PREDATOR-PREY INTERACTIONS IN A BENTHIC STREAM COMMUNITY - A FIELD-TEST OF FLOW-MEDIATED REFUGES

Ecological theory suggests that the impact of predation can be strongl y modified by the existence of regions of the environment in which pre y are less accessible to predators, which underscores the need for emp irical studies examining the factors influencing the availability and importance of such prey refuges. Our study tested whether benthic micr ohabitats with high flows provide suspension-feeding larval black flie s (Simulium vittatum) with a spatial refuge in which the negative impa ct of predatory flatworms (Dugesia dorotocephala) is reduced. We condu cted a short-term field experiment in Chester Creek (southeastern Penn sylvania, United States) to examine how the number of black fly larvae inhabiting tile substrates responded to manipulated variations in fla tworm abundance and current speed. The abundance of flatworms declined with increasing current speed, thereby creating the potential for sit es with high flows to provide larvae with a refuge from these predator s. Multiple regression analysis revealed that the final abundance of l arvae exhibited a significant negative relationship to flatworm abunda nce and a significant positive relationship to current speed. After ad justing for variations in elapsed time and initial larval abundance, f low and predators explained 38% of the variation in the rate of change in larval abundance. The positive correlation between larval abundanc e and flow had two components: a positive, direct effect of flow on la rvae, which arises because these food-limited consumers prefer to resi de within sites with faster flows where they can feed at higher rates; and a negative effect of flow on predators, and of predators on larva e, which combine to yield a positive indirect effect of flow on larvae . This indirect effect demonstrates the existence of flow-mediated ref uges (i.e., microhabitats in which the impact of predation is reduced due to high flows), although the effect accounts for a small proportio n of total variation in larval abundance. A consideration of biomechan ical relationships suggests that microhabitats with high flows are lik ely to create prey refuges in a wide range of freshwater and marine be nthic environments. In particular, predators will often experience gre ater dislodgement forces than prey because of their larger size and be cause they project farther above the bed where current speeds are fast er. Moreover, the ability to resist a given dislodgement force may be greater for many prey, especially those that are sessile or semi-sessi le.