EFFECTS OF AN INVERTEBRATE GRAZER ON THE SPATIAL ARRANGEMENT OF A BENTHIC MICROHABITAT

We demonstrated the effect of an aquatic herbivore on the spatial arra ngement of benthic algal biomass within artificial stream channels. Tr ansects of ceramic tiles were exposed to a gradient of snail (Physella ) densities in a 30 d experiment. We observed positive effects of snai ls on the mean abundance of ''overstory'' algae (the filamentous chlor ophyte Cladophora and associated epiphytes), an important benthic micr ohabitat in streams. Snails altered several aspects of the spatial arr angement of overstory algae. Snails reduced the strength of downstream gradients in overstory biomass, as well as residual variability aroun d these gradients. Geostatistical analysis revealed that snails also r educed the strength of spatial dependence, and so reduced spatial hete rogeneity of the overstory, at small scales (<40 cm). As a result, org anisms inhabiting the overstory might experience a more fragmented hab itat landscape at high snail densities. In addition, snails increased the scale of spatial dependence in understory algal biomass (algae rem aining on tiles after overstory was removed) from 10 cm to 40 cm. Cons umer effects on the spatial arrangement of a microhabitat argue for th e inclusion of feedbacks between the biota and the environment in spat ially-explicit models.