Showing papers in "The American Naturalist in 2003"

The Ecology of Individuals: Incidence and Implications of Individual Specialization

Abstract: Most empirical and theoretical studies of resource use and population dynamics treat conspecific individuals as ecologically equivalent. This simplification is only justified if interindividual niche variation is rare, weak, or has a trivial effect on ecological processes. This article reviews the incidence, degree, causes, and implications of individual-level niche variation to challenge these simplifications. Evidence for individual specialization is available for 93 species dis- tributed across a broad range of taxonomic groups. Although few studies have quantified the degree to which individuals are specialized relative to their population, between-individual variation can some- times comprise the majority of the population's niche width. The degree of individual specialization varies widely among species and among populations, reflecting a diverse array of physiological, be- havioral, and ecological mechanisms that can generate intrapopu- lation variation. Finally, individual specialization has potentially im- portant ecological, evolutionary, and conservation implications. Theory suggests that niche variation facilitates frequency-dependent interactions that can profoundly affect the population's stability, the amount of intraspecific competition, fitness-function shapes, and the population's capacity to diversify and speciate rapidly. Our collection of case studies suggests that individual specialization is a widespread but underappreciated phenomenon that poses many important but unanswered questions.

Community Patterns in Source‐Sink Metacommunities

Abstract: We present a model of a source‐sink competitive metacommunity, defined as a regional set of communities in which local diversity is maintained by dispersal. Although the conditions of local and regional coexistence have been well defined in such systems, no study has attempted to provide clear predictions of classical community‐wide patterns. Here we provide predictions for species richness, species relative abundances, and community‐level functional properties (productivity and space occupation) at the local and regional scales as functions of the proportion of dispersal between communities. Local (α) diversity is maximal at an intermediate level of dispersal, whereas between‐community (β) and regional (γ) diversity decline as dispersal increases because of increased homogenization of the metacommunity. The relationships between local and regional species richness and the species rank abundance distributions are strongly affected by the level of dispersal. Local productivity and space occupatio...

Behavioral drive versus behavioral inertia in evolution: a null model approach.

Abstract: Some biologists embrace the classical view that changes in behavior inevitably initiate or drive evolutionary changes in other traits, yet others note that behavior sometimes inhibits evolutionary changes. Here we develop a null model that quantifies the impact of regulatory behaviors (specifically, thermoregulatory behaviors) on body temperature and on performance of ectotherms. We apply the model to data on a lizard (Anolis cristatellus) and show that ther- moregulatory behaviors likely inhibit selection for evolutionary shifts in thermal physiology with altitude. Because behavioral adjustments are commonly used by ectotherms to regulate physiological per- formance, regulatory behaviors should generally constrain rather than drive evolution, a phenomenon we call the "Bogert effect." We briefly review a few other examples that contradict the classical view of behavior as the inevitable driving force in evolution. Overall, our analysis and brief review challenge the classical view that behavior is invariably the driving force in evolution, and instead our work supports the alternative view that behavior has diverse—and some- times conflicting—effects on the directions and rates at which other traits evolve.

The temperature-size rule in ectotherms: Simple evolutionary explanations may not be general

Abstract: In many organisms, individuals in colder environments grow more slowly but are larger as adults. This widespread pattern is embodied by two well‐established rules: Bergmann’s rule, which describes the association between temperature and body size in natural environments, and the temperature‐size rule, which describes reaction norms relating temperature to body size in laboratory experiments. Theory predicts that organisms should grow to be larger in colder environments when growth efficiency decreases with increasing environmental temperature. Using data from 97 laboratory experiments, including 58 species of ectotherms, we found little evidence that growth efficiency is negatively related to environmental temperature within the thermal range that is relevant to the temperature‐size rule. Instead, growth efficiency was either positively related or insensitive to environmental temperature in the majority of cases (73 of 89 cases for gross growth efficiency and 18 of 24 cases for net growth effici...

Partitioning Species Diversity across Landscapes and Regions: A Hierarchical Analysis of α, β, and γ Diversity

Abstract: Species diversity may be additively partitioned within and among samples (alpha and beta diversity) from hierarchically scaled studies to assess the proportion of the total diversity (gamma) found in different habitats, landscapes, or regions. We developed a statistical approach for testing null hypotheses that observed partitions of species richness or diversity indices differed from those expected by chance, and we illustrate these tests using data from a hierarchical study of forest-canopy beetles. Two null hypotheses were implemented using individual- and sample-based randomization tests to generate null distributions for alpha and beta components of diversity at multiple sampling scales. The two tests differed in their null distributions and power to detect statistically significant diversity components. Individual-based randomization was more powerful at all hierarchical levels and was sensitive to departures between observed and null partitions due to intraspecific aggregation of individuals. Sample-based randomization had less power but still may be useful for determining whether different habitats show a higher degree of differentiation in species diversity compared with random samples from the landscape. Null hypothesis tests provide a basis for inferences on partitions of species richness or diversity indices at multiple sampling levels, thereby increasing our understanding of how alpha and beta diversity change across spatial scales.

Assessing the Cost of Mounting an Immune Response

Abstract: The evolution of parasite resistance has often been assumed to be governed by antagonistic selection pressures. Defense against pathogens, by mounting an immune response, confers evident benefits but may also incur costs, so that the optimal level of defense is expected to depend on the balance between benefits and costs. Although the benefits of immune surveillance are well known, estimates of costs are still equivocal. Here we studied the behavioral and physiological modifications associated with exposure to a nonreplicating antigen (lipopolysaccharide [LPS] of Escherichia coli) in a passerine species, the house sparrow (Passer domesticus). We further investigated whether the behavioral and physiological changes provoked by LPS induced measurable repercussions on life-history traits, such as the breeding effort and reproductive success. Finally, we tested whether the trade-off between immune activation and breeding effort was modulated by the workload required to feed the brood. Exposure to LPS reduced activity and increased body mass loss of captive individuals; similarly, LPS injection induced a dramatic drop in feeding rate and reproductive success of breeding females. However, this reduction depended on brood size, suggesting that the strength of the trade-off between immune activation and reproduction was affected by the workload required to feed the brood. Overall, this study stresses the magnitude of costs associated with mounting immune responses and the ecological and evolutionary consequences for natural populations.

A globally consistent richness-climate relationship for angiosperms.

Abstract: Species richness, the simplest index of biodiversity, varies greatly over broad spatial scales. Richness-climate relationships often account for >80% of the spatial variance in richness. However, it has been suggested that richness-climate relationships differ significantly among geographic regions and that there is no globally consistent relationship. This study investigated the global patterns of species and family richness of angiosperms in relation to climate. We found that models relating angiosperm richness to mean annual temperature, annual water deficit, and their interaction or models relating richness to annual potential evapotranspiration and water deficit are both globally consistent and very strong and are independent of the diverse evolutionary histories and functional assemblages of plants in different parts of the world. Thus, effects of other factors such as evolutionary history, postglacial dispersal, soil nutrients, topography, or other climatic variables either must be quite minor over broad scales (because there is little residual variation left to explain) or they must be strongly collinear with global patterns of climate. The correlations shown here must be predicted by any successful hypothesis of mechanisms controlling richness patterns.

Carotenoids, immunocompetence, and the information content of sexual colors: an experimental test.

Abstract: Many male birds use carotenoid pigments to acquire brilliant colors that advertise their health and condition to prospective mates. The direct means by which the most colorful males achieve superior health has been debated, however. One hypothesis, based on studies of carotenoids as antioxidants in humans and other animals, is that carotenoids directly boost the immune system of colorful birds. We studied the relationship between carotenoid pigments, immune function, and sexual coloration in zebra finches (Taeniopygia guttata), a species in which males incorporate carotenoid pigments into their beak to attract mates. We tested the hypotheses that increased dietary carotenoid intake enhances immunocompetence in male zebra finches and that levels of carotenoids circulating in blood, which also determine beak coloration, directly predict the immune response of individuals. We experimentally supplemented captive finches with two common dietary carotenoid pigments (lutein and zeaxanthin) and measured...

Toward a Mechanistic Understanding and Prediction of Biotic Homogenization

Abstract: The widespread replacement of native species with cosmopolitan, nonnative species is homogenizing the global fauna and flora. While the empirical study of biotic homogenization is substantial and growing, theoretical aspects have yet to be explored. Consequently, the breadth of possible ecological mechanisms that can shape current and future patterns and rates of homogenization remain largely unknown. Here, we develop a conceptual model that describes 14 potential scenarios by which species invasions and/or extinctions can lead to various trajectories of biotic homogenization (increased community similarity) or differentiation (decreased community similarity); we then use a simulation approach to explore the model’s predictions. We found changes in community similarity to vary with the type and number of nonnative and native species, the historical degree of similarity among the communities, and, to a lesser degree, the richness of the recipient communities. Homogenization is greatest when simil...

Multispecies microbial mutualisms on coral reefs: the host as a habitat.

Abstract: Reef‐building corals associate with a diverse array of eukaryotic and noneukaryotic microbes. Best known are dinoflagellates in the genus Symbiodinium (“zooxanthellae”), which are photosynthetic symbionts found in all reef‐building corals. Once considered a single species, they are now recognized as several large, genetically diverse groups that often co‐occur within a single host species or colony. Variation among Symbiodinium in host identities, tolerance to stress, and ability to colonize hosts has been documented, but there is little information on the ecology of zooxanthellar free‐living stages and how different zooxanthellae perform as partners. Other microbial associates of reef corals are much less well known, but studies indicate that individual coral colonies host diverse assemblages of bacteria, some of which seem to have species‐specific associations. This diversity of microbial associates has important evolutionary and ecological implications. Most mutualisms evolve as balanced reci...

The effect of energy and seasonality on avian species richness and community composition.

Abstract: We analyzed geographic patterns of richness in both the breeding and winter season in relation to a remotely sensed index of seasonal production (normalized difference vegetation index [NDVI]) and to measures of habitat heterogeneity at four different spatial resolutions. The relationship between avian richness and NDVI was consistent between seasons, suggesting that the way in which available energy is converted to bird species is similar at these ecologically distinct times of year. The number and proportion of migrant species in breeding communities also increased predictably with the degree of seasonality. The NDVI was a much better predictor of seasonal richness at finer spatial scales, whereas habitat heterogeneity best predicted richness at coarser spatial resolutions. While we find strong support for a positive relationship between available energy and species richness, seasonal NDVI explained at most 61% of the variation in richness. Seasonal NDVI and habitat heterogeneity together expl...

Comparative tests of parasite species richness in primates.

Abstract: Some hosts harbor diverse parasite communities, whereas others are relatively parasite free. Many factors have been proposed to account for patterns of parasite species richness, but few studies have investigated competing hypotheses among multiple parasite communities in the same host clade. We used a comparative data set of 941 host-parasite combinations, representing 101 anthropoid primate species and 231 parasite taxa, to test the relative importance of four sets of variables that have been proposed as determinants of parasite community diversity in primates: host body mass and life history, social contact and population density, diet, and habitat diversity. We defined parasites broadly to include not only parasitic helminths and arthropods but also viruses, bacteria, fungi, and protozoa, and we controlled for effects of uneven sampling effort on per-host measures of parasite diversity. In nonphylogenetic tests, body mass was correlated with total parasite diversity and the diversity of helminths and viruses. When phylogeny was taken into account, however, body mass became nonsignificant. Host population density, a key determinant of parasite spread in many epidemiological models, was associated consistently with total parasite species richness and the diversity of helminths, protozoa, and viruses tested separately. Geographic range size and day range length explained significant variation in the diversity of viruses.

Biological correlates of extinction risk in bats.

Abstract: We investigated patterns and processes of extinction and threat in bats using a multivariate phylogenetic comparative approach. Of nearly 1,000 species worldwide, 239 are considered threatened by the International Union for Conservation of Nature and Natural Resources (IUCN) and 12 are extinct. Small geographic ranges and low wing aspect ratios are independently found to predict extinction risk in bats, which explains 48% of the total variance in IUCN assessments of threat. The pattern and correlates of extinction risk in the two bat suborders are significantly different. A higher proportion (4%) of megachiropteran species have gone extinct in the last 500 years than microchiropteran bats (0.3%), and a higher proportion is currently at risk of extinction (Megachiroptera: 34%; Microchiroptera: 22%). While correlates of microchiropteran extinction risk are the same as in the order as a whole, megachiropteran extinction is correlated more with reproductive rate and less with wing morphology. Bat extinction risk is not randomly distributed phylogenetically: closely related species have more similar levels of threat than would be expected if extinction risk were random. Given the unbalanced nature of the evolutionary diversification of bats, it is probable that the amount of phylogenetic diversity lost if currently threatened taxa disappear may be greater than in other clades with numerically more threatened species.

Inferring process from pattern in plant invasions: a semimechanistic model incorporating propagule pressure and environmental factors.

Abstract: Propagule pressure is intuitively a key factor in biological invasions: increased availability of propagules increases the chances of establishment, persistence, naturalization, and invasion. The role of propagule pressure relative to disturbance and various environmental factors is, however, difficult to quantify. We explored the relative importance of factors driving invasions using detailed data on the distribution and percentage cover of alien tree species on South Africa’s Agulhas Plain (2,160 km2). Classification trees based on geology, climate, land use, and topography adequately explained distribution but not abundance (canopy cover) of three widespread invasive species (Acacia cyclops, Acacia saligna, and Pinus pinaster). A semimechanistic model was then developed to quantify the roles of propagule pressure and environmental heterogeneity in structuring invasion patterns. The intensity of propagule pressure (approximated by the distance from putative invasion foci) was a much better pre...

Explaining species richness from continents to communities: the time-for-speciation effect in emydid turtles.

Abstract: Speciation is the process that ultimately generates species richness. However, the time required for speciation to build up diversity in a region is rarely considered as an explanation for patterns of species richness. We explored this “time‐for‐speciation effect” on patterns of species richness in emydid turtles. Emydids show a striking pattern of high species richness in eastern North America (especially the southeast) and low diversity in other regions. At the continental scale, species richness is positively correlated with the amount of time emydids have been present and speciating in each region, with eastern North America being the ancestral region. Within eastern North America, higher regional species richness in the southeast is associated with smaller geographic range sizes and not greater local species richness in southern communities. We suggest that these patterns of geographic range size variation and local and regional species richness in eastern North America are caused by glacia...

Interacting Guilds: Moving beyond the Pairwise Perspective on Mutualisms

Abstract: Most textbook treatments imply, and almost all theoretical analyses assume, that mutualistic interactions take place between a single pair of interacting partner species. A major goal of this symposium is to broaden and shift this pairwise perspective and make it concordant with the emerging view that locally exclusive mutualisms between just two species are the exception and that many communities include guilds of mutualistic species on one or both sides of the interaction. Many pollination and seed‐dispersal mutualisms have long been recognized as diffuse, but recent molecular analyses are revealing unrecognized partner diversity in mutualistic interactions previously thought to be locally species specific. Co‐occurring species within a mutualist guild are unlikely to be ecologically equivalent in the way they locate, compete for, and/or reward partners, and so intraguild interactions have the potential to influence population dynamics and patterns of selection in species on both sides of the ...

Measuring Nonlinear Selection

Abstract: A recent comprehensive review of empirical studies that measured the strength of selection concluded that there was little evidence for strong nonlinear selection in natural populations (Kingsolver et al. 2001). The median quadratic selection gradient identified by Kingsolver et al. (2001) was only 0.1, and gradients consistent with stabilizing or disruptive selection were found at a similar frequency and to be of similar magnitude. Stabilizing selection in particular is an important premise in many areas of evolutionary biology (Travis 1989), so this finding challenges our current understanding of how selection may operate in the wild (Conner 2001; Kingsolver et al. 2001). There is already some evidence that the strength and frequency of nonlinear selection identified in the review of Kingsolver et al. (2001) has influenced how evolutionary biologists view the potential role of stabilizing selection (Barton and Keightley 2002; Zhang et al. 2002). Kingsolver et al. (2001) suggest a number of reasons why empirical studies may not find or may underestimate nonlinear selection, including an empirical bias toward selecting experimental systems that are likely to show directional selection. In addition, nonlinear selection gradients are often able to be estimated from data sets but are not published (Kingsolver et al. 2001), probably as a consequence of the paucity of significant individual gradients and the difficulty of interpreting the overall pattern of nonlinear selection from the large number of estimated gradients. Here, we demonstrate that the strength of non-

Does Herbivore Diversity Depend on Plant Diversity? The Case of California Butterflies

Abstract: It is widely believed that the diversity of plants influences the diversity of animals, and this should be particularly true of herbivores. We examine this supposition at a moderate spatial extent by comparing the richness patterns of the 217 butterfly species resident in California to those of plants, including all 5,902 vascular plant species and the 552 species known to be fed on by caterpillars. We also examine the relationships between plant/butterfly richness and 20 environmental variables. We found that although plant and butterfly diversities are positively correlated, multiple regression, path models, and spatial analysis indicate that once primary productivity (estimated by a water‐energy variable, actual evapotranspiration) and topographical variability are incorporated into models, neither measure of plant richness has any relationship with butterfly richness. To examine whether butterflies with the most specialized diets follow the pattern found across all butterflies, we repeated t...

Body condition, migration, and timing of reproduction in snow geese: a test of the condition-dependent model of optimal clutch size.

Abstract: The seasonal decline of avian clutch size may result from the conflict between the advantage of early breeding (greater offspring value) and the advantage of a delay in lay date (improved body condition and hence clutch size). We tested predictions of a condition‐dependent individual optimization model based on this trade‐off (Rowe et al. 1994) in a long‐distance migrant, the greater snow goose (Chen caerulescens atlantica), using data on condition, migration, and reproductive decisions of individuals. We closely tracked radio‐marked females at their main spring staging area and on their breeding grounds. Our results were consistent with predictions of the model. Early‐arriving females had a longer prelaying period and initiated their nests earlier than late arrivals. After controlling statistically for arrival date, we determined that females with high premigration condition had an earlier lay date than those in low condition. After controlling for the seasonal decline (i.e., lay date), we obse...

History and the global ecology of squamate reptiles

Abstract: The structure of communities may be largely a result of evolutionary changes that occurred many millions of years ago. We explore the historical ecology of squamates (lizards and snakes), iden- tify historically derived differences among clades, and examine how this history has affected present-day squamate assemblages globally. A dietary shift occurred in the evolutionary history of squamates. Iguanian diets contain large proportions of ants, other hymenop- terans, and beetles, whereas these are minor prey in scleroglossan lizards. A preponderance of termites, grasshoppers, spiders, and in- sect larvae in their diets suggests that scleroglossan lizards harvest higher energy prey or avoid prey containing noxious chemicals. The success of this dietary shift is suggested by dominance of sclero- glossans in lizard assemblages throughout the world. One sclero- glossan clade, Autarchoglossa, combined an advanced vomeronasal chemosensory system with jaw prehension and increased activity levels. We suggest these traits provided them a competitive advantage during the day in terrestrial habitats. Iguanians and gekkotans shifted to elevated microhabitats historically, and gekkotans shifted activity to nighttime. These historically derived niche differences are apparent in extant lizard assemblages and account for some observed structure. These patterns occur in a variety of habitats at both regional and local levels throughout the world.

The many growth rates and elasticities of populations in random environments.

Abstract: Despite considerable interest in the dynamics of populations subject to temporally varying environments, alternate population growth rates and their sensitivities remain incompletely understood. For a Markovian environment, we compare and contrast the meanings of the stochastic growth rate ( \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} ewcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} ormalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape $\lambda _{\mathrm{S}\,}$ \end{document} ), the growth rate of average population ( \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm...

Herbivore responses to plant secondary compounds: a test of phytochemical coevolution theory.

Abstract: Literature data were collected on the floristic distribution and toxicity of phytochemicals to herbivores and on herbivore specialization in order to test phytochemical coevolution theory. The theory makes four predictions that can be tested with this information. Herbivores can adapt to novel, more toxic chemicals by becoming specialists, or they can become generalists but at the cost of lower feeding success on any particular host. Thus, the first two predictions are as follows: herbivores should do better on chemicals that are present in their normal host, and this pattern should be stronger for specialists than for generalists. The "escape and radiation" aspect of the theory holds that if a plant taxon with a novel defense chemical diversifies, the chemical will become widespread. Eventually, herbivores will adapt to and disarm it. So the third prediction is that more widespread chemicals are less toxic than more narrowly distributed ones. Because generalists should not do as well as specialists on chemicals disarmed by the latter, the fourth prediction is that the third prediction should be more true for generalists than specialists and should depend on presence/absence of the chemical in the normal host. Multiple regressions of toxicity (herbivore mortality and final weight) on three predictor variables (chemical presence/absence in the normal host, specialism, and chemical floristic distribution) and relevant interactions were used to test these predictions. Chemical presence/absence in the normal host, the interaction between this variable and specialism, and chemical floristic distribution had significant effects on both measures of toxicity, supporting the first three predictions of the model. Support for the fourth prediction (a three-way interaction among all predictor variables) was evident for final weight but not mortality, perhaps because growth is more responsive to toxicity differences than survival. In short, the phytochemistry literature provides broad support for the phytochemical coevolution model.

Least‐Cost Input Mixtures of Water and Nitrogen for Photosynthesis

Abstract: In microeconomics, a standard framework is used for determining the optimal input mix for a two‐input production process. Here we adapt this framework for understanding the way plants use water and nitrogen (N) in photosynthesis. The least‐cost input mixture for generating a given output depends on the relative cost of procuring and using nitrogen versus water. This way of considering the issue integrates concepts such as water‐use efficiency and photosynthetic nitrogen‐use efficiency into the more inclusive objective of optimizing the input mix for a given situation. We explore the implications of deploying alternative combinations of leaf nitrogen concentration and stomatal conductance to water, focusing on comparing hypothetical species occurring in low‐ versus high‐humidity habitats. We then present data from sites in both the United States and Australia and show that low‐rainfall species operate with substantially higher leaf N concentration per unit leaf area. The extra protein reflected i...

The relative importance of life-history variables to population growth rate in mammals: Cole's prediction revisited.

Abstract: The relative importance of life‐history variables to population growth rate (λ) has substantial consequences for the study of life‐history evolution and for the dynamics of biological populations. Using life‐history data for 142 natural populations of mammals, we estimated the elasticity of λ to changes in age at maturity (α), age at last reproduction (ω), juvenile survival (Pj), adult survival (Pa), and fertility (F). Elasticities were then used to quantify the relative importance of α, ω, Pj, Pa, and F to λ and to test theoretical predictions regarding the relative influence on λ of changes in life‐history variables. Neither α nor any other single life‐history variable had the largest relative influence on λ in the majority of the populations, and this pattern did not change substantially when effects of phylogeny and body size were statistically removed. Empirical support for theoretical predictions was poor at best. However, analyses of elasticities on the basis of the magnitude (F) and onse...

Dispersal rates affect species composition in metacommunities of Sarracenia purpurea inquilines.

Abstract: Dispersal among local communities can have a variety of effects on species composition and diversity at local and regional scales. Local conditions (e.g., resource and predator densities) can have independent effects, as well as interact with dispersal, to alter these patterns. Based on metacommunity models, we predicted that local diversity would show a unimodal relationship with dispersal frequency. We manipulated dispersal frequencies, resource levels, and the presence of predators (mosquito larvae) among communities found in the water‐filled leaves of the pitcher plant Sarracenia purpurea. Diversity and abundance of species of the middle trophic level, protozoa and rotifers, were measured. Increased dispersal frequencies significantly increased regional species richness and protozoan abundance while decreasing the variance among local communities. Dispersal frequency interacted with predation at the local community scale to produce patterns of diversity consistent with the model. When predat...

Herbivory Reduces the Strength of Pollinator-Mediated Selection in the Mediterranean Herb Erysimum mediohispanicum: Consequences for Plant Specialization

Abstract: In this study, I tested whether selection occurring on several morphological and floral traits in Erysimum mediohispanicum (Cruciferae) is modified by the effects of herbivores. Six plots were established in 1997 in the Sierra Nevada, Spain; three were fenced to exclude native ungulates, and the remaining were open to ungulates. I determined pollinator and ungulate preferences for plant traits and their effect on plant fecundity. Then I compared the selection regimes between plants excluded from herbivores and plants open to them. When ungulates were absent, I found significant selection on flower number, reproductive stalk height, basal diameter of the stalks, petal length, and inner diameter of the flowers. When ungulates were present, selection on floral traits completely disappeared, and selection strength on flower number and morphological traits decreased. This effect was due to the ungulate preference for larger plants and the phenotypic correlations between plant size and floral traits. ...

Spatial Dynamics in Model Plant Communities: What Do We Really Know?

Abstract: A variety of models have shown that spatial dynamics and small‐scale endogenous heterogeneity (e.g., forest gaps or local resource depletion zones) can change the rate and outcome of competition in communities of plants or other sessile organisms. However, the theory appears complicated and hard to connect to real systems. We synthesize results from three different kinds of models: interacting particle systems, moment equations for spatial point processes, and metapopulation or patch models. Studies using all three frameworks agree that spatial dynamics need not enhance coexistence nor slow down dynamics; their effects depend on the underlying competitive interactions in the community. When similar species would coexist in a nonspatial habitat, endogenous spatial structure inhibits coexistence and slows dynamics. When a dominant species disperses poorly and the weaker species has higher fecundity or better dispersal, competition‐colonization trade‐offs enhance coexistence. Even when species have...

Molecular Systematic Analysis Reveals Cryptic Tertiary Diversification of a Widespread Tropical Rain Forest Tree

Abstract: The broad geographic range of many Neotropical rain forest tree species implies excellent dispersal abilities or range establishment that preceded the formation of current dispersal barriers. In order to initiate historical analyses of such widespread Neotropical trees, we sequenced the nuclear ribosomal spacer (ITS) region of Symphonia globulifera L. f. (Clusiaceae) from populations spanning the Neotropics and western Africa. This rain forest tree has left unmistakable Miocene fossils in Mesoamerica (15.5–18.2 Ma) and in South America (∼15 Ma). Although marine dispersal of S. globulifera is considered improbable, our study establishes three marine dispersal events leading to the colonization of Mesoamerica, the Amazon basin, and the West Indies, thus supporting the paleontological data. Our phylogeographic analysis revealed the spatial extent of the three Neotropical S. globulifera clades, which represent trans‐Andes ( \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage...

Mutualistic asexual endophytes in a native grass are usually parasitic.

Abstract: Asexual systemic fungi that live symbiotically within grasses are viewed as strong mutualists on the basis of theory and empirical studies of introduced agronomic grasses. Evolutionary theory predicts that microbial symbionts that lose sexuality and rely on propagules of their hosts for transmission should evolve to benefit their hosts. Fungal endophytes of some cultivated turf and pasture grasses are well known for increasing plant performance and competitive abilities, especially under stress, and increasing resistance to herbivores, pathogens, and root‐feeders by virtue of fungal alkaloids. The assumption of mutualism, however, has rarely been tested in native grasses, which often harbor high but variable frequencies of systemic asexual endophytes. We tested the effect of Neotyphodium infections for the native grass Arizona fescue in a 3‐yr field experiment. We strictly controlled host genotype and manipulated soil moisture and nutrients. Infection generally decreased host growth in terms of ...