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Urban natural areas are often heavily invaded by non-native plants. To restore the biodiversity and ecosystem function of these areas, land managers commonly remove invasive species. Partnering with Portland Parks and Recreation, I studied a management-scale removal of Hedera spp., Clematis vitalba, Ilex aquifolium and Prunus laurocerasus from a 59 ha forested natural area in Portland, OR. Over four years, I compared the community composition of this area with contiguous control areas where invasive species remained intact. Hedera, the most abundant invader, was dramatically reduced in removal areas, along with Clematis and Ilex. Both the abundance and the diversity of native herbs and woody seedlings were substantially higher in removal areas, with Claytonia sibirica, Galium spp., Hydrophyllum tenuipes, Trillium ovatum, and Acer macrophyllum seedlings all showing significant increases. By 2015, the minimum native species richness for removal areas was 37, as determined by the Chao 2 estimator, versus 10 native species for control areas. In contrast, native shrubs and ferns showed no increase in response to invasive removal; in fact, control plots typically contained significantly higher stem densities of native shrubs and ferns than removal plots did. Few non-native species recolonized removal areas, except for seedlings of Clematis. Collectively, these results indicate that passive recovery of the herbaceous layer is possible at large scales following invasive species removal, but that active replanting of woody species may be needed. Partnerships between land managers and academic researchers are key to documenting the effects of management-scale removals.

Urban forest fragments are vulnerable to invasion by non-native species, and invaded forests are increasingly targeted for invasive species removals. Our goal was to determine the extent to which persistent seed banks can contribute to the recruitment of native forest species into urban forest fragments from which invasive plant species have been removed. In a greenhouse, we germinated seeds from soil samples from three forest fragments in Portland, Oregon, US. All sites had been invaded by Hedera hibernica (Irish ivy) and H. helix (English ivy), Clematis vitalba (virgin's bower), Ilex aquifolium (English holly), and Prunus laurocerasus (English laurel). At one site, these species had been removed three years prior to our study. Emergents represented 53 taxa, classified as: native forest species, native non-forest species, and non-native species. We observed few native forest species (5–12/site); 29–83% of samples contained these species, at median densities of 0–2 seeds/sample/site. Non-native species were more diverse (12–17/site), more frequent (75–89% of samples), and denser (median = 2–5 seeds/sample/site). Clematis vitalba seeds were especially abundant. Invasive removal had little effect on the persistent seed bank; however, the least-invaded site had the highest richness, frequency, and density of native forest species. The low richness and abundance of native forest species is not necessarily a concern, because many forest species do not form persistent seed banks. The annual seed rain can regenerate native species whose density has been diminished by invasive plant species and their removal. However, managers wishing to restore absent species should plan to follow removal efforts with active revegetation.

Studies of at-risk species are likely to have greater conservation impact if they: involve managers in question formulation, set specific management or restoration goals, and are based on detailed knowledge of species’ resource needs. These ideas guided our investigation of the foraging behavior of larvae of a US federally threatened butterfly, the Oregon silverspot (Speyeria zerene hippolyta), with the goal of making management recommendations for habitat restoration. S. z. hippolyta larvae feed exclusively on Viola adunca and must consume multiple individuals to pupate successfully. Larvae forage at random through meadow vegetation to locate host plants. Observations of larvae foraging under field conditions revealed that larger, older larvae move more rapidly and turn less acutely than smaller, younger larvae; the consequence of these developmental differences is that younger larvae tend to remain in one place while older larvae tend to range more widely, presumably in search of new host plants. Results from a simulation model initialized with these data suggested that a host plant density of at least four V. adunca plants/m2 (depending on predation intensity to which larvae are exposed) is required in order for 4th instar larvae to have a 10 % chance of survival to pupation. These findings are being used to guide a violet restoration program for this sensitive species.

The genetic structure of lineages can provide important information for delineating “evolutionarily significant units” (ESUs) for conservation, and for planning actions to protect and restore taxa threatened with extinction. Speyeria zerene hippolyta, the Oregon silverspot butterfly, is a U.S.A. federally threatened subspecies that is the focus of considerable conservation effort, but whose evolutionary relationships with other Speyeria taxa are not well-understood. We conducted a genetic analysis of nine Speyeria species and 25 subspecies from western U.S.A., using both mitochondrial and nuclear markers. Our goal was to determine whether such data supported (a) S. z. hippolyta’s designation as an ESU, and (b) the current morphologically-based taxonomy of Speyeria spp. Our data for S. z. hippolyta were equivocal; while nuclear markers resolved all these individuals into a single clade, mtDNA data suggested the existence of two clades. Aside from S. cybele, which was consistently supported as monophyletic, our data provided little support for most of the species currently recognized for western U.S. Speyeria, including S. zerene, and even less for the many subspecies designations. These genetic findings stand in contrast to the morphological differences recognized by experts, and suggest a relatively recent origin for many of these taxa. Two of 66 individuals screened for Wolbachia infection tested positive for this symbiont. Our results provide no persuasive evidence that S. z. hippolyta should lose its status as an ESU, but they have important implications for ongoing management actions such as population augmentation.

Uncertainty associated with ecological forecasts has long been recognized, but forecast accuracy is rarely quantified. We evaluated how well data on 82 populations of 20 species of plants spanning ...

Matrix projection models are among the most widely used tools in plant ecology. However, the way in which plant ecologists use and interpret these models differs from the way in which they are presented in the broader academic literature. In contrast to calls from earlier reviews, most studies of plant populations are based on < 5 matrices and present simple metrics such as deterministic population growth rates. However, plant ecologists also cautioned against literal interpretation of model predictions. Although academic studies have emphasized testing quantitative model predictions, such forecasts are not the way in which plant ecologists find matrix models to be most useful. Improving forecasting ability would necessitate increased model complexity and longer studies. Therefore, in addition to longer term studies with better links to environmental drivers, priorities for research include critically evaluating relative/comparative uses of matrix models and asking how we can use many short-term studies to understand long-term population dynamics.

1. Relatively few studies of the host-finding ability of specialised, phytophagous insects involve direct observations of individual insects moving among intact hosts and non-hosts. Information from such studies can inform the design of restoration programmes for species of conservation concern. 2. The movement of caterpillars of the threatened Oregon silverspot butterfly, Speyeria zerene hippolyta (Edwards) (Lepidoptera: Nymphalidae) was studied in the field in cleared arenas with 10 cm radii. Caterpillars were placed in the centre, surrounded by three individuals of their host, Viola adunca, and three different non-host individuals, separated by bare ground. In a second experiment, second instars were placed between a host and a non-host, 3–6 cm away. Caterpillars were observed to determine if they walked to their host more often than expected by chance. 3. Caterpillars walked to vegetation significantly more often than expected by chance. They did not, however, reach their hosts more often than expected, based on plant availability. 4. It is concluded that S. z. hippolyta caterpillars can distinguish vegetation from bare ground from 10 cm away. There is no evidence that they can distinguish their host plant from other herbaceous species at distances of 3 cm.

To reduce soil erosion after fires, forest managers commonly employ rehabilitation treatments, including application of grass seed or mulching with straw. In 1999 the Megram fire burned 24,000 ha of mixed conifer/tanoak forest in northern California, USA. In the subsequent year, some areas were aerially seeded with barley and others were mulched with rice straw; some areas received both treatments. We investigated how these treatments affected these aspects of ecological recovery: (a) the introduction of non-native plants, (b) percent vegetative cover, (c) the diversity and density of regenerating native plants, and (d) the density and frequency of conifer seedlings. We found that mulched quadrats had a significantly higher occurrence of non-native species than untreated quadrats did. In addition, we found little evidence that mulching or mulching+seeding facilitated recovery of the native plant community. Percent vegetative cover was no higher in treated quadrats than in untreated quadrats. While annuals were present in significantly higher densities in the mulched+seeded quadrats compared to the other treatments, perennial density and overall plant density were not affected by treatment. Mulching and mulching+seeding significantly reduced the density and frequency of conifer seedlings. While we found similar native species richness on a per-quadrat basis for all three treatments, the cumulative number of native species in untreated quadrats taken as a whole exceeded the numbers found in mulched-only and mulched+seeded quadrats. We recommend that managers take steps to assure that mulch is free of non-native seed, and that further efforts be made to investigate systematically aspects of ecological recovery after the application of rehabilitation treatments.

Although competitive interactions within predator populations are known to depend on their size structure, we understand less about how these interactions are influenced by prey characteristics. Most studies of such interactions for tiger salamander (Ambystoma tigrinum nebulosum) larvae have used small zooplankton prey. We investigate the potential of exploitation and interference competition to influence the success of tiger salamander larvae feeding on relatively large prey, mayfly and damselfly larvae. We measured salamander foraging efficiency for a range of salamander and prey sizes and observed aggression levels of salamanders of varying size housed together. Exploitative foraging efficiency (captures per attempts) increased with salamander size but was better predicted by relative prey size (prey size as a percentage of salamander snout–vent length) than by salamander size alone; it also depended significantly on prey type. Aggression (interference) levels were higher when prey were present, and larger salamanders were more aggressive than smaller ones but did not consume more mayfly prey. Our results suggest that investigating the environmental conditions, particularly the prey characteristics, that influence size-based competitive advantages will lead to a better understanding of predator population dynamics.

Motivated by data demonstrating fluctuating relative and absolute fitnesses for white- versus blue-flowered morphs of the desert annual Linanthus parryae, we present conditions under which temporally fluctuating selection and fluctuating contributions to a persistent seed bank will maintain a stable single-locus polymorphism. In L. parryae, blue flower color is determined by a single dominant allele. To disentangle the underlying diversity-maintaining mechanism from the mathematical complications associated with departures from Hardy-Weinberg genotype frequencies and dominance, we successively analyze a haploid model, a diploid model with three distinguishable genotypes, and a diploid model with complete dominance. For each model, we present conditions for the maintenance of a stable polymorphism, then use a diffusion approximation to describe the long-term fluctuations associated with these polymorphisms. Our protected polymorphism analyses show that a genotype whose arithmetic and geometric mean relative fitnesses are both less than one can persist if its relative fitness exceeds one in years that produce the most offspring. This condition is met by data from a population of L. parryae whose white morph has higher fitness (seed set) only in years of relatively heavy rain fall. The data suggest that the observed polymorphism may be explained by fluctuating selection. However, the yearly variation in flower color frequencies cannot be fully explained by our simple models, which ignore age structure and possible selection in the seed bank. We address two additional questions--one mathematical, the other biological--concerning the applicability of diffusion approximations to intense selection and the applicability of long-term predictions to datasets spanning decades for populations with long-lived seed banks.

Analyses of population projection models are increasingly being used by conservation biologists and land managers to assess the health of sensitive species and to evaluate the likely effects of management strategies, harvesting, grazing, or other manipulations. Here I describe some of the limitations of this approach and illustrate how these limitations may affect its usefulness. I do this by comparing the results of such an analysis, performed in 1979 on two populations of a perennial plant, Arisaema triphyllum, with new information about the size and structure of these same populations gathered in 1994, 15 years later. While one population changed as the model projected it would, the other behaved quite differently from the projection. Instead of increasing in size, this population decreased between 1979 and 1994. Possible shortcomings in the data and in the model include: too few plants to provide accurate transition probabilities; too few years to capture accurately the complete range of year-to-year ...

Matrix projection models are among the most widely used tools in plant ecology. However, the way in which plant ecologists use and interpret these models differs from the way in which they are presented in the broader academic literature. In contrast to calls from earlier reviews, most studies of plant populations are based on5 matrices and present simple metrics such as deterministic population growth rates. However, plant ecologists also cautioned against literal interpretation of model predictions. Although academic studies have emphasized testing quantitative model predictions, such forecasts are not the way in which plant ecologists find matrix models to be most useful. Improving forecasting ability would necessitate increased model complexity and longer studies. Therefore, in addition to longer term studies with better links to environmental drivers, priorities for research include critically evaluating relative/comparative uses of matrix models and asking how we can use many short-term studies to understand long-term population dynamics.

Understanding the evolutionary mechanisms that contribute to the local genetic differentiation of populations is a major goal of evolutionary biology, and debate continues regarding the relative importance of natural selection and random genetic drift to population differentiation. The desert plant Linanthus parryae has played a prominent role in these debates, with nearly six decades of empirical and theoretical work into the causes of spatial differentiation for flower color. Plants produce either blue or white flowers, and local populations often differ greatly in the frequencies of the two color morphs. Sewall Wright first applied his model of "isolation by distance" to investigate spatial patterns of flower color in Linanthus. He concluded that the distribution of flower color morphs was due to random genetic drift, and that Linanthus provided an example of his shifting balance theory of evolution. Our results from comprehensive field studies do not support this view. We studied an area in which flower color changed abruptly from all-blue to all-white across a shallow ravine. Allozyme markers sampled across these regions showed no evidence of spatial differentiation, reciprocal transplant experiments revealed natural selection favoring the resident morph, and soils and the dominant members of the plant community differed between regions. These results support the hypothesis that local differences in flower color are due to natural selection, not due to genetic drift.

Linanthus parryae, a diminutive desert annual with white or blue flowers, has been the focus of a long-standing debate among evolutionary biologists. At issue is whether the flower color polymorphism in this species is the product of random genetic drift, as Sewall Wright argued, or of natural selection, as proposed by Carl Epling and his colleagues. Our long-term studies of three polymorphic populations in the Mojave Desert demonstrate that flower color is subject to selection that varies in both time and space in its direction and magnitude. For all sites taken together, blue-flowered plants produced more seeds than white-flowered plants in years of relatively low seed production, whereas white-flowered plants had higher fitness in years of high seed production. Evidence of selection on flower color was found in two of the three study sites. Differences in fitness between the color morphs were sometimes large, with selection coefficients as high as 0.60 in some years. Our longest period of observations was at Pearblossom site 1, where plants reached appreciable densities in seven of the 11 years of study. Here we found significant differences in the seed production of the color morphs in six years, with three years of blue advantage and three years of white advantage. For all sites taken together, total spring precipitation (March and April) was positively correlated with both absolute and relative seed production of the color morphs. At Pearblossom site 1, blue-flowered plants typically had a fitness advantage in years of low spring precipitation, whereas white-flowered plants had a fitness advantage in years of high spring precipitation. This temporal variation in selection may contribute to the maintenance of the flower-color polymorphism at Pearblossom site 1, whereas gene flow from neighboring populations is proposed as the principal factor maintaining the polymorphism at the other study sites. We found no significant differences between the color morphs in pollinator visitation rate or in their carbon isotope ratio, a measure of water-use efficiency. Although the mechanism of selection remains elusive, our results refute Wright's conclusion that the flower color polymorphism in L. parryae is an example of isolation by distance, a key component of his shifting balance theory of evolution.

Demographic transition matrices are among the most commonly applied population models for both basic and applied ecological research. The relatively simple framework of these models and simple, easily interpretable summary statistics they produce have prompted the wide use of these models across an exceptionally broad range of taxa. Here, we provide annual transition matrices and observed stage structures/population sizes for 20 perennial plant species which have been the focal species for long-term demographic monitoring. These data were assembled as part of the “Testing Matrix Models” working group through the National Center for Ecological Analysis and Synthesis (NCEAS). The data represent 82 populations with >460 total population-years of observations. It is our hope that making these data available will help promote and improve our ability to monitor and understand plant population dynamics. The complete data sets corresponding to abstracts published in the Data Papers section of the journal are publ...

Few studies have examined the potential for pathogens with complex life cycles to cause selection on their required alternate (=intermediate) hosts. Here we examine the effects of two fungal pathogens on an herbaceous mustard, Arabis holboellii. One pathogen species uses A. holboellii as a primary host, the other uses it as an alternate host. This plant-pathogen system is especially interesting because the host, A. holboellii, is apomictic; thus individuals reproduce exact copies of themselves. Despite this mode of reproduction, A. holboellii populations are surprisingly genetically diverse. Could frequency dependent selection by pathogens be maintaining clonal diversity? This study assesses the potential for selection by pathogens. In a controlled greehouse experiment we show that there is heritable variation in A. holboellii's resistance to the rust, Puccinia monoica, and that host fitness is severely reduced by P. monoica infection in both the greenhouse and under natural conditions. Field observations indicate that host clones are also differentially susceptible to the short-cycled rust, P. thlaspeos, and that host fitness is reduced by infection to this pathogen as well. Although the preconditions for pathogen-mediated selection are present, frequency-dependent selection by pathogens is unlikely to be important in structuring populations of Arabis holboellii because multiple host genotypes are susceptible to the same inoculum and the pathogen has a long generation time.

A size-classified transition matrix model of the demography of Arisaema triphyllum, a perennial herb of temperate deciduous forest, was constructed from 3 yr of field data from two different sites. Arisaema is quite unusual in that individuals change sex in a pattern determined by their sizes: small individuals do not flower, larger individuals reproduce as males, and the largest as females. As plants increase or decrease in size, they also may change sex. Though demographic parameters varied with the site and with the year, some consistent patterns were seen. Mortality rates ranged from 20 to 40%/yr for small, nonflowering plants, but decreased to much lower levels as plants reached reproductive size. For these large plants, mortality rates did not differ between sexes or among size-classes. Because rates of sexual reproduction were severely pollinator limited, levels of seed production were low (averaging 7 seeds female-1 yr-1) and were not correlated with plant size. All plants engaged in clonal reproduction, regardless of their size. Transition matrices based on these data were compared statistically and were found to vary in time, as a result of year-to-year environmental variability. No population had achieved a stable size distribution. The classical method of calculating rate of increase, X, was therefore rejected, and an alternative method incorporating temporal stochasticity substituted. A sensitivity analysis was per- formed to evaluate the effect on population growth of changing various life history parameters. Differences in rates of individual growth and of clonal reproduction were found to be largely respon- sible for the relative success of the two populations, one of which was increasing, the other declining. Arisaema's status as a climax-forest herb suggests that temporally varying transition matrices might be common even in environments traditionally regarded as "stable." The prevalence of tem- poral variability has consequences both for the future direction of demographic theory and for the design and analysis of field studies.

Summary Information is reviewed on the life histories and demographic patterns of shade-tolerant herbs of temperate deciduous forest. Most of the species that have been studied are deciduous perennials. These species generally have a juvenile (non-flowering) period of several years followed by an extended period of flower and seed production. About half the species reviewed are self-incompatible, the others are at least partially self-compatible. Although vegetative reproduction is common, replacement does not always occur by vegetative spread; many species appear to be able to replace themselves by seed. Sexual reproductive effort ranges from 1–5 to 50 % among the species reviewed; seed weight averages 2–6 mg. Short dormancy periods (one season) are most common, with germination rates generally below 50%. Mortality rates of small plants are high, but decline as plant size increases. ‘Ageing’ does not appear to occur. No information exists on population growth rates of temperate deciduous forest herbs, or on the temporal stability of their population sizes or structures. The transition matrix model is suggested as a promising approach to future studies in plant demography.

Plant-pollinator relationships have been recognized as excellent systems in which to test optimal foraging theory (Pyke et al., 1977). In particular, Pyke (1978a, 1978b, 1978c, 1979) has considered the movements of hummingbirds and bumblebees within and among inflorescences to determine the extent to which the behavior of these organisms can be considered "optimal," i.e., that the net rate of energy gain is maximized. Pyke's (1978c, 1979) predictions of how an optimally foraging bee should behave have been based on general theoretical arguments rather than on precise quantitative considerations of the energetic "costs" and "rewards" of foraging. In addition, recent work by Waddington and Heinrich (1979) has revealed new information about the flexibility of response of foraging bees to changes in the presentation of rewards. In this paper we examine the relationship between Digitalis purpurea (Scrophulariaceae) and its most frequent visitor, Bombus flavifrons dimidiatus. We construct a quantitative model for the foraging behavior of energy-maximizing bees on Digitalis, predicting where on the inflorescence bees will begin foraging, the direction of their movement, and at what flower position they will leave. We then seek to answer the following questions, raised by the model: 1) Is the spatial arrangement of nectar rewards within the inflorescence exploited by foraging bees in the manner predicted by our model? 2) Does the plant's pattern of nectar rewards

Several recent studies have shown that males and females in some populations of dioecious plants are spatially segregated with respect to an environmental gradient. The inference is often made that such spatial segregation of the sexes (SSS) is favored by selection because it reduces competition between individuals of opposite sex (sexual "niche partitioning"). This paper was written to clarify the evolutionary interpretation of cases of SSS. We describe the possible proximate mechanisms capable of producing SSS and evaluate their plausibility. Then, we identify the selective factors that could favor the evolution of SSS. We argue that SSS can be favored if male fitness and female fitness respond differently across environments (because of differences in reproductive biology), that a reduction in competition between males and females is unlikely to be an evolutionary cause of SSS, and that differential mortality is unlikely to evolve as a proximate mechanism for achieving adaptive SSS. Such a pattern is r...

Arisaema triphyllum plants change sex: small individuals are males while large plants are females. Contrary to the expectations generated by the size-advantage model, though, reproductive success is independent of size for existing males and females, because it is pollinator-limited rather than resource-limited. Estimates of the likely reproductive effort that would be borne by nonexistent small females suggest, however, that there is a size threshold below which female reproduction is prohibitively costly. This threshold creates selection for sex-changing, but because of temporal fluctuations in seed production and sex ratio, the magnitude and even the direction of this selection are not constant. Such fluctuations make it unlikely that jack-in-the-pulpits ever attain the simple sort of optimum sex-changing strategy described by theory predicated on assumptions of population stability.

Plant taxa that reproduce asexually display some distinct geographical and ecological patterns. A literature review reveals that such taxa 1) tend to have larger ranges, 2) tend to range into higher latitudes, and 3) tend to range to higher elevations than do their sexual relatives. Asexual taxa have a greater tendency than sexual taxa do to colonize once-glaciated areas. These trends have previously been identified as characteristic of parthenogenetic animals as well. While many authors have interpreted these trends as providing support for the ‘biotic uncertainty’ hypothesis for the maintenance of sex, these trends are consistent with several other interpretations as well. Furthermore, all of these interpretations have ignored the positive correlation that exists between ploidy level and breeding system: asexual plant and animal taxa are generally polyploid, while their sexual relatives are generally diploid. Evidence is presented for plants, and by extension for animals as well, that high ploidy levels alone could (independent of breeding system) endow individuals with the ability to tolerate these ‘extreme’ environments. For this reason, it appears premature to interpret observed distribution patterns as evidence to support hypotheses about what forces maintain sexual reproduction. Only experimental tests, using sexuals and asexuals of comparable ploidy levels, can permit us to discriminate among the alternatives.

Several recent reviews have described the ecological and geographical patterns of parthenogenesis in animals (Bell, 1982; Glesener and Tilman, 1978) in an effort to determine which selective forces best explain the maintenance of sexual reproduction. These reviews conclude that parthenogenetic lineages tend to be found at higher latitudes, at higher elevations, in more xeric conditions, in more disturbed habitats, and on more island-like habitats than their sexual relatives. In general, it is argued that asexual taxa are more widely distributed than are their sexual relatives, with sexuals limited to habitats in which the important selective forces are principally biotic rather than physical.

Theoretical biologists have responded enthusiastically to the challenge of resolving the paradox of sex: why is sexual reproduction so widespread even though its practitioners suffer a twofold numerical disadvantage? At least five books (Ghiselin, 1974; Williams, 1975; Maynard Smith, 1978; Bell, 1982; Shields, 1982) and a myriad of papers have sought to identify conditions that could mitigate the twofold disadvantage of sex. Curiously, however, even though over a decade has elapsed since the publication of the earliest of these books (time enough for two generations of doctoral theses), this flurry of theoretical work has not been followed by anything near an equivalent amount of experimental activity.

Jack-in-the-pulpit, Arisaema triphyllum, is a perennial forest herb with the ability to change sex. At two sites in upstate New York, plant sex was correlated with plant size: males were smaller than females, nonflowering plants were smaller than males. Changes in plant size were accompanied by changes in sex. Sex change occurred quite frequently; at one site, 8% of nonflowering plants, 64% of males, and 63% of females changed their sex from one season to the next. The probability that a plant will change size and sex between years was altered by artificial defoliation and by the production of seeds, but was not affected by supplementing plants with nutrient fertilizer. Discriminant analysis indicated that several historical factors significantly affected plant sex: a model including the variables of current plant size, previous year's plant size, and previous year's sex was significantly better at predicting the current sex of individuals than was a model containing only current plant size. However, even the consideration of these three variables left up to a third of the plants misclassified with respect to gender. This analysis explains in part why plants at the two sites changed sex at different sizes, but it is likely that other factors-e.g. genetic differences-are involved.

It has been suggested that Asclepias curassavica, Lantana camara, and Epidendrum radicans, three unrelated tropical "weeds" with very similar flowers, comprise a floral mimicry complex. Asclepias and Lantana are presumed to be Miillerian mimics of each other while the nectarless Epidendrum is thought to be a Batesian mimic of the first two. Although the three species have overlapping ranges and share pollinators, visitation frequencies in stands of different floral composition do not support this hypothesis. Flowers in high-density stands of the "Miillerian mimics" are not visited more often than flowers in low-density stands, so apparent increase in population density through mimicry does not appear to confer an advantage. The "Batesian mimic" is not visited more often when interspersed with the model than when alone. Other possible reasons for the resemblance are discussed. Two OF THE MOST CONSPICUOUS ROADSIDE "WEEDS" in tropical America are Asclepias curassavica (Asclepiadaceae) and Lantana camara (Verbenaceae). Their red and yellow inflorescences are strikingly similar. Both species produce nectar and attract many species of potential pollinators. Epidendrum radicans, a terrestrial orchid that also bears clusters of bright red and yellow flowers, is somewhat less common. Epidendrum produces no floral nectar. Since the pollen grains of orchids are massed into pollinia that adhere to an insect's back, legs, head, or proboscis, and are thus unavailable to it as food, biologists have wondered how Epidendrum secures pollinators. An unpublished but widely circulated hypothesis is that these three plant species form a floral mimicry complex. Asclepias and Lantana are thought to be Miillerian mimics whose resemblance allows them to attract more pollinator visits, through the combined impact of their floral displays, than either would solely on the basis of its own abundance. Epidendrum, on the other hand, might be a Batesian mimic, obtaining visits by resembling the nectar-producers, Asclepias and Lantana. It has been demonstrated theoretically that the fitness of a rare plant species can be increased by the addition of another species with similar flowers, if pollinators do not distinguish between the two (Bobisud and Neuhaus 1975). In this case the advantage arises because the apparent increase in population size which results from the addition of the second species makes it more likely that a pollinator will specialize on that "'morphospecies" (actually both species combined). The existence of a number of similar species pairs has been so explained (Schelpe 1966, Yeo 1968, Macior 1970 and 1971, Proctor and 1Present address: Department of Biology, Pomona College, Claremont, California 91711, U.S.A. 54 REPRODUCTIVE BOTANY 54-58 1981 Yeo 1972, Gentry 1974, Heinrich 1975, Lack 1976, Dafni and Ivri 1979), but without proof. Asclepias, Lantana, and Epidendrum comprise another such

The fact that apomictic taxa typically occupy a wider range of environments than their sexual relatives has generated the hypothesis that apomicts are more likely to possess "general-purpose genotypes," i.e., genotypes whose performance is relatively insensitive to changes in environmental conditions. This hypothesis was tested by cloning sexual and apomictic females of Antennaria parvifolia (Asteraceae) and growing each genotype in six growth-chamber environments varying in temperature and moisture levels. A joint regression analysis revealed that the survival of apomictic genotypes was significantly less sensitive to environmental conditions than that of sexual genotypes but demonstrated no differences with regard to flowering or biomass. However, the coefficient of variation in biomass across the six environments was significantly lower for apomicts than for sexuals, and the geometric mean of survival over the six environments was significantly higher for apomicts. Apomicts significantly exceeded sexuals in mean survival, mean flower-head production, and mean biomass. These results support the hypothesis that apomictic genotypes are more "general-purpose" than sexuals, and increase the difficulty of explaining the persistence of sexual reproduction in A. parvifolia.