Introduction

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.

If an actual number of individuals cannot be determined, scientists have used indices of abundance, such as changes through time, percentage cover or harvested biomass (e.g., for plants), and catch per unit effort (e.g., for fishes). Ecologists are always striving for an accurate and precise estimate of abundance; therefore, a thorough knowledge of the organism and its environment is necessary to design the proper sample unit and best allocation ofthat sample unit in space and time. Estimates of abundance can be made more accurate or at least accuracy assessed by eliminating or decreasing biases and by using various methods to determine abundance. Determining whether there is an accurate estimate of abundance is difficult because usually the true abundance is unknown and the estimate may contain unknown biases. Being aware of the potential biases and striving to minimize and investigate biases will increase the chances of an accurate abundance estimate. Different sampling designs will help ensure a representative sample is obtained that also will increase accuracy. Variability in the estimate of abundance, or precision, is affected by the natural variability in abundance among the samples and by the number of samples. Because natural variability cannot be controlled, the single best means of increasing the precision of the abundance estimate is to increase sample size (e.g., number of transects, cores, marked individuals). An understanding of sampling design (or the observation of sample units in space and time) can help determine whether there are enough independent and representative sample units to provide an accurate and precise estimate of abundance.

The spatial and temporal patterns of abundance (i.e., dispersion) often indicate fluctuations in physical or biological factors. Abundance is a measure of how many organisms are within an area whereas dispersion is how those organisms are arranged within the area. We usually recognize three basic patterns of abundance in space or time: uniform, random, or aggregated (Figure 1). A uniform abundance in space or time is one where the organism is spaced evenly. Rarely is this the case for organisms because biological factors (e.g., attraction, aggression, competition) will cause nonuniformity and most environmental

Uniform Random Aggregated

Figure 1 Various forms of dispersion in space (uniform, random, and aggregated) are depicted using red stars as individual organisms. In reality, there is a continuum of dispersion from the extremely uniform to greatly aggregated, with random in between these two forms.

features that affect organisms are not uniformly distributed. With a random distribution we assume that the probability that an organism can inhabit any location or time is equal. This also is rare for the same reasons that organisms are not uniformly distributed. Finally, organisms can be aggregated if they occupy very specific locations, such that in some locations or times the probability of encountering that organism is nearly one and in other locations or times it is zero. At some spatial scale all organisms are grouped or aggregated. Sea bird nests may be uniformly distributed within a colony because the birds place their nests just far enough apart to not be pecked by their neighbor, but the nests are very much aggregated because some sea birds only nest on isolated islands. If you focused your attention at the scale of the colony, the bird's nests would be distributed uniformly, but at the scale of the world, the nests are aggregated. Aggregations typically occur where local conditions are optimum for survival and reproduction. For instance, certain plants require specific soil types, light exposure, moisture, and nutrients. Through adaptive radiation, species have evolved specific requirements; hence, they cannot live just anywhere. The aggregated distribution of organisms implies that individuals of a population will be abundant in some locations and rare or absent from other locations. The same patterns and rationale can be used to assess abundance patterns in time. Certain periods of time are more conducive to some species; hence they are more abundant, than other times. The timescales that affect abundance can be days for short-lived organisms like insects, or thousands of years like large trees. Natural and anthropogenic changes in environmental conditions will cause changes in abundance for all organisms, and these changes can be predicted using mathematical models of population dynamics. The actual patterns of dispersion in space and time form a continuum, where populations can have varying levels of uniform, random, or aggregated patterns. Because organisms in space and time have an infinite array of patterns it makes it difficult to accurately model and test patterns of dispersion.

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