General Characteristics of Stream Organisms – Essay

Comparisons have been made between stream organisms and their marine counterparts (Patrick 1975). In general, stream organisms as a group are smaller than their marine relatives. For example, porpoises of the Amazon, though fairly large for stream organisms, are smaller than marine porpoises.

Also, it has been observed that fish living in large rivers are usually larger than fish inhabiting small streams. The small size of stream organisms is probably related to the conditions found in the small streams. Narrow channels and seasonal fluctuations promote the survival of small as opposed to large animals.

Small streams undergo greater fluctuations in water due to small catchment area and lack of storage capacity. Large numbers of small animals have a greater likelihood of surviving the inevitable fluctuation of small streams. Under these unstable conditions small numbers of large freshwater animals would be comparatively prone to extinction.

In one way, freshwater environments of the Earth are remarkably similar. They support the same four types of insects: water striders, whirligig beetles, backswimmers, and springtails (Milne & Milne 1978).

Water striders glide or dart across the water surface by rowing with their middle pair of legs. Gerris is a genus found on freshwater surfaces in all parts of the Earth. Whirligig beetles (Gyrinues and Dineules) live half in the water and half out, set up ripples as they move, and can dive or fly, and crawl on solid surfaces. Backswimmers (Nolonecta) live in ponds and the relatively quiet pools of streams.

They swim belly-up and live on the underside of the surface film of the water. Their major food is the midge or mosquito.

The fourth cosmopolitan surface-film aquatic insect is the springtail (Fodura and Sminthurus). These latter are the most ancient surface-water insects and first appeared about 375 Ma ago. They alone inhabited the surface film until 250 Ma ago. Springtails feed on plant spores that settle on to the water surface.

Stream organisms generally have short life cycles compared with deep-lake or marine organisms. Many species have life cycle that last only one day. Since life cycles are short, the reproductive rate tends to be very high.

Stream communities tend to have long food chains that include four or five transfers of energy. Algae are important both as a food source and as a substrate for the growth of other epiphytic food sources. Rooted plants are not as important as a food source as they are as a substrate or shelter for other organisms. They thus serve to increase habitat diversity (Patrick 1975).

Streams commonly contain very large numbers of species. Many species have developed specialized organs or appendages for surviving in currents (Vogel 1981). Hard-water streams tend to support fewer species of algae, larger number of protozoa and insects, and slightly fewer species of fish than soft-water streams (Patrick 1975). Along a single river, the number of species in common during a single collection time between different collection sites tends to be higher than the number of species in common found at a single site during different collection times (Patrick 1975). Overall, there is a high species turnover in time and space. Examination of the physical characteristics that influence biotic qualities of streams provides insight into the biogeographic patterns of the stream habitat.