Beyond the ecological: biological invasions alter natural selection on a native plant species

Biological invasions can have strong ecological effects on native communities by altering ecosystem functions, species interactions, and community composition. Even though these ecological effects frequently impact the population dynamics and fitness of native species, the evolutionary consequences of biological invasions have received relatively little attention. Here, I show that invasions impose novel selective pressures on a native plant species. By experimentally manipulating community composition, I found that the exotic plant Medicago polymorpha and the exotic herbivore Hypera brunneipennis alter the strength and, in some instances, the direction of natural selection on the competitive ability and anti-herbivore defenses of the native plant Lotus wrangelianus. Furthermore, the community composition of exotics influenced which traits were favored. For example, high densities of the exotic herbivore Hypera selected for increased resistance to herbivores in the native Lotus; however, when Medicago also was present, selection on this defense was eliminated. In contrast, selection on tolerance, another plant defense trait, was highest when both Hypera and Medicago were present at high densities. Thus, multiple exotic species may interact to influence the evolutionary trajectories of native plant populations, and patterns of selection may change as additional exotic species invade the community.     

A model of ecological and evolutionary consequences of color polymorphism

We summarize direct and indirect effects on fitness components of animal color pattern and present a synthesis of theories concerning the ecological and evolutionary dynamics of chromatic multiple niche polymorphisms. Previous endeavors have aimed primarily at identifying conditions that promote the evolution and maintenance of polymorphisms. We consider in a conceptual model also the reciprocal influence of color polymorphism on population processes and propose that polymorphism entails selective advantages that may promote the ecological success of polymorphic species. The model begins with an evolutionary branching event from mono- to polymorphic condition that, under the influence of correlational selection, is predicted to promote the evolution of physical integration of coloration and other traits (cf. multi-trait coevolution and complex phenotypes). We propose that the coexistence within a population of alternative ecomorphs with coadapted gene complexes promotes utilization of diverse environmental resources, population stability and persistence, colonization success, and range expansions, and enhances the evolutionary potential and speciation. Conversely, we predict polymorphic populations to be less vulnerable to environmental change and at lower risk of range contractions and extinctions compared with monomorphic populations. We offer brief suggestions as to how these falsifiable predictions may be tested.     

Coexistence of the niche and neutral perspectives in community ecology

The neutral theory for community structure and biodiversity is dependent on the assumption that species are equivalent to each other in all important ecological respects. We explore what this concept of equivalence means in ecological communities, how such species may arise evolutionarily, and how the possibility of ecological equivalents relates to previous ideas about niche differentiation. We also show that the co-occurrence of ecologically similar or equivalent species is not incompatible with niche theory as has been supposed, because niche relations can sometimes favor coexistence of similar species. We argue that both evolutionary and ecological processes operate to promote the introduction and to sustain the persistence of ecologically similar and in many cases nearly equivalent species embedded in highly structured food webs. Future work should focus on synthesizing niche and neutral perspectives rather than dichotomously debating whether neutral or niche models provide better explanations for community structure and biodiversity.     

The growth-predation risk trade-off under a growing gape-limited predation threat

Growth is a critical ecological trait because it can determine population demography, evolution, and community interactions. Predation risk frequently induces decreased foraging and slow growth in prey. However, such strategies may not always be favored when prey can outgrow a predator's hunting ability. At the same time, a growing gape-limited predator broadens its hunting ability through time by expanding its gape and thereby creates a moving size refuge for susceptible prey. Here, I explore the ramifications of growing gape-limited predators for adaptive prey growth. A discrete demographic model for optimal foraging/growth strategies was derived under the realistic scenario of gape-limited and gape-unconstrained predation threats. Analytic and numerical results demonstrate a novel fitness minimum just above the growth rate of the gape-limited predator. This local fitness minimum separates a slow growth strategy that forages infrequently and accumulates low but constant predation risk from a fast growth strategy that forages frequently and experiences a high early predation risk in return for lower future predation risk and enhanced fecundity. Slow strategies generally were advantageous in communities dominated by gape-unconstrained predators whereas fast strategies were advantageous in gape-limited predator communities. Results were sensitive to the assumed relationships between prey size and fecundity and between prey growth and predation risk. Predator growth increased the parameter space favoring fast prey strategies. The model makes the testable predictions that prey should not grow at the same rate as their gape-limited predator and generally should grow faster than the fastest growing gape-limited predator. By focusing on predator constraints on prey capture, these results integrate the ecological and evolutionary implications of prey growth in diverse predator communities and offer an explanation for empirical growth patterns previously viewed to be anomalies.     

Evolving human landscapes: a virtual laboratory approach

ABSTRACT

Different human societies shape landscapes differently. Anthroecology theory explains this long-term differential shaping of landscapes as the product of sociocultural niche construction (SNC): an evolutionary theory coupling social change with ecosystem engineering. The evolutionary mechanisms underpinning this theory cannot be tested without experimental approaches capable of reproducing emergent selection processes acting on the combined suite of cultural, material, and ecological inheritances that determine the adaptive fitness of human individuals, groups, and societies. Agent-based modeling, as a ‘generative social science’ tool, appears ideal for this. Here we propose an agent-based virtual laboratory (ABVL) approach to generating and testing basic hypotheses on SNC as a general mechanism capable of producing the broadly varied anthroecological forms and dynamics of human landscapes from prehistory to present. While major challenges must still be overcome, a prospective modeling framework specification, guiding questions, and illustrative examples demonstrate clear potential for an ABVL to test predictions of anthroecology theory through generative social simulation.     

Behavioral type–environment correlations in the field: a study of three-spined stickleback

Behavioral type–environment correlations occur when specific behavioral types of individuals are more common in certain environments. Behavioral type–environment correlations can be generated by several different mechanisms that are probably very common such as niche construction and phenotypic plasticity. Moreover, behavioral type–environment correlations have important ecological and evolutionary implications. However, few studies have examined behavioral type–environment correlations in natural populations. In this study, we asked whether some behavioral types of three-spined stickleback were more likely to occur in certain social environments (alone or in a shoal with other stickleback) or in certain microhabitats in a river (in the open or under cover). We found that individuals that were in shoals with other stickleback at the time of collection from the field emerged from a refuge more quickly compared to individuals that were found alone. In addition, fish that were alone in an open microhabitat explored more of a pool compared to fish that were alone in cover, but this difference did not occur among fish that were in shoals at the time of collection. Subsequent analyses of gut contents suggested that differences in microhabitat use were consistent over time. Our study provides some of the first evidence for behavioral type–environment correlations in a natural population of non-human animals.     

What does ecological modelling model? A proposed classification of ecological niche models based on their underlying methods

Species distribution model is the term most frequently used in ecological modelling, but other authors used instead predictive habitat distribution model or species-habitat models. A consensual ecological modelling terminology that avoids misunderstandings and takes into account the ecological niche theory does not exist at present. Moreover, different studies differ in the type of niche that is represented by similar distribution models. I propose to use as standard ecological modelling terminology the terms “ecological niche”, “potential niche”, “realized niche” models (for modelling their respective niches), and “habitat suitability map” (for the output of the niche models). Therefore, the user can understand more easily that models always forecast species’ niche and relate more closely the different types of niche models.     

Niche and fitness differences relate the maintenance of diversity to ecosystem function

The frequently observed positive correlation between species diversity and community biomass is thought to depend on both the degree of resource partitioning and on competitive dominance between consumers, two properties that are also central to theories of species coexistence. To make an explicit link between theory on the causes and consequences of biodiversity, we define in a precise way two kinds of differences among species: niche differences, which promote coexistence, and relative fitness differences, which promote competitive exclusion. In a classic model of exploitative competition, promoting coexistence by increasing niche differences typically, although not universally, increases the "relative yield total", a measure of diversity's effect on the biomass of competitors. In addition, however, we show that promoting coexistence by decreasing relative fitness differences also increases the relative yield total. Thus, two fundamentally different mechanisms of species coexistence both strengthen the influence of diversity on biomass yield. The model and our analysis also yield insight on the interpretation of experimental diversity manipulations. Specifically, the frequently reported "complementarity effect" appears to give a largely skewed estimate of resource partitioning. Likewise, the "selection effect" does not seem to isolate biomass changes attributable to species composition rather than species richness, as is commonly presumed. We conclude that past inferences about the cause of observed diversity-function relationships may be unreliable, and that new empirical estimates of niche and relative fitness differences are necessary to uncover the ecological mechanisms responsible for diversity-function relationships.     

Habitat patch shape, not corridors, determines herbivory and fruit production of an annual plant

Habitat corridors confer many conservation benefits by increasing movement of organisms between habitat patches, but the benefits for some species may exact costs for others. For example, corridors may increase the abundance of consumers in a habitat to the detriment of the species they consume. In this study we assessed the impact of corridors on insect herbivory of a native plant, Solanum americanum, in large-scale, experimentally fragmented landscapes. We quantified leaf herbivory and assessed fruit production as a proxy for plant fitness. We also conducted field surveys of grasshoppers (Orthoptera), a group of abundant, generalist herbivores that feed on S. americanum, and we used exclosure cages to explicitly link grasshopper herbivory to fruit production of individual S. americanum. The presence of corridors did not increase herbivory or decrease plant fruit production. Likewise, corridors did not increase grasshopper abundance. Instead, patches in our landscapes with the least amount of edge habitat and the greatest amount of warmer "core" area had the highest levels of herbivory, the largest cost to plant fruit production as a result of herbivory, and the most grasshoppers. Thus habitat quality, governed by patch shape, can be more important than connectivity for determining levels of herbivory and the impact of herbivory on plant fitness in fragmented landscapes.     

Ecological determinism increases with organism size

After much debate, there is an emerging consensus that the composition of many ecological communities is determined both by species traits, as proposed by niche theory, as well as by chance events. A critical question for ecology is, therefore, which attributes of species predict the dominance of deterministic or stochastic processes. We outline two hypotheses by which organism size could determine which processes structure ecological communities, and we test these hypotheses by comparing the community structure in bromeliad phytotelmata of three groups of organisms (bacteria, zooplankton, and macroinvertebrates) that encompass a 10 000-fold gradient in body size, but live in the same habitat. Bacteria had no habitat associations, as would be expected from trait-neutral stochastic processes, but still showed exclusion among species pairs, as would be expected from niche-based processes. Macroinvertebrates had strong habitat and species associations, indicating niche-based processes. Zooplankton, with body size between bacteria and macroinvertebrates, showed intermediate habitat associations. We concluded that a key niche process, habitat filtering, strengthened with organism size, possibly because larger organisms are both less plastic in their fundamental niches and more able to be selective in dispersal. These results suggest that the relative importance of deterministic and stochastic processes may be predictable from organism size.     

Individual specialization in the hunting wasp <Emphasis Type="Italic">Trypoxylon</Emphasis> (<Emphasis Type="Italic">Trypargilum</Emphasis>) <Emphasis Type="Italic">albonigrum</Emphasis> (Hymenoptera,Crabronidae)

Individual-level variation in resource use occurs in a broad array of vertebrate and invertebrate taxa and may have important ecological and evolutionary implications. In this study, we measured the degree of individual-level variation in prey preference of the hunting wasp Trypoxylon albonigrum, which inhabits the Atlantic Forest in southeastern Brazil. This wasp captures several orb-weaving spider genera to provision nests. Individuals consistently specialized on a narrow subset of the prey taxa consumed by the population, indicating the existence of significant individual-level variation in prey preferences. The population niche was broader in the wet season in terms of both prey size and taxa. In the case of prey size, the population niche expansion was achieved via increased individual niche breadths, whereas in the case of prey taxa, individual niches remained relatively constrained, and the population niche expanded via increased interindividual variation. The observed pattern suggests the possibility of functional trade-offs associated with the taxon of the consumed prey. The nature of the trade-offs remains unknown, but they are likely related to learning in searching and/or handling prey. We hypothesize that by specializing on specific prey taxa, individuals increase foraging efficiency, reducing foraging time and ultimately increasing reproductive success.     

Nonadditive effects of floral display and spur length on reproductive success in a deceptive orchid

Pollinators may mediate selection on traits affecting pollinator attraction and effectiveness, and while nonadditive effects of traits influencing the two components of pollination success are expected when seed production is pollen limited, they have been little studied. In a factorial design, we manipulated one putative attraction trait (number of flowers) and one putative efficiency trait (spur length) previously shown to be subject to pollinator-mediated selection in the deceptive orchid Dactylorhiza lapponica. Removal of half of the flowers reduced pollen removal, proportion of flowers receiving pollen, fruit set, and fruit mass compared to unmanipulated plants, while spur-tip removal increased fruit set and fruit mass but did not affect pollen removal or proportion of flowers receiving pollen. The effect of spur-tip removal on fruit mass was stronger among plants with intact number of flowers compared to plants with experimentally reduced number of flowers. The results demonstrate that number of flowers and spur length are direct targets of selection and may affect female fitness nonadditively. More generally, they show that the adaptive value of a given trait can depend on floral context and illustrate how experimental approaches can advance our understanding of the evolution of trait combinations.     

A trait-based test for habitat filtering: convex hull volume

Community assembly theory suggests that two processes affect the distribution of trait values within communities: competition and habitat filtering. Within a local community, competition leads to ecological differentiation of coexisting species, while habitat filtering reduces the spread of trait values, reflecting shared ecological tolerances. Many statistical tests for the effects of competition exist in the literature, but measures of habitat filtering are less well-developed. Here, we present convex hull volume, a construct from computational geometry, which provides an n-dimensional measure of the volume of trait space occupied by species in a community. Combined with ecological null models, this measure offers a useful test for habitat filtering. We use convex hull volume and a null model to analyze California woody-plant trait and community data. Our results show that observed plant communities occupy less trait space than expected from random assembly, a result consistent with habitat filtering.     

The consequences of genetic diversity in competitive communities

Several lines of evidence suggest that the species diversity and composition of communities should depend on genetic diversity within component species, but there has been very little effort to directly assess this possibility. Here I use models of competition among genotypes and species to demonstrate a strong positive effect of the number of genotypes per species on species diversity across a range of conditions. Genetic diversity allows species to respond to selection imposed by competition, resulting in both functional convergence and divergence among species depending on their initial niche positions. This ability to respond to selection promotes species coexistence and contributes to a reduction in variation in species composition among communities. These models suggest that whenever individual fitness depends on the degree of functional similarity between a focal individual and its competitors, genetic diversity should promote species coexistence; this prediction is consistent with the few relevant empirical data collected to date. The results point to the importance of considering the genetic origin and diversity of material used in ecological experiments and in restoration efforts, in addition to highlighting potentially important community consequences of the loss of genetic diversity in natural populations.     

A trait specific model of competition in a spatially structured plant community

The non-random dispersal of plant propagules is thought to counter competitive exclusion and thus promote the survival of competitively inferior species. We investigated this process by modelling the outcome of interactions between species with competitive ability defined as a function of both life-history traits and the environment with both random and clustered dispersal strategies and in environmentally homogeneous and heterogeneous environments. Four main results emerged: (1) environmental heterogeneity was seen to promote co-existence in conjunction with associated trait variation for tolerance to the environmental variable and where this trait variation was effectively limited by ‘trade-off’ such that no single species had an overall competitive advantage, (2) consistent with theory, random dispersal appeared to enhance the likelihood of competitive exclusion, whereas clustering favoured co-existence, (3) the ecological outcome of interactions between dispersal and competitive relationships varied as a function of the trait determining the competitive advantage within a particular environment, and (4) promotion of co-existence by clustered dispersal was most marked when associated with environmental heterogeneity. It is argued that these results suggest that current ecological models of species interactions may need to be modified to incorporate a more realistic understanding of competitive ability if we are to better understand the factors effecting species co-existence.     

Mechanisms of nonlethal predator effect on cohort size variation: ecological and evolutionary implications

Understanding the factors responsible for generating size variation in cohorts of organisms is important for predicting their population and evolutionary dynamics. We group these factors into two broad classes: those due to scaling relationships between growth and size (size-dependent factors), and those due to individual trait differences other than size (size-independent factors; e.g., morphology, behavior, etc.). We develop a framework predicting that the nonlethal presence of predators can have a strong effect on size variation, the magnitude and sign of which depend on the relative influence of both factors. We present experimental results showing that size-independent factors can strongly contribute to size variation in anuran larvae, and that the presence of a larval dragonfly predator reduced expression of these size-independent factors. Further, a review of a number of experiments shows that the effect of this predator on relative size variation of a cohort ranged from negative at low growth rates to positive at high growth rates. At high growth rates, effects of size-dependent factors predominate, and predator presence causes an increase in the scaling of growth rate with size (larger individuals respond less strongly to predator presence than small individuals). Thus predator presence led to an increase in size variation. In contrast, at low growth rates, size-independent factors were relatively more important, and predator presence reduced expression of these size-independent factors. Consequently, predator presence led to a decrease in size variation. Our results therefore indicate a further mechanism whereby nonlethal predator effects can be manifest on prey species performance. These results have strong implications for both ecological and evolutionary processes. Theoretical studies indicate that changes in cohort size variation can have profound effects on population dynamics and stability, and therefore the mere presence of a predator could have important ecological consequences. Further, changes in cohort size variation can have important evolutionary implications through changes in trait heritability.     

半干旱区人工封育草场植被群落生态位研究——以宁夏盐池县长期定位监测点为例

人工封育是草场恢复和重建的一种主要措施。为研究半干旱区人工封育草地植物种群生态位,以宁夏盐池县人工封育区3种不同处理方式（长期完全封育、短期完全封育、短期不完全封育）为研究对象,将封育区分为封育核心区（E区）、封育边缘区（E1区）和封育外围区（E2区）,通过分析在该地区出现的27种植物种群的Levins生态位宽度指数和Pianka生态位重叠指数,揭示植物种群生态位在干旱荒漠环境下的变化规律,为进一步分析该地区群落稳定性提供依据。研究结果表明：1）核心区和外围区生态位宽度最大的都是刺沙蓬Salsola ruthenica lljin,分别为0.832和0.810,而边缘区生态位宽度最大的是丝叶山苦荬Lxeris chinensis var.graminifolia,为0.742,这2种植物的生物学特性决定了它们能够很好的适应封育区的环境,并很好的利用封育区的资源,结果是它们的生态位宽度明显高于其他物种。2）对三区内不同植物种群的生态位宽度分析结果显示,三区的群落稳定性表现为E1区〉E2区〉E区;长期的完全封育并不利于封育区内植物群落的稳定;而短期的完全封育措施比不完全封育措施更有利于植被群落的稳定。3）通过分析三区内不同植物种之间的生态位重叠得出,E区生态位重叠最大值出现在草地风毛菊Saussurea amara、苦豆子Sophora alopecuroides L、沙芦草Agropyron mongolicum Keng与臭蒿Artemisia hedinii之间,都是0.98。E1区生态位重叠最大值是达乌里胡枝子Lespedeza davurica和草木樨状黄芪Astragalus melilotoides之间（0.82）。E2区生态位重叠最大的一对是草木樨状黄芪和冰草Agropyron cristatum（0.99）。相关分析表明：生态位宽度与生态位重叠之间并没有明确的线性关系。生态位宽度窄的物种之间可能有较高的生态位重叠,反之,则较低,但并不是完全如此。     

Photosynthetic resource-use efficiency and demographic variability in desert winter annual plants

We studied a guild of desert winter annual plants that differ in long-term variation in per capita reproductive success (lb, the product of per capita survival from germination to reproduction, l, times per capita reproduction of survivors, b) to relate individual function to population and community dynamics. We hypothesized that variation in lb should be related to species' positions along a trade-off between relative growth rate (RGR) and photosynthetic water-use efficiency (WUE) because lb is a species-specific function of growing-season precipitation. We found that demographically variable species have greater RGR and greater leaf carbon isotope discrimination (Delta, a proxy inversely related to WUE). We examined leaf nitrogen and photosynthetic characteristics and found that, in this system, variation in Delta is a function of photosynthetic demand rather than stomatal regulation of water loss. The physiological characteristics that result in low Delta in some species may confer greater photosynthetic performance during the reliably moist but low temperature periods that immediately follow winter rainfall in the Sonoran Desert or alternatively during cool periods of the day or early growing season. Conversely, while species with high Delta and high RGR exhibit low leaf N, they have high biomass allocation to canopy leaf area display. Such trait associations may allow for greater performance during the infrequent conditions where high soil moisture persists into warmer conditions, resulting in high demographic variance. Alternatively, high variance could arise from specialization to warm periods of the day or season. Population dynamic buffering via stress tolerance (low RGR and Delta) correlates negatively with buffering via seed banks, as predicted by bet-hedging theory. By merging analyses of population dynamics with functional trait relationships, we develop a deeper understanding of the physiological, ecological, and evolutionary mechanisms involved in population and community dynamics.     

A model for the evolution of communal foraging in hierarchically structured populations

A kin selection model is described for populations in which groups of interacting individuals (trait groups, sensu Wilson 1975) are spatially situated within larger aggregations. The model predicts the optimal foraging strategy when resources are shared with other trait group members and there is an individual risk in foraging. The ecological mechanism of variation in group fitness, differential resource accumulation, is explicitly incorporated into the model. The optimal foraging rate obtained from this model depends on the product of a benefit-to-cost ratio and a relatedness parameter. The appropriate definition of relatedness for the evolution of communal foraging is determined by the details of the ecological interaction between consumers and resources. When competition is purely intra-specific, the genetic correlation among interactants relative to other members of the local aggregation defines the relatedness parameter applicable to selection on foraging propensity. When competition is primarily inter-specific, the genetic correlation among trait group members relative to the entire population defines relatedness.     

Neutral theory and the evolution of ecological equivalence

Since the publication of the unified neutral theory in 2001, there has been much discussion of the theory, pro and con. The hypothesis of ecological equivalence is the fundamental yet controversial idea behind neutral theory. Assuming trophically similar species are demographically alike (symmetric) on a per capita basis is only an approximation, but it is equivalent to asking: How many of the patterns of ecological communities are the result of species similarities, rather than of species differences? The strategy behind neutral theory is to see how far one can get with the simplification of assuming ecological equivalence before introducing more complexity. In another paper, I review the empirical evidence that led me to hypothesize ecological equivalence among many of the tree species in the species-rich tropical forest on Barro Colorado Island (BCI). In this paper, I develop a simple model for the evolution of ecological equivalence or niche convergence, using as an example evolution of the suite of life history traits characteristic of shade tolerant tropical tree species. Although the model is simple, the conclusions from it seem likely to be robust. I conclude that ecological equivalence for resource use are likely to evolve easily and often, especially in species-rich communities that are dispersal and recruitment limited. In the case of the BCI forest, tree species are strongly dispersal- and recruitment-limited, not only because of restricted seed dispersal, but also because of low recruitment success due to heavy losses of the seedling stages to predators and pathogens and other abiotic stresses such as drought. These factors and the high species richness of the community strongly reduce the potential for competitive exclusion of functionally equivalent or nearly equivalent species.