Species Diversity of Riffle Fish
Tessellated Darter, Etheostoma olmstedii
In this exercise we will investigate some of the factors that may influence the number of species of fish that characteristically inhabit the riffle portions of a stream.
We can compare riffles to islands in the ocean. Like an island, a riffle is a specialized habitat that is separated from other such habitats by environments that are unfavorable to riffle-dwellers. An oceanic island is separated from other islands by water; pools and areas of deeper, more stable water flow separate riffles. (Can you suggest environmental parameters that may differ between riffles, runs and pools?)
MacArthur and Wilsonís (1967) theory of island biogeography suggests that the number of species on any island represents a dynamic equilibrium between rates of immigration and extinction. Consider two oceanic islands, one large one and a small one set at the same distance from a continent. Both are devoid of life. New species will invade the island from the mainland. The rate at which they arrive will be dependent upon island size. A large island is a bigger target than a small one, so the larger island will accumulate species faster. Once a species arrives, it can establish itself or go extinct. The rate of extinction should be higher on a small island because I will represent a poorer mix of environments. Also, the smaller populations on a smaller island are more susceptible to stochastic causes of extinction. When the extinction rate is equal to the immigration rate the number of species on the island will stabilize. Alterations in either immigration or extinction rates will affect the equilibrium number of species.
In studying the communities of fishes inhabiting riffles, we will sample the fish populations by seining or electrofishing. We can also measure the physical and biological attributes of the riffle ďislandĒ:
1.) area of the riffle
2.) depth of the riffle
3.) flow rate
4.) substrate diversity
5.) food abundance
6.) stream gradient
7.) distance from the stream mouth (the large, nearby source of species)
You will perform a data analysis of field data collected from a large stream in central Ohio. Here, the riffles are considered to be islands, and MacArthur and Wilsonís ideas can be tested. This procedure is explained on following pages.
Eight well-spaced riffles were seined for all species of fish. The following environmental factors were also measured.
Table 1. Data collected from Big Darby Creek, Ohio.
Analysis: We will use a statistical technique known as Spearmanís rank correlation to determine whether there is any correlation between these variables and species number.
References and resources:
Brown, J. 1971. Mammals on mountaintops: nonequilibrium insular biogeography. American Naturalist 105: 467-489.
MacArthur, R. and E. O. Wilson. 1967. The theory of island biogeography. Princeton University Press, Princeton, NJ.
Sheldon, A. L. 1968. Species diversity and longitudinal succession in stream fishes. Ecology 49 (2): 194-198.