Importance of Movement–
Population Continuity

Although movement and migration present obvious advantages for organisms, individual animals live and die. It is populations, operating in the context of ecosystems, which persist over time.

Populations are groups of organisms arranged in such a way that they can regularly interact and interbreed. Animal movements are necessary to maintain continuous populations and constraints on movement often delineate one population from another. The ability of a population to remain genetically viable and persist over time is related to its size and the degree of interaction with other populations of the same species.

Because smaller and more isolated populations are vulnerable to extinction, conservation biologists use general rules of thumb for populations that are likely to remain viable in the short-term and over the long-term. “Minimum viable population” is a concept based on computer models of population change over time. Over the short-term, depending on a species' life history characteristics, the minimum viable population size ranges from 50 to 200 or more individuals. For long-term viability, estimates of minimum population size range from 500 to 5000 or more individuals. Given the narrow, linear configuration of streams and rivers, animal movements are critical for maintaining populations large enough to remain viable.

Smaller populations may be able to persist, despite their small size, if they are connected to larger, regional populations. Connections occur when individuals move from one population to another. For some species, dispersing juveniles are responsible for these movements between populations. For other species dispersal occurs via adults. Such movements maintain gene flow among populations, helping to maintain genetic health. They may also represent movements of surplus animals from one population to another, perhaps one that could not support itself on its own reproduction. This supplementation of failing populations from “source” populations is referred to as “the rescue effect.” Finally, areas of appropriate habitat that may be temporarily vacant due to local extinction can be re-colonized by individuals from populations nearby.