Predator-mediated coexistence of epiphytic grass shrimps that compete for refuges

Epiphytic grass shrimps, Latreutes acicularis and Heptacarpus geniculatus, are often found to coexist in seagrass beds in Japanese shallow seas. The seagrass provides effective refuges by its complex vegetation. A laboratory experiment showed that the competitive interactions between the two shrimps were qualitatively altered in the presence of a predator. H. geniculatus was competitively superior in the absence of the predator, while the presence of the predator acted to lower activities of both species, resulting in a reduced intra- and interspecific interference competition. Based on the experimental data on the behavioral changes, we constructed a model for population dynamics of the shrimp community, and examined the conditions for coexistence of the prey species in terms of their recruitment rates and death rates in the presence and absence of predator. The results show that predator-induced behavioral changes in competitive ability make coexistence significantly easier to achieve.