106 
John R. Holsinger and David C. Culver 
found it to be important in cave beetles, and Cooper (1975) discussed 
the importance of competition among cave crayfish. The most extensive 
study of competition has concerned peracarid crustacean communities 
in southwest Virginia and east Tennessee caves (Culver 1973a, 1976, 
1981, 1982; Dickson 1976, 1977a; Dickson and Holsinger 1981; Estes 
1978; Estes and Holsinger 1982). Various combinations of three species 
are present: Cacidotea recurvata, Crangonyx antennatus, and Lirceus 
usdagalun. 
The basis for competition is that, in the stream, amphipods and 
isopods need a place to avoid the brunt of the current and a place to 
feed on detritus washed into the cave. Because riffles are food-rich and 
well oxygenated, amphipods and isopods congregate there, even though 
high current velocities increase the risk of being washed out. Although 
many stones in a riffle are unoccupied (Culver 1973a), amphipods and 
isopods frequently meet because of jostling about by the current and 
directed movement toward food. When two individuals meet, one is 
almost invariably washed downstream. Some of these individuals die; 
all are removed from the riffle. 
Since washout is the basis of competition, it was possible to derive 
a formula for measuring the competition coefficients and to measure 
washout in an artificial stream in the laboratory. Competition coeffi- 
cients (cxij) for all pairs of the three species studied were as follows: 
C.a. 
C.r. 
L.u. 
Crangonyx antennatus 
\ > 
0.99 
1.32"] 
Caecidotea recurvata 
0.32 
1 
1.29 
Lirceus usdagalun 
L 1.16 
0.49 
i J 
If these are adequate measures of competition and our reasoning above 
has been correct, then the greater the competition, the greater the 
microhabitat separation. Crangonyx antennatus and L. usdagalun, 
which show the strongest competition, barely coexist in the same cave; 
C. recurvata and L. usdagalun occupy different riffles; and C. antenna- 
tus and C. recurvata occupy different-sized rocks in the same riffle. 
Estes (1978) has closely examined microhabitat niche differences 
among the three species and determined that L. usdagalun was almost 
always found in current with a velocity/ depth ratio greater than 0.67, 
whereas the other two species were found over a greater range of cur- 
rent velocities. In addition, relative densities on different rock sizes var- 
ied (Table 4). The large C. recurvata was under large rocks, and the 
small C. antennatus was in gravel. Lirceus usdagalun was on an 
intermediate substrate. 
There is evidence that the intensity of competition between a pair 
of species declines through evolutionary time. Culver (1976) showed 
