisms over space and time. Species effects act in concert with the effects of climate, resource availability, and disturbance regimes in influencing ecosystem properties. Human activities can modify all of the above factors; here we focus on modification of these biotic controls. The scientific community has come to a broad consensus on many aspects of the relationship between biodiversity and ecosystem functioning, including many points relevant to management of ecosystems. Further progress will require integration of knowledge about biotic and abiotic controls on ecosystem properties, how ecological communities are structured, and the forces driving species extinctions and invasions. To strengthen links to policy and management, we also need to integrate our ecological knowledge with understanding of the social and economic constraints of potential management practices. Understanding this complexity, while taking strong steps to minimize current losses of species, is necessary for responsible management of Earth's ecosystems and the diverse biota they contain. Based on our review of the scientific literature, we are certain of the following conclusions: 1) Species' functional characteristics strongly influence ecosystem properties. Functional characteristics operate in a variety of contexts, including effects of dominant species, keystone species, ecological engineers, and interactions among species (e.g., competition, facilitation, mutualism, disease, and predation). Relative abundance alone is not always a good predictor of the ecosystem-level importance of a species, as even relatively rare species (e.g., a keystone predator) can strongly influence pathways of energy and material flows. 2) Alteration of biota in ecosystems via species invasions and extinctions caused by human activities has altered ecosystem goods and services in many well-documented cases. Many of these changes are difficult, expensive, or impossible to reverse or fix with technological solutions. 3) The effects of species loss of changes in composition, and the mechanisms by which the effects manifest themselves, can differ among ecosystem properties, ecosystem types, and pathways of potential community change. 4) Some ecosystem properties are initially insensitive to species loss because (a) ecosystems may have multiple species that carry out similar functional roles, (b) some species may contribute relatively little to ecosystem properties, or (c) properties may be primarily controlled by abiotic environmental conditions. 5) More species are needed to insure a stable supply of ecosystem goods and services as spatial and temporal variability increases, which typically occurs as longer time periods and larger areas are considered. We have high confidence in the following conclusions: 1) Certain combinations of species are complementary in their patterns of resource use and can increase average rates of productivity and nutrient retention. At the same time, environmental conditions can influence the importance of complementarity in structuring communities. Identification of which and how many species act in a complementary way in complex communities is just beginning. 2) Susceptibility to invasion by exotic species is strongly influenced by species composition and, under similar environmental conditions, generally decreases with increasing species richness. However, several other factors, such as propagule pressure, disturbance regime, and resource availability also strongly influence invasion success and often override effects of species richness in comparisons across different sites or ecosystems. 3) Having a range of species that respond differently to different environmental perturbations can stabilize ecosystem process rates in response to disturbances and variation in abiotic conditions. Using practices that maintain a diversity of organisms of different functional effect and functional response types will help preserve a range of management options. Uncertainties remain and further research is necessary in the following areas: 1) Further resolution of the relationships among taxonomic diversity, functional diversity, and community structure is important for identifying mechanisms of biodiversity effects. 2) Multiple trophic levels are common to ecosystems but have been understudied in biodiversity/ecosystem functioning research. The response of ecosystem properties to varying composition and diversity of consumer organisms is much more complex than responses seen in experiments that vary only the diversity of primary producers. 3) Theoretical work on stability has outpaced experimental work, especially field research. We need long-term experiments to be able to assess temporal stability, as well as experimental perturbations to assess response to and recovery from a variety of disturbances. Design and analysis of such experiments must account for several factors that covary with species diversity. 4) Because biodiversity both responds to and influences ecosystem properties, understanding the feedbacks involved is necessary to integrate results from experimental communities with patterns seen at broader scales. Likely patterns of extinction and invasion need to be linked to different drivers of global change, the forces that structure communities, and controls on ecosystem properties for the development of effective management and conservation strategies. 5) This paper focuses primarily on terrestrial systems, with some coverage of freshwater systems, because that is where most empirical and theoretical study has focused. While the fundamental principles described here should apply to marine systems, further study of that realm is necessary. Despite some uncertainties about the mechanisms and circumstances under which diversity influences ecosystem properties, incorporating diversity effects into policy and management is essential, especially in making decisions involving large temporal and spatial scales. Sacrificing those aspects of ecosystems that are difficult or impossible to reconstruct, such as diversity, simply because we are not yet certain about the extent and mechanisms by which they affect ecosystem properties, will restrict future management options even further. It is incumbent upon ecologists to communicate this need, and the values that can derive from such a perspective, to those charged with economic and policy decision-making. Key words: biodiversity; complementary resource use; ecosystem goods and services; ecosystem processes; ecosystem properties; functional characteristics; functional diversity; net primary production; sampling effect; species extinction; species invasions; species richness; stability.

Few invaded ecosystems are free from habitat loss and disturbance, leading to uncertainty whether dominant invasive species are driving community change or are passengers along for the environmental ride. The ''driver' model predicts that invaded communities are highly interactive, with subordinate native species being limited or excluded by competition from the exotic dominants. The 'passenger' model predicts that invaded communities are primarily structured by noninteractive factors (environmental change, dispersal limitation) that are less constraining on the exotics, which thus dominate. We tested these alternative hypotheses in an invaded, fragmented, and fire-suppressed oak savanna. We examined the impact of two invasive dominant perennial grasses on community structure using a reduction (mowing of aboveground biomass) and removal (weeding of above- and belowground biomass) experiment conducted at different seasons and soil depths. We examined the relative importance of competition vs. dispersal limitation with experimental seed additions. Competition by the dominants limits the abundance and reproduction of many native and exotic species based on their increased performance with removals and mowing. The treatments resulted in increased light availability and bare soil: soil moisture and N were unaffected. Although competition was limiting for some, 36 of 79 species did not respond to the treatments or declined in the absence of grass cover. Seed additions revealed that some subordinates are dispersal limited: competition alone was insufficient to explain their rarity even though it does exacerbate dispersal inefficiencies by lowering reproduction. While the net effects of the dominants were negative, their presence restricted woody plants, facilitated seedling survival with moderate disturbance (i.e., treatments applied in the fall), or was not the primary limiting factor for the occurrence of some species. Finally, the species most functionally distinct from the dominants (forbs, woody plants) responded most significantly to the treatments. This suggests that relative abundance is determined more by trade-offs relating to environmental conditions (long-term fire suppression) than to traits relating to resource capture (which should most impact functionally similar species). This points toward the passenger model as the underlying cause of exotic dominance, although their combined effects (suppressive and facilitative) on community structure are substantial. Key words: British Columbia, Canada, dominance; facilitation; functional redundancy; oak savanna; plant competition; plant invasion; recruitment limitation; removal experiment; repeated-measures ANOVA.

A widespread but controversial idea in ecology states that the number of invaders of a species assemblage depends on its species richness. Both negative and positive relationships have been reported. We examined whether a simple neutral model where assemblages are generated by drawing individuals from two pools of identical species (native and alien) can predict this relationship. We performed a meta-analysis of published data on this relationship. The neutral model showed that in communities with low and fixed numbers of individuals, the relationship between the number of aliens and native species is strong and negative. This becomes weaker as the proportion of species from both pools already present in the community increases. The relationship between alien and native species richness becomes positive when the number of individuals is allowed to vary, because the richness depends on number of individuals and/or area sampled. The meta-analysis showed that scale-dependence of the relationship between alien and native species richness is universal and compatible with the neutral model. Unless more evidence is available to refute it, the relationship between native and alien species richness should be considered a result of a neutral processes due to constraints on the number of individuals in the community. Key words: alien species; disturbance rate; meta-analysis; neutral process; scale; species--area relationship; species richness; transient dynamics.

Surveys of natural habitats often indicate that native and exotic species richness are positively correlated at large scales and negatively correlated at small scales. The small-scale relationship is often presented as evidence that native richness can repel invasion or conversely that exotic invasions can reduce native diversity. The larger scale pattern has been interpreted as evidence of the importance of facilitation, variable habitat quality, propagule supply, and other ecological phenomena. However, these explanations fail to consider expected native-exotic richness relationships under a null model assuming no species interactions. We show via simulation that the null expectation for a randomly assembled community is a negative relationship between native and exotic species richness at the smallest scales and, when plots vary in total richness, a positive relationship at larger scales. We outline a procedure to compare observational data to this null expectation using a permutation test of labels of species origin (native or exotic). Our use of this technique on plant community data indicates that patterns of native and exotic richness are remarkably similar to those generated by a null model. We argue that a null model approach is needed to evaluate whether observed native-exotic richness data deviate significantly from expected patterns generated by sampling and statistical artifacts. Key words: assembly rules; ecosystem functioning; invasion biology; invasion frequency; permutation test; plant diversity; riparian plant communities; scale dependence; strandline plant communities.

The damage caused by biological invasions has traditionally been thought to result from alien species taking advantage of ecological differences between the native and introduced ranges. In contrast, the role of evolutionary forces has received relatively little attention. Our results show that evolutionary change in Silene latifolia, a North American weed that was introduced from Europe about 200 years ago, can help explain the plant's successful North American invasion. By growing plants from seed collected in 40 populations from Europe and North America under common garden greenhouse and field conditions, we found significant genetic differences in life history, reproductive, and defensive characters. In general, morphological traits and competitive ability remained unchanged, while North American plants germinated earlier, grew faster, produced more flowers, had greater survival, and invested less into defensive traits (trichomes, fruit capsule) than their European conspecifics. We suggest that as S. latifolia escaped a suite of specialist enemies, natural selection favored individuals that invest more in growth and reproduction and less in defense. Key words: alien plant: biological invasion: common garden; EICA hypothesis; escape from enemies; Silene latifolia; trichome.

The demography of the fresh water fish species vendace during its development over a decade, from first occurrence to an established population, in the Pasvik watercourse is documented. The invasive ability of vendace appears to result from a combination of high fecundity and strong competitive ability in a disturbed environment.

The discovery record of introduced species reflects a combination of the introduction process and the discovery process. For this reason, the discovery record does not provide a direct proxy for the record of introductions. We describe a general method for estimating the rate of introductions from the discovery record. The method is based on a statistical model of the discovery record that includes both the introduction and discovery processes. The method is illustrated using the discovery record of introduced species in the San Francisco estuary (California, USA). The estimated mean rate of introductions increases from 0.3 introductions in 1850 to 2.3 introductions in 1995. Key words: maximum likelihood estimation; Poisson distribution; species introductions, estimating rate.

Understanding biological invasions requires information on the history of spatial spread, as well as measures of landscape and biotic features that control habitat invasibility. Because invasive species often spread quickly over large areas, attaining these two sets of information simultaneously is uncommon. We studied the spread of a fatal nonnative root pathogen, Phytophthora lateralis, across a heterogeneous landscape of its host, Port Orford cedar (Chamaecyparis lawsoniana). Within our 37-[km.sub.2] study area in southwestern Oregon and northwest California Port Orford cedar populations are generally restricted to riparian zones along creeks. The pathogen is spread between watersheds in two ways: (1) by spore-infested material being dislodged from vehicles, and (2) by animals or people moving infested mud (i.e., via foot traffic). Using dendrochronological techniques, we determined the date of infection for dead cedars and reconstructed spread history across our study area from 1977 to 1999. Twenty-six of the 36 (72%) separate infection events we identified were caused by dispersal via vehicles along roads, and the remainder by foot traffic. Survival analysis demonstrated that cedar populations in creeks crossed by roads were more likely to be infected than those creeks that were not crossed by roads. Also, a comparison of minimum dispersal distances showed infections that moved via road moved significantly farther than those vectored by foot traffic, and the distance infection traveled declined significantly through time. We also coupled our spread history with measures of landscape and host features, including abundance of potential host trees, the distance from the road surface to the nearest potential host, length of road in immediate contact with the riparian zone, catchment area (a measure of stream flow), elevation, slope, and solar radiation. Our results show that catchment area, host abundance, and proximity to the nearest tree are significantly and positively associated with infection risk. Our study demonstrates that increased connectivity between invasible sites created by the presence of roads can increase invasion success of a plant pathogen. We also document that successful pathogen invasion can be governed by both physical landscape features and attributes of host plant populations. Key words: Chamaecyparis lawsoniana; dendrochronology; disease ecology; forest pathogen; invasibility; invasion ecology; landscape heterogeneity; Phytophthora lateralis; plant pathogen; Port Orford cedar; spatial spread.

The potential benefits of two restoration scenarios on Greater Sage-Grouse in the interior Columbia Basin and adjacent portions of the Great Basin of the western United States, is evaluated. Expensive and sustained habitat restoration can maintain desired conditions and reduce future extirpation risk for sage grouse on FS-BLM lands in western North America, as per suggestion of the results. A formidable challenge to successful restoration and warrants substantial research and management attention is presented by the continued spread of exotic plants.

1 A long-term experiment was established in 1990 in which seeds of 54 native species, not originally present at the site, were sown into a fertility x disturbance matrix established in unproductive limestone grassland at the Buxton Climate Change Impacts Laboratory (BCCIL). The objective was to examine the roles of productivity and disturbance as major factors controlling the invasibility of plant communities, and to identify the functional characteristics of successful invaders in response to different types of invasion opportunity. The results of the first 2 years of the study have already been published. 2 After 2 years, invasion was strongly promoted by disturbance and less so by increased fertility. Three years later the cover of invaders had declined over most of the matrix, and the greatest cover of sown species was where the highest levels of fertility and disturbance coincided. However, no part of the fertility--disturbance matrix was immune to invasion and the area of the matrix occupied by each of the sown species that successfully established was unique. Abundance of invaders was reduced by low soil pH. 3 The identity and distribution of the successful invaders changed as the early stages of invasion gave way to a later stage of consolidation. After 2 years regenerative traits (seed mass and germination characteristics) were the best predictors of success. After 5 years these traits were unrelated to success of the invaders, the most successful invaders were perennial grasses, and no single trait was a good predictor of invasiveness. 4 Our results are consistent with the hypothesis that invasions are promoted by an increase in the availability of resources, either through addition of extra resources or a reduction in their use by the resident vegetation. Key-words: BCCIL, disturbance, fertility, functional types, herbivory, invasibility, limestone grassland, plant traits, seed size

Research indicates that the reed Phragmites australis uses clonal integration to expand from high marsh to lower, less favorable marshes, as opposed to using rhizome fragments or seeds. This finding has implications for management strategies of invasive plants.