To manage a species, whether common or rare, one needs to have a firm understanding of the factors that influence population size. Ecologists achieve this by monitoring population size, understanding the factors limiting the population, and using mathematical models to predict the future dynamics of a population.
A population is a collection of individuals from the same species that occupies some specified area. Populations change in abundance due to intraspecific interactions (interactions between individuals of the same species-competition for example), interspecific interactions (interactions between individuals of different species-predation and herbivory for example), and environmental factors such as temperature, rainfall, and nutrient availability. Populations also undergo random fluctuations this is especially important for small populations.
Changes in population size can also be thought of as density independent (changes occur independent of the size of the population for example weather-induced population declines) or density dependent (changes are a function of the size of the population itself for example when a population gets so large that it reduces the availability of a needed resource, and thus declines). In a density-dependent situation, mortality, reproduction and other factors may change concurrently with changes in population density, such that as population density becomes more extreme (high or low) a greater proportion of the population is influenced.
Ideally, a measure of the size of a population would be obtained by counting all the individuals, with the knowledge that none are missed in such a count. For many organisms, however, estimates of population size by direct counts are impractical. For mobile or secretive animals, for example, it is difficult to obtain direct estimates of population density, even in very small areas. However, animal population estimates can also be made using a variety of indirect techniques such as counting signs of animals (e.g. tracks, feces, vocalizations) and by estimates derived from observing a subset of animals and extrapolating to estimate the size of the entire population. Transect estimates and mark-recapture techniques are examples of the later. The mark and recapture technique can provide a robust estimate of population size by marking a segment of the population on one occasion and then resampling the population at a later time (often more than one occasion) to quantify numbers of previously marked plus new unmarked animals. The ratio of marked and unmarked animals is used to estimate total population size.
Many variables interact to determine the size of a population and its persistence. Often the only way to gain insight is to develop a mathematical model of the population and use it to perform a Population Viability Analysis, or PVA. PVAs are models that relate a dependent variable (in our case, population size) to the independent variables that influence it (for example, mortality, harvesting, and weather). A PVA estimates the probability of a population persisting in an environment. It is a form of risk analysis in which one is interested not only in the average size of a population at some time in the future, but perhaps more importantly in the range of possible future values. A robust PVA incorporates density dependent and density independent events, as well as the role of random fluctuations.
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