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The keystone roles of mega-fauna in many terrestrial ecosystems have been lost to defaunation. Large predators and herbivores often play keystone roles in their native ranges, and some have established invasive populations in new biogeographic regions. However, few empirical examples are available to guide expectations about how mega-fauna affect ecosystems in novel environmental and evolutionary contexts. We examined the impacts on aquatic ecosystems of an emerging population of hippopotamus (Hippopotamus amphibus) that has been growing in Colombia over the last 25 yr. Hippos in Africa fertilize lakes and rivers by grazing on land and excreting wastes in the water. Stable isotopes indicate that terrestrial sources contribute more carbon in Colombian lakes containing hippo populations, and daily dissolved oxygen cycles suggest that their presence stimulates ecosystem metabolism. Phytoplankton communities were more dominated by cyanobacteria in lakes with hippos, and bacteria, zooplankton, and benthic invertebrate communities were similar regardless of hippo presence. Our results suggest that hippos recapitulate their role as ecosystem engineers in Colombia, importing terrestrial organic matter and nutrients with detectable impacts on ecosystem metabolism and community structure in the early stages of invasion. Ongoing range expansion may pose a threat to water resources.

Populations at risk of extinction due to climate change may be rescued by adaptive evolution or plasticity. Selective agents, such as introduced predators, may enhance or constrain plastic or adaptive responses to temperature. We tested responses of Daphnia to temperature by collecting populations from lakes across an elevational gradient in the presence and absence of fish predators (long-term selection). We subsequently grew these populations at two elevations in field mesocosms over two years (short-term selection), followed by a common-garden experiment at two temperatures in the lab to measure life-history traits. Both long-term and short-term selection affected traits, suggesting that genetic variation of plasticity within populations enabled individuals to rapidly evolve plasticity in response to high temperature. We found that short-term selection by high temperature increased plasticity for growth rate in all populations. Fecundity was higher in populations from fishless lakes and body size showed greater plasticity in populations from warm lakes (long-term selection). Neither body size nor fecundity were affected by short-term thermal selection. These results demonstrate that plasticity is an important component of the life-history response of Daphnia, and that genetic variation within populations enabled rapid evolution of plasticity in response to selection by temperature.

The strength of species interactions often varies geographically and locally with environmental conditions. Competitive interactions are predicted to be stronger in benign environments while facilitation is expected to be stronger in harsh ones. We tested these ideas with an aboveground neighbor removal experiment at six salt marshes along the California coast. We determined the effect of removals of either the dominant species, Salicornia pacifica, or the subordinate species on plant cover, aboveground biomass and community composition, as well as soil salinity and moisture. We found that S. pacifica consistently competed with the subordinate species and that the strength of competition varied among sites. In contrast with other studies showing that dominant species facilitate subordinates by moderating physical stress, here the subordinate species facilitated S. pacifica shortly after removal treatments were imposed, but the effect disappeared over time. Contrary to expectations based on patterns observed in east coast salt marshes, we did not see patterns in species interactions in relation to latitude, climate, or soil edaphic characteristics. Our results suggest that variation in interactions among salt marsh plants may be influenced by local‐scale site differences such as nutrients more than broad latitudinal gradients.

Interactions among microbes determine the prevalence of harmful algal blooms that threaten water quality. These interactions can be indirectly mediated by shared resources or consumers, or through interference by the production of allelochemicals. Allelopathic interactions and resource competition have been shown to occur among algae and associated microbes. However, little work has considered seasonal influences on ecosystem structure and function. Here, we report results of our investigations on seasonal changes in the interactions between benthic microbial assemblies and the bloom forming cyanobacterium Microcystis aeruginosa. We show that phosphorus (P) competition and allelopathy by the microbial assembly vary seasonally and inhibit growth of M. aeruginosa. The interactions per unit biomass of the microbial assembly are stronger under winter than summer conditions and inhibit the recruitment of the cyanobacteria, thereby preventing the reoccurrence of cyanobacterial blooms in the following summer. The seasonality of these interactions correlates with changes in composition, metabolic activity and functional diversity of the microbial assembly. Our findings highlight the importance of competitive and allelopathic interactions in regulating the occurrence of harmful algal blooms. Our results also imply that seasonal variation of competition and allelopathy of the microbial assembly might be beneficial to adjust aquatic ecosystem structure and function.

Trade‐offs among traits that shape different components of fitness determine the potential for species to coexist in communities and the regulation of ecosystem productivity by consumers vs. resources. We compared functional and structural traits of 14 species of freshwater phytoplankton to test the influence of life history (exponential growth rate, r, and asymptotic density, K), cell volume, cellular fatty acid (FA) biochemistry, and stoichiometry (cellular C:N and C:P ratios) on grazing and growth rate of Daphnia consumers. Grazers exerted the strongest top‐down effects on algae with the highest C:N and C:P, and lower levels of FA unsaturation. In contrast, the grazer's growth rate was most associated with phytoplankton species’ FA unsaturation and chain length. Different traits were related to grazing rate and trophic transfer efficiency, and as a result, the grazing rate on a particular phytoplankton taxon was unrelated to the grazers’ growth rate. The best nutrient competitors (those with high C:N, C:P, and K, which previous experiments showed had lower minimum requirements for nutrients) were most heavily affected by grazing, but did not contribute the most to grazer growth. Our results indicate that Daphnia increase their food consumption rate in order to compensate for low stoichiometric food quality. Strong competitors with low nutrient demands may therefore be subject to the most intense grazing by individual consumers but not support greater production of higher trophic levels.

The relationship between biodiversity and productivity has emerged as a central theme in ecology. Mechanistic explanations for this relationship suggest that the role organisms play in the ecosystem (i.e., niches or functional traits) is a better predictor of ecosystem stability and productivity than taxonomic richness. Here, we tested the capacity of functional diversity in nitrogen uptake in experimental microalgal communities to predict the complementarity effect (CE) and selection effect (SE) of biodiversity on productivity. We grew five algal species as monocultures and as polycultures in pairwise combinations in homogeneous (ammonium, nitrate, or urea alone) and heterogeneous nitrogen (mixed nitrogen) environments to determine whether complementarity between species may be enhanced in heterogeneous environments. We show that the positive diversity effects on productivity in heterogeneous environments resulted from complementarity effects with no positive contribution by species‐specific SEs. Positive biodiversity effects in homogeneous environments, when present (nitrate and urea treatments but not ammonium), were driven both by CE and SE. Our results suggest that functional diversity increases species complementarity and productivity mainly in heterogeneous resource environments. These results provide evidence that the positive effect of functional diversity on community productivity depends on the diversity of resources present in the environment.

© The Ecological Society of America. Global demand for transportation fuels will increase rapidly during the upcoming decades, and concerns about fossil-fuel consumption have stimulated research on renewable biofuels that can be sustainably produced from biological feedstocks. However, if unchecked, pathogens and parasites are likely to infect these cultivated biofuel feedstocks, greatly reducing crop yields and potentially threatening the sustainability of renewable bioenergy production efforts. In particular, clonal biofuel crops grown as monocultures at industrial scales will be confronted both by an accumulation of specialist pathogens over time, and by the rapid evolution of those pathogens. We propose possible solutions to these important sustainability problems, with a focus on managing disease risk through crop rotations and the cultivation of multi-species polycultures.

Sea otters are a classic example of a predator controlling ecosystem productivity through cascading effects on basal, habitat-forming kelp species. However, their indirect effects on other kelp-associated taxa like fishes are poorly understood. We examined the effects of sea otter (Enhydra lutris) reintroduction along the west coast of Vancouver Island, Canada on giant kelp (Macrocystis pyrifera) distributions and the trophic niches and growth of two common kelp forest fishes, black (Sebastes melanops) and copper (S. caurinus) rockfishes. We sampled 47 kelp forests, and found that red sea urchins (Strongylocentrotus franciscanus) were eliminated in the presence of otters, and that kelp forests were 3.7 times deeper and 18.8 times larger. Despite order-of-magnitude differences in kelp forest size, adult black and copper rockfishes contained less kelp-derived carbon in their tissues (as measured by stable isotopes of C and N) in regions with otters. Adults of both species had higher mean trophic positions in the presence of otters, indicating more frequent consumption of higher trophic level prey such as fishes. Smaller trophic niche space of rockfishes in the presence of otters indicated a higher degree of trophic specialization. Juvenile black rockfishes rapidly shifted to higher kelp-carbon contents, trophic positions, and body condition factors after settling in kelp forests. The relationships of growth to length, percentage of kelp carbon, and trophic position varied between the two regions, indicating that potential effects of kelp forest size on trophic ontogeny may also affect individual performance. Our results provide evidence that the indirect effects of otters on rockfishes arise largely through the creation of habitat for fishes and other prey rather than a direct trophic connection through invertebrates or other consumers of kelp productivity.

© 2015 by the Ecological Society of America. Understanding the evolutionary potential of organisms to adapt to a changing climate, and the fitness consequences of temperature fluctuations, are critical to forecasting the future of biodiversity. Geographic variation among populations in life history response to temperature mean and variability offers one view of the potential for local adaptation to broaden the thermal niche. We used laboratory growth experiments to examine the effects of temperatures between 138C and 308C on five life history traits and the intrinsic rate of increase for 15 Tigriopus californicus populations distributed over 178 of latitude. Different life history stages showed distinct latitudinal shifts in thermal response, while the temperature of peak population growth consistently declined with increasing latitude. In addition, high-latitude populations grew faster at optimal temperatures but showed steeper fitness declines at high temperature. To test geographic population variation in response to the amplitude of daily thermal fluctuations, we grew three northern and three southern populations and manipulated nightly low and daily high temperatures. We found the lowest fitness overall in the treatment with the highest mean temperature, and the treatment with the greatest variability showed high fitness despite an 88C greater daily range. Population responses to daily thermal variability were unrelated to latitude of origin. Our results indicate that trade-offs between adaptation to high vs. low temperature, and between growth and maturation vs. survival and fecundity, govern local adaptation along the latitudinal gradient. They also indicate that T. californicus populations can maintain fitness over a wide range of daily variability but are more sensitive to small changes in the mean temperature.

Changing temperature can substantially shift ecological communities by altering the strength and stability of trophic interactions. Because many ecological rates are constrained by temperature, new approaches are required to understand how simultaneous changes in multiple rates alter the relative performance of species and their trophic interactions. We develop an energetic approach to identify the relationship between biomass fluxes and standing biomass across trophic levels. Our approach links ecological rates and trophic dynamics to measure temperature-dependent changes to the strength of trophic interactions and determine how these changes alter food web stability. It accomplishes this by using biomass as a common energetic currency and isolating three temperature-dependent processes that are common to all consumer-resource interactions: biomass accumulation of the resource, resource consumption and consumer mortality. Using this framework, we clarify when and how temperature alters consumer to resource biomass ratios, equilibrium resilience, consumer variability, extinction risk and transient vs. equilibrium dynamics. Finally, we characterise key asymmetries in species responses to temperature that produce these distinct dynamic behaviours and identify when they are likely to emerge. Overall, our framework provides a mechanistic and more unified understanding of the temperature dependence of trophic dynamics in terms of ecological rates, biomass ratios and stability. © 2014 John Wiley & Sons Ltd/CNRS.

Understanding the factors that influence larval dispersal and connectivity among marine populations is critical to the conservation and sustainable management of marine resources. We assessed genetic subdivision among ten populations of copper rockfish (Sebastes caurinus) representing paired samples of outer coast and the heads of inlets in five replicate sounds on the west coast of Vancouver Island, British Columbia, using 17 microsatellite DNA loci. Overall, subdivision (F(ST)) was low (F(ST) = 0.031, P0.001), but consistently higher between paired coast and head of inlet sites (mean FST = 0.047, P0.001) compared to among the five coast sites (mean F(ST) = -0.001, P0.5) or among the five head of inlet sites (mean F(ST) = 0.026, P0.001). Heterozygosity, allelic richness and estimates of effective population size were also lower in head of inlet sites than in coast sites. Bayesian analysis identified two genetic groups across all samples, a single genetic group among only coast samples, two genetic groups among head of inlet samples and two genetic groups within each sound analysed separately. Head of inlet copper rockfish tended to be shorter with lower condition factors and grew more slowly than coast sites fish. Reduced physical connectivity and selection against immigrants in contrasting outer coast-head of inlet environments likely contribute to the evolution of population structure of copper rockfish. Based on genetic connectivity, coast sites appear to be better served by existing marine protected areas than are head of inlet sites.

Microalgae represent one of the most promising groups of candidate organisms for replacing fossil fuels with contemporary primary production as a renewable source of energy. Algae can produce many times more biomass per unit area than terrestrial crop plants, easing the competing demands for land with food crops and native ecosystems. However, several aspects of algal biology present unique challenges to the industrial-scale aquaculture of photosynthetic microorganisms. These include high susceptibility to invading aquatic consumers and weeds, as well as prodigious requirements for nutrients that may compete with the fertiliser demands of other crops. Most research on algal biofuel technologies approaches these problems from a cellular or genetic perspective, attempting either to engineer or select algal strains with particular traits. However, inherent functional trade-offs may limit the capacity of genetic selection or synthetic biology to simultaneously optimise multiple functional traits for biofuel productivity and resilience. We argue that a community engineering approach that manages microalgal diversity, species composition and environmental conditions may lead to more robust and productive biofuel ecosystems. We review evidence for trade-offs, challenges and opportunities in algal biofuel cultivation with a goal of guiding research towards intensifying bioenergy production using established principles of community and ecosystem ecology.

Conceptual models of adaptive radiation predict that competitive interactions among species will result in an early burst of speciation and trait evolution followed by a slowdown in diversification rates. Empirical studies often show early accumulation of lineages in phylogenetic trees, but usually fail to detect early bursts of phenotypic evolution. We use an evolutionary simulation model to assemble food webs through adaptive radiation, and examine patterns in the resulting phylogenetic trees and species’ traits (body size and trophic position). We find that when foraging trade-offs result in food webs where all species occupy integer trophic levels, lineage diversity and trait disparity are concentrated early in the tree, consistent with the early burst model. In contrast, in food webs in which many omnivorous species feed at multiple trophic levels, high levels of turnover of species’ identities and traits tend to eliminate the early burst signal. These results suggest testable predictions about how the niche structure of ecological communities may be reflected by macroevolutionary patterns.

The exchange of organisms and energy among ecosystems has major impacts on food web structure and dynamics, yet little is known about how climate warming combines with other pervasive anthropogenic perturbations to affect such exchanges. We used an outdoor freshwater mesocosm experiment to investigate the interactive effects of warming, eutrophication, and changes in top predators on the flux of biomass between aquatic and terrestrial ecosystems. We demonstrated that predatory fish decoupled aquatic and terrestrial ecosystems by reducing the emergence of aquatic organisms and suppressing the decomposition of terrestrial plant detritus. In contrast, warming and nutrients enhanced cross-ecosystem exchanges by increasing emergence and decomposition, and these effects were strongest in the absence of predators. Furthermore, we found that warming advanced while predators delayed the phenology of insect emergence. Our results demonstrate that anthropogenic perturbations may extend well beyond ecosystem boundaries by influencing cross-ecosystem subsidies. We find that these changes are sufficient to substantially impact recipient communities and potentially alter the carbon balance between aquatic and terrestrial ecosystems and the atmosphere.

1. Climate change and other human-driven environmental perturbations are causing reductions in biodiversity and impacting the functioning of ecosystems on a global scale. Metacommunity theory suggests that ecosystem connectivity may reduce the magnitude of these impacts if the regional species pool contains functionally redundant species that differ in their environmental tolerances. Dispersal may increase the resistance of local ecosystems to environmental stress by providing regional species with traits adapted to novel conditions. 2. We tested this theory by subjecting freshwater zooplankton communities in mesocosms that were either connected to or isolated from the larger regional species pool to a factorial manipulation of experimental warming and increased salinity. 3. Compensation by regional taxa depended on the source of stress. Warming tolerant regional taxa partially compensated for reductions in heat sensitive local taxa but similar compensation did not occur under increased salinity. 4. Dispersal-mediated species invasions dampened the effects of warming on summer net ecosystem productivity. However, this buffering effect did not occur in the fall or for periphyton growth, the only other ecosystem function affected by the stress treatments. 5. The results indicate that regional biodiversity can provide insurance in a dynamic environment but that the buffering capacity is limited to some ecosystem processes and sources of stress. Maintaining regional biodiversity and habitat connectivity may therefore provide some limited insurance for local ecosystems in changing environments, but is unable to impart resistance against all sources of environmental stress.

Ecology Letters (2011) 14: 852–862 Abstract Synergistic interactions between multiple limiting resources are common, highlighting the importance of co-limitation as a constraint on primary production. Our concept of resource limitation has shifted over the past two decades from an earlier paradigm of single-resource limitation towards concepts of co-limitation by multiple resources, which are predicted by various theories. Herein, we summarise multiple-resource limitation responses in plant communities using a dataset of 641 studies that applied factorial addition of nitrogen (N) and phosphorus (P) in freshwater, marine and terrestrial systems. We found that more than half of the studies displayed some type of synergistic response to N and P addition. We found support for strict definitions of co-limitation in 28% of the studies: i.e. community biomass responded to only combined N and P addition, or to both N and P when added separately. Our results highlight the importance of interactions between N and P in regulating primary producer community biomass and point to the need for future studies that address the multiple mechanisms that could lead to different types of co-limitation.

Species interactions come in a variety of forms, from weak to strong, and negative or positive, each with unique consequences for local community structure. However, interactions depend on several biotic, abiotic and scale-dependent variables that make their magnitude and direction difficult to predict. Here, we quantify the relative impacts of multiple factors on species interactions for a diverse array of intertidal organisms, using our own experiments across a range of environments in New Zealand and North America. Interaction strengths are related to organism body size and trophic level, but are relatively insensitive to environmental conditions associated with tidal height. Although species at higher trophic levels exert stronger per-capita effects on other taxa, their population-level impacts are equivalent to basal trophic groups. This indicates that interaction intensity is largely based on requirements for resources, such as food or space, that follow allometric scaling rules. These results demonstrate the potential to predict interactivity based on simple criteria without detailed information on particular species or communities.

If natural communities are assembled according to deterministic rules, coexisting species will represent a nonrandom subset of the potential species pool. We tested for signatures of assembly rules in the distribution of species' traits in Pacific rockfish (Sebastes spp.) assemblages. We used morphology, dietary niche (estimated with stable nitrogen isotopes), and distribution data to identify traits that relate to local-scale resource use (the alpha-niche) and to environmental gradients (the beta-niche). We showed that gill raker morphology was related to trophic position (an alpha-niche axis), while relative eye size was associated with depth habitat (a beta-niche axis). We therefore hypothesized that, within assemblages of coexisting rockfish species, the gill raker trait would be overdispersed (evenly spaced) due to limiting similarity, while relative eye size would be clustered due to environmental filtering. We examined the evolutionary relatedness of coexisting species to ask whether phylogenetic community structure and trait distributions gave similar indications about the roles of assembly processes. We tested the trait distributions and phylogenetic structure of 30 published rockfish assemblages against a null model of random community assembly. As predicted, the gill raker trait tended to be more evenly spaced than expected by chance, as did overall body size, while relative eye size was more clustered than expected. Phylogenetic community structure appeared to reflect historical dispersal and speciation and did not provide consistent support for assembly rules. Our results indicate that rockfish community assembly is nonrandom with regard to species' traits and show how distinguishing traits related to the alpha- and beta-niches and incorporating functional morphology can provide for powerful tests of assembly rules.

Plant-herbivore interactions mediate the trophic structure of ecosystems. We use a comprehensive data set extracted from the literature to test the relative explanatory power of two contrasting bodies of ecological theory, the metabolic theory of ecology (MTE) and ecological stoichiometry (ES), for per-capita and population-level rates of herbivory across ecosystems. We found that ambient temperature and herbivore body size (MTE) as well as stoichiometric mismatch (ES) both constrained herbivory, but at different scales of biological organization. Herbivore body size, which varied over 11 orders of magnitude, was the primary factor explaining variation in per-capita rates of herbivory. Stoichiometric mismatch explained more variation in population-level herbivory rates and also in per-capita rates when we examined data from within functionally similar trophic groups (e.g. zooplankton). Thus, predictions from metabolic and stoichiometric theories offer complementary explanations for patterns of herbivory that operate at different scales of biological organization.

The cycles of the key nutrient elements nitrogen (N) and phosphorus (P) have been massively altered by anthropogenic activities. Thus, it is essential to understand how photosynthetic production across diverse ecosystems is, or is not, limited by N and P. Via a large-scale meta-analysis of experimental enrichments, we show that P limitation is equally strong across these major habitats and that N and P limitation are equivalent within both terrestrial and freshwater systems. Furthermore, simultaneous N and P enrichment produces strongly positive synergistic responses in all three environments. Thus, contrary to some prevailing paradigms, freshwater, marine and terrestrial ecosystems are surprisingly similar in terms of N and P limitation.

Recent meta-analyses confirm that the strength of trophic cascades (indirect positive effects of predators on plant biomass through control of herbivores) varies among ecosystem types. In particula ...

Most experimental studies of diversity effects on invasibility have reported negative relationships while observational studies have often found positive correlations between the numbers of exotic and native taxa. Nearly all of these studies have been done with terrestrial plants or aquatic invertebrates. We investigated effects of native macroalgal diversity on invasion success of the introduced macroalga Sargassum muticum (Yendo) Fensholt (Phaeophyceae: Fucales) on the west coast of Vancouver Island. We conducted both observational field surveys of the correlation between native diversity and exotic cover, and experimental manipulations of native diversity in constructed 25 � 25 cm communities. Field surveys found higher cover of S. muticum in plots with low native diversity, suggesting a negative relationship between diversity and invasibility at the neighbourhood scale. The experiment found initial cover of S. muticum germlings was highest in plots with greater diversity. Over the duration of the experiment cover of settled germlings increased fastest in the low diversity plots, so that there was a weak negative effect of diversity on final cover of the invader after 77 days. The slope of the relationship reversed over time, with field patterns and experimental results converging at the end of the experiment. Our results suggest native diversity has contrasting effects on different stages of invasion. Diversity facilitates invader recruitment of S. muticum but decreases growth and or survivorship.

Humans are rapidly altering the diversity and composition of ecological communities by accelerating rates of species extinctions and introductions. These changes in diversity are not random and disproportionately involve the addition or extinction of predators. Theoretical and microcosm studies suggest predator removal may either increase or decrease ecosystem stability. Here we test whether the addition or removal of predators affects aggregate biomass stability in 40 experiments carried out in six different ecosystems. Predators did not alter the temporal variability of autotroph biomass, but significantly destabilized herbivore biomass. The effects of predators on herbivore biomass stability varied significantly among ecosystems, with benthic and pelagic lake systems showing the greatest shifts. Consequently, the addition of predators to communities, as occurs in many conservation efforts, biological control programmes and species introductions, may lead to more variable system dynamics.

The metacommunity concept is an important way to think about linkages between different spatial scales in ecology. Here we review current understanding about this concept. We first investigate issues related to its definition as a set of local communities that are linked by dispersal of multiple potentially interacting species. We then identify four paradigms for metacommunities: the patch-dynamic view, the species-sorting view, the mass effects view and the neutral view, that each emphasizes different processes of potential importance in metacommunities. These have somewhat distinct intellectual histories and we discuss elements related to their potential future synthesis. We then use this framework to discuss why the concept is useful in modifying existing ecological thinking and illustrate this with a number of both theoretical and empirical examples. As ecologists strive to understand increasingly complex mechanisms and strive to work across multiple scales of spatio-temporal organization, concepts like the metacommunity can provide important insights that frequently contrast with those that would be obtained with more conventional approaches based on local communities alone.

The distribution of organisms can be regulated by local environmental factors and regional processes such as dispersal. Here, we review recent work on the role of dispersal for generating population and community structure in freshwater zooplankton. We examine evidence for different mechanisms of dispersal among lakes, for the effects of dispersal limitations on populations and communities, and for the effects of spatial scale on dispersal rates. Zooplankton move via human or animal vectors, flowing surface waters, and wind; the relative importance of the different modes of transport is poorly understood. Several lines of evidence suggest that dispersal among lakes separated over short spatial scales (,10 km) is sufficiently rapid that local interactions should limit species diversity and composition more than the supply of colonists. However, dispersal limitation over broad scales (tens to thousands of kilometers) might constrain geographic ranges and influence community structure. The current explosion in the incidence of exotic species indicates that such global- or continental-scale dispersal was limiting in the past. The spread of exotic species also provides opportunities to study the scale dependence of zooplankton dispersal. We show how patterns of range expansion can be used to estimate the change in invasion likelihood with distance to a source population. Such dispersal functions provide a crucial link between small-scale experimental studies and broad-scale geographic patterns. Theories of geographic structure in communities and populations come from two broad schools of thought. The regional approach examines the importance of movement of individuals and the colonization of isolated patches of habitat separated over broad spatial scales. Parallel theories have been developed for regional control of both genetic variation in populations (Slatkin 1985) and species diversity in communities (MacArthur and Wilson 1963). The regional approach often emphasizes neutral or drift processes such as colonization and extinction and ignores differences among species or environments (Hubbell 2001). By contrast, the local approach examines the response of species to conditions that influence population growth rates within relatively small, homogeneous patches of habitat (MacArthur 1972). Local-scale ecology focuses on differences among organisms or habitats that allow coexistence of multiple species or genotypes. The local and regional views have both enjoyed 1

Communities of organisms form as a result of both interspecific and abiotic interactions within local habitat patches and dispersal among patches in a region. Local processes are expected to play a dominant role when dispersal occurs much more often than extinction. We performed two field experiments to examine rates and mechanisms of dispersal in freshwater pond zooplankton communities. First, we tested the effect of distance from a source on the rate of colonization of artificial habitat by placing wading pools at 5, 10, 30 and 60 m from two natural fishless ponds and observing the succession of zooplankton. Seventy-eight percent of the species in the source ponds that were capable of living in the pools colonized at least once during the experiment. A new species was found in the pools on average once every four days, suggesting that colonization events occur on the order of days to weeks for many species. Colonization rates declined further from the source at one pond but not the other, and the effect of distance was relatively weak at both ponds. This suggests that many species disperse broadly over short distances. The second experiment tested the role of animal vectors for zooplankton dispersal by restricting access to the pools. Eight treatments were imposed that excluded potential animal vectors along a body size gradient from large mammals to small insects. While the treatments affected zooplankton colonization, many species invaded even when all animals larger than 1 mm were excluded. Animal vectors may therefore be less important for dispersal than wind. Our results suggest that zooplankton are highly effective dispersers over short distances, and can disperse via several mechanisms. Local interactions should therefore play a dominant role in structuring these communities at small regional scales.

A new analysis of the nearly century-old Lotka-Volterra theory allows us to link species interactions to biodiversity patterns, including: species abundance distributions, estimates of total community size, patterns of community invasibility, and predicted responses to disturbance. Based on a few restrictive assumptions about species interactions, our calculations require only that the community is sufficiently large to allow a mean-field approximation. We develop this analysis to show how an initial assemblage of species with varying interaction strengths is predicted to sort out into the final community based on the species' predicted target densities. The sorting process yields predictions of covarying patterns of species abundance, community size, and species interaction strengths. These predictions can be tested using enrichment experiments, examination of latitudinal and productivity gradients, and features of community assembly. (Less)

The spread of exotic species can be limited by dispersal or by constraints imposed by the local environment. Using data collected from 152 Missouri (USA) lakes over seven years, we asked whether models based on dispersal or local-scale processes best predicted invasion by the exotic cladoceran Daphnia lumholtzi. We used multiple logistic regression to test the relative importance of 10 local physicochemical features and proximity to all known potential source populations for predicting which lakes were invaded. The decline in invasion likelihood with distance to source populations was used to estimate the shape of the dispersal kernel. Between 1992 and 1998 the cumulative prevalence of D. lumholtzi increased from 6% to 34% of lakes sampled, with frequent appearances of pop- ulations in new watersheds. Spatial position and physical factors were both important for predicting the new colonization events. The probability of colonization increased with lake surface area and epilimnetic temperature, declined with increasing conductivity, and was unaffected by variation in lake fertility. Invasion likelihood declined sharply as a nonlinear function of distance to source populations up to around 30 km, and was relatively constant at greater distances. The results suggest that dispersal and local abiotic constraints jointly limit the spread of D. lumholtzi. This approach illustrates how range expansion can be used to estimate dispersal rates at broad spatial scales.

Although trophic cascades (indirect effects of predators on plants via herbivores) occur in a wide variety of food webs, the magnitudes of their effects are often quite variable. We compared the responses of herbivore and plant communities to predator manipulations in 102 field experiments in six different ecosystems: lentic ( lake and pond), marine, and stream benthos, lentic and marine plankton, and terrestrial (grasslands and agricultural fields). Predator effects varied considerably among systems and were strongest in lentic and marine benthos and weakest in marine plankton and terrestrial food webs. Predator effects on herbivores were generally larger and more variable than on plants, suggesting that cascades often become attenuated at the plant‐ herbivore interface. Top-down control of plant biomass was stronger in water than on land; however, the differences among the five aquatic food webs were as great as those between wet and dry systems.

The topological analysis of food webs is a recently reinvigorated and rapidly growing area of inquiry in community ecology (Williams and Martinez 2000, Sole and Montoya 2001, Camacho et al. 2002, Montoya and Sole 2002). Topological studies assess system properties using the number and distribution of connections among nodes in an interconnected network. Analysis of a food web as a network of links (feeding relationships) and nodes (species) is not a new approach (MacArthur 1955, Gardner and Ashby 1970, Pimm 1979); however recent advances by physicists in the study of complex networks have revived this area of theoretical community ecology. Here we discuss a few characteristics of food webs that may cause them to respond differently to node loss than other types of networks. We also suggest ways in which empiricists can provide data to test predictions derived from complex network theory. Complex network analysis has shown that the sensitivity of a network to node loss depends on the frequency distribution of connections among nodes (Albert et al. 2000). Current theoretical developments suggest that networks can be classified into two broad categories based on the frequency distribution of links: exponential or scale-free. Each node in an exponential network has a similar number of links to other nodes. The frequency distribution of the number of links per node in this type of network has an exponential decay (Albert et al. 2000). Because nodes in an exponential network have similar numbers of links, the loss of any given node from this type of network causes a monotonic increase in the number of links required to connect any two nodes in the network. In contrast, a scale-free network has a few nodes with a large number of links and many nodes with only a few links. Although connectivity in scale-free networks does not decrease with the random loss of nodes as in an exponential network, scale-free networks are extremely sensitive to the loss of highly connected "hub" nodes (Albert et al. 2000). In recent work, researchers have used frequency distribution models to describe the properties of a broad array of complex networks such as social networks (Watts and Strogatz 1998), the World Wide Web (Albert et al. 1999), transportation networks (Banavar et al. 1999), and enzymatic pathways (Jeong et al. 2000). The obvious extension of this theory to food web dynamics has renewed interest in the topology of trophic webs. Williams and Martinez (2000) show that a simple topological model can reproduce properties of complex food webs. But when we examine empirical webs, does the frequency distribution of trophic connections show a general pattern? If so, is the structure of food webs generally exponential or scale-free? A recent assessment of several food webs suggests that they share the scalefree properties of many other complex networks, implying that their structure should be resistant to random attacks but quite sensitive to loss of "hub" species (Sole and Montoya 2001). The general applicability of recent findings in complex network theory to food web studies depends on whether topological descriptions capture key aspects of communities and ecosystems such as the flow of energy or factors regulating populations. An increase in biological realism may be merged with suitable data in future food web analyses to better understand processes structuring communities. Further work in this area will allow us to determine the appropriate role of complex network theory in the modeling and conservation of ecological communities.

Communities of coexisting organisms are formed by both dispersal among habitat patches in a region and local interactions within patches. The supply of colonists from the regional pool has the potential to alter the outcome of local interactions by providing species that fill different ecological roles. I examined the consequences of invasion by fish (juvenile Lepomis macrochirus) and insect (Notonecta undulata) predators on ex- perimental plankton communities that were either connected to or isolated from the regional pool of zooplankton species. The effects of predators on zooplankton composition and diversity depended on dispersal by members of the regional zooplankton species pool. Fish and notonectids reduced the abundance of large zooplankton and thereby facilitated invasion by new species from the surrounding region. Both predators reduced zooplankton species richness in the absence of dispersal, while fish had positive effects and notonectids had no effects with rapid dispersal. Fish facilitated invasion by more species than they excluded, while similar numbers of species invaded with notonectids as were driven extinct. The effects of predators on planktonic trophic structure (zooplankton size structure and biomass, phytoplankton biomass) were independent of the level of dispersal. Both predators reduced zooplankton mean body size, but only fish enhanced phytoplankton density. Invasion by members of the regional species pool slightly increased zooplankton mean body size and reduced phytoplankton biomass; however, the effects were small and independent of the predator treatment. Predator-facilitated invasion of new species altered the effects of pred- ators on zooplankton species composition and diversity, but not on plankton trophic struc- ture. The results suggest that spatial heterogeneity in predator abundance promotes regional coexistence among zooplankton. Connection to a diverse regional species pool via dispersal may therefore be important for determining the impact of local interactions such as predation on communities.

Assemblages of coexisting species are formed by immigration from a re- gional pool of colonists and local interactions among species and with the physical envi- ronment. Theory suggests that the shape of the relationship between regional and local species richness may indicate the relative roles of dispersal and local interactions in limiting local diversity. Here we examine patterns of regional and local species richness in freshwater crustacean zooplankton to test whether linear (suggesting dispersal limitation) or curvilinear (suggesting saturation, via strong local control) functions best fit the data. Local richness appeared saturated when regions of different spatial extents were included on the same graph. However, this pattern was influenced by differences in scale among surveys. We corrected for the effects of regional scale by plotting mean local richness against the residuals of the species-landscape area relations. Controlling for the extent of the regional scale produced much more linear patterns, suggesting strong dispersal limitation. We present a simple graphical model to explain how variation among surveys in the geographic size of regions can produce apparent saturation of local diversity even if the underlying pattern of local and regional richness is linear. We also compare the predictive power of residual regional richness on local richness with that of several local features in a multiple regression model. Local richness exhibits strong relationships with both residual regional richness and pH. We argue that the relative strengths of local and regional processes depend on the definition of the regional scale. A variety of evidence suggests that local processes play a major role in generating differences in zooplankton diversity among lakes within a bio- geographic region. Evidence for the importance of dispersal limitation comes largely from comparisons of lakes across very large scales. Our analysis suggests that linear patterns of local and regional diversity are not incompatible with strong local interactions.

In many cladoceran zooplankton, females divide their reproductive effort among females, males, and resting eggs depending on a suite of environmental conditions. Having alternative life-history strategies allows cladocerans such as Daphnia to achieve high reproductive rates asexually when conditions are favorable and to produce offspring sexually for survival when the environment becomes unsuitable. Because of their rapid reproduction, Daphnia and other cladocerans are useful model organisms in aquatic toxicology. Bioassays using Daphnia serve two functions, to estimate the ecosystem level risk posed by specific chemicals and to quickly detect contaminated waters. Bioassays examine effects in terms of life history features such as growth, survival, and asexual fecundity. We studied the effects of toxic stress on Daphnia galeata mendotae grown under conditions in which it produces female, male, and ephippial offspring. Fecundities in terms of the three types of offspring showed different sensitivities to chemical stress. The stressors were a toxic strain of the cyanobacterium Microcystis aeruginosa and nonylphenol, a plasticizer, which is a surfactant that interferes with endocrine function in vertebrates. Production of resting eggs and female offspring were affected in both experiments, but the experiments showed different dose responses. Production of males was less sensitive. Exposure to nonylphenol also produced a characteristic developmental abnormality at environmentally relevant concentrations. Life table analysis showed that stress during the sexual phase of the life cycle of Daphnia reduces short-term population growth and as well as affecting the stock of dormant individuals.