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Ecology of Populations
- Esa Ranta, Per Lundberg, Veijo Kaitala
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The theme of the book is the distribution and abundance of organisms in space and time. The core of the book lies in how local births and deaths are tied to emigration and immigration processes, and how environmental variability at different scales affects population dynamics with stochastic processes and spatial structure and shows how elementary analytical tools can be used to understand population fluctuations, synchrony, processes underlying range distributions and community structure and species coexistence. The book also shows how spatial population dynamics models can be used to understand life history evolution and aspects of evolutionary game theory. Although primarily based on analytical and numerical analyses of spatial population processes, data from several study systems are also dealt with.
Frontmatter
- Esa Ranta, University of Helsinki, Per Lundberg, Lunds Universitet, Sweden, Veijo Kaitala, University of Jyväskylä, Finland
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- Ecology of Populations
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10 - Resource matching
- Esa Ranta, University of Helsinki, Per Lundberg, Lunds Universitet, Sweden, Veijo Kaitala, University of Jyväskylä, Finland
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- Ecology of Populations
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- 06 February 2005, pp 237-266
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Summary
This chapter addresses the problem of how individuals are distributed in space and time. Space is assumed to consist of areas differing in terms of profitability, some being more productive or otherwise of higher quality than others. The theory of ideal free distribution (IDF), or in more general terms resource matching, was developed to address the issue of how individuals are expected to be distributed across areas differing in availability of relevant resources. We shall first discuss resource matching in terms of distribution of foragers over their renewable resources under various circumstances. We end by extending our exploration at the level of population dynamics in areas with differing carrying capacities.
Ideal free distribution
Ecology is the scientific exploration of the distribution of individuals and species in space and time (Krebs 1972). This is also the central theme of this chapter. We are specifically addressing the following question: how should individuals be distributed in an environment consisting of a number of habitat patches varying in resource availability? This is a question studied in the framework of the ideal free distribution (Fretwell and Lucas 1970; Fretwell 1972; or the theory on resource/habitat matching in general, Parker 1974; Morris 1994). According to the IFD theory (fig. 10.1), assuming virgin habitats, the first arriving individual should occupy the most rewarding area. From then on, its presence and activity there devalue that particular habitat patch. The next arrival should also go to a place where the highest reward can be extracted.
2 - Population renewal
- Esa Ranta, University of Helsinki, Per Lundberg, Lunds Universitet, Sweden, Veijo Kaitala, University of Jyväskylä, Finland
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Population renewal is about how births and deaths of individuals are translated into population level dynamics. Here, we are reviewing some basic concepts and models of population renewal, disregarding both spatial processes (immigration and emigration) as well as interactions with other populations. Those extensions will be addressed in subsequent chapters. We are also briefly reviewing some statistical building blocks necessary for understanding population dynamics as a stochastic process and not only a deterministic route to persistence or extinction. This includes primarily the time series approach to population dynamics. We conclude this chapter by highlighting some very important and disturbing problems when confronting models with data (and the reverse), especially when trying to disentangle the demographic skeleton from “noise.”
There is really nothing more to population ecology than births and deaths. If the number of individuals born exceeds the number that dies, the population size increases; should deaths exceed births, the population size decreases. If that simple, how is it so difficult to predict the population size in the future, and to determine what limits – or even regulates – the distribution and abundance of organisms in natural systems? We could argue that it is because the models we inevitably need to perform the above exercises are not good enough. One could also say that the task is difficult because it is not so easy to measure things accurately in nature. It is even problematic to determine what a population really is.
Preface
- Esa Ranta, University of Helsinki, Per Lundberg, Lunds Universitet, Sweden, Veijo Kaitala, University of Jyväskylä, Finland
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The ecology of populations is the study of the patterns of distribution and abundance of organisms. It also goes beyond mere description and seeks the evolutionary forces that might produce such patterns, and their ecological constraints. This book is on the ecology of populations but it is not a population ecology textbook. Therefore, there will be much standard textbook material left out. This book does not attempt to establish a new field, or summarize or synthesize an old one. Neither does it provide the student of population biology with all the necessary tools for further exploration, nor does it review the entire discipline, or parts of it. So what does this book do? As with most books, it presents an idiosyncratic world-view. We hope that some well-known problems and phenomena are getting a fresh and novel approach. We also hope that applying basically the same analytical tool to a number of seemingly disparate problems in population biology will be convincing enough to make others do likewise. By using rather simple models of population change to a large number of problems, we hope that conceptual unification will be promoted. Science becomes more and more specialized with the risk of losing track of the bigger picture. Although no bigger picture is presented here in a coherent way, the approach we have taken to address problems in population ecology aims at getting to that more synthetic understanding.
1 - Introduction
- Esa Ranta, University of Helsinki, Per Lundberg, Lunds Universitet, Sweden, Veijo Kaitala, University of Jyväskylä, Finland
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… there is nothing so practical as good theory.
Richard FeynmanThe scope of this book is almost as wide as it gets. It touches upon a range of topics in ecology and evolution found in many modern textbooks. Instead of going into considerable depth in any one topic, we have chosen to cover quite a few in order to show that the same basic (and well-known) tools are applicable to a wide variety of ecological and evolutionary problems in population biology. However, this is also a narrow-minded book in the sense that it is very “theoretical,” i.e., full of mathematical expressions and computer simulation results. We believe ecology becomes a healthier science if it appreciates and acknowledges its strong quantitative and more rigorous nature. It is also narrow-minded in the sense that it reflects our own interests in population ecology without attempting to cover all aspects of the ecology of populations. Yet, the scope remains wide and possibly shallow. We believe that ecology and evolutionary biology have to become far more integrated than the fragmented and disparate impression they give today. We think that this can be done by going back to very simple first principles of births and deaths, immigration, and emigration. From those “simple” entities, we can derive virtually everything that plants and animals do in nature. To do so, however, requires a common thread of theory, the seeds of which at least we believe exist. Extensions of that theory will also be dealt with in this book.
9 - Population harvesting and management
- Esa Ranta, University of Helsinki, Per Lundberg, Lunds Universitet, Sweden, Veijo Kaitala, University of Jyväskylä, Finland
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- Ecology of Populations
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Apart from giving us a general and fundamental understanding of dynamic processes at the population and community level, population ecology also has obvious practical connections and associations. It helps us to analyze and manage natural populations. In this chapter, we address selected aspects of population management. We first consider problems related to conservation issues, i.e., dealing with small and/or declining populations. The other side of the coin is pest management, not dealt with here however. We then focus on exploited populations, where there is generally a trade-off between the number of individuals (or amount of biomass) removed and the viability of the target population, i.e., the problem of sustainability. We do this by emphasizing the need for rigorous risk analysis procedures.
From a conservation point of view, we become concerned when the number of individuals of a species declines to low numbers, or when a particular population of a species becomes small. There is, of course, an ongoing debate about what the appropriate conservation units really are – should we, e.g., only focus on species preservation when there is so much biological diversity (genetic and phenotypic) also within a species? We take no stand in this debate here, and we boldly neglect the concern about genetic diversity. Here, it will be assumed that the population is a useful and reasonably well-defined entity, and that the population is an appropriate management unit.
6 - Structured populations
- Esa Ranta, University of Helsinki, Per Lundberg, Lunds Universitet, Sweden, Veijo Kaitala, University of Jyväskylä, Finland
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Populations are not collections of identical individuals, albeit such approximations are often useful. In many cases, however, a higher resolution is needed to understand population and community processes. In this chapter, we introduce some more details by letting the population be divided into age or stage classes, all of them possibly with their specific vital rates. This can be achieved by rather small modifications of the simple models, yet qualitatively and quantitatively new phenomena will emerge.
The state of a population is usually thought of as its size or density. As we have seen, it is in fact the density-dependent feedback that is assumed to be important at the population level. The implicit assumption behind this is that all individuals are more or less identical when it comes to their demographic effects: their contributions to births and deaths (and immigration and emigration). This is, of course, not the case in most natural populations. Young individuals are often more susceptible to death than adults, and they often contribute less to reproduction. Individuals at some intermediate adult stage face less risk of dying and are the ones that reproduce successfully. Such differences are not necessarily only attributed to age, but also body size (often co-varying with age). For example, in most species with indeterminate growth, as in fish and reptiles, fecundity and survival are strongly dependent on size, rather than on age per se (Roff 1992; Stearns 1992).
Contents
- Esa Ranta, University of Helsinki, Per Lundberg, Lunds Universitet, Sweden, Veijo Kaitala, University of Jyväskylä, Finland
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- Ecology of Populations
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3 - Population dynamics in space – the first step
- Esa Ranta, University of Helsinki, Per Lundberg, Lunds Universitet, Sweden, Veijo Kaitala, University of Jyväskylä, Finland
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- Ecology of Populations
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The population renewal processes discussed in the previous chapter assumed spatially homogenous environments. For natural systems this mostly fails to be true. Here, we extend the population renewal in space and will hence come back to the problem of including emigration and immigration in population dynamics. We will do so by arbitrarily delimiting the landscape into well-defined habitat patches connected by redistribution of individuals. This simplified representation of spatial structure and population dynamics allows us to analyze and interpret a wide range of single-species phenomena. Spatial structure can alter the population dynamics significantly and produce emergent phenomena such as synchrony and complex dynamics. This chapter sets the theoretical and conceptual stage for such problems dealt with in more detail in the coming chapters.
In the previous chapter, we deliberately overlooked the important and natural aspect of the import and export of individuals to and from a given focal population. For some populations, ignoring dispersal may be a fair approximation. Most extreme examples of this might be experimental populations of fruitflies in a single container or small aquatic microcosms of protozoans. In such cases, it would be natural to assume that a complete mixing occurs in the whole population. Most populations, however, are spatially structured and the exchange of individuals between landscape elements is an integral part of the dynamics (Hastings 1990; Kareiva 1990; Bascompte and Solé 1997; Tilman and Kareiva 1997).
4 - Synchronicity
- Esa Ranta, University of Helsinki, Per Lundberg, Lunds Universitet, Sweden, Veijo Kaitala, University of Jyväskylä, Finland
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Charles Elton (1924) was very well aware of the fact that many populations of a given species display large-scale temporal match in their population fluctuations. He was also among the first to propose that this, almost ubiquitous phenomenon, is due to environmental forcing, redistribution of individuals between breeding seasons, or biotic interactions of some kind. In this chapter, we shall first describe, with a set of examples familiar to us, patterns of synchronicity in various taxa. In these data, one new feature emerges that Elton did not mention: often the degree of coherent temporal population fluctuations is high among nearby populations but levels off with increasing distance. In the second part we shall address the question of how to analyze synchrony patterns. Finally we will turn to the different major explanations provided to understand large-scale synchronous fluctuations in population features of animals and plants.
Natural populations live in patchy environments. The distribution area of any given species should not be viewed as a continuous uniformly spread population, evenly painted over the landscape. Rather, the environment is composed of a network of habitable areas differing in profitability and of areas less suitable for population renewal. Even in pristine habitats, individuals are not distributed evenly all over the range. Our dogma is that natural populations are composed of local populations of varying size and quality. The independence of these units may vary: some of them can be entirely isolated while most population subunits are linked to other similar units via dispersing individuals.
8 - Habitat loss
- Esa Ranta, University of Helsinki, Per Lundberg, Lunds Universitet, Sweden, Veijo Kaitala, University of Jyväskylä, Finland
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- Ecology of Populations
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Our focus is on population fluctuations, extinction risks, and on species coexistence in a fragmented landscape. As this is an entirely theoretical enterprise, we can control for various aspects (constant carrying capacity, fixed density of fragments per unit area, increasing isolation, etc.) while altering others. This is often impossible to achieve with natural systems. The concept of refugee-arrival (former inhabitants of habitats lost) caused perturbations in local populations will be introduced. In addition, we shall explore population fluctuations in the center and border of a species' distribution range. Finally, we shall provide a few explanations as to why species with periodic multi-annual dynamics may lose the cycle.
What is meant by habitat loss?
The issue of habitat loss brings into mind various aspects of changes over time in pristine habitats in nature. Often we tend to associate habitat loss with human-caused consequences: intensifying agriculture initially, timber logging for sawmills and pulp mills of the paper industry, not forgetting urban development (fig. 8.1). The picture that we tend to have in mind when somebody mentions “habitat loss” is that in the beginning there was large widespread homogeneous coverage of uniform habitat areas all over a given biome. Ever since those times everything has deteriorated, fragmented, and we have lost habitats. Areas suitable for breeding of a given species have become isolated from each other. Many species have become extinct and numerous others have become threatened. This has all happened in a very short historical time.
Index
- Esa Ranta, University of Helsinki, Per Lundberg, Lunds Universitet, Sweden, Veijo Kaitala, University of Jyväskylä, Finland
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7 - Biodiversity and community structure
- Esa Ranta, University of Helsinki, Per Lundberg, Lunds Universitet, Sweden, Veijo Kaitala, University of Jyväskylä, Finland
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- Ecology of Populations
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This chapter is devoted to biodiversity, viz. the local and global number of coexisting species. We first show how community assembly critically depends on the interaction terms αij of the community matrix A. In an isolated community, elimination of a single species easily leads to cascading extinctions. Attempts to reintroduce the species lost may not always succeed, and may even lead to further extinctions. Extending community assembly into space enhances local and global diversity but too much dispersal among communities may considerably reduce maximum achievable species richness. We also suggest that harvesting species from a community with strong interactions may result in unexpected extinction cascades.
Community assembly
The issue of species richness ultimately translates into the concept of the ecological niche, defining the resource utilization profile of any single species either when alone or in a network of interactions with other species with closely matching profiles (e.g., Levins 1968; Emlen 1984; Lundberg et al. 2000b). The classic question now becomes how many and how similar species can (locally) coexist. This problem of local species richness crystallizes into a simple set of questions (Diamond 1975):
To what extent are the component species in a given locality or community mutually selected from a larger species pool to fit with each other?
Does the resulting constellation resist invasion?
If so, how?
To what extent is the final species composition of a community uniquely specified by the properties of the physical environment, and to what extent does it depend on chance events?
5 - Order–disorder in space and time
- Esa Ranta, University of Helsinki, Per Lundberg, Lunds Universitet, Sweden, Veijo Kaitala, University of Jyväskylä, Finland
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- Ecology of Populations
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In this chapter, we shall continue exploring how large-scale ecological processes may be fundamentally important for our understanding of emergent phenomena in various population systems. The modern ecological literature on spatial population dynamics has drawn our attention to various captivating configurations, such as traveling waves, suggesting that spatially structured populations may become self-organized. Spatial interactions in population dynamics can create a variety of spatial-temporal patterns. Spatial self-organization was first demonstrated in dispersal-coupled predator–prey and host–parasitoid models and we will discuss them first. We then proceed to expand the self- organization on a large scale of redistribution-coupled population processes. We shall finish by discussing data that support some of the theoretical findings.
Spatial interactions in population dynamics can generate a rich ensemble of spatial patterns. One example is traveling waves (Shigesada et al. 1986; Kot 1992; Ranta and Kaitala 1997; Shikesada and Kawasaki 1997; Kaitala and Ranta 1998) and they may appear in the form of wave fronts, periodic waves, spirals, and rings. Other eye-catching patterns are represented by crystal lattices, patches, and spatial chaos (Hassell et al. 1991, 1994; Solé and Valls 1991; Comins et al. 1992; Solé et al. 1992a, b; Solé and Bascompte 1993; Hassell 2000; Bjørnstad and Bascompte 2001). There are no strict definitions for the different spatial configurations emerging due to spatially coupled population renewal, although some attempts to derive more formal approaches to identify the patterns have been presented (Bjørnstad and Bascompte 2001; Bjørnstad et al 2002a; Kaitala 2002).
12 - Evolutionary population dynamics
- Esa Ranta, University of Helsinki, Per Lundberg, Lunds Universitet, Sweden, Veijo Kaitala, University of Jyväskylä, Finland
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- Ecology of Populations
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The interface between the evolution of life history traits and population dynamics in temporally and spatially variable environments is the topic of this chapter. Thus, the frame for the life history processes is set by spatial and temporal fluctuations in population density. Here, we will focus primarily on modes of reproduction and we are especially interested in whether alternative reproductive strategies can co-exist in a population. We show that spatially structured populations may allow co-existence of various life history strategies that do not easily co-exist in a nonstructured environment. Also, intrinsic and external temporal fluctuations in the environment tend to enhance polymorphism in certain traits, e.g., iteroparity versus semelparity and whether monogamy or polygamy are favored reproductive strategies. In this chapter, we largely omit genetics, but a short comment on that aspect is found towards the end.
All life history problems are related to optimizing reproduction. One central question is how individuals allocate resources to survival and reproduction, and, for example, how offspring number, size, and sex are decided. When we put the evolution of life histories and optimizing behavioral decisions into the context of population dynamics, we will change our focus from optimizing to evolutionary stability. We may introduce, say, different reproductive behavioral patterns in our population models and ask which one of them will be an ESS. Technically, as is the tradition in the ESS literature, we will assume two or more distinct phenotypes competing (Maynard Smith 1982; Bulmer 1994).
11 - Spatial games
- Esa Ranta, University of Helsinki, Per Lundberg, Lunds Universitet, Sweden, Veijo Kaitala, University of Jyväskylä, Finland
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- Ecology of Populations
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Individuals in natural populations encounter each other in numerous different ways. Such encounters include mating, conflicts over food or other resources, or the joint and co-operative acquiring of resources. The behavioral adaptations to such situations are often studied by evolutionary game theory. In this chapter, we will review some classic behavioral games: the Hawk–Dove, the Prisoner's Dilemma (including the evolution of co-operation), and the somewhat more obscure Rock–Scissors–Paper game. We also extend those problems to spatially heterogeneous environments. Towards the end of this chapter, we will combine the game theoretical analyses with dispersal-coupled population models.
Many, but far from all, encounters between individuals are pairwise. If the encounter involves a conflict, there is generally a winner and a loser. Take, e.g., two male deer fighting for the chance of mating with a female. The fight may be furious and last for a long time, possibly resulting in injuries to one or both contestants. Eventually one of the males will retreat and the winner will gain the mating. Such behavioral and ecological problems have inspired the development of evolutionary game theory (Maynard Smith and Price 1973; Maynard Smith 1982).
Most evolutionary theory assumes selfishness-driven adaptations (Dawkins 1976). It does not pay an individual to be nice or altruistic and helpful towards others unless there is a guarantee for not being cheated. Hence, altruistic behaviors are susceptible to selfish cheaters and will disappear from the population.
13 - Epilogue
- Esa Ranta, University of Helsinki, Per Lundberg, Lunds Universitet, Sweden, Veijo Kaitala, University of Jyväskylä, Finland
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Twelve short chapters, some of them rather superficial, is, of course, not much for a book on the ecology of populations. In most chapters, we have caught glimpses of intriguing and sometimes unexpected population phenomena; in others, we have been able to reach more definitive and firm conclusions and somewhat deeper understanding. This book does not, however, primarily summarize and synthesize; rather, it illustrates a set of approaches and points of departure for studies and analyses yet to be done. This book is a manifestation of ecological and evolutionary significance of dispersal-linkage in spatially structured populations. If the book serves its purpose as a source of inspiration, we have performed well.
The power of modern computers has made it easy to simulate complicated population processes with various sources of environmental stochasticity, population structure, and spatial heterogeneity. That is not to say that we are therefore necessarily closer to a more robust understanding of the ecology of populations, but it helps. Real understanding can only be achieved if there is a theory to aid us in obtaining insights. Such a theory does arguably exist for the temporal structure of population abundance (Turchin 1999; Berryman and Turchin 2001). We have the data, means to analyze them, and the theory to interpret the results for single-population dynamics in uniform space.
References
- Esa Ranta, University of Helsinki, Per Lundberg, Lunds Universitet, Sweden, Veijo Kaitala, University of Jyväskylä, Finland
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From arctic lemmings to adaptive dynamics: Charles Elton's legacy in population ecology
- JAN LINDSTRÖM, ESA RANTA, HANNA KOKKO, PER LUNDBERG, VEIJO KAITALA
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- Biological Reviews / Volume 76 / Issue 1 / February 2001
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- 27 March 2001, pp. 129-158
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- February 2001
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We shall examine the impact of Charles S. Elton's 1924 article on periodic fluctuations in animal populations on the development of modern population ecology. We argue that his impact has been substantial and that during the past 75 years of research on multi-annual periodic fluctuations in numbers of voles, lemmings, hares, lynx and game animals he has contributed much to the contemporary understanding of the causes and consequences of population regulation. Elton was convinced that the cause of the regular fluctuations was climatic variation. To support this conclusion, he examined long-term population data then available. Despite his firm belief in a climatic cause of the self-repeating periodic dynamics which many species display, Elton was insightful and far-sighted enough to outline many of the other hypotheses since put forward as an explanation for the enigmatic long-term dynamics of some animal populations. An interesting, but largely neglected aspect in Elton's paper is that it ends with speculation regarding the evolutionary consequences of periodic population fluctuations. The modern understanding of these issues will also be scrutinised here. In population ecology, Elton's 1924 paper has spawned a whole industry of research on populations displaying multi-annual periodicity. Despite the efforts of numerous research teams and individuals focusing on the origins of multi-annual population cycles, and despite the early availability of different explanatory hypotheses, we are still lacking rigorous tests of some of these hypotheses and, consequently, a consensus of the causes of periodic fluctuations in animal populations. Although Elton would have been happy to see so much effort spent on cyclic populations, we also argue that it is unfortunate if this focus on a special case of population dynamics should distract our attention from more general problems in population and community dynamics.