Importance of Movement–
Changes in Habitat Conditions

Within the course of a single year or over the course of several years or decades, rivers and streams experience broad ranges of environmental conditions that affect habitat suitability for fish and wildlife. These include changes in water depth, velocity, turbulence, turbidity, temperature, dissolved oxygen, substrate suitability, availability of food and cover, and the distribution of specific habitat units such as pools, riffles, runs, rapids, and backwaters. In some cases aquatic organism may be able to hunker down and wait out unfavorable conditions. In other cases aquatic organisms will need to move in order to survive.

In many stream systems where natural disturbances cause significant habitat variability access to refuge habitat is especially important. Disturbances that require fish to seek refuge habitats can also be human-caused. For example, many streams are paralleled by major highways, and toxic spills in streams are not uncommon. When these occur, fish must have the ability to move to unaffected habitats.

Extreme events such as floods, landslides, and drought may force entire populations to move to avoid unfavorable conditions. Provided that there are no barriers preventing the movement of individual animals back into the areas, populations will reoccupy the habitat once conditions have improved.

In the intermittent Colorado plains streams that provide habitat for the Arkansas darter, habitat changes seasonally with regular wet and dry cycles. During dry periods darters rely on groundwater-fed refuge pools. The number, distribution and quality of these pools change in response to drought, winter conditions (pool freezing), and flooding that occur on average every few years or decades. Occasional flash floods scour out new pools and fill others. In order to persist in these streams Arkansas darters must rely on long distance movements to locate and colonize pools in this ever-changing landscape (Labbe & Fausch, 2000. Ecological Applications 10:1774-1791).