Population ecology
Population ecology is a field of ecology that deals with the dynamics of species populations and how these populations interact with the environment, such as birth and death rates, and by immigration and emigration.
The discipline is important in conservation biology, especially in the development of population viability analysis which makes it possible to predict the long-term probability of a species persisting in a given patch of habitat. Although population ecology is a subfield of biology, it provides interesting problems for mathematicians and statisticians who work in population dynamics.
History
In the 1940s, ecology was divided into autecology—the study of individual species in relation to the environment—and synecology—the study of groups of species in relation to the environment. The term autecology, refers to roughly the same field of study as concepts such as life cycles and behaviour as adaptations to the environment by individual organisms. Eugene Odum, writing in 1953, considered that synecology should be divided into population ecology, community ecology and ecosystem ecology, renaming autecology as 'species ecology', thus that there were four subdivisions of ecology. In 1946 Thomas Park named four people for establishing the field of population ecology: Carl Semper for noting how the organs of organizations are specialized to their environments; Karl Möbius for developing the biocoenosis concept; Stephen Alfred Forbes for the ideas in his work, "The Lake as a microcosm"; and C.G. Johannes Petersen for effectively applying quantitative methods to fish populations.Terminology
A population is defined as a group of interacting organisms of the same species. A demographic structure of a population is how populations are often quantified. The total number of individuals in a population is defined as a population size, and how dense these individuals are is defined as population density. There is also a population's geographic range, which has limits that a species can tolerate.Population size can be influenced by the per capita population growth rate Births, deaths, emigration, and immigration rates all play a significant role in growth rate. The maximum per capita growth rate for a population is known as the intrinsic rate of increase.
In a population, carrying capacity is known as the maximum population size of the species that the environment can sustain, which is determined by resources available. In many classic population models, r is represented as the intrinsic growth rate, where K is the carrying capacity, and N0 is the initial population size.
| Term | Definition |
| Species population | All individuals of a species. |
| Metapopulation | A set of spatially disjunct populations, among which there is some migration. |
| 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 investigator-delimited area smaller than the geographic range of the species and often within a population. A local population could be a disjunct population as well. |
| Subpopulation | An arbitrary spatially delimited subset of individuals from within a population. |
| Immigration | The number of individuals that join a population over time. |
| Emigration | The number of individuals that leave a population over time. |
Population dynamics
The development of population ecology owes much to the mathematical models known as population dynamics, which were originally formulae derived from demography at the end of the 18th and beginning of 19th century.The beginning of population dynamics is widely regarded as the work of Malthus, formulated as the Malthusian growth model. According to Malthus, assuming that the conditions remain constant, a population will grow exponentially. This principle provided the basis for the subsequent predictive theories, such as the demographic studies such as the work of Benjamin Gompertz and Pierre François Verhulst in the early 19th century, who refined and adjusted the Malthusian demographic model.
A more general model formulation was proposed by F. J. Richards in 1959, further expanded by Simon Hopkins, in which the models of Gompertz, Verhulst and also Ludwig von Bertalanffy are covered as special cases of the general formulation. The Lotka–Volterra predator-prey equations are another famous example, as well as the alternative Arditi–Ginzburg equations.
Exponential vs. logistic growth
When describing growth models, there are two main types of models that are most commonly used: exponential and logistic growth.When the per capita rate of increase takes the same positive value regardless of population size, the graph shows exponential growth. Exponential growth takes on the assumption that there is unlimited resources and no predation. An example of exponential population growth is that of the Monk Parakeets in the United States. Originally from South America, Monk Parakeets were either released or escaped from people who owned them. These birds experienced exponential growth from the years 1975-1994 and grew about 55 times their population size from 1975. This growth is likely due to reproduction within their population, as opposed to the addition of more birds from South America.
When the per capita rate of increase decreases as the population increases towards the maximum limit, or carrying capacity, the graph shows logistic growth. Environmental and social variables, along with many others, impact the carrying capacity of a population, meaning that it has the ability to change.
Fisheries and wildlife management
In fisheries and wildlife management, population is affected by three dynamic rate functions.- Natality or birth rate, often recruitment, which means reaching a certain size or reproductive stage. Usually refers to the age a fish can be caught and counted in nets.
- Population growth rate, which measures the growth of individuals in size and length. More important in fisheries, where population is often measured in biomass.
- Mortality, which includes harvest mortality and natural mortality. Natural mortality includes non-human predation, disease and old age.
where N0 is the number of individuals at time 0, B is the number of individuals born, D the number that died, I the number that immigrated, and E the number that emigrated between time 0 and time 1.
If we measure these rates over many time intervals, we can determine how a population's density changes over time. Immigration and emigration are present, but are usually not measured.
All of these are measured to determine the harvestable surplus, which is the number of individuals that can be harvested from a population without affecting long-term population stability or average population size. The harvest within the harvestable surplus is termed "compensatory" mortality, where the harvest deaths are substituted for the deaths that would have occurred naturally. Harvest above that level is termed "additive" mortality, because it adds to the number of deaths that would have occurred naturally. These terms are not necessarily judged as "good" and "bad," respectively, in population management. For example, a fish & game agency might aim to reduce the size of a deer population through additive mortality. Bucks might be targeted to increase buck competition, or does might be targeted to reduce reproduction and thus overall population size.
For the management of many fish and other wildlife populations, the goal is often to achieve the largest possible long-run sustainable harvest, also known as maximum sustainable yield. Given a population dynamic model, such as any of the ones above, it is possible to calculate the population size that produces the largest harvestable surplus at equilibrium. While the use of population dynamic models along with statistics and optimization to set harvest limits for fish and game is controversial among some scientists, it has been shown to be more effective than the use of human judgment in computer experiments where both incorrect models and natural resource management students competed to maximize yield in two hypothetical fisheries. To give an example of a non-intuitive result, fisheries produce more fish when there is a nearby refuge from human predation in the form of a nature reserve, resulting in higher catches than if the whole area was open to fishing.
r/K selection
An important concept in population ecology is the r/K selection theory. For example, if an animal has the choice of producing one or a few offspring, or to put a lot of effort or little effort in offspring—these are all examples of trade-offs. In order for species to thrive, they must choose what is best for them, leading to a clear distinction between r and K selected species.The first variable is r and the second variable is K.
It is important to understand the difference between density-independent factors when selecting the intrinsic rate and density-dependent for the selection of the carrying capacity. Carrying capacity is only found during a density-dependent population. Density-dependent factors influence the carrying capacity are predation, harvest, and genetics, so when selecting the carrying capacity it is important to understand to look at the predation or harvest rates that influence the population.
An r-selected species 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 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. K-selected species generally experience stronger competition, where populations generally live near carrying capacity. These species have heavy investment in offspring, resulting in longer lived organisms, and longer period of maturation. Offspring of K-selected species generally have a higher probability of survival, due to heavy parental care and nurturing.