The effects of area and surface complexity on the structure and formation of stream benthic communities

Rocks, situated in a stream riffle, can.be interpreted as islands. A sampling of macroinvertebrate communities found on a series of rocks 31 to 1000 cm² in surface area, produced a Log species- Log area curve with a slope of 0.352. This slope could not be compared to Preston’s prediction of a slope value of 0.17 or 0.27 because individuals in invertebrate communities on rocks are not lognormally distributed nor are densities of individuals (per cm²) constant with area. The physical topography of the rock surface affected species numbers. Rocks with a complex surface (havin crevices, smooth areas, pores, etc.) surfaces; Log species- Log area curves were steeper for a series of simple rock communities. Most taxa were not restricted to specific rock sizes or complexities, though both factors did affect population sizes.

The dynamics of stream rock invertebrate community formation was examined as were the effects of area and complexity on formation. Larger and more complex substrates had higher colonization, immigration, invasion, and replacement rates during the first 30 days of colonization than smaller substrates. Extinction rate was independent of substrate area (assuming constant turnover). A simulation of colonization, using random immigration and extinction, produced a colonization curve that was very similar to the actual colonization curve, though relative abundances of most taxa were altered. It was hypothesized that stream invertebrates communities are a product of random immigration and of a combination of random and deterministic extinction. Methods for monitoring the activities of individuals are needed to estimate turnover; only by assessing turnover can the importance of extinction be determined.