LECTURE 1: Introduction and Brief History of Population Ecology

Transcription

1 WMAN 512 SPRING 2010 ADV WILDL POP ECOL LECTURE 1: Introduction and Brief History of Population Ecology Cappuccino, N Novel approaches to the study of population dynamics. pp 2-16 in Population Dynamics: New Approaches and Synthesis. Eds. N. Cappuccino and P. W. Price. Academic Press. San Diego, CA. Graham, M. H. and P. K. Dayton On the evolution of ecological ideas: paradigms and scientific progress. Ecology 83: Hixon, M. A., S. W. Pacala, and S. A. Sandin Population regulation: historical context and contemporary challenges of open vs. closed systems. Ecology 83: Murdoch, W. M Population regulation in theory and practice. Ecology 75: O Neill, R. V Is it time to bury the ecosystem concept? (With full military honors of course!). Ecology 82: Paine, R. T Advances in ecological understanding: by Kuhnian revolution or conceptual evolution? Ecology 83: Sale, P. F., and N. Tolimieri Density dependence at some time and place? Oecologia 124: Sinclair, A. R. E Population regulation in animals. Pages in J. M. Cherret, editor. Ecological concepts. Blackwell Scientific Publishers, Oxford, UK. Turchin, P Complex population dynamics: A theoretical / empirical synthesis. Princeton University Press. 456 pgs. Turchin, P Population regulation: old arguments and a new synthesis. pp in Population Dynamics: New Approaches and Synthesis. Eds. N. Cappuccino and P. W. Price. Academic Press. San Diego, CA Williams, B. K., J. D. Nichols, and M. J. Conroy Introduction to population ecology. Chapter 1 (pages 3-9) in Analysis and management of animal populations. Academic Press. San Diego, CA. 1

2 I. A Few Definitions 1. Population: A group of organisms of the same species, occupying a particular space, at a particular time, with the potential to breed with each other. The spatial boundaries defining populations sometimes are easily identified, but usually not. Consequently, boundaries often are defined by an investigator. Examples. 2. Population Ecology: Study of how physical, chemical, and biological processes interact to influence the distribution and abundance of individuals in a population over time and space. 3. Population Dynamics: Study of short- and long-term changes in the size and age composition of populations and the biological and environmental processes influencing those changes. Population dynamics deals with the way populations are affected by birth and death rates and by immigration and emigration. 4. Conservation Biology: Study of the nature and status of Earth s biodiversity and the effects of humans on the environment with the aim of protecting species, their habitats, and ecosystems from excessive rates of extinction. II. Aims of Population Ecology 1. Elucidate general principles explaining dynamic patterns of population size, age composition, and spatial distributions. 2. Integrate these principles with mechanistic models and theories of community and ecosystem dynamics. 3. Apply these principles to the management and conservation of natural populations. III. Populations can be described quantitatively Characteristics Individuals Size Age Sex Nutritional condition Behavioral tendencies Populations Number / Density / Biomass Age composition Sex ratio Spatial distribution Geographic range Habitat associations Processes Development Growth Energetics Reproductive behavior Foraging behavior Movement behavior Habitat selection Birth Death Production Population growth rate Change in age composition Change in distribution Mortality rates Recruitment rates Immigration rates Emigration rates 2

3 IV. Fundamental Question of Population Ecology / Population Dynamics WHAT CONTROLS THE PERSISTENCE AND STABILITY OF POPULATIONS? persistence - the tendency for populations to not go extinct, stability - the tendency for populations to not increase to infinity. In other words, Population Sizes tend to be bounded by 0 and Infinity. Why? V. Related Questions Population Ecologists Should Address 1. Which population processes (e.g., birth, death, dispersal) are the most important determinants of change in populations from season to season, or year to year? 2. Are biotic factors more or less important than abiotic factors in controlling population dynamics? 3. Are intraspecific or interspecific interactions more important? 4. Do populations tend to be influenced by bottom-up forces or top-down forces? 5. To what extent does harvest (selective and non-selective) influence the dynamics of a population? 6. What are the chances that a given population of an endangered species will persist 100 years? VI. An Historical Perspective 1. Malthus (1798) Malthusian Principle: Populations increase at a rate greater than their food supply. 2. Rise of Mathematical Ecology (early 1900 s) Raymond Pearl Gause Volterra Lotka Nicholson The activities of these guys led to: 1) Development of Equilibrium Theory Idea that populations tend to return to an equilibrium point following a perturbation. 2) Ideological / Dogmatic View that Populations are Regulated by Density Dependent Processes D-D mechanisms provide a negative feedback mechanism that tends to keep populations near an equilibrium point. Both of these ideas are extensions of the Logistic Model, which was presented by Pearl in the 20 s as a Law of Populations. 3

4 Both of these ideas were embedded in the reigning Balance of Nature Paradigm, which had reigned supreme since antiquity. This view was referred to as the Oeconomia Naturae by Linneaus (1749). 3. Nicholson (early 1933) Extremely outspoken proponent of equilibrium theory and D-D regulation 1 st to insist and show mathematically that in order for populations to be regulated they must be subject to density-dependent factors. 4. Early Empirical Approaches (mid 1900 s) 1) Analyses of Long-Term Abundance Data (full of statistical issues) 2) Measures of Mortality Factors, i.e., k-factors (argued by many to be meaningless with regard to testing for the importance of d-d regulation of population dynamics). 5. Debate of the Past Four Decades - centered on the issue of density-dependent regulation: how frequent, how strong, and whether or not it is necessary to explain the persistence of populations. 6. Thompson (1930 s 40 s) Very outspoken critic of the Equilibrium School and specifically of Nicholson. One of the 1 st biologists to speak out against the uprise of mathematical ecology. Believed that people had stopped discriminating between mathematical figments and biological facts. (too much toward the Population Mathematics end of the spectrum). Tendency for people to believe that models must be true (mathematics produced dogma). Example: Nicholson populations persisted and were regulated in his models only when density-dependence was included, therefore, density-dependent mechanisms are the only possible explanation for why populations persist in nature. Unfortunately, Thompson was not an accomplished mathematician and really all he did was complain. 7. Andrewartha and Birch (1954) Published a textbook titled, The Distribution and Abundance of Animals. 4

5 An extremely influential book that spawned a debate in Population Ecology that has raged to this date. A&B severely criticized the Equilibrium View and argued Density-Dependent factors are not a General Theory of populations because they do not describe any substantial body of facts. A&B were biologists with a lot of data on insects and birds and the wherewithal to be able to interpret the mathematicians models. They presented table and figure upon table and figure that basically showed two things: 1) that populations of birds and insects are extremely variable; and 2) that much of this variation is independent of density and controlled primarily by climatic factors, such as rainfall. Repeatedly argued that the Persistence and Stability of natural populations did not require density dependent mechanisms. Proposed the idea of Density-Limitation by density-independent (exogenous) factors. D- I factors keep populations from increasing to extremely high levels. Their ideas led to a remarkable, POLARIZING debate: 1) What does population regulation mean? 2) Are population regulation and limitation the same thing? 3) Are d-d factors a necessary condition for regulation and persistence? 4) How frequent is d-d regulation? 5) Can we detect d-d regulation? 6) To what degree do d-i and d-d factors interact to regulate populations? 8. Population Ecology Lies Dormant (1970 s) People grew weary. There were no new ideas, only the same arguments. And the ideas of an unassuming man named MacArthur and the field of Community Ecology reigned supreme (early ideas that fit the Balance of Nature mold very well). 9. Rise of Non-Equilibrium Theory (1980 s) Made possible through improved computational abilities and ability to model random variables. In its purest form a population not subject to d-d factors that return their densities toward an equilibrium. (e.g., Random Walk populations). In its most fascinating form - population dynamics in which a local population does not trend toward a point equilibrium (though not truly non-equilibrial). Density Vagueness (Strong 1984) 5

6 Stochastic Boundedness (Chesson) In a spatially derived form - Metapopulation Dynamics - require no explicit assumption of local density dependence 10. The New Synthesis ( Present) It is generally accepted that population regulation does occur in many natural populations and that it can only occur in the presence of density-dependent factors. It also is generally recognized that populations are controlled simultaneously by d-d and d-i processes. The result is population dynamics that can be described as a noisy equilibrium. Peter Turchin We also have arrived at 3 good definitions: 1) Population Regulation a. Process by which a population returns to an equilibrium size CONCEPTUAL b. Presence of a long-term stationary probability distribution of population densities (i.e., there is some mean level of density around which a regulated population fluctuates and the variance of population density is bounded) OPERATIONAL 2) Density Dependence a. Functional dependence of a vital population rate (e.g., survivorship) on population abundance or density. CONCEPTUAL AND OPERATIONAL b. Functional dependence of per capita population growth rate on present and/or past population densities. CONCEPTUAL AND OPERATIONAL 3) Population Limitation a. Process which sets the equilibrium size or the long-term stationary probability distribution of population densities. CONCEPTUAL b. Population stability in the absence of d-d regulation OPERATIONAL 6

7 11. Remaining Questions 1) How often is regulation brought about by local processes and how often is it the result of metapopulation or source-sink processes (i.e., to what extent does spatial population structure regulate population dynamics)? 2) What is the relative importance of top-down (i.e., predators) vs. bottom-up (i.e., resources) forces in regulating natural populations? 3) What are the behavioral mechanisms underlying population regulation (dispersal, habitat selection, territoriality)? 4) What are the exact mechanisms by which d-i and d-d factors interact? 5) At what spatial scale does population regulation occur? (this is where A&B and others have gotten into trouble) (this is also the central crux of the matter) 6) Can empirical facts about populations continue to be incorporated into new mathematical theories? 12. Final Paragraph in Turchin s Chapter Finally, we should approach ecological questions with a synthetic approach that blends together statistical analyses of time series data, experiments, and mathematical models. The density-dependence debate was to a large degree driven by mutual misunderstandings between theoretically minded and empirical ecologists. To bridge this gap, empirical ecologists should become more theoretical and make an effort to understand theoretical papers. Theoretical ecologists, on the other hand, should become more empirical and make an effort to understand and address in their models the concerns of empiricists. One of the hopeful signs is that graduate training in ecology today places a greater emphasis than before on quantitative skills. In a recent poll of readers of Trends in Ecology and Evolution, readers under 36 showed greater enthusiasm for mathematical ecology than the general sample. The gap between theory and empiricism in population ecology may soon be bridged. Turchin, P. (1995) 7