In dynamic environments like rivers and streams, the location and quality of habitats are ever-changing. Flooding and woody debris work together to shape river and stream channels, water depth, and flow characteristics, creating a shifting mosaic of habitats within riverine systems. Organisms need to move to survive in these dynamic river and stream ecosystems. These range from regular movements necessary to access food, shelter, mates, nesting areas, or other resources, to significant shifts in response to extreme conditions brought about by natural or human-caused disturbance.
Survival of individual animals, facilitation of reproduction, and the maintenance of population continuity are important functions of movement at a population level.
Daily Movements
Animals move through rivers and streams for a variety of reasons. Some are regular daily movements to find food and avoid predators. It is not unusual for aquatic animals to forage at night and seek shelter during the day. Examples include juvenile bull trout and Atlantic salmon, American eel, hellbenders and many other species of stream salamanders. Many fish species are active during the day and seek shelter at night.
Seasonal Movements
Some animal movements within streams and rivers are seasonal in nature and represent adjustments to seasonal changes in habitat conditions or movements linked to the life cycle needs of the species.
Changes in habitat conditions, such as temperature, water depth or flow velocity, may require organisms to move to areas with more favorable conditions. During the summer, for example, many salmonid species move up into cool headwater streams to avoid temperature stress in mainstem waterways. When conditions become too dry, these animals shift to areas with suitable water. Floodplain side channels and sidewall channels that are fed by groundwater also provide thermal refuges for fish and other aquatic organisms.
As organisms move through their various life stages they need access to areas that meet a variety of habitat requirements, requirements that may change as these organisms grow and develop. Sometimes spawning habitat doubles as nursery habitat for juvenile fish or larval amphibians. In other cases the survival needs of eggs (e.g. cool temperatures, specific substrates, or well-oxygenated water) may be quite different from those required by juveniles or larvae (appropriate cover, more persistent hydrology, lower flow velocities, or adequate food supplies). Adult fish may require deeper water and larger cover objects.
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 organisms 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 variabilit,y 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.
For example, 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).
Reproduction
During the breeding season animals move to find mates and smaller individuals may have to move to avoid areas dominated by larger, territorial adults. A common strategy among river and stream fishes is to segregate habitats used by adults from those used by juvenile fish. Adult fish typically use habitats in areas of deeper water and more stable hydrology than those in which they spawn. They migrate to spawning areas that have higher productivity or fewer predators such as floodplains and headwater streams. In these areas, recently hatched fish can take advantage of decreased predation or higher productivity with the large number of juveniles compensating for the risks inherent in these more variable habitats (Hall, C.A. 1972. Ecology 53(4):585-604).
The most dramatic examples of breeding movements are the long-range migrations of anadromous fish – species that live much of their adult lives in the ocean, yet return to freshwater streams and rivers to breed. These include various species of salmon, sea-run trout, shad and other herring species, sturgeons, and other fish. The common eel is a catadromous species, living as adults in freshwater and migrating to the ocean to breed.
Adult salmon live in the ocean until the breeding season when they migrate long distances to reach spawning streams. As they become larger, juvenile salmon hatched in these streams make their way downstream to the ocean, where the large marine food base can support much higher growth rates than can be supported in freshwater environments. Other fish species make similar, but less dramatic migrations to reach spawning habitats. Pike and pickerel move into vegetated floodplains to spawn. Many “non-migratory” fish (e.g. some species of trout, suckers, and freshwater minnows) utilize headwater streams as spawning and nursery habitat.
In contrast to fish, many stream salamanders utilize intermittent headwater streams as adults, but deposit their eggs in more perennial areas of the stream. The semi-aquatic adults can readily move up into headwaters to exploit the productivity of these areas. Their less mobile larvae are aquatic and need areas of more reliable, year-round hydrology.
Exploit Vacant Habitat
In dynamic environments like rivers and streams, the location and quality of habitats are ever-changing. For a time, fisheries biologists thought that fish like trout generally stayed put, except for specific periods of movement for breeding or to avoid unfavorable conditions. However, it now appears that a significant proportion of these fish make regular movements (ranging behavior) that allow individuals to locate and exploit favorable habitat within these ever-shifting mosaics (Gowan, et al. 1994. Can. J. Fish. Aquat. Sci. 51:2626-2637).
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.
Dispersal
Dispersal of individuals provides a mechanism for regulating population density. These dispersing individuals maintain gene flow among populations and may supplement populations where recruitment is unable to keep pace with the loss of individuals. For many small species (especially invertebrates), dispersal of individuals provides a mechanism for colonizing habitat, allowing local populations to come and go as habitat is created or eliminated, while maintaining viable regional populations.
Among aquatic communities, the movement of animals helps maintain the balance between predators and prey and facilitates more efficient utilization of food-based energy within the system.