This is a risk now on many rivers and canal systems in Illinois and Missouri be
This is a risk now on many rivers and canal systems in Illinois and Missouri because of Asian carp. This fish—a group of species including the silver, black, grass, and big-head carp—has been farmed and eaten in China for over 1,000 years. It is one of the most important aquaculture food resources worldwide. In the United States, however, Asian carp is considered a dangerous invasive species that disrupt ecological community structure to threaten native species. The effects of invasive species (such as the Asian carp, kudzu vine, predatory snakehead fish, and zebra mussel) are just one aspect of what ecologists study to understand how populations interact within ecological communities and what impact natural and human-induced disturbances have on the characteristics of communities.
Populations are dynamic entities. Their size and composition fluctuate due to numerous factors, including seasonal and yearly environmental changes, natural disasters such as forest fires and volcanic eruptions, and competition for resources between and within species. The study of populations is called demography.
Population Size and Density
Populations are characterized by population size (total number of individuals) and population density (number of individuals per unit area). A population may have a large number of individuals that are distributed densely or sparsely. There are also populations with small numbers of individuals that may be dense or very sparsely distributed in a local area. Population size can affect the potential for adaptation because it affects the amount of genetic variation present in the population. Density can affect interactions within a population, such as competition for food and the ability of individuals to find a mate. Smaller organisms are more densely distributed than larger ones (Figure 1).
Estimating Population Size
The most accurate way to determine population size is to count all individuals within the area. However, this method is usually not logistically or economically feasible, especially when studying large areas. Thus, scientists usually study populations by sampling a representative portion of each habitat and use this sample to make inferences about the population as a whole. The methods used to sample populations to determine their size and density are typically tailored to the characteristics of the organism being studied. A quadrat may be used for immobile organisms such as plants or very small and slow-moving organisms. A quadrat is a square structure that is randomly located on the ground and used to count the number of individuals that lie within its boundaries. To obtain an accurate count using this method, the square must be placed at random locations within the habitat enough times to produce an accurate estimate.
For smaller mobile organisms, such as mammals, a technique called mark and recapture is often used. This method involves marking captured animals and releasing them back into the environment to mix wi