Acerca del autor

Andre M. de Roos is professor of theoretical ecology at the University of Amsterdam. Lennart Persson is professor of aquatic ecology at Umea University in Sweden.

De la contraportada

"This important and timely book is the best discussion of structured population modeling currently available. De Roos and Persson are true experts in this field and their arguments have particularly significant implications in both applied and basic ecology. Very few others could write such a book."--Alan Hastings, University of California, Davis

"This is probably the most important new book on animal population dynamics to appear in a decade. It provides a lucid exposition of a coherent, individual-based approach to population dynamics based on fundamental bioenergetic principles. This book has the potential to become a classic."--Roger M. Nisbet, coauthor of Consumer-Resource Dynamics

De la solapa interior

"This important and timely book is the best discussion of structured population modeling currently available. De Roos and Persson are true experts in this field and their arguments have particularly significant implications in both applied and basic ecology. Very few others could write such a book."--Alan Hastings, University of California, Davis

"This is probably the most important new book on animal population dynamics to appear in a decade. It provides a lucid exposition of a coherent, individual-based approach to population dynamics based on fundamental bioenergetic principles. This book has the potential to become a classic."--Roger M. Nisbet, coauthor of Consumer-Resource Dynamics

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Population and Community Ecology of Ontogenetic Development

PRINCETON UNIVERSITY PRESS

Contents

Preface............................................................................................................................................................ix1. Summary: A Bird's-Eye View of Community and Population Effects of Ontogenetic Development.......................................................................32. Life History Processes, Ontogenetic Development, and Density Dependence.........................................................................................243. Biomass Overcompensation........................................................................................................................................494. Emergent Allee Effects through Biomass Overcompensation.........................................................................................................1155. Emergent Facilitation among Predators on Size-Structured Prey...................................................................................................1656. Ontogenetic Niche Shifts........................................................................................................................................1967. Mixed Interactions..............................................................................................................................................2538. Ontogenetic Niche Shifts, Predators, and Coexistence among Consumer Species.....................................................................................2969. Dynamics of Consumer-Resource Systems...........................................................................................................................32910. Dynamics of Consumer-Resource Systems with Discrete Reproduction: Multiple Resources and Confronting Model Predictions with Empirical Data.....................36111. Cannibalism in Size-Structured Systems.........................................................................................................................39112. Demand-Driven Systems, Model Hierarchies, and Ontogenetic Asymmetry............................................................................................425References.........................................................................................................................................................505Index..............................................................................................................................................................525

Chapter One

A Bird's-Eye View of Community and Population Effects of Ontogenetic Development

Why start with summarizing the contents of a book? In the present case we see at least two good reasons. First, the amount of information provided in our book is without doubt quite massive. A summary provides an overview of the many topics that we cover and, we hope, reduces the risk that the reader will get lost in details and can no longer see the forest through the trees. Second, and partly related to the first reason, a number of, in our mind, novel and fundamental insights (some of them were not even known to us when we started to write the book!) are advanced. Here a summary serves the purpose of clearly showing how different chapters fit together in a general framework with respect to model approaches as well as results obtained. Reading this summary chapter will show you the different types of community modules that we analyze (summarized in figure 1.3) and give you a clear impression of the results and insights that we present in this book. Most of all, we hope it will serve as an encouragement to delve in more detail into the chapters that follow.

HISTORICAL BACKGROUND

Ecology has a long tradition of building theory about the dynamics of populations and the structure of communities that emerge from it. The early foundations for this theory were established by the pioneering work of Alfred Lotka and Vito Volterra, who formulated the most simple models for the dynamics of populations engaged in competitive and predator-prey interactions. Lotka-Volterra models have been used widely and form the basis of most, if not all, of our current theory on population and community processes. Because of their simplicity Lotka-Volterra models have often been viewed as leading to predictions that are general and representative for many different systems. And yet, these models completely ignore one of the most important processes in an individual's life history—one that is, moreover, unique to biology: development.

Lotka-Volterra models use population abundance as the state variable to characterize a population and describe changes therein using rather abstract demographic and community parameters such as population growth rates and coefficients of interaction between species. The dynamics of a population is described as the balance between reproduction and mortality, which increase and decrease abundance, respectively. In essence, these two processes are not unique to biological systems, as synthesis of particles from substituent elements and degradation of particles also occurs in, for example, chemical systems. Analogously, increases and decreases in population abundance result from the reproduction and mortality, respectively, of individual organisms. However, no individual organism of any species can reproduce right after it has been born, nor is its chance to die the same throughout all stages of its life history. Individual organisms go through an ontogenetic development, which is a major component of their life history. In fact, given that many individuals die before they manage to reproduce, ontogenetic development can be considered the most prominent life history process after mortality.

The intricacies of an individual's life history, including its ontogenetic development, have forever fascinated ecologists and have been studied extensively. Nonetheless, the absence of ontogenetic development as an important life history process from the core models of ecological theory has not been cause for major concern, nor has it been considered an important omission. The progression through different life stages has been accounted for in matrix models that focus on the potential of population growth of single populations, but when considering interactions between species and models of larger communities, the simplification of the population to a number of individuals without distinguishing between these individuals has been the rule. Basic ecological models thus ignore without much qualm the most prominent process in an individual's life history, one that is unique to biological systems and has no counterpart in physical and chemical systems.

The most important aspect of ontogenetic development in virtually all species is an increase in body size. Body size, in turn, determines to a large extent an individual's ecology in terms of its feeding, growth and reproduction rate, the food sources it can exploit, and the predators to which it is exposed. Moreover, in most species growth in body size is plastic, dependent on the environmental conditions the individual experiences, in particular, the availability of food resources that are required for its maintenance, growth, and reproduction. The consequences of such plastic and food-dependent ontogenetic development for the dynamics of populations and the structure of communities are the subject of this book. We investigate these population and community consequences using a collection of models that differ in the amount of detail with which the individual life history is represented (figure 1.1). All models account for growth in body size as the most important aspect of ontogenetic development and rigorously adhere to a mass-conservation principle. Thus, they explicitly account for the processes by which energy is acquired (feeding) and through which energy is spent (maintenance, growth, and reproduction). We also confront the predictions derived from these models as much as possible with experimental and empirical data that we either collected ourselves or that are available in the literature. More specifically, we present experimental and/ or empirical evidence for the majority of phenomena that we deduce from our model analysis.

We use two basic model formulations: a fully size-structured model formulation with an explicit handling of continuous size distributions and a stage-based formulation with two stages (juveniles, adults). Most important, the latter can be shown to be a faithful approximation to the fully size-structured model formulation, as model results of the two formulations are completely identical under equilibrium conditions. In the stage-based formulation, the juvenile stage increases in biomass as a result of adult reproduction and somatic growth of juvenile individuals, whereas adult biomass increases as a result of maturation of juvenile biomass into adult biomass (figure 1.1, top panel). Significantly, all these rates are dependent on the resource biomass level. Biomass is lost from the system through mortality and energetic costs for maintenance. In the fully size-structured model formulation, individuals grow in body size over ontogeny to reach maturity at a specific size (figure 1.1, bottom panel). The rate of increase in size is a function of net intake (intake minus maintenance costs). Net intake and reproduction rates are also in this case dependent on resource biomass density.

A crucial question to ask when considering community effects of ontogenetic development is whether it matters that basic ecological models ignore this process when modeling population dynamics. In our opinion, the answer is obviously yes, as we otherwise would not have written this book. The book underpins this assertion by presenting the major consequences of ontogenetic development for community structure and population dynamics. Furthermore, we will show that the conditions under which development does not have a significant community influence represent only a limiting case. We hence postulate that the ecological theory based on Lotka-Volterra models is all but a limiting case of a more general population and community theory.

BIOMASS OVERCOMPENSATION

Arguably the most important finding presented in the book is that an increase in mortality of a population can lead to an increase in its biomass, which runs counter to all our intuitive ideas about the consequences of mortality. Figure 1.2 illustrates this phenomenon using the stage-based formulation schematically presented above for a consumer population, but in contrast to what is assumed in figure 1.1, juvenile and adult individuals forage on two different resources with otherwise exactly identical mass-specific feeding rates (this model is analyzed further in chapter 6). When individual consumers forage to an equal extent on both resources (50 percent foraging effort on both resource 1 and 2, respectively), irrespective of whether they are juvenile or adult, an increase in mortality of all consumers leads to the intuitively obvious result that total consumer biomass decreases (figure 1.2, left). Furthermore, the increase in mortality does not change the ratio between juvenile and adult consumer biomass in equilibrium. The relative composition of the population hence remains the same. Note that we have assumed in this model that individual consumers do vary in their body size as they grow from their size at birth to mature at an adult body size, but that per unit of biomass, the rates of feeding, maintenance, and mortality are the same for all consumers. The fact that both juveniles and adults exploit both resources to an equal extent makes them identical on a mass-specific basis.

When juvenile consumers exclusively feed on resource 1 and at maturation switch to exclusively feeding on resource 2, the response of the population to an increase in mortality of all individuals is quite different: a higher mortality translates into a higher total biomass of the population, mainly because the biomass density of juvenile consumers increases significantly, whereas adult consumer biomass only decreases with mortality, as before (figure 1.2, right). This positive relationship between mortality and biomass density in equilibrium we refer to as biomass overcompensation. Biomass overcompensation comes about because the relative composition of the population changes with changing conditions, that is, mortality. Note that we refer to an increase in standing stock equilibrium biomass and not to an increase in a population dynamic rate process, such as the population reproduction rate, with increasing mortality. The latter is in fact more understandable and has been discussed before in the literature.

ONTOGENETIC (A)SYMMETRY IN ENERGETICS

When does biomass overcompensation occur? This question brings us to another important concept presented in the book, that of ontogenetic symmetry in energetics. In classical Lotka-Volterra models all individuals are by definition the same and thus characterized by equal rates of feeding, maintenance, reproduction, and mortality. As we will show, the population dynamics models that account for individual growth in body size throughout life history and hence for the population size structure, presented in figure 1.1 above, can under specific conditions be simplified to a single ordinary differential equation for the changes in total population biomass over time. More specifically, the classical, bioenergetics model presented by Yodzis and Innes (1992) represents an example of such a model, which looks like an unstructured population model that characterizes the population with a single state variable: population biomass. At the same time, the Yodzis and Innes model can also be shown to exactly describe the dynamics of a fully size-structured population, in which all individuals experience the same mortality and per unit of body mass produce the same amount of new biomass through either somatic growth in body size or production of offspring. The condition that allows for simplifying a size-structured model to a simple model for total population biomass we refer to as ontogenetic symmetry in energetics, because of the symmetry between individuals of different body sizes (i.e., in different stages in their development) in the efficiency with which they convert ingested food into new biomass. Under conditions of ontogenetic symmetry, size-structured population dynamics do not differ from the dynamics of unstructured populations: increases in mortality lead to the expected decrease in total population biomass, and changing external conditions such as mortality or food supply do not change the relative population composition.

The results shown in the right panel of figure 1.2 hence occur because of ontogenetic asymmetry between juvenile and adult consumers. In particular, juveniles and adults differ because we have assumed that they forage on different resources that are produced at different rates. Figure 1.2 represents a situation with a high productivity of resource 1, on which the juveniles forage, whereas resource 2, exclusive to adult consumers, is in short supply. Juvenile and adult consumers thus experience very different feeding regimes. At low mortality this leads to a domination of the population by adults, because adult resource is limited in equilibrium. Adult consumers then use most of their intake to cover their maintenance costs, and reproduction is low. At the same time, juvenile resource density is high, leading to rapid growth of juveniles from their size at birth to their maturation size and early maturation.

Hence, at low mortality a bottleneck occurs in the adult stage of the consumer life history. An increase in mortality relaxes this bottleneck, as it decreases adult biomass and increases the availability of resource for the surviving adults. The latter, positive effect is so substantial that total reproduction by all adults together increases with increasing mortality. In turn, this increase in total reproduction translates into an increase in biomass of juveniles, which continue to experience high food availability and can cash in on the high supply of their resource. The biomass overcompensation hence occurs because individual consumers differ in their energetics, which results in a differential change in maturation and reproduction rate when an increase in mortality relaxes the intraspecific competition and resource densities increase. The differential change in maturation and reproduction rate leads to a change in population composition and ultimately to a more efficient use of the supplied resources.

Biomass overcompensation occurs when the ontogenetic symmetry in energetics is broken. In physics, symmetry breaking is known to bring a system from a disorderly state, which occurs in the symmetric case, into one of two definite states that are robust against small changes as opposed to the symmetric state. Analogously, we can identify two distinct regimes of ontogenetic asymmetry in energetics. First, if juveniles are energetically less efficient than adults, a bottleneck in development during the juvenile stage will mostly limit the population at equilibrium when mortality is low. Then, the population tends to be dominated at equilibrium by juveniles. Increases in mortality will generically decrease juvenile biomass but increase adult biomass. Second, if adults are energetically less efficient, a bottleneck in reproduction will limit the population at equilibrium when mortality is low, leading to adults making up the largest part of population biomass under these circumstances. Increases in mortality will then decrease adult biomass but increase juvenile biomass. These two regimes of ontogenetic asymmetry, either juvenile development or adult reproduction limiting a population at equilibrium, are like two sides of a coin. A state of ontogenetic symmetry represents the dividing line between them. Just as in physics, the system states that are observed in the symmetric case, which in our case refers to the type and nature of community equilibria or the type of population dynamics predicted by a model, are sensitive to small changes in the energetic status of juveniles and adults. This is the reason why we postulate that classic ecological theory based on Lotka-Volterra models only covers a limiting case of the more general theory, which also encompasses cases with asymmetric conditions of individual energetics.

Biomass overcompensation occurs for almost all parameter combinations in different size-structured, consumer population models irrespective of the precise details of the individual life history that are accounted for in these models. Figure 1.2 does represent an extreme example, in that even total population biomass increases with increasing mortality. In general, however, it is more likely to find that total population biomass decreases and only the biomass density of juveniles or that of adults increases in response to mortality. Stage-specific biomass overcompensation, for example, occurs when adults and juveniles share the same resource but differ in their mass specific intake rate (figure 1.3, top row, left module). It is hence a generic phenomenon, although the precise form of biomass overcompensation may depend on model details (see chapter 3 for details). Crucial for its occurrence is the fact that individuals require energy to cover the costs for maintaining themselves, an aspect of individual life history that is often ignored in Lotka-Volterra models.

(Continues...)

Excerpted from Population and Community Ecology of Ontogenetic Developmentby ANDRÉ M. DE ROOS LENNART PERSSON Copyright © 2013 by Princeton University Press. Excerpted by permission of PRINCETON UNIVERSITY PRESS. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
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