Species Diversity of Riffle Fish
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 Wilson’s (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 “island”:
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 Wilson’s 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.
Riffle / # of Species / Riffle Area (m2) / Depth (cm) / Flow Rate(cm/sec) / Gradient
(ft/mi) / Distance from Mouth (rank)
A / 11 / 7155 / 22.6 / 58.4 / 7.7 / 2
B / 14 / 6175 / 33.8 / 60.0 / 7.7 / 3
C / 11 / 8250 / 30.3 / 88.1 / 7.7 / 1 (closest)
D / 9 / 2837 / 20.4 / 62.7 / 8.3 / 6
E / 6 / 1834 / 22.4 / 35.1 / 8.3 / 7
F / 8 / 2015 / 14.2 / 19.0 / 4.5 / 8 (farthest)
G / 15 / 5887 / 20.4 / 53.9 / 4.8 / 4
H / 11 / 3800 / 25.7 / 35.0 / 6.9 / 5
Analysis: We will use a non-parametric statistical technique known as Spearman’s rank correlation to determine whether there is a correlation between these variables and species number.
To do so:
- rank each variable from lowest to highest. If there are ties (like in depth) average the tied ranks. For instance, if 3 values are tied for the lowest, they share ranks 1, 2, & 3, so they all get the average rank of (1 + 2 + 3) = 6; 6/3 = 2.
- calculate Spearman’s rho statistic, r
where D = the difference between ranks of the two variables being compared.
N = number of pairs of data values.
The value of rho obtained will range from -1 to +1. Zero values indicate no correlation Positive values indicate a positive correlation. That is, as one value increases, so does the other. A negative value indicates an inverse relationship.
3. determine whether this correlation is significant at α = 0.05 from the attached table. In this table, v = N.
4. using common sense, briefly discuss why these variables are/aren’t correlated with species richness. Can you think of any other stream parameters that might be related, either positively or inversely?
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.