Effects of Microhabitat Size and 
Competitor Size on Two Cave Isopods 
David C. Culver 
AND 
Timothy J. Ehlinger 
Department of Biological Sciences, 
Northwestern University, Evanston, Illinois 60201 
ABSTRACT. isopods are common in cave streams in northern 
West Wxfmxdi—Caecidotea cannulus (Steeves) and Caecidotea holsingeri 
(Steeves). Laboratory experiments using an artificial stream demon- 
strated that (1) for any given size of isopod, washout rate depended 
on the size of gravels in the stream bed, i.e., larger isopods had low 
washout rates in larger gravels and vice versa, and (2) competition (as 
reflected in washout rate) decreased as size differences among competi- 
tors increased. Field evidence provided support for the first result. In 
particular, there was a concordance between isopod size and gravel size 
distributions. There was no evidence for character displacement. 
Appalachian cave stream communities are relatively simple systems, 
dominated by one to three isopod and amphipod species (Culver 1976). 
The limited amount of utilizable habitat is of great importance in struc- 
turing these communities. Utilizable habitat is limited to the underside of 
stream gravels, which provide concentrations of detritus, the major food 
source, and hiding places from the brunt of the current (Culver 1971, 
Estes 1978). Dislodgment from a rock often follows the encounter of two 
individuals and results in significant mortality (Culver 1973). Since 
washout is density-dependent, competition results, and is important in 
determining annual population size fluctuations (Culver 1971, Estes 
1978), microhabitat separation (Culver 1973), and resistance of com- 
munities to invasion by other species (Culver 1976). 
In this study we examined the effect of size on competition in two 
species of isopods found in northern West Virginia caves. We first tested 
the hypothesis that washout rates of different size isopods and gravel size 
are correlated. This should result in a “match” between isopod sizes and 
gravel sizes in cave streams. Second, we tested the hypothesis that in- 
terspecific competition is reduced by size divergence of the two species. 
This should result in a “mismatch” between isopod size and gravel size. 
Using an artificial stream in the laboratory, we showed that washout rate 
depends on both gravel and isopod size, and that size differences among 
isopods reduces competition. Then, using data on the size distribution of 
isopods and stream gravels in various caves, we assessed the importance 
of these two factors. 
Brimleyana No. 4; 103-113. December 1980. 
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