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This chapter introduces the concept of the consumer-resource link, the idea that each species in a community consumes resources and is itself consumed by other species. The consumer–resource link is the fundamental building block from which more-complex food chains and food webs are constructed. The chapter continues by exploring what is arguably the simplest consumer–resource interaction—one predator species feeding on one species of prey. Important topics discussed in the context of predator–prey interactions are the predator’s functional response, the Lotka–Volterra predator–prey model, the Rosenzweig–MacArthur predator–prey model, and the suppression-stability trade-off. Isocline analysis is introduced as a method for visualizing the outcome of species interactions at steady-state or equilibrium. Herbivory and parasitism are briefly discussed within the context of general predator–prey models.

This chapter is the essential beginner’s guide to the functional response, its derivation, the various forms, its connection to other models in the literature, and what the parameters mean. It is the ground floor for the rest of the book, covering the four main types of functional response, what the parameters mean in biological terms, and how we arrived at these equations. Surprisingly, our understanding of the functional response as represented in the literature is quite muddled, with confusion ranging from the terminology used, to the various mathematical forms the functional response takes, to the biological interpretation of functional response model parameters. I provide a summary and forward-looking perspective on these issues.

In this chapter I cover some key issues in fitting functional response models to data and determining the values of parameters. Because some of these issues have been covered elsewhere, here I focus on the nature of foraging trial data and why noise, stochasticity, and individual variation pose particular challenges for understanding functional responses. I examine several data sets to illustrate methods of determining differences in functional response parameters and types. I also show through simulations that individual variation in functional response parameters may account for the noisiness of foraging data and also lead to underestimates of both space clearance rate and handling time in curve-fitting approaches.

Being recognized for more than 70 years and estimated thousands of times, with numerous analyses of compilations, it would seem there is a lot we should know about functional responses. Indeed, we know some of the ways in which functional responses vary, how foraging mechanisms combine to determine, to at least some extent, functional response parameters, and how functional responses influence community interactions from biocontrol impacts to invasive predators to food webs. I suggest, however, that there remains a considerable amount that we do not know, in particular for field-based functional responses, multi-species functional responses, individual variation, behavioral mechanisms, and the impact and evolution of underlying traits. I suggest these areas should be high priorities for future work on functional responses.

Predator–prey interactions represent an essential component of natural systems. By consuming other organisms, predators transfer energy from lower trophic levels to higher trophic levels, simultaneously altering the abundance of prey and fueling growth of the predator population. The functional response describes the rate of foraging as a function of prey abundance, connecting predator–prey pairs in food webs. The functional response integrates nearly all aspects of biology, including genetics, morphology, behavior, parasites and disease, risk, and the abiotic environment. As a result, the functional response is a core construct that is essential for understanding and predicting the structure and dynamics of ecological systems. Because the functional response responds to temperature and other changes in the environment, the functional response is also essential for predicting the effects of climate change, managing and conserving species, and the evolution of interacting species.

The parameters of the functional response are not traits. They represent processes such as hunting and digesting prey. Thus, all the traits that influence the way predators and prey encounter each other in space and the morphologies and behaviors that influence capture, evasion, or digestion are all potential sources of variation in the functional response parameters. In this chapter, I cover how we break the parameters down mathematically so that the connection between the parameters and traits is more transparent. I review the empirical evidence for the dependence of functional response parameters on phenotypic traits, temperature, and habitats, and I showcase some examples of these effects.

In this chapter I review the many ways that functional responses may show a sigmoidal shape rather than the simpler asymptotic shape. I break down the potential for prey dependence of the space clearance rate through effects on each of the component mechanisms. Given the emergent nature of the functional response, type III curves can arise through density dependence of the probability of successful capture, prey detectability, and predator–prey encounter rates. Given the variety of mechanisms, it may be possible that there are really multiple types of type III curve. I also raise some concerns with the standard type III model and offer an alternative model that gets around these problems.

Historically, studies of the population ecology of mutualism have been largely divorced from the key principles (e.g., density dependence, functional responses, consumer–resource theory) that have shaped our understanding of the dynamics of predation and competition. The population ecology of mutualism has been slow to mature, largely due to unrealistic Lotka–Volterra theory. This chapter summarizes shortcomings of such historical models that stifled theory for decades, reviews the new generation of consumer–resource theory for the population dynamics of mutualism, synthesizes empirical studies of the density dependence of mutualism, and proposes future research directions. Contemporary advances in theory are emphasized, including mechanisms for the density dependence of mutualism for which research at the interface of theory and empiricism will be critical to future progress. The chapter builds a foundation for future theoretical and empirical work to enhance our understanding of the short-term transient dynamics and long-term stability properties of mutualistic interactions.

Climate change is likely to impact all trophic levels, although the response of communities and ecosystems to it has only recently received considerable attention. Further, it is expected to affect the magnitude of species interactions themselves. In this chapter, we summarize why and how climate change could affect predator–prey interactions, then review the literature about its impact on predator–prey relationships in birds, and provide prospects for future studies. Expected effects on prey or predators may include changes in the following: distribution, phenology, population density, behaviour, morphology, or physiology. We review the currently available information concerning particular key topics: top-down versus bottom-up control, specialist versus generalist predators, functional versus numerical responses, trophic cascades and regime shifts, and lastly adaptation and selection. Finally, we focus our review on two well-studied bird examples: seabirds and raptors. Key future topics include long-term studies, modelling and experimental studies, evolutionary questions, and conservation issues.

Predation and parasitism are dominant forms of interspecific interactions in aquatic ecosystems. Predation effects have been more commonly quantified in aquatic ecosystems than disease. Diet studies documenting predation are substantially more common that routine monitoring for disease in aquaculture systems. The simplest predator–prey models predict lagged cycles of prey and their predators. Density-dependent regulation of the prey or predator population is required for stable coexistence of predator and prey populations. Predator–prey models are extended with the incorporation of non-linear functional responses, which can result in multiple equilibria. The behavior and dynamics of natural predators hold important insights in our consideration of human predation on aquatic resource species. Disease outbreaks have wrought tremendous impacts on a very broad spectrum of aquatic species. For economically important species, these impacts include significant economic costs to fishing communities and aquaculture facilities.

This chapter starts with proving the inevitability of population growth regulation, and concludes with an explanation of the exclusive resource limitation principle that, through determining community structure, shapes the landscape surrounding us. Population regulation may be mediated by resource-limitation (Tilman model) or site-limitation (Levins model), or natural enemies like predators, parasites, or parasitoids. Food chain length sometimes determines the main quantitative features of complete communities through top-down regulation. All regulatory mechanisms share one feature: they feed back population abundance on population growth, ultimately setting strict limits on population growth and fluctuations, even if facilitation-induced positive ecological feedbacks (e.g. Allee effects) may act at low population sizes. The way of modelling interactions between individuals (e.g., functional responses) is explained and illustrated by examples. The relations of explicit (logistic) and implicit (process-based) models of population dynamics and some model-based interpretations of case studies and experimental results are shown.

In this chapter I show why there should be selection on traits associated with functional response parameters. I describe this using standard quantitative genetics techniques to show how a classic evolutionary arms race arises and how it depends on key features of the functional response. I suggest this arms race is more aptly described as a tug-of-war. I then show that selection on the predator and prey components of space clearance rate is synchronous for predator and prey through population cycles but alternating between predator and prey for handling time. I suggest that trade-offs, ecological pleiotropy, and phenotypic plasticity can slow natural selection on traits that influence functional response parameters.

In this chapter I consider the question of whether predators switch their preference for different types of prey as those prey change in abundance. There are numerous experiments in the literature focusing on this, but generally they have focused on a simplified analysis that ignores the functional response. Here I show why the functional response is crucial for understanding prey choice, and I show that null expectations considering a multi-species functional response lead to different interpretations than standard null expectations. I also derive null expectations for the proportion of prey consumed given the single-species functional response.

This chapter discusses the most widely applied field methods used in studies of predator food habits, and some commonly used approaches to make ecological inference from these data. It first describes the various field procedures used to quantify predator diet, and the assumptions and biases inherent in the different methods. The chapter then presents methods to make further inferences with these data, such as niche breadth, prey selection and estimation of kill rates, and functional responses; and more recently developed methods based on stable isotopes and studies of non-lethal effects of predation. Finally, further avenues for predator–prey studies and how to achieve them are discussed. The importance of studies combining methods based on predator diet and food habits with analysis of prey demography to gain further insight into predator–prey dynamics is underscored.

In this chapter, I show how the functional response can drive predator–prey cycles (and dynamics more generally). I introduce predator–prey differential equation models and fit them to real dynamic data on classic predator–prey systems (lynx–hare and Daphnia–algae). This coupling achieves two things. First, it allows me to demonstrate that the models are capable of describing real predator–prey dynamics and that the functional response really does have a role in driving predator–prey cycles (even if it is not the driver of all cycles). Second, it allows me, from an empirically grounded starting point, to vary the parameters of the functional response to show how changes in the functional response parameters change the dynamics.

In this chapter, I extend the standard functional response model to communities in which predators are foraging on more than one kind of prey. This is an essential component of real foraging scenarios that is not yet widely represented in the functional response literature but is crucial to understanding food webs generally. Here I develop the multi-species functional response and describe it using my particular perspective on how we understand these functions in general. I review the importance of considering multiple prey types and the limited empirical work estimating multi-species functional responses.

This chapter is a refresher on the prey model of classic optimal foraging theory through the lens of this book. I build on the multi-species functional response, the selection ideas, and the parameter breakdown presented in the preceding chapters to argue for how optimal foraging might arise. I rederive the models and suggest that optimal foraging theory may still be relevant to understanding predator–prey interactions, in particular in the context of multi-species functional responses. I also address the possibility that predators mostly have broad diets because they experience low prey abundances most of the time in nature.

Foraging is the set of processes by which organisms acquire energy and nutrients, whether the food is directly consumed (feeding), stored for later consumption (hoarding), or given to other individuals (provisioning). Foraging behavior plays an important role in evolutionary biology, not only because it is a major determinant of the survival, growth, and reproductive success of foragers but also because of its impact on predator avoidance, pollination, and dispersal adaptations of potential food organisms. From a contemporary perspective, it is surprising how generally the fundamental role of behavior was neglected in early-20th-century studies of evolution and ecology. Following the development of quantitative techniques and field-oriented approaches by European ethologists, however, interest in foraging, along with other aspects of behavior grew rapidly. Most of this research has sought to describe, explain, and predict foraging behavior quantitatively. The development of an a priori predictive approach using optimality theory, in particular, has revealed a richness and complexity in the patterns of foraging that could not have been imagined only a few decades ago. My goal in this chapter is to provide a brief overview of the main issues in foraging behavior and the logical basis of current approaches. I wish to highlight the successes and potential value of these approaches, while recognizing the gaps and challenges for future research. Contemporary studies of foraging by evolutionary ecologists are based on the synthesis of two research traditions, both emerging during the 1960s. The ethological approach to behavior is illustrated by the research of K. von Frisch and his associates on honeybee foraging and N. Tinbergen and his group on searching behavior of birds. The ethologists’ recognition of behavior as an evolved phenotype, their emphasis on its ecological context, and their careful quantitative and experimental fieldwork set the stage for behavioral ecology (Curio 1976). They classified the behavioral components of foraging, an important contribution to much of the ecological work that followed, and identified a number of widespread characteristics such as localized search following the discovery of a prey (“area-restricted search”) and enhanced detection following experience of a particular prey type (“search image”). The theoretical approach to population ecology was foreshadowed by the Russian V. S. Ivlev.