Population ecology is a major subfield of ecology that deals with the dynamics of species populations and how these populations interact with the environment.^{[1]}
The first journal publication of the Society of Population Ecology, titled Population Ecology (originally called Researches on Population Ecology), was released in 1952.[1] Population ecology is concerned with the study of groups of organisms that live together in time and space. One of the first laws of population ecology is the Thomas Malthus' exponential law of population growth.^{[2]} This law states that:
"...a population will grow (or decline) exponentially as long as the environment experienced by all individuals in the population remains constant."^{[2]}^{:18}
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 principal: two consumers consuming the same resource.^{[3]}^{:222}
This premise in population ecology provides the basis for formulating predictive theories and tests that follow. 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."^{[4]} 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:
dN / dT = B − D = bN − dN = (b − d)N = rN,
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).^{[2]}^{[3]}
Using these techniques, Malthus' population principal of growth was later transformed into a mathematical model known as the logistic equation:
dN / dT = aN(1 − N / K),
where N is the biomass density, a is the maximum percapita 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 (1N/K). From these basic mathematical principals 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 ^{[3]}^{[5]}
Term  Definition 

Species population  All individuals of a species. 
Metapopulation  A set of spatially disjunct populations, among which there is some immigration. 
Population  A group of conspecific individuals that is demographically, genetically, or spatially disjunct from other groups of individuals. 
Aggregation  A spatially clustered group of individuals. 
Deme  A group of individuals more genetically similar to each other than to other individuals, usually with some degree of spatial isolation as well. 
Local population  A group of individuals within an investigatordelimited 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. 
Subpopulation  An arbitrary spatiallydelimited subset of individuals from within a population (as defined above). 
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).^{[7]} An rselected species (e.g., many kinds of insects, such as aphids^{[8]}) 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 rselected species. In contrast, a Kselected 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 Kselected species favors efficiency in the conversion of more resources into fewer offspring.^{[9]}^{[10]}
Populations are also studied and conceptualized through the metapopulation concept. The metapopulation concept was introduced in 1969^{[11]}:
"as a population of populations which go extinct locally and recolonize."^{[12]}^{:105}
Metapopulation ecology is a simplified model of the landscape into patches of varying levels of quality.^{[13]} 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.^{[14]}
The older term, autecology (from Greek: αὐτο, auto, "self"; οίκος, oikos, "household"; and λόγος, logos, "knowledge") refers to roughly the same field of study, coming from the division of ecology into autecology—the study of individual species in relation to the environment—and 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."^{[1]} 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 life history and behavior as adaptations to the environment of individual organisms or species—as one of four subdivisions of ecology.
The development of the field of population ecology owes much to the science of demography and the use of actuarial life tables. Population ecology has also played an important role in the development of the field of conservation biology especially in the development of population viability analysis (PVA) which makes it possible to predict the longterm probability of a species persisting in a given habitat patch (e.g., a national park).
While essentially a subfield of biology, population ecology provides many interesting problems for mathematicians and statisticians, who work mainly in the study of population dynamics.
Contents 
Scientific literature on population ecology can be found in The Journal of Animal Ecology, Oikos and other journals.

