Laboratory experiments were conducted to determine the effects of various factors on interactions for refuge sites of two stream-dwelling benthic organisms. The effect of a piscivore, the smallmouth bass (Micropterus dolomieu), on intraspecific and interspecific interactions involving crayfish (Crustacea: Cambaridae: Cambarus longulus) and hellgrammites (Insecta: Megaloptera: Corydalus cornutus) was examined. Three possible interactions were recognized in all experiments: (1) the refuge occupant was evicted by an intruder; (2) the refuge occupant excluded the intruder from the refuge; (3) the occupant and the intruder shared the refuge.

The size of both crayfish and hellgrammites had a statistically significant effect on the outcome of interspecific and intraspecific interactions, larger individuals being more successful at excluding or evicting smaller individuals from a refuge. The effect of size on the interactions, however, was tempered by the effects of prior occupancy of the refuge (owner-intruder models), allowing smaller individuals of both species to maintain occupancy of the refuge when faced with a larger intruder in about one-third of all trials. There was no significant effect of gender on the outcome of interactions between male and female crayfish. The presence of a fish (Micropterus dolomieu) significantly altered the outcome of interspecific and intraspecific interactions. Not only were both species more likely to gain access to the refuge as an intruder, but the occupant was less likely to be evicted, resulting in a greatly increased incidence of sharing of the refuge in the presence of the fish. Refuge sharing by crayfish never occurred unless a fish was present. Refuge sharing by hellgrammites was an especially important interaction in the presence of fish.