L. Rowe et Js. Richardson, Community responses to experimental food depletion: resource tracking by stream invertebrates, OECOLOGIA, 129(3), 2001, pp. 473-480
The regulation of population processes for most organisms depends upon the
strength and rate of feedback between resources and consumers. We conducted
an experimental manipulation of leaf packs in stream channels, a patchy an
d ephemeral resource, which is consumed by a number of detritivorous invert
ebrates. We reduced the number of available food patches (red alder leaf pa
cks) by half and then measured a variety of community responses, including
emigration rate, aggregation on remaining food patches, decomposition rate
of food patches, and species-specific differences in these responses. Repla
cement of removed leaf packs with polyester mimics resulted in no statistic
al difference in emigration rates or aggregation on remaining resources whe
n compared to those removal channels without replacement. These results ind
icate that leaf packs are not used primarily for refuge. In the removal cha
nnels (including those with leaf pack mimics) emigration rate nearly double
d relative to control channels. Those invertebrates that did not emigrate f
rom removal channels aggregated on remaining leaf packs, which led to more
rapid decomposition of leaf packs relative to control channels. The increas
e in emigration rate only became apparent 2-3 days after the manipulation,
presumably because animals colonized the remaining leaf packs and did not e
migrate until food patch value per individual had been reduced by higher de
nsities or due to increased discharge. Discharge through the channels incre
ased slightly starting 3 days after the manipulation, resulting in increase
d emigration rates in all channels. Despite the increase in discharge, the
effect of the manipulation remained strong. These results show that stream
invertebrates colonizing leaf packs responded in predictable ways to a shor
t-term reduction in food resources which would be adaptive in a system whic
h is heterogeneous in space and time.