Population Regulation

12 Key excerpts on "Population Regulation"

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  The Ecological World View

  • Charles Krebs(Author)
  • 2008(Publication Date)
  • CSIRO PUBLISHING

    (Publisher)

  Figure 10.4 The modern view of Population Regulation processes. The population being studied is indicated by the blue circle. Extrinsic processes like predators and disease interact with the variable properties of individuals that make up the population (intrinsic processes), so that Population Regulation emerges as a result of interplay between these two kinds of processes: extrinsic and intrinsic. We cannot assume that extrinsic agents like diseases or predators act on individuals of different age or sex in the same way, so we must take into account the composition of the population when we study Population Regulation.

  Are populations that fluctuate widely in abundance still regulated?

  The simple model of Population Regulation shown in Figure 10.2 is focused on the concept of equilibrium, and we must begin by asking whether natural populations can be in equilibrium. Recent work on ecological stability has given us a more comprehensive view of the factors that affect the tendency of populations to reach a stable equilibrium (Figure 10.5 ). There is no reason to expect all populations to show stable equilibria. There are two sources of instability in populations. Strong environmental fluctuations in weather can produce instability, but strong biotic interactions may also promote instability. Time lags also can affect population stability. We should expect real world populations to cover the whole spectrum from those showing stable, equilibrium dynamics to those showing unstable, non-equilibrium dynamics. The simple model shown in Figure 10.2 will be difficult to detect in a real population that shows unstable dynamics over time.

  Populations and Metapopulations

  A second consideration in Population Regulation is the recognition of habitat variation in space. It is important in studying population dynamics to consider the spatial scale of the study because it can be important in measuring stability. If you study a very small population on a small area, it may fluctuate widely and even become extinct. A large population on a large study area may, at the same time, have a stable density. The important concept here is that local populations may be linked together through dispersal into metapopulations (Figure 10.6

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  Animal Population Ecology

  An Analytical Approach

  • T. Royama(Author)
  • 2021(Publication Date)
  • Cambridge University Press

    (Publisher)

  7  Regulation of Populations: Its Myths and Real Nature 7.1 A Brief History During the 1950s, my apprenticeship years, hot debates were ongoing in animal ecology as to what factors control or regulate animal populations. Prominent ecologists in the world across Europe, North America, and Down Under (the hot bed, in fact) were divided into two schools of thought: one emphasizing the essential roles played by biotic factors, the other stressing the importance of physical factors, like climatic influences. However, the origin of the debates goes way back to the early 1900s, when two American entomologists, L. O. Howard and W. F. Fiske, recognized two distinct categories of mortality factors, ‘facultative’ and ‘catastrophic’, in their 1911 article. A couple of decades later, H. S. Smith (1935), a colleague of Howard’s, coined the terms that are now univer- sally used in ecology: ‘density-dependent’ and ‘density-independent’ factors. The focus of the Great Debates was to answer the question: ‘How are the populations of organisms so controlled or regulated as to persist?’ In both schools, it was intuitively conceived that, for a given population to persist for a sufficiently long time, there must be a mechanism that would, on the one hand, discourage a high population to increase further and would, on the other, prevent a low population from continuing to decrease to extinction. From this intuitive perception of Population Regulation stemmed the notion that the action of the regulatory mech- anism should necessarily depend on population density. This intuitive notion was the basis of the contention for the biotic-factor school. In contrast, the climatic-control school emphasized the importance of environmental (density-independent, sensu H. S. Smith) factors on the basis of their perception that climatic influences appeared to be dominant factors in determining many insect population processes.

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  Population Ecology of Individuals

  • Adam Lomnicki(Author)
  • 2020(Publication Date)
  • Princeton University Press

    (Publisher)

  For the theoretical models presented above, no good empirical confirmations are available. It is well known that animals exhibit a lower level of aggression toward their kin, but there are no data to show that contest competition can be a result of kin selection. 7.2. SELF-REGULATION IN CONFINED LABORATORY POPULATIONS Old textbooks of ecology, for example Allee et al. (1950), extensively discussed population growth as a function of density (i.e. the number of individuals per unit area or per unit volume) and often disregarded limiting factors other than space. In the ecological literature, there are plenty of data concerning pop-ulation growth in limited space, This chapter discusses only those in which space is the only limiting factor, while all other resources that are required for survival and reproduction are in excess. To make the arguments clear, I will put aside all data on density in relation to available food, nesting sites, concen-tration of oxygen or carbon dioxide in water, as well as data on the density of sedentary organisms, for which an area of available substratum space can be a real limiting factor. 132 SELF-REGULATION OF POPULATION SIZE A separate problem is that population density can manifest itself by the concentration of metabolic products or other chem-ical substances that are secreted by organisms into water, soil, or other media in which they live. This is so-called water con-ditioning (or as in the case of Tribolium populations, flour con-ditioning), which may exert either positive or negative effects on population growth. Putting aside the positive effect of con-ditioning, the negative effect can be interpreted in the following two ways.

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  Consumer-Resource Dynamics

  • William W. Murdoch, Cheryl J. Briggs, Roger M. Nisbet(Authors)
  • 2013(Publication Date)
  • Princeton University Press

    (Publisher)

  Put less technically, a population is regulated if the amplitude or intensity of its fluctuations does not grow without limit. For further details see Nisbet and Gurney 1982. Even this definition of regulation, however, is impractical for use on real populations. It turns out that population variance appears bounded in a substantial fraction of long-run simulations of unregulated, random-walk populations, a result we would mistakenly take as evidence of regulation. (This happens because the variance of population variance itself increases rapidly with time: whereas some runs accumulate vari-ance extremely rapidly, the variance in others appears bounded over ecologically substantial periods of time; Murdoch 1994.) One solution is to create a deterministic approximation to a stochastic model of the real population being studied. We then say the popula-tion is regulated if solutions of this deterministic model are bounded (Ellner et al. 1998; Kendall et al. 1999). This allows us to use the unambiguous deterministic definitions of boundedness: mathematically bounded deterministic trajectories include the approach to a stable equilibrium, cycles, and chaos. Alternatively, one could create a full stochastic model and see if it was stochastically bounded (P. L. Chesson, pers. comm.). 2 4 C H A P T E R 2 Strong and Weak Coupling, and Model Simplification Throughout this book we look at any one time at interactions among only a few species. In doing so, we follow a hallowed ecological tradition that implicitly asserts that we can understand the dynamics of some particu-lar consumer-resource interaction with little regard to the other species in the community. This is essentially a claim that real ecological com-munities can be decomposed into components, and that a component is dynamically sufficiently independent that we can, as a first approxima-tion, treat the rest of the community simply as part of the environment of the component of interest.

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  Ecology

  Principles and Applications

  • J. L. Chapman, M. J. Reiss(Authors)
  • 1998(Publication Date)
  • Cambridge University Press

    (Publisher)

  Factors which do not show this effect are said to be density independent and include fire, volcanic eruption and other catastrophes, extreme weather conditions like heavy frosts and high winds, and toxic pollution. Some of the major factors which alter population size, both unpredictable hazards and population regulators, are discussed in this section. Spatial heterogeneity may mean that more than one factor regulates a single population and that density-inde- pendent factors can be important in Population Regulation. Experiments or observations of natural populations are included which give examples of how these factors can regulate populations. Although these are examples where one main factor is involved in regulation, the size of most populations is influ- enced by a whole complex of interactive factors. Different factors may be important at different times of year, in different years, or on individuals of differ- ent ages and in different parts of the geographical range of the population. 40 5.2 Factors which regulate population size 5.2.2 Space The most basic requirement an organism has is space. Protoctists and most animals, except for stationary ones like scale insects, corals or barnacles, are not restricted by lack of physical space. They may have problems when finding the right place to breed, but this is a form of population restriction which falls under the heading of territories (see Section 5.2.4). Plants, on the other hand, are often crowded closely together. As each individual grows it spreads out roots and branches and takes up more space. Unless it is an isolated plant, at some point in growth it starts to interfere with or be interfered with by neighbouring plants of the same or different species. Thus a mature plant will probably have displaced several others. Tree plantations are good examples of groups of plants of the same age and species growing up and competing for space.

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  Natural Regulation of Animal Populations

  • Ian A. McLaren(Author)
  • 2017(Publication Date)
  • Routledge

    (Publisher)

  The importance of the genetic feedback mechanism as a regulatory system in communities is substantiated by its wide application to such diverse interacting population systems as herbivore and plant, parasite and host, predator and prey, and interspecific competitor systems. The real significance of this mechanism for Population Regulation lies in the fact that it has its foundation in evolution. Population Regulation by genetic feedback supports Emerson’s (36) view that evolution in natural populations is toward homeostasis (balance) within populations, communities. and ecosystems.

  Students of population ecology and especially of parasitology and epidemiology generally accept the fact that evolutionary trends in relationships of parasite and host are toward balance. The deductive basis for this generalization rests on the ecological principle that disharmony results in serious losses to both parasite and host. Large numbers of fatal infections in the host population eventually lead to host extinction which in turn brings about the extinction of the parasite. The success of any living population is measured by its relative abundance and distribution as well as its ability to survive in time.

  Homeostasis, in herbivore-plant, parasite-host, and predator-prey species and among other community members in general. results in improved survival of the community system. The evolved balance in supply and demand achieved by the feeding species and its host establishes a sound economy for the community. This, of course, enables the community to make effective use of the resources available to it.

  Increased species diversity in a community is due in part to community homeostasis. The genetic integration of interspecific competitors which makes possible the use of the same resource by competing species and enables them to occupy the same niche contributes to greater species diversity. The increased network of interactions within the community, resulting from a greater number of species present, further contributes to community homeostasis.

  With more knowledge concerning the regulation of natural populations, man will be in a better position to control the pests on his food crops and the parasitic diseases of mankind. This will also help conserve the millions of living species which are vital for the functioning of the vast living system of which he is a part.

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  The Environment

  Science, Issues, and Solutions

  • Mohan K. Wali, Fatih Evrendilek, M. Siobhan Fennessy(Authors)
  • 2009(Publication Date)
  • CRC Press

    (Publisher)

  This, in turn, may lead to decreased survival and reproduction within the population. As a con-sequence, population growth will slow or may even become negative. Such populations could be regarded as self-regulating systems. Historically, nearly all of the population ecology concepts have come from the studies conducted by animal ecologists. It was rela-tively recently that these concepts have been applied to terrestrial plant ecology, which will be discussed in Chapter 9. Population Growth All organisms, without exception, have a pro-pensity to produce as many offspring as they can; if they do so, this leads to the intrinsic rate of population growth, also known as the “biotic potential” of the species. Thus, if envi-ronmental resources and other ecological con-ditions do not constrain population growth, exponential growth will occur. For example, if a single cell of baker’s yeast ( Saccharomyces cervisiae ) were to reproduce unconstrained, it is estimated that in just 2 weeks it could form a layer around the Earth that would be 3 m deep (Hofman 1992). A pair of fruit flies may in one year produce a mind-boggling 10 41 descendants (Evans 1995). Similarly, if the biotic potential of a mating pair of houseflies were actually reached, the number of flies in the seventh generation would be 5.7 trillion (Table 8.1) (Kormondy 1996). These stupendous numbers illustrate the power of exponential growth (See Box 8.1). If a population were to achieve exponential growth with unlimited availability of resources such Table 8.1 Biotic Potential of House Flies ( Musca domestica ) in 1 Year a Generation Total Population If All Survive 1 Year But Reproduce Only Once 1 120 2 7,320 3 439,520 4 26,359,320 5 1,581,559,320 6 94,893559,320 7 5,693,623,559,320 Source: From Kormondy, E. J. 1996. Concepts of Ecology, 4th ed. Upper Saddle River, NJ: Prentice Hall.

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  Population Dynamics

  Alternative Models

  • Bertram G. Jr. Murray(Author)
  • 2013(Publication Date)
  • Academic Press

    (Publisher)

  Others (Skutch, 1949, 1967; Wynne-Edwards, 1962) have suggested the opposite, that reproductive rates change in response to environmentally induced mortality. Emigration (i.e., dispersal) is sometimes considered a 38 3: LIMITATIONS TO POPULATION GROWTH means of preventing overpopulation (Lidieker, 1962; Wynne-Edwards, 1962). The alternatives to the density-dependent regulation theories, the theories of Thompson (1939, 1956), A'ndrewartha and Birch (1954), Schwerdtfeger (1958), and in part Milne (1962), suggest that population growth is limited by the availability of resources or of time in which to reproduce and that fluctuations are often the result of density-independent changes in the environment. Neither the regulation theories nor the limitation theories have been developed in terms of the interrelated effects of one population parameter on the others. As indicated earlier, for example^ a change in age-specific death rates affects the l x schedule, stable age distribution, and replace-ment rate, and it can affect the birth rate as well. The following sections of this chapter discuss the cause and effect relationships of these parameters with respect to space-limited, time-limited, food-limited, and predator-limited populations. SPACEA SA LIMITIN GFACTO R A factor limiting annual recruitment in some populations is territorial behavior. Because there seems to be a limit to which territories can be compressed (Huxley, 1934), only so many individuals can establish ter-ritories within the available habitat and successfully breed. Territorial aggression, then, limits recruitment by limiting the number of individuals that can establish territories and by limiting the number of births. A numerical example will show how a sigmoid-shaped growth curve can be generated in a territorial population even if no change occurs in either age-specific death rates (i.e., survivorship is constant at all densities) or individual fecundity.

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  Regulation of Parasite Populations

  • Gerald Esch(Author)
  • 2012(Publication Date)
  • Academic Press

    (Publisher)

  An unstable population is one that is unregulated and so is in constant danger of extinction; it is con-strained only by density-independent factors. This REGULATION OF FISH PARASITE POPULATIONS 65 concept of stability does not imply that in a stable parasite population all individual host-parasite inter-actions are stable. It has often been assumed that because parasite life cycles are very complex, involving in some cases as many as six infrapopulations of parasites, e.g. eggs, miracidia, sporocysts, cercariae, metacercariae and adults, and at least three host populations, this complexity alone is sufficient to impart stability to the parasite population. This is not the case. May (1973) has shown convincingly that in model ecosystems complexity alone is not sufficient to ensure stability, and that increasing complexity may even decrease sta-bility. This has been confirmed in respect of parasite populations in the models of Anderson (1974a; in press), who has gone on to show that the complexity of parasite life cycles is important in that it increases the num-ber of stages or places at which density-dependent con-trols can act. The more complex the life cycle, the more the opportunities for such action. Nevertheless, even though several controls may operate at different stages in the life cycle, Anderson (in press) has also demonstrated that only a single negative feedback con-trol mechanism operating in a single host species can be sufficient to regulate the whole parasite suprapopu-lation. Complexity of life cycle is important in another respect also. Both Anderson (in press) and May, et al. (1974) are agreed that the more complex the life cycle, the more opportunities there are for time lags in the system, and that, in general, time lags destabilize the system. REGULATORY MECHANISMS In a discussion of parasite population stability, Bradley (1974) recognized three basic types of regula-tion of parasite population numbers.

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  Ecology and Applied Environmental Science

  • Kimon Hadjibiros(Author)
  • 2013(Publication Date)
  • CRC Press

    (Publisher)

  Its purpose is to examine the way in which population sizes change through time, along with the analysis of the causes of these variations. This study is of great practical importance given that humans are interested in the prediction, or even control, of the size of various human or other animal, plant or micro-organism populations. Population dynamics is a mathematical branch of ecology that uses vari-ous simple or complex models. Mathematical analysis of human popula-tions is similar to the analysis of other animal populations with overlapping generations. In population dynamics, deterministic or stochastic models, differential equations, or other mathematical tools such as matrices are used. In simpler models, a basic assumption stating that all population individuals are identical is followed; therefore the population state can be described with only one variable—its size, N . Population percentage Population percentage Population percentage Age 80 + 75 – 79 70 – 74 65 – 69 60 – 64 55 – 59 50 – 54 45 – 49 40 – 44 35 – 39 30 – 34 25 – 29 20 – 24 15 – 19 10 – 14 5 – 9 0 – 4 Figure 3.4 Pyramids of three human populations, with increase rates 2.1%, 0.6%, and 0.0% per year. (From Hadjibiros 2007. Ecology. Ecosystems and Environmental Protection, 3rd edition. Symmetria, Athens. With permission.) Organisation at Population Level 19 © 2010 Taylor & Francis Group, LLC A central issue for ecology is the search for the causes of ecosystem sta-bility, which are considered to be connected with the interactions between populations. The explanation of ecosystem stability represents a main theo-retical and practical challenge for population dynamics. The existence of stability, a precondition for persistence, is an important criterion for evalu-ating models used to describe ecological phenomena. However, the high complexity of ecosystems does not allow, at least for the time being, the formulation of a stability theory.

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  Foundations of Mathematical Biology

  Supercellular Systems

  • Robert J. Rosen(Author)
  • 2013(Publication Date)
  • Academic Press

    (Publisher)

  It is then meaningless to 7. Physiological Regulation and Control 67 ask for the direction of causality in the ecology's complex food web and other interspecies relations. Certainly none of the subsystems can be considered to exist for the purpose of other subsystems unless one conceptually assumes the role of a godly observer. More than this, it now becomes impossible to isolate active control pathways by which any such control could be exerted in the time-variable adaptive way described for homeostasis. On the other hand, ecologies do tend to show dynamics characteristic of control systems, and this control might perhaps best be called passive, or perhaps evolu-tionary, in a context we shall discuss shortly. Of course, it is tempting to postulate that prédation is a direct control mechanism by which the popula-tion levels of the various prey species are effectively controlled. However, it seems increasingly accepted that the effective population stability which is typically observed in mature ecologies is not crucially dependent upon préda-tion, but rather upon some intraspecies passive mechanisms which keep the birth rate at a small but appropriate fraction of that potentially avail-able. As an example, territoriality is one well-publicized mechanism. Stated more directly, there do not seem to be any direct control pathways which adaptively control ecological population, and in particular, not through prédation. The stable population behavior that is certainly observed in typical mature ecologies, however, must therefore be implemented through other mechanisms. In fact, the most that should be claimed is that populations are stabilized as if there were control pathways at work.

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  CLEP® Biology Book + Online

  • Laurie Ann Callihan(Author)
  • 2013(Publication Date)
  • Research & Education Association

    (Publisher)

  CHAPTER 7 Population Biology CHAPTER 7 POPULATION BIOLOGY ECOLOGY

  Ecology is the study of how organisms interact with other organisms and how they influence or are influenced by their physical environment . The word “ecology” is derived from the Greek term oikos (meaning “home” or “place to live”) and ology (meaning “the study of ”), so ecology is a study of organisms in their home. This study has revealed a number of patterns and principles that help us understand how organisms relate to their environment. First, however, it is important to grasp some basic vocabulary used in ecology.

  The study of ecology centers on the ecosystem. An ecosystem is a group of populations found within a given locality, including the abiotic environment around those populations. A population is the total number of a single species of organism found in a given ecosystem. Typically, there are many populations of different species within a particular ecosystem. The term organism refers to an individual of a particular species. Each species is a distinct group of individuals that are able to interbreed (mate), producing viable offspring. Although species are defined by their ability to reproduce, they are usually described by their morphology (their anatomical features).

  Populations that interact with each other in a particular ecosystem are collectively termed a community . For instance, a temperate forest community includes pine trees, oaks, shrubs, lichen, mosses, ferns, squirrels, deer, insects, owls, bacteria, fungi, etc.

  The part of the Earth that includes all living things is called the biosphere . The biosphere also includes the atmosphere (air), the lithosphere (ground), and the hydrosphere (water).

  A habitat refers to the physical place where a species lives. A species’ habitat must include all the factors that will support its life and reproduction. These factors may be biotic (i.e., living—food source, predators, etc.) and abiotic

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