Dd. Hart et Ra. Merz, PREDATOR-PREY INTERACTIONS IN A BENTHIC STREAM COMMUNITY - A FIELD-TEST OF FLOW-MEDIATED REFUGES, Oecologia, 114(2), 1998, pp. 263-273
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.