The abundance of an organism, often considered as total population size or the number of organisms in a particular area (density), is one of the basic measures in ecology. Ecologists often are interested in the abundance and distribution of organisms because the number and spatial extent of an organism reflects the influences of many factors such as patterns in nutrients (fuel), predators or herbivores, competitors, dispersal, and physical conditions. Organisms generally are more abundant where conditions are favorable, such as locations with sufficient quantity and quality of food or nutrients, fewer herbivores or predators, fewer competitors, and optimal physical features. The physical features that affect abundance could be substrate type, moisture, light, temperature, pH, salinity, oxygen or CO2, wind, or currents. Ultimately, the abundance of an organism is dependent on the number of individuals that survive and reproduce. Therefore, any factors that affect survival or reproduction will affect abundance.
Abundance can be measured at many levels, such as the number of individuals of a certain sex or age within a population, the number in a certain geographical region, the number in a certain population (possibly defined as the interbreeding individuals of the same species in a certain geographical area), or the number of individuals of a certain species. Species or populations have different levels of abundance and different population dynamics because of inherent biological characteristics (vital rates), such as the number of young produced per individual, longevity, and survival, and because the species may be adapted and exposed to various environmental conditions. Estimating abundance, however, can be difficult depending on the distribution, visibility, density, and behaviors of the organism.
Estimates of abundance can be obtained by counting all individuals in the population or sampling some portion of the population. A census or total count of all individuals is a common technique used to assess abundance of organisms that are relatively rare and easily observed. If the organism is too numerous or not easily counted then a representative portion of the population is sampled using various techniques such as (1) counts within randomly selected sampling units (e.g., quadrats, cores, nets, or traps); (2) mark-recapture; (3) strip or line transects, which is essentially sampling a long thin quadrat; and (4) distance methods (e.g., nearest neighbor). Most of these methods have a well-developed theoretical and analytical basis. Based on whether the organism is numerous and relatively stationary (e.g., plants), or rare and mobile (e.g., many vertebrates) certain techniques are appropriate. Numbers of individuals within a sample can be determined directly by visually counting individuals or indirectly using acoustics, such as hydroacoustics for assessing fishes or counting calls of bird or whales. Other indirect methods include counting the number of eggs or juveniles, which is an indication of the number of adults (sometimes used to assess fish abundance) or counting nests (such as used for birds). Recently the amount of genetic variation in a population has been used to estimate abundance.
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