Geoscience Reference
In-Depth Information
River ecology
A great diversity of creatures makes up the ecology of rivers, an interconnected web of
life that ranges from microscopic algae to huge fish larger than a human being. Their di-
verse communities reflect the great array of running water environments that vary from
large lowland rivers occupying basins on a subcontinental scale to small, turbulent moun-
tain brooks. The physical structure of the river is one set of influences on its ecology, but
chemical and biological attributes are also important, and all are to some degree related.
The water's oxygen content, acidity or alkalinity, nutrients, metals, and other constituents
are all determined largely by the types of soil and rock that make up the drainage basin, but
also in part by interactions with plants and animals both in the water and on land.
The organisms found in fluvial ecosystems are commonly classified according to the meth-
ods they use to gather food and feed. 'Shredders' are organisms that consume small sec-
tions of leaves; 'grazers' and 'scrapers' consume algae from the surfaces of objects such as
stones and large plants; 'collectors' feed on fine organic matter produced by the breakdown
of other once-living things; and 'predators' eat other living creatures. The relative import-
ance of these groups of creatures typically changes as one moves from the headwaters of
a river to stretches further downstream, reflecting physical factors such as channel width,
shading by trees, and the velocity of the water. This is the 'river continuum concept' which
describes a continuum of changes that integrate energy sources, food webs, and stream or-
der in an essentially linear way. Hence, small headwater streams are often shaded by over-
hanging vegetation which limits sunlight and photosynthesis but contributes organic mat-
ter by leaf fall. Shredders and collectors typically dominate in these stretches, but further
downstream, where the river is wider and thus receives more sunlight and less leaf fall, the
situation is quite different. Here, food chains are typically based on living plant material
rather than leaf fall, so there are few shredders and probably more predators.
The river continuum concept is a popular model that has influenced many studies of fluvial
ecosystems, but it is not the only one. Another important model used to study river eco-
logy stresses the importance of the annual pulse of floodwaters that extends many rivers in
temperate and tropical regions on to their floodplains. The 'flood-pulse concept' broadens
the focus beyond the main river channel and puts much greater emphasis on interactions
with a greater variety of habitats such as the marshes and lakes typically found on flood-
plains. These habitats are broadly synonymous with the river's 'riparian' zone (from the
Latin word
ripa
, a bank), made up of any land that adjoins, regularly influences, or is influ-
enced by a body of water. Vegetation in the riparian zone helps to maintain the condition of
