Showing papers in "The American Naturalist in 1999"
TL;DR: This analysis suggests that an animal should exhibit its greatest antipredator behavior in high‐risk situations that are brief and infrequent, and suggests that a common experimental protocol is likely to overestimate the intensity of antipredators behavior expected under field situations or chronic exposure to high risk.
Abstract: The rapid response of animals to changes in predation risk has allowed behavioral ecologists to learn much about antipredator decision making. A largely unappreciated aspect of such decision making, however, is that it may be fundamentally driven by the very thing that allows it to be so readily studied: temporal variation in risk. We show theoretically that temporal variability in risk leaves animals with the problem of allocating feeding and antipredator efforts across different risk situations. Our analysis suggests that an animal should exhibit its greatest antipredator behavior in high‐risk situations that are brief and infrequent. An animal should also allocate more antipredator effort to high‐risk situations and more feeding to low‐risk situations, with an increase in the relative degree of risk in high‐risk situations. However, the need to feed leaves an animal with little choice but to decrease its allocation of antipredator effort to high‐risk situations as they become more frequent or...
1,283 citations
TL;DR: Interspecific killing among mammalian carnivores is common in nature and accounts for up to 68% of known mortalities in some species, and may have important implications for conservation and management of carnivores and their prey.
Abstract: Interspecific killing among mammalian carnivores is common in nature and accounts for up to 68% of known mortalities in some species. Interactions may be symmetrical (both species kill each other) or asymmetrical (one species kills the other), and in some interactions adults of one species kill young but not adults of the other. There is a positive significant relationship between the body masses of solitary killer species and body masses of their victim species, and grouping species kill larger victims than solitary species. Interactions and consumption of the victim appear more common when food is scarce or disputed. In response to killers, victim species may alter their use of space, activity patterns, and form groups. Consequences of interspecific killing include population reduction or even extinction, and reduction and enhancement of prey populations, and may therefore have important implications for conservation and management of carnivores and their prey.
958 citations
TL;DR: It is proposed that MHC‐dependent mating preferences enable hosts to provide a “moving target” against rapidly evolving parasites that escape immune recognition (the Red Queen hypothesis) and may also function to avoid inbreeding.
Abstract: House mice prefer mates genetically dissimilar at the major histocompatibility complex (MHC). The highly polymorphic MHC genes control immunological self/nonself recognition; therefore, this mating preference may function to provide “good genes” for an individual's offspring. However, the evidence for MHC‐dependent mating preferences is controversial, and its function remains unclear. Here we provide a critical review of the studies on MHC‐dependent mating preferences in mice, sheep, and humans and the possible functions of this behavior. There are three adaptive hypotheses for MHC‐dependent mating preferences. First, MHC‐disassortative mating preferences produce MHC‐heterozygous offspring that may have enhanced immunocompetence. Although this hypothesis is not supported by tests of single parasites, MHC heterozygotes may be resistant to multiple parasites. Second, we propose that MHC‐dependent mating preferences enable hosts to provide a “moving target” against rapidly evolving parasites that e...
754 citations
TL;DR: Long‐term field studies of the same interaction across multiple communities and spatially structured mathematical models are together beginning to show that coevolution may be a more important ongoing process than had been indicated by earlier empirical and theoretical studies lacking a geographic perspective.
Abstract: Coevolution is one of the major processes organizing the earth's biodiversity. The need to understand coevolution as an ongoing process has grown as ecological concerns have risen over the dynamics of rapidly changing biological communities, the conservation of genetic diversity, and the population biology of diseases. The biggest current challenge is to understand how coevolution operates across broad geographic landscapes, linking local ecological processes with phylogeographic patterns. The geographic mosaic theory of coevolution provides a framework for asking how coevolution continually reshapes interactions across different spatial and temporal scales. It produces specific hypotheses on how geographically structured coevolution differs from coevolution at the local scale. It also provides a framework for understanding how local maladaptation can result from coevolution and why coevolved interactions may rarely produce long lists of coevolved traits that become fixed within species. Long‐te...
648 citations
TL;DR: This work synthesizes recent empirical results concerning the effects of habitat edges on population dynamics with contemporary theoretical developments to outline the ways in which species interactions, and the dynamics of the communities in which they are embedded, can be changed by habitat edges.
Abstract: Traditionally, ecologists interested in habitat edges have focused on edge‐related gradients in patterns of species richness or abiotic variables. Here, however, we take a different perspective, attempting to synthesize recent empirical results concerning the effects of habitat edges on population dynamics with contemporary theoretical developments to outline the ways in which species interactions, and the dynamics of the communities in which they are embedded, can be changed by habitat edges. We find a striking convergence between empirical notions of a patch's core area and analytical results from partial differential equation models. A review of both empirical and theoretical studies suggests four general classes of mechanisms through which habitat edges can have similar impacts on dissimilar types of species interactions. Specifically, we focus on edges' roles as dispersal barriers or filters, edges' influences on mortality, edges' involvement in spatial subsidies (in which dispersers' intra...
615 citations
TL;DR: Conceptual issues in evolutionary biology from an endocrinological perspective are approached, noting that single hormones typically act on several target tissues and thereby mediate suites of correlated phenotypic traits.
Abstract: We approach conceptual issues in evolutionary biology from an endocrinological perspective, noting that single hormones typically act on several target tissues and thereby mediate suites of correlated phenotypic traits. When several components of such a suite are beneficial, an important evolutionary question is whether all are adaptations or some are exaptations. The answer may depend on whether the traits arose in response to selection on variation in systemic levels of the hormone on variation in responsiveness of target tissues to invariant levels of the hormone. If the former, selection probably acted directly on fewer than all traits; beneficial traits arising indirectly would be exaptations. In contrast, multiple beneficial traits that arose out of independent changes in target‐tissue sensitivity to invariant hormone levels could all be adaptations. Knowledge of specific hormonal mechanisms as well as of historical selective regimes will be necessary to draw such distinctions. Endocrine c...
588 citations
TL;DR: The model shows that specializations for colonization, exploitation, and tolerance are all possible, and these are the only possible spatial strategies; among them, they partition all of the endogenous spatial structure in the environment.
Abstract: A plant lineage can compete for resources in a spatially variable environment by colonizing new areas, exploiting resources in those areas quickly before other plants arrive to compete with it, or tolerating competition once other plants do arrive. These specializations are ubiquitous in plant communities, but all three have never been derived from a spatial model of community dynamics—instead, the possibility of rapid exploitation has been either overlooked or confounded with colonization. We use moment equations, equations for the mean densities and spatial covariance of competing plant populations, to characterize these strategies in a fully spatial stochastic model. The moment equations predict endogenous spatial pattern formation and the efficacy of spatial strategies under different conditions. The model shows that specializations for colonization, exploitation, and tolerance are all possible, and these are the only possible spatial strategies; among them, they partition all of the endogen...
579 citations
TL;DR: Although dispersal had the strongest effect on the predicted spread rate, it was showed that dispersal interacts strongly with plant life history, disturbance, and habitat loss in influencing the predicted rate of spread.
Abstract: Models of plant migration based on estimates of biological parameters severely underestimate the rate of spread when compared to empirical estimates of plant migration rates. This is disturbing, since an ability to predict migration and colonization rates is needed for predicting how native species will distribute themselves in response to habitat loss and climate change and how rapidly invasive species will spread. Part of the problem is the difficulty of formally including rare long‐distance dispersal events in spread models. In this article, we explore the process of making predictions about plant migration rates. In particular, we examine the links between data, statistical models, and ecological predictions. We fit mixtures of Weibull distributions to several dispersal data sets and show that statistical and biological criteria for selecting the most appropriate statistical model conflict. Fitting a two‐component mixture model to the same data increases the spread‐rate prediction by an aver...
577 citations
TL;DR: The results suggest plant palatability as a main limiting factor of consumer metabolical and feeding rates across communities and offer a basis to predict the effects of widespread enhancement of plant nutritional quality due to large‐scale anthropogenic eutrophication on carbon balances in ecosystems.
Abstract: I examine, through an extensive compilation of published reports, the nature and variability of carbon flow (i.e., primary production, herbivory, detrital production, decomposition, export, and biomass and detrital storage) in a range of aquatic and terrestrial plant communities. Communities composed of more nutritional plants (i.e., higher nutrient concentrations) lose higher percentages of production to herbivores, channel lower percentages as detritus, experience faster decomposition rates, and, as a result, store smaller carbon pools. These results suggest plant palatability as a main limiting factor of consumer metabolical and feeding rates across communities. Hence, across communities, plant nutritional quality may be regarded as a descriptor of the importance of herbivore control on plant biomass (“top‐down” control), the rapidity of nutrient and energy recycling, and the magnitude of carbon storage. These results contribute to an understanding of how much and why the trophic routes of ca...
572 citations
TL;DR: A model of social selection acting on interacting phenotype that can be evaluated independently from the genetics of interacting phenotypes is presented, analogous to covariance models of other forms of selection.
Abstract: Social interactions often affect the fitness of interactants. Because of this, social selection has been described as a process distinct from other forms of natural selection. Social selection has been predicted to result in different evolutionary dynamics for interacting phenotypes, including rapid or extreme evolution and evolution of altruism. Despite the critical role that social selection plays in theories of social evolution, few studies have measured the force of social selection or the conditions under which this force changes. Here we present a model of social selection acting on interacting phenotypes that can be evaluated independently from the genetics of interacting phenotypes. Our model of social selection is analogous to covariance models of other forms of selection. We observe that an opportunity for social selection exists whenever individual fitness varies as a result of interactions with conspecifics. Social selection occurs, therefore, when variation in fitness due to interac...
477 citations
TL;DR: This study revealed previously unnoticed sex differences in plumage coloration and the nature of iridescent and noniridescent sex differences, which has important implications for classifications of animals as mono‐ or dimorphic and for taxonomic and conservation purposes.
Abstract: Assessment of color using human vision (or standards based thereon) is central to tests of many evolutionary hypotheses. Yet fundamental differences in color vision between humans and other animals call this approach into question. Here we use techniques for objectively assessing color patterns that avoid reliance on species‐specific (e.g., human) perception. Reflectance spectra are the invariant features that we expect the animal's color cognition to have evolved to extract. We performed multivariate analyses on principal components derived from >2,600 reflectance spectra (300–720 nm) sampled in a stratified random design from different body regions of male and female starlings in breeding plumage. Starlings possess spatially complex plumage patterns and extensive areas of iridescence. Our study revealed previously unnoticed sex differences in plumage coloration and the nature of iridescent and noniridescent sex differences. Sex differences occurred in some body regions but not others, were mor...
TL;DR: The eukaryotic crown group Alveolata has a particularly complex history of plastid acquisition, and may or may not be descended from a single endosymbiotic event.
Abstract: Plastids (chloroplasts) are endosymbiotic organelles derived from previously free-living cyanobacteria. They are dependent on their host cell to the degree that the majority of the proteins expressed in the plastid are encoded in the nuclear genome of the host cell, and it is this genetic dependency that distinguishes organelles from obligate endosymbionts. Reduction in the size of the plastid genome has occurred via gene loss, substitution of nuclear genes, and gene transfer. The plastids of Chlorophyta and plants, Rhodophyta, and Glaucocystophyta are primary plastids (i.e., derived directly from a cyanobacterium). These three lineages may or may not be descended from a single endosymbiotic event. All other lineages of plastids have acquired their plastids by secondary (or tertiary) endosymbiosis, in which a eukaryote already equipped with plastids is preyed upon by a second eukaryote. Considerable gene transfer has occurred among genomes and, at times, between organisms. The eukaryotic crown group Alveolata has a particularly complex history of plastid acquisition.
TL;DR: This study quantifies trophic relationships and food chain length in 14 Ontario and Quebec lakes and argues that productive space (productivity by area) is a more improved need for food web descriptions that improve the relative importance of information pathways.
Abstract: Food web structure is paramount in regulating a variety of ecologic patterns and processes, although food web studies are limited by poor empirical descriptions of inherently complex systems. In this study, stable isotope ratios (δ15N and δ13C) were used to quantify trophic relationships and food chain length (measured as a continuous variable) in 14 Ontario and Quebec lakes. All lakes contained lake trout as the top predator, although lakes differed in the presumed number of trophic levels leading to this species. The presumed number of trophic levels was correlated with food chain length and explained 40% of the among‐lake variation. Food chain length was most closely related to fish species richness ( \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}
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TL;DR: A simple theoretical model is presented to explore the influence of immigration from an external source on local coexistence, species abundance patterns, and ecosystem processes in plant communities and shows that immigration can have a huge effect on local species diversity in competitive communities where competition for space would lead to the exclusion of all but one species if the community were closed.
Abstract: Explaining the maintenance of high local species diversity in communities governed by competition for space has been a long‐standing problem in ecology. We present a simple theoretical model to explore the influence of immigration from an external source on local coexistence, species abundance patterns, and ecosystem processes in plant communities. The model is built after classical metapopulation models but is applied to competition for space between individuals and includes immigration by a propagule rain and an extinction threshold for rare species. Our model shows that immigration can have a huge effect on local species diversity in competitive communities where competition for space would lead to the exclusion of all but one species if the community were closed. Local species richness is expected to increase strongly when immigration intensity increases beyond the threshold required for the successful establishment of one or a few individuals. Community structure and species relative abunda...
TL;DR: It is suggested that high virulence and low migration contribute to the strength of local adaptation by parasites, and parasite local adaptation was not affected by giving the parasite a longer generation time than the host, but differences in local selection intensities had a dramatic effect.
Abstract: The geographic mosaic theory of coevolution is predicated on structured populations of interacting species where gene flow and the force of selection can vary among populations, leading to a mosaic of traits in space. Here, I briefly review some recent studies of adaptation by a sterilizing parasite to structured populations of a freshwater snail. The results show geographic structure as expected under the geographic mosaic model. I then consider the effects of virulence and migration on local adaptation by parasites using a computer simulation. The results suggest that high virulence and low migration contribute to the strength of local adaptation by parasites. Highly virulent parasites showed adaptation to local hosts for migration rates of up to 10% of the population per generation. In addition, because of the dynamic nature of host‐parasite coevolution, the magnitude of local adaptation fluctuates over time. During some points in the cycle, parasites may be no more effective at infecting ind...
TL;DR: This catalog includes all known major elements of the comprehensive evolutionary tree of protists and eukaryotes and introduces the concept of “extended in‐group” to refer to in‐groups and the most proximate sister group and to assist in identifying the hierarchical location of taxa.
Abstract: The discipline of evolutionary protistology has emerged in the past 30 yr. There is as yet no agreed view of how protists are interrelated or how they should be classified. The foundations of a stable taxonomic superstructure for the protists and other eukaryotes lie in cataloging the diversity of the major monophyletic lineages of these organisms. The use of common patterns of cell organization (ultrastructural identity) seems to provide us with the most robust hypotheses of such lineages. These lineages are placed in 71 groups without identifiable sister taxa. These groups are here referred to as “major building blocks.” For the first time, the compositions, ultrastructural identities, synapomorphies (where available), and subgroups of the major building blocks are summarized. More than 200 further lineages without clear identities are listed. This catalog includes all known major elements of the comprehensive evolutionary tree of protists and eukaryotes. Different approaches among protistolog...
TL;DR: This study suggests that corridors direct movements of habitat‐restricted species and that local behaviors may be used to predict the conservation potential of corridors in fragmented landscapes.
Abstract: Through empirical studies and simulation, I demonstrate how simple behaviors can be used in lieu of detailed dispersal studies to predict the effects of corridors on interpatch movements. Movement paths of three butterfly species were measured in large (1.64 ha) experimental patches of open habitat, some of which were connected by corridors. Butterflies that “reflected” off boundaries between open patches and the surrounding forest also emigrated from patches through corridors at rates higher than expected from random movement. This was observed for two open‐habitat species, Eurema nicippe and Phoebis sennae; however, edges and corridors had no effect on a habitat generalist, Papilio troilus. Behaviorally based simulation models, which departed from correlated random walks only at habitat boundaries, predicted that corridors increase interpatch movement rates of both open‐habitat species. Models also predicted that corridors have proportionately greater effects as corridor width increases, that ...
TL;DR: The landscape indices, which describe scale‐dependent correlation between and within habitat types, were able to explain variations in variables of population dynamics caused by different landscape structure.
Abstract: We construct and explore a general modeling framework that allows for a systematic investigation of the impact of changes in landscape structure on population dynamics. The essential parts of the framework are a landscape generator with independent control over landscape composition and physiognomy, an individual‐based spatially explicit population model that simulates population dynamics within heterogeneous landscapes, and scale‐dependent landscape indices that depict the essential aspects of landscape that interact with dispersal and demographic processes. Landscape maps are represented by a grid of \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}
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TL;DR: A study of long‐tailed tits Aegithalos caudatus showed that both parental strategies may occur within a species, depending on the number of helpers, and there was some evidence that males and females had different investment rules.
Abstract: An individual's optimal investment in young depends partly on the number of individuals caring for the same brood. In cooperative breeders, the investment strategy of parents with helpers is variable. When parents maintain the same effort regardless of helper number, helper care is additive. When parents fully compensate for the care of helpers by decreasing their own effort, total care does not increase. A study of long‐tailed tits Aegithalos caudatus showed that both parental strategies may occur within a species, depending on the number of helpers. A comparative analysis of 27 cooperative breeders was conducted to test the predictions of a graphical model that care is additive when nestling starvation is frequent and parents exhibit compensatory reductions in care when starvation is rare. Both predictions were supported. In this interspecific comparison, a species' mean group size was not associated with compensatory responses by parents. There was some evidence that males and females had dif...
TL;DR: Investigation of the nuclear genomes of entamoebae, Microsporidia, parabasalids, and diplomonads suggests that these organisms likely descend from mitochondrion‐bearing ancestors, and most evidence suggests that the mitochondrial endosymbiosis took place prior to the divergence of all extant eukaryotes.
Abstract: Resolving the order of events that occurred during the transition from prokaryotic to eukaryotic cells remains one of the greatest problems in cell evolution. One view, the Archezoa hypothesis, proposes that the endosymbiotic origin of mitochondria occurred relatively late in eukaryotic evolution and that several mitochondrion‐lacking protist groups diverged before the establishment of the organelle. Phylogenies based on small subunit ribosomal RNA and several protein‐coding genes supported this proposal, placing amitochondriate protists such as diplomonads, parabasalids, and Microsporidia as the earliest diverging eukaryotic lineages. However, trees of other molecules, such as tubulins, heat shock protein 70, TATA box‐binding protein, and the largest subunit of RNA polymerase II, indicate that Microsporidia are not deeply branching eukaryotes but instead are close relatives of the Fungi. Furthermore, recent discoveries of mitochondrion‐derived genes in the nuclear genomes of entamoebae, Microsp...
TL;DR: The issue of selective reporting has received little attention in recent meta-analyses of ecological and evolutionary patterns, and the inflation of average effect size due to selective reporting may imply a conscious intent to deceive.
Abstract: Those familiar with human nature and the publication process acknowledge that biases due to selective reporting of results are likely widespread in all fields of academic inquiry that depend on tools of statistical inference (e.g., see Begg and Berlin 1988; Iyengar and Greenhouse 1988; and the extensive discussion following each). However, although qualitative and quantitative methods exist for assessing the prevalence of selective reporting, and selective reporting has been studied in the medical and social sciences (Begg 1994, and references therein), the issue has received little attention in recent meta-analyses of ecological and evolutionary patterns. Clearly, “if publication bias is present, and if it operates in the same direction for all studies (as is likely), then [meta-analysis] is likely not only to produce biased summary estimates but also to produce estimates which are apparently precise and accurate leading to conclusions which may not only be wrong but appear convincing” (Begg and Berlin 1988, p. 437). To biologists unacquainted with the formal study of publication patterns, the terms “selective reporting” (statistical significance of an outcome influences its likelihood of being reported or published) and “publication bias” (the inflation of average effect size due to selective reporting) may imply a conscious intent to deceive, but this
TL;DR: A linearized model for small fluctuations shows that dispersal can substantially increase the scale of population synchrony for weakly regulated populations.
Abstract: A stochastic model is developed to analyze the equilibrium spatial pattern of population synchrony, the correlation of temporal fluctuations in population density between localities. The expected population dynamics and the distribution of individual dispersal distance are homogeneous in space. Environmental stochasticity is caused by temporal fluctuations in the intrinsic rate of increase and/or carrying capacity of local populations that are correlated in space (but not time), the environmental correlation decreasing with distance. We analyze a linearized model for small fluctuations. Employing the standard deviation of a function in a given direction as a measure of scale, the spatial scale of population synchrony, lρ, is related to the spatial scales of environmental correlation, le, and individual dispersal, l, by the simple general formula \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepack...
TL;DR: Only population abundance and habitat specialization explained a significant amount of the observed variation in species extinction rates and body mass itself did not explain variation in extinction rates, although it was strongly correlated with abundance.
Abstract: One of the central questions of conservation biology is what life‐history traits render a species prone to extinction. We addressed this problem by calculating extinction rates for 35 species of turtles and squamates (lizards and snakes) occurring on 87 land‐bridge islands in the Mediterranean Sea. We calculated extinction rates in two ways: first, by incorporating the known sequence of historical island separations and second by ignoring history and assuming that the islands became isolated simultaneously. The second procedure is simpler and more frequently used in the literature and produces estimates of extinction rates that are similar to the first, more complex procedure. We then determined the relationship between extinction rates (calculated using both methods) and body mass, longevity, habitat specialization, and population abundance using two methods: first, by accounting for the phylogenetic relationships among species and, second, by ignoring them. Only population abundance and habita...
TL;DR: The model makes quantitative predictions on the rate of dispersal by the dispersing sex and suggests that inbreeding avoidance may only rarely be the sole reason for dispersal, underline the necessity to control for phylogenetic relationships when investigating empirical trends in dispersal.
Abstract: Using a game‐theoretical approach, we investigate the dispersal patterns expected if inbreeding avoidance were the only reason for dispersal. The evolutionary outcome is always complete philopatry by one sex. The rate of dispersal by the other sex depends on patch size and mating system, as well as inbreeding and dispersal costs. If such costs are sex independent, then two stable equilibria coexist (male or female philopatry), with symmetric domains of attraction. Which sex disperses is determined entirely by history, genetic drift, and gene flow. An asymmetry in costs makes one domain of attraction extend at the expense of the other. In such a case, the dispersing sex might also be, paradoxically, the one that incurs the higher dispersal costs. As asymmetry increases, one equilibrium eventually disappears, which may result in a sudden evolutionary shift in the identity of the dispersing sex. Our results underline the necessity to control for phylogenetic relationships (e.g., through the use of ...
TL;DR: The genetic and physiological mechanisms underlying “shade avoidance” responses in plants are reviewed, how genetic manipulation can elucidate their adaptive value is illustrated, and the use of physiological manipulation to measure natural selection on plasticity in the wild is discussed.
Abstract: Phenotypic plasticity is often assumed to be adaptive, but this hypothesis has rarely been tested. To support the hypothesis, it is necessary to demonstrate that the phenotype induced in each relevant environment confers high fitness in that environment, relative to alternative phenotypes. Unfortunately, such tests are difficult to perform because plasticity prevents the expression of "inappropriate" phenotypes within each environment. Genetic and physiological manipulation can be used very effectively to extend the range of phenotypes within environments and thus provide powerful tools for testing the adaptive plasticity hypothesis. The expression of specific genes involved in cue perception or signal transduction can be altered by mutation or the introduction of transgenes, thus altering the plastic response of an organism to environmental cues. It is also possible to alter the cue itself or to manipulate the developmental response physiologically so as to obtain alternative phenotypes. The re...
TL;DR: Both hypotheses were confirmed: species with high values of sprint sensitivity avoided using perches on which their maximal sprinting abilities are impaired, whereas species with low sprint sensitivity used such “submaximal” surfaces more frequently.
Abstract: Recent years have seen an increased emphasis on measuring ecologically relevant performance capabilities to understand associations between morphology and habitat use. Such studies presume that performance is invariant, but in eight Caribbean Anolis lizard species, we found that maximal sprinting ability depends on surface diameter. Moreover, these species differ in the degree to which sprint speed declines with decreasing surface diameter, defined as “sprint sensitivity” (high sprint \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}
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TL;DR: This article examines the degree of among‐population asynchrony in disease presence/absence and abundance, and the extent to which this is a function of isolation, and evaluates some predictions for numerical and genetic patterns at local and regional scales.
Abstract: Spatial structuring is important in understanding the ecological and evolutionary dynamics of natural populations since local demes are rarely, if ever, completely isolated from neighboring demes. Plant host-pathogen interactions provide good examples of coevolutionary systems where both numerical and genetic dynamics have been explicitly investigated in a spatial context and where genes under selection can be unambiguously identified. In this article, we focus on long-term studies of several natural host-pathogen interactions that span a range of life histories and taxa. We use these studies to evaluate some predictions for numerical and genetic patterns at local and regional scales. Specifically, we examine the degree of among-population asynchrony in disease presence/absence and abundance, and the extent to which this is a function of isolation. For one host-pathogen interaction (Linum-Melampsora), we focus on whether there is local correspondence between resistance and virulence genes (as would be predicted by single-population coevolutionary models) or whether such correspondence occurs at larger spatial scales. Finally, we discuss the implications of these studies with respect to the impact of host and pathogen life-history variation on the spatial scale of coevolutionary interactions. Understanding coevolutionary interactions in nature requires a multidisciplinary approach, including long-term empirical studies of multiple populations and computer modeling.
TL;DR: A reaction‐diffusion model is used to investigate the implications of mixing processes for the dynamics and species composition of phytoplankton blooms and predicts a low species diversity.
Abstract: With the eutrophication of many freshwaters and coastal environments, phytoplankton blooms have become a common phenomenon. This article uses a reaction-diffusion model to investigate the implications of mixing processes for the dynamics and species composition of phytoplankton blooms. The model identifies four key parameters for bloom development: incident light intensity, background turbidity, water column depth, and turbulent mixing rates. The model predicts that the turbulent mixing rate is a major determinant of the species composition of phytoplankton blooms. In well-mixed environments, the species with lowest "critical light intensity" should become dominant. But at low mixing rates, the species with lowest critical light intensity is displaced if other species obtain a better position in the light gradient. Instead of a gradual change in species composition, the model predicts steep transitions between the dominance regions of the various species. The model predicts a low species diversity: phytoplankton blooms in eutrophic environments should be dominated by one or a few species only. The model predictions are consistent with laboratory competition experiments and many existing field data. We recommend examining competition in phytoplankton blooms under well-controlled laboratory conditions, and we derive scaling rules that facilitate translation from the laboratory to the field.
TL;DR: No evidence that tolerance to apical meristem damage or tolerance to folivory was correlated with resistance, although these two types of tolerance were positively correlated with one another, and fluctuations from fluctuating herbivore loads may be responsible for maintaining tolerance at an intermediate level.
Abstract: Tolerance to herbivory minimizes the effects of herbivory on plant fitness. In the presence of herbivores, maximal levels of tolerance may be expected to evolve. In many plant species, however, tolerance is found at an intermediate level. Tolerance may be prevented from evolving to a maximal level by genetic constraints or stabilizing selection. We report on a field study of Ipomoea purpurea, the common morning glory, in which we measured three types of costs that may be associated with tolerance and the pattern of selection acting on tolerance to two types of herbivore damage: apical meristem damage and folivory. We used genetic correlations to test for the presence of three types of costs: a trade-off between tolerance and fitness in the absence of herbivore damage, a trade-off between tolerance and resistance, and genetic covariances among tolerance to different types of damage. We found no evidence that tolerance to apical meristem damage or tolerance to folivory was correlated with resistance, although these two types of tolerance were positively correlated with one another. Tolerance to both types of damage involved costs of lower fitness in the absence of herbivory. Selection acting on tolerance to either type of herbivory was not detected at natural levels of herbivory. Selection is expected to act against tolerance at reduced levels of herbivory and favor tolerance at elevated levels of herbivory. In addition, significant correlational selection gradients indicate that the pattern of selection acting on tolerance depends on values of resistance. Although we found no evidence for stabilizing selection, fluctuating selection resulting from fluctuating herbivore loads may be responsible for maintaining tolerance at an intermediate level.
TL;DR: It is found that linkage disequilibria may tend to increase, rather than decrease, additive genetic variance, which is consistent with the idea that selection for recombination is mediated by fluctuating epistasis.
Abstract: Host‐parasite coevolution has been shown to provide an advantage to recombination, but the selective mechanism underlying this advantage is unclear. One possibility is that recombination increases the frequency of advantageous genotypes that are disproportionately rare because of fluctuating epistasis. However, for this mechanism to work, epistasis for fitness must fluctuate over a very narrow timescale: two to five generations. Alternatively, recombination may speed up the response to directional selection by breaking up linkage disequilibria that decrease additive genetic variance. Here we analyze the results of a numerical simulation of host‐parasite coevolution to assess the importance of these two mechanisms. We find that linkage disequilibria may tend to increase, rather than decrease, additive genetic variance. In addition, the sign of epistasis changes every two to five generations under several of the parameter values investigated, and epistasis and linkage disequilibrium are frequently...