Population Ecology
Population ecology provides scientists and managers with information on populations of species that can indicate that population’s sustainability over time and the degree to which the species utilizes the available habitat.
Section 1: Demography
A population is the total of the individuals of a given species living in a given area. Examples are the population of sage grouse in one hundred acres of sagebrush steppe or all the individuals of the Gunnison sage grouse throughout the western United States. The study of how a population of a given species changes over time is referred to as demography. Changes studied can include population size, population density, species distribution, sex ratios, births, deaths, or any characteristic that describes the entire population under study or management.
Population Size and Density
Population size is the number of individuals of a species in a given area. Population density is the number of individuals per unit area. For plant species and many animal species, these numbers are estimates obtained using sampling methods. For example, if we want to know the population density of big basin sagebrush (Artemisia tridentata) we might lay out a quadrat (an area of a given size, such as a meter or an acre) and count all the individuals of the species within that quadrat. This sampling method is conducted randomly throughout the study area, then statistics are used to estimate the population. If we want to know the population size of pronghorn, we might use visual observation counts from random sites or from the same site over a period of time (days to weeks) and then use statistical analysis to estimate the population size. Another common method in wildlife population counts is mark and recapture.
Population and density size tell us several things about the species and the ecosystem such as the following:
- The population’s degree of stability. Larger populations tend to be more stable as they can absorb a certain threshold of increase or decrease in population size. Larger populations also have a greater degree of phenotypic plasticity and a greater ability to adapt to environmental shifts.
- Habitat availability in some instances can be one indicator of the ecosystem function and structure in a given area.
- Population size is directly dependent on the carrying capacity of a given area.
- Among wildlife and with livestock, population size informs dietary overlap.
- In general, humans are most interested in species’ population size in relation to humans or human activity, such as conservation efforts, hunting, endangered species survival, and so on.
Species Distribution
Species distribution is how individuals of a population are distributed across a given area and reflects species dispersal methods and population interactions. There are three general distribution types:
In uniform distribution, individuals are spaced uniformly across a given area. It is fairly easy to recognize a population of cr