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| '''Population ecology''' is a sub-field of [[ecology]] that deals with the dynamics of [[species]] [[population]]s and how these populations interact with the [[natural environment|environment]].<ref name="Odum1959">{{cite book | last =Odum | first =Eugene P. | authorlink =Eugene Odum | title =Fundamentals of Ecology | edition=Second| publisher =W. B. Saunders Co. | year =1959 | location =Philadelphia and London | isbn = 9780721669410|oclc = 554879 | page =546 p}}</ref> It is the study of how the [[population size]]s of species living together in groups change over time and space.
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| The development of population ecology owes much to [[demography]] and [[actuary|actuarial]] [[life table]]s. Population ecology is important in [[conservation biology]], especially in the development of [[population viability analysis]] (PVA) which makes it possible to predict the long-term probability of a species persisting in a given habitat patch, such as a national park. Although population ecology is a subfield of [[biology]], it provides interesting problems for [[mathematics|mathematicians]] and [[statistics|statisticians]] who work in [[population dynamics]].
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| ==Fundamentals==
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| The most fundamental law of population ecology is [[Thomas Malthus]]' exponential law of population growth.<ref name="Turchin01">{{Cite journal | last = Turchin | first = P. | title = Does Population Ecology Have General Laws? | journal = Oikos
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| | volume = 94 | issue = 1 | pages = 17–26 | year = 2001 | doi = 10.1034/j.1600-0706.2001.11310.x | postscript = <!--None-->}}</ref>
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| {| class="wikitable" style="float:right; border:0.5px solid grey; margin: 10px; text-align:middle; background-color: #F8F8F8; font-size:11px;"
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| |+ Terms used to describe natural groups of individuals in ecological studies<ref name="Wells95">Terms and definitions directly quoted from: {{Cite journal | last = Wells | first = J. V. | last2 = Richmond | first2 = M. E. | title = Populations, metapopulations, and species populations: What are they and who should care? | journal = Wildlife Society Bulletin | volume = 23 | issue = 3 | pages = 458–462 | year = 1995 | url = http://www.uoguelph.ca/zoology/courses/BIOL3130/wells11.pdf | format = }} {{dead link|date=June 2010}}</ref>
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| !width="120"|Term
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| !! !|Definition
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| |'''Species population''' || All individuals of a species.
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| |'''Metapopulation''' || A set of spatially disjunct populations, among which there is some immigration.
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| |'''Population''' || A group of conspecific individuals that is demographically, genetically, or spatially disjunct from other groups of individuals.
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| | '''Aggregation''' || A spatially clustered group of individuals.
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| | '''Deme''' || A group of individuals more genetically similar to each other than to other individuals, usually with some degree of spatial isolation as well.
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| | '''Local population''' || A group of individuals within an investigator-delimited area smaller than the geographic range of the species and often within a population (as defined above). A local population could be a disjunct population as well.
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| |'''Subpopulation''' || An arbitrary spatially delimited subset of individuals from within a population (as defined above).
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| |}
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| <blockquote>
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| ''A population will grow (or decline) exponentially as long as the environment experienced by all individuals in the population remains constant.''<ref name="Turchin01" />{{rp|18}}
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| </blockquote>
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| This principle in population ecology provides the basis for formulating predictive theories and tests that follow.
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| Simplified population models usually start with four key variables including death, birth, immigration, and emigration. Mathematical models used to calculate changes in population demographics and evolution hold the assumption (or [[null hypothesis]]) of no external influence. Models can be more mathematically complex where "...several competing hypotheses are simultaneously confronted with the data."<ref name="Johnson04">{{Cite journal | last = Johnson | first = J. B. | last2 = Omland | first2 = K. S. | title = Model selection in ecology and evolution. | journal = Trends in Ecology and Evolution | volume = 19 | issue = 2 | pages = 101–108 | year = 2004 | url = http://www.usm.maine.edu/bio/courses/bio621/model_selection.pdf | doi = 10.1016/j.tree.2003.10.013 | pmid = 16701236 | postscript = <!--None-->}}</ref> For example, in a closed system where immigration and emigration does not take place, the per capita rates of change in a population can be described as:
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| : <math>\frac{dN}{dT} = B - D = bN - dN = (b - d)N = rN,</math>
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| where ''N'' is the total number of individuals in the population, ''B'' is the number of births, ''D'' is the number of deaths, ''b'' and ''d'' are the per capita rates of birth and death respectively, and ''r'' is the per capita rate of population change. This formula can be read as the rate of change in the population (''dN/dT'') is equal to births minus deaths (B - D).<ref name="Turchin01" /><ref name="Vandermeer03">{{Cite book | last = Vandermeer | first = J. H. | last2 = Goldberg | first2 = D. E. | title = Population ecology: First principles | place = Woodstock, Oxfordshire | publisher = Princeton University Press | year = 2003 | isbn = 0-691-11440-4 | postscript = <!--None-->}}</ref>
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| Using these techniques, Malthus' population principle of growth was later transformed into a mathematical model known as the [[Logistic function#In ecology: modeling population growth|logistic equation]]:
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| : <math>\frac{dN}{dT} = aN \left( 1 - \frac{N}{K} \right),</math>
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| where ''N'' is the biomass density, ''a'' is the maximum per-capita rate of change, and ''K'' is the [[carrying capacity]] of the population. The formula can be read as follows: the rate of change in the population (''dN/dT'') is equal to growth (''aN'') that is limited by carrying capacity ''(1-N/K)''. From these basic mathematical principles the discipline of population ecology expands into a field of investigation that queries the [[demographics]] of real populations and tests these results against the statistical models. The field of population ecology often uses data on life history and matrix algebra to develop projection matrices on fecundity and survivorship. This information is used for managing wildlife stocks and setting harvest quotas <ref name="Vandermeer03" /><ref name="Berryman92">{{Cite journal | last = Berryman | first = A. A. | title = The Origins and Evolution of Predator-Prey Theory | journal = Ecology | volume = 73 | issue = 5 | pages = 1530–1535 | year = 1992 | doi = 10.2307/1940005 | publisher = Ecology, Vol. 73, No. 5 | jstor = 1940005}}</ref>
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| {{clear}}
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| ==r/K selection==
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| {{main|r/K selection}}
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| {{quote box
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| | quote = At its most elementary level, interspecific competition involves two species utilizing a similar resource. It rapidly gets more complicated, but stripping the phenomenon of all its complications, this is the basic principle: two consumers consuming the same resource.<ref name="Vandermeer03" />{{rp|222}}
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| An important concept in population ecology is the [[r/K selection]] theory. The first variable is ''r'' (the intrinsic rate of natural increase in population size, density independent) and the second variable is ''K'' (the carrying capacity of a population, density dependent).<ref name="Begon06">{{Cite book
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| | last = Begon
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| | first = M.
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| | last2 = Townsend
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| | first2 = C. R.
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| | last3 = Harper
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| | first3 = J. L.
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| | title = Ecology: From Individuals to Ecosystems
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| | place = Oxford, UK
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| | publisher = Blackwell Publishing
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| | year = 2006
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| | edition = 4th
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| | url = http://books.google.com/?id=Lsf1lkYKoHEC&printsec=frontcover&dq=ecology&cd=1#v=onepage&q=
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| | isbn = 978-1-4051-1117-1
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| | postscript = <!--None-->}}</ref>
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| An ''r''-selected species (e.g., many kinds of insects, such as aphids<ref name="Whitman78">{{Cite journal | last = Whitham | first = T. G. | title = Habitat Selection by Pemphigus Aphids in Response to Response Limitation and Competition | journal = Ecology | volume = 59 | issue = 6 | pages = 1164–1176 | year = 1978 | doi = 10.2307/1938230 | publisher = Ecology, Vol. 59, No. 6 | jstor = 1938230}}</ref>) is one that has high rates of fecundity, low levels of parental investment in the young, and high rates of mortality before individuals reach maturity. Evolution favors productivity in r-selected species. In contrast, a ''K''-selected species (such as humans) has low rates of fecundity, high levels of parental investment in the young, and low rates of mortality as individuals mature. Evolution in ''K''-selected species favors efficiency in the conversion of more resources into fewer offspring.<ref name="MacArthur67">{{Cite journal | last = MacArthur | first = R. | last2 = Wilson | first2 = E. O. | title = The Theory of Island Biogeography | place = Princeton, NJ | publisher = Princeton University Press | year = 1967 | postscript = <!--None-->}}</ref><ref name="Pianka72">{{Cite journal | last = Pianka | first = E. R. | title = r and K Selection or b and d Selection? | journal = The American Naturalist | volume = 106 | issue = 951 | pages = 581–588 | year = 1972 | doi = 10.1086/282798}}</ref>
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| ==Metapopulation==
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| {{main|Metapopulation}}
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| Populations are also studied and conceptualized through the "[[metapopulation]]" concept. The metapopulation concept was introduced in 1969:<ref name="Levins69">{{Cite journal | last = Levins | first = R. | title = Some demographic and genetic consequences of environmental heterogeneity for biological control | journal = Bulletin of the Entomological Society of America | volume = 15 | pages = 237–240 | year = 1969 | url = http://books.google.com/?id=8jfmor8wVG4C&pg=PA162&q= | publisher = Columbia University Press | isbn = 978-0-231-12680-9}}</ref><blockquote> "as a population of populations which go extinct locally and recolonize."<ref name="Levins70">{{Cite book | last = Levins | first = R. | editor-last = Gerstenhaber | editor-first = M. | title = Extinction. In: Some Mathematical Questions in Biology | year = 1970 | pages = 77–107 | url = http://books.google.com/?id=CfZHU1aZqJsC&dq=Some+Mathematical+Questions+in+Biology&printsec=frontcover&q= | publisher = AMS Bookstore | isbn = 978-0-8218-1152-8}}</ref>{{rp|105}}</blockquote> Metapopulation ecology is a simplified model of the landscape into patches of varying levels of quality.<ref name="Hanski98">{{Cite journal | last = Hanski | first = I. | title = Metapopulation dynamics | journal = Nature | volume = 396 | pages = 41–49 | year = 1998 | url = http://www.helsinki.fi/~ihanski/Articles/Nature%201998%20Hanski.pdf | doi = 10.1038/23876 | issue=6706}}</ref> Patches are either occupied or they are not. Migrants moving among the patches are structured into metapopulations either as sources or sinks. Source patches are productive sites that generate a seasonal supply of migrants to other patch locations. Sink patches are unproductive sites that only receive migrants. In metapopulation terminology there are emigrants (individuals that leave a patch) and immigrants (individuals that move into a patch). Metapopulation models examine patch dynamics over time to answer questions about spatial and demographic ecology. An important concept in metapopulation ecology is the rescue effect, where small patches of lower quality (i.e., sinks) are maintained by a seasonal influx of new immigrants. Metapopulation structure evolves from year to year, where some patches are sinks, such as dry years, and become sources when conditions are more favorable. Ecologists utilize a mixture of computer models and field studies to explain metapopulation structure.<ref name="Hanski04">{{Cite book | editor-last = Hanski | editor-first = I. | editor2-last = Gaggiotti | editor2-first = O. E. | title = Ecology, genetics and evolution of metapopulations. | publisher = Elsevier Academic Press | year = 2004 | location = Burlington, MA | url = http://books.google.com/?id=EP8TAQAAIAAJ&q=ecology,+genetics,+and+evolution+of+metapopulations&dq=ecology,+genetics,+and+evolution+of+metapopulations&cd=1 | isbn = 0-12-323448-4}}</ref>
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| ==History==
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| The older term, autecology (from Greek: αὐτο, ''auto'', "self"; οίκος, oikos, "household"; and λόγος, logos, "knowledge"), refers to roughly the same field of study as population ecology. It derives from the division of ecology into autecology—the study of individual species in relation to the environment—and [[Community ecology|synecology]]—the study of groups of organisms in relation to the environment—or community ecology. Odum (1959, p. 8) considered that synecology should be divided into population ecology, community ecology, and ecosystem ecology, defining autecology as essentially "species ecology."<ref name="Odum1959"/> However, for some time biologists have recognized that the more significant level of organization of a species is a population, because at this level the species gene pool is most coherent. In fact, Odum regarded "autecology" as no longer a "present tendency" in ecology (i.e., an archaic term), although included "species ecology"—studies emphasizing [[Biological life cycle|life history]] and behavior as adaptations to the environment of individual organisms or species—as one of four subdivisions of ecology.
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| ==Journals==
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| The first journal publication of the Society of Population Ecology, titled ''Population Ecology'' (originally called ''Researches on Population Ecology'') was released in 1952.<ref>{{cite web|url= http://www.springerlink.com/content/1438-3896?sortorder=asc&p=93932389f9764a2aadcbe167b466fcef&o=0|title= Population Ecology}}</ref>
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| Scientific articles on population ecology can also be found in the ''[[Journal of Animal Ecology]]'', ''[[Oikos (journal)|Oikos]]'' and other journals.
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| ==See also==
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| {{Refbegin|3}}
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| *[[Deep ecology]]
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| *[[Density-dependent inhibition]]
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| *[[Irruptive growth]]
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| *[[Lists of organisms by population]]
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| *[[Overpopulation (biology)|Overpopulation]]
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| *[[Overpopulation in companion animals]]
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| *[[Overshoot (population)]]
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| *[[Population density]]
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| *[[Population distribution]]
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| *[[Population dynamics]]
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| *[[Population dynamics of fisheries]]
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| *[[Population genetics]]
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| *[[Population growth]]
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| *[[Theoretical ecology]]
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| {{Refend}}
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| ==References==
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| {{reflist|2}}
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| ==Further reading==
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| * {{cite book | last = Kareiva | first = Peter | authorlink = | editor = Roughgarden J., R.M. May and S. A. Levin | title = Perspectives in ecological theory | edition = | date = | year = 1989
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| | month = | publisher = Princeton University Press| location = New Jersey | chapter = Renewing the Dialogue between Theory and Experiments in Population Ecology | page = 394 p}}
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| * {{cite book | last =Odum | first =Eugene P. | authorlink =Eugene Odum | title =Fundamentals of Ecology | edition=Second| publisher =W. B. Saunders Co. | year =1959 | location =Philadelphia and London | isbn = 9780721669410|oclc = 554879 | page =546 p}}
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| *{{cite journal | last =Smith | first =Frederick E. | authorlink =| title =Experimental methods in population dynamics: a critique | journal =Ecology | volume =33 | issue = 4| pages =441–450
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| | year =1952| doi = 10.2307/1931519| id = | publisher =Ecology, Vol. 33, No. 4 | jstor = 1931519 }}
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| ==External links==
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| *[http://atlas.aaas.org/ AAAS Atlas of Population and Environment]
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| *[http://depositfiles.com/rmv/0140641254884570]
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| {{modelling ecosystems|expanded=other}}
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| {{Population}}
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| {{DEFAULTSORT:Population Ecology}}
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| [[Category:Population ecology|*]]
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| [[Category:Demography]]
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| [[Category:Fields of application of statistics]]
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| [[Category:Population]]
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| [[Category:Subfields of ecology]]
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