Dr. Ray L. Winstead
Professor of Biology, Indiana University of
Two major habitats exist in streams - rapids and pools, and therefore two different
communities of organisms are present. Communities in these two basic habitats are further
determined by the type of bottom substrate that is present (e.g., sand, pebbles, clay,
medium rocks, large rocks).
Current is the most important factor that determines where many animals are found in
streams and therefore govern differences in communities in various parts of a given
stream. Velocity of the current is determined by the steepness of the surface, roughness
of the stream bed, and depth and width of the stream bed. Mechanical forces of the
current put a great deal of stress on organisms. Current velocity is less at the bottom
of the stream. Current brings food to animals. Slower pool waters are favorable for
algae, small swimming animals, and burrowing animals.
Adaptations for Maintaining Position in Swift Water (rapids):
1) Permanent attachment to a firm substrate, e.g., insect cases cemented to rocks.
2) Hooks and suckers, e.g., many insect larvae.
3) Sticky undersurfaces, e.g., snails.
4) Streamlined bodies, e.g., fish and most other stream animals.
5) Flattened bodies - for refuge in rock crevices; water flows over and presses
animal down onto substrate.
6) Continual swimming against the current.
7) Clinging close to a substrate surface.
Temperature is less variable in water than in air. However, aquatic organisms often
have narrow ranges of tolerances for temperature. Changing temperatures in a stream will
alter the communities present, so thermal pollution by Humans is important.
Streams usually contain abundant oxygen because of shallow depth, large surface area
exposed to the air, and constant motion. Stream animals have narrow tolerances for oxygen
and are very sensitive to reduced oxygen. Therefore, organic pollution, which reduces
oxygen levels because of increased bacteria activity, is especially damaging.
The basic energy supply of a stream comes from the surrounding land. Plant
production in a stream is not enough to support all the animals in the stream. In streams
many detritus feeders feed on organic material that falls into or is swept into the stream
from the land. The stream ecosystem is an open ecosystem, i.e., energy and minerals are
not circulated primarily within the system. A stream exports energy in the form of
emerging insects and prey taken by air-breathing predators (e.g., a raccoon eating a
fish). Insects usually are the most abundant and have the greatest diversity of species.
More insects are found in the stream in the winter, since many have matured and left the
stream to go into the surrounding area as flying adults in other seasons.
A population is a group of individual organisms of the same species in a specified area,
e.g., all Mayfly nymphs in a stream.
A community is a group of different populations of organisms in a specified area, e.g.,
all plants and animals in a particular stream.
An ecosystem is a dynamic system composed of the community of all living organisms, the
abiotic (nonliving) environment in the area, and all relationships and interactions
among the biotic (living) and abiotic components of the system.
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Dr. Ray L.
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