Ecological boundaries and constraints on viable eco-evolutionary pathways [Pre-print]

Evolutionary dynamics are subject to constraints ranging from limitations on what is physically possible to limitations on the pathways that evolution can take. One set of evolutionary constraints, known as ‘demographic constraints’, constrain what can occur evolutionarily due to the population demographic or population dynamical consequences of evolution leading to conditions that make populations susceptible to extinction. These demographic constraints can limit the strength of selection or rates of environmental change populations can experience while remaining extant and the trait values a population can express. Here we further hypothesize that the population demographic and population dynamical consequences of evolution also can constrain the eco-evolutionary pathways that populations can traverse by defining ecological boundaries represented by areas of likely extinction. We illustrate this process using a model of predator evolution. Our results show that the populations that persist over time tend to be those whose eco-evolutionary dynamics have avoided ecological boundaries representing areas of likely extinction due to stochastic deviations from a deterministic eco-evolutionary expectation. We term this subset of persisting pathways viable eco-evolutionary pathways. The potential existence of ecological boundaries constraining evolutionary pathways has important implications for predicting evolutionary dynamics, interpreting past evolution, and understanding the role of stochasticity and ecological constraints on eco-evolutionary dynamics

Predator-Dependent Functional Responses Alter the Coexistence and Indirect Effects among Prey that Share a Predator

Predator functional responses describe predator feeding rates as a function of prey abundance and are central to pred-ator–prey theory. Despite ample evidence that functional responses also depend on predator abundance, theory incor-porating predator-dependent functional responses has focused almost exclusively on specialist predator–prey pairs or linear food chains. This leaves a large gap in our knowledge as many predators feed on multiple prey, and in so doing, generate indirect effects among prey that can alter their coexistence. Here we investigate how predator-dependent functional responses in a one predator–two prey model alter the coexistence among prey and their net effects on one another. We use two different functional response forms (the Beddington–DeAngelis and Crowley–Martin functional responses) and consider situations in which the prey do not directly interact and in which they directly compete with one another. We find that predator dependence can facilitate, hinder, or have no effect on prey coexistence depending on whether prey compete directly and the role of predation in mediating coexistence among the prey in the absence of predator dependence. We also show that the negative net effects of prey on one another are generally weakened by predator dependence and can become positive under the Crowley–Martin functional response. Together, these results suggest that predator dependence may have widespread effects on ecological communities by altering the coexistence among prey species and the strength and signs of the interactions among them

Ecological boundaries and constraints on viable eco-evolutionary pathways

Evolutionary dynamics are subject to constraints ranging from limitations on what is physically possible to limitations on the pathways that evolution can take. One set of evolutionary constraints, known as ‘demographic constraints’, constrain what can occur evolutionarily due to the demographic or dynamical consequences of evolution leading to conditions that make populations susceptible to extinction. These demographic constraints can limit the strength of selection or the rates of environmental change populations can experience while remaining extant and the trait values a population can express. Here we further hypothesize that the population demographic and dynamic consequences of evolution also can constrain the eco-evolutionary pathways that populations can traverse by defining ecological boundaries represented by areas of likely extinction. We illustrate this process using a model of predator evolution. Our results show that the populations that persist over time tend to be those whose eco-evolutionary dynamics have avoided ecological boundaries representing areas of likely extinction due to stochastic deviations from a deterministic eco-evolutionary expectation. We term this subset of persisting pathways viable eco-evolutionary pathways. The potential existence of ecological boundaries constraining evolutionary pathways has important implications for predicting evolutionary dynamics, interpreting past evolution, and understanding the role of stochasticity and ecological constraints on eco-evolutionary dynamics

Ecological boundaries and constraints on viable eco-evolutionary pathways

Evolutionary dynamics are subject to constraints ranging from limitations on what is physically possible to limitations on the pathways that evolution can take. One set of evolutionary constraints, known as ‘demographic constraints’, constrain what can occur evolutionarily due to the demographic or dynamical consequences of evolution leading to conditions that make populations susceptible to extinction. These demographic constraints can limit the strength of selection or the rates of environmental change populations can experience while remaining extant and the trait values a population can express. Here we further hypothesize that the population demographic and dynamic consequences of evolution also can constrain the eco-evolutionary pathways that populations can traverse by defining ecological boundaries represented by areas of likely extinction. We illustrate this process using a model of predator evolution. Our results show that the populations that persist over time tend to be those whose eco-evolutionary dynamics have avoided ecological boundaries representing areas of likely extinction due to stochastic deviations from a deterministic eco-evolutionary expectation. We term this subset of persisting pathways viable eco-evolutionary pathways. The potential existence of ecological boundaries constraining evolutionary pathways has important implications for predicting evolutionary dynamics, interpreting past evolution, and understanding the role of stochasticity and ecological constraints on eco-evolutionary dynamics

Predator feeding rates may often be unsaturated under typical prey densities

Predator feeding rates (described by their functional response) must saturate at high prey densities. Although thousands of manipulative functional response experiments show feeding rate saturation at high densities under controlled conditions, it remains unclear how saturated feeding rates are at natural prey densities. The general degree of feeding rate saturation has important implications for the processes determining feeding rates and how they respond to changes in prey density. To address this, we linked two databases—one of functional response parameters and one on mass–abundance scaling—through prey mass to calculate a feeding rate saturation index. We find that: (1) feeding rates may commonly be unsaturated and (2) the degree of saturation varies with predator and prey taxonomic identities and body sizes, habitat, interaction dimension and temperature. These results reshape our conceptualization of predator–prey interactions in nature and suggest new research on the ecological and evolutionary implications of unsaturated feeding rates

Candidate Causes, Consequences, and Estimation of Individual Diet Specialization

Diet variation among individuals within populations is widespread. Often diet differences among individuals are attributable to obvious differences among individuals such as age, sex, or morphology. However, growing evidence suggests that individual diet variation is also common among seemingly identical individuals within populations. This phenomenon has been termed individual diet specialization. Individual diet specialization has been documented across a variety of taxa and biomes and theory suggests that diet specialization can potentially alter the structure and strength of predator-prey interactions. This raises two important questions: 1) What are the causes of individual diet specialization?, and 2) What are the potential consequences of diet specialization for populations and communities? In this dissertation, I attempt to address these two questions by combining mathematical theory, the novel application of statistical methods, and field and laboratory experiments with the intertidal whelk, extit{Nucella ostrina}. A potential ultimate cause of variation among individuals is disruptive selection in which natural selection favors individuals with more extreme trait values over individuals with intermediate trait values. Theory has suggested that the availability of alternative resources and intraspecific competition for those resources can drive disruptive selection in consumers and lead to increased diet variation. However, this theory makes several ecologically unrealistic assumptions. In particular, this theory assumes that consumers have linear functional responses and that the trait of the consumer under selection only influences the consumer's attack rates on resources. In Chapter 2, I alleviate these assumptions and show that nonlinear functional responses and traits influencing multiple functional response parameters simultaneously can influence the strength and likelihood of disruptive selection. My results suggest the characteristics of consumers in which disruptive selection in resource-use traits may occur and diet specialization through this mechanism may be most likely. To empirically evaluate hypotheses on the causes and consequences of individual diet specialization, we need to be able to accurately quantify diet specialization. In Chapter 3, I apply Bayesian hierarchical models to the problem of estimating diet specialization and compare the performance of the Bayesian hierarchical models to currently used methods for estimating diet specialization. Currently used methods infer individual prey preferences using the observed proportion of prey in individuals' diets whereas the Bayesian hierarchical models instead estimate these proportions. I find that the currently used approach tends to overestimate diet specialization compared to the Bayesian hierarchical approach. This is especially the case when sample sizes per individual are low or heterogeneous. In addition, the Bayesian hierarchical approach provides estimates of prey proportions, their variability, and the uncertainty on these estimates in ways that are inaccessible to current methods. These results suggest that the Bayesian hierarchical method can provide an improved method for quantifying diet specialization. In Chapter 4, I present the results from a field caging experiment examining the proximate ecological mechanisms determining individual diet specialization and its consequences in the intertidal whelk, extit{Nucella ostrina}. Many of the hypotheses on the ecological causes of diet specialization assume that individuals differ from one another in their prey preferences. However, these hypotheses ignore the potential influence of stochasticity in the foraging process in generating diet variation among individuals. The results of this chapter suggest that changes in the magnitude of diet variation with changes in prey community composition in Nucella ostrina can largely be explained by stochastic foraging by individuals with shared prey preferences. In this chapter, I also estimate the consequences of this diet variation for estimates of predator feeding rates through nonlinear averaging (Jensen's inequality) of predator attack rates. The results suggest that nonlinear averaging alters the perceived strength of predator-prey interactions in this system providing one of the first empirical estimates of the potential consequences of diet variation. Overall, my dissertation provides several insights into the potential causes of diet specialization, provides one of the first empirical estimates of a possibly widespread consequence of diet specialization for populations and communities, and suggests improved statistical methods for quantifying diet specialization. I believe that this dissertation will lead to a critical assessment of definitions of individual diet specialization, provide guidance towards systems in which diet specialization is the most likely to occur, and encourage further empirical research estimating the effects of diet specialization on populations and communities

Quantifying predator functional responses under field conditions reveals interactive effects of temperature and interference with sex and stage

Predator functional responses describe predator feeding rates and are central to predator–prey theory. Originally defined as the relationship between predator feeding rates and prey densities, it is now well known that functional responses are shaped by a multitude of factors. However, much of our knowledge about how these factors influence functional responses is based on laboratory studies that are generally logistically constrained to examining only a few factors simultaneously and that have unclear links to the conditions organisms experience in the field. We apply an observational approach for measuring functional responses to understand how sex/stage differences, temperature and predator densities interact to influence the functional response of zebra jumping spiders on midges under natural conditions. We used field surveys of jumping spiders to infer their feeding rates and examine the relationships between feeding rates, sex/stage, midge density, predator density and temperature using generalized additive models. We then used the relationships supported by the models to fit parametric functional responses to the data. We find that feeding rates of zebra jumping spiders follow some expectations from previous laboratory studies such as increasing feeding rates with body size and decreasing feeding rates with predator densities. However, in contrast to previous results, our results also show a lack of temperature response in spider feeding rates and differential decreases in the feeding rates of females and juveniles with densities of different spider sexes/stages. Our results illustrate the multidimensional nature of functional responses in natural settings and reveal how factors influencing functional responses can interact with one another through behaviour and morphology. Further studies investigating the influence of multiple mechanisms on predator functional responses under field conditions will increase our understanding of the drivers of predator–prey interaction strengths and their consequences for communities and ecosystems

Influence of Sediment Characteristics on the Composition of Soft-Sediment Intertidal Communities in the Northern Gulf of Mexico

Benthic infaunal communities are important components of coastal ecosystems. Understanding the relationships between the structure of these communities and characteristics of the habitat in which they live is becoming progressively more important as coastal systems face increasing stress from anthropogenic impacts and changes in climate. To examine how sediment characteristics and infaunal community composition were related along the northern Gulf of Mexico coast, we sampled intertidal infaunal communities at seven sites covering common habitat types at a regional scale. Across 69 samples, the communities clustered into four distinct groups on the basis of faunal composition. Nearly 70% of the variation in the composition of the communities was explained by salinity, median grain size, and total organic content. Our results suggest that at a regional level coarse habitat characteristics are able to explain a large amount of the variation among sites in infaunal community structure. By examining the relationships between infaunal communities and their sedimentary habitats, we take a necessary first step that will allow the exploration of how changes in habitat and community composition influence higher trophic levels and ecosystem scale processes

Influence of Sediment Characteristics on the Composition of Soft-Sediment Intertidal Communities in the Northern Gulf of Mexico

Benthic infaunal communities are important components of coastal ecosystems. Understanding the relationships between the structure of these communities and characteristics of the habitat in which they live is becoming progressively more important as coastal systems face increasing stress from anthropogenic impacts and changes in climate. To examine how sediment characteristics and infaunal community composition were related along the northern Gulf of Mexico coast, we sampled intertidal infaunal communities at seven sites covering common habitat types at a regional scale. Across 69 samples, the communities clustered into four distinct groups on the basis of faunal composition. Nearly 70% of the variation in the composition of the communities was explained by salinity, median grain size, and total organic content. Our results suggest that at a regional level coarse habitat characteristics are able to explain a large amount of the variation among sites in infaunal community structure. By examining the relationships between infaunal communities and their sedimentary habitats, we take a necessary first step that will allow the exploration of how changes in habitat and community composition influence higher trophic levels and ecosystem scale processes

Quantifying predator dependence in the functional response of generalist predators

A long-standing debate concerns how functional responses are best described. Theory suggests that ratio dependence is consistent with many food web patterns left unexplained by the simplest prey-dependent models. However, for logistical reasons, ratio dependence and predator dependence more generally have seen infrequent empirical evaluation and then only so in specialist predators, which are rare in nature. Here we develop an approach to simultaneously estimate the prey-specific attack rates and predator-specific interference (facilitation) rates of predators interacting with arbitrary numbers of prey and predator species in the field. We apply the approach to surveys and experiments involving two intertidal whelks and their full suite of potential prey. Our study provides strong evidence for predator dependence that is poorly described by the ratio dependent model over manipulated and natural ranges of species abundances. It also indicates how, for generalist predators, even the qualitative nature of predator dependence can be prey-specific