Intra- and interspecific competition among coexisting lotic snails

The competitive interactions of two lotic snails, Elimia cahawbensis and Elimia carinifera were examined in a second-order spring-fed stream. We first demonstrated food limitation in laboratory microcosms adhere snails grew faster hen exposed to enhanced periphyton levels. We then tested the magnitude and relative strengths of intra- and interspecific competition in similar stream and laboratory mesocosm experiments, Treatments ere maintained in Plexiglas enclosures over a 7-week period with 0.1 x. 2 x and 4 x ambient biomass of each species alone, as well as mixed species treatments at 2 x and 4 x ambient. Snail responses to treatments were almost identical in field and laboratory experiments, Growth rates of both species were reduced by increased density of snails indicating strong intra- and interspecific competition among E. cahawbensis, and E. carinifera. An analysis of the strengths of intra- and interspecific competition indicated minimal differences for either species. implying a lack of competitive dominance, Although periphyton biomass was generally highest without snails. there was little difference in periphyton biomass and snail production over the four-fold density range. regardless of species composition. These results suggest that E. cahawbensis si, and E. carinifera are functionally redundant with density-dependent responses in growth rate resulting in similar grazing pressure across a density gradient. This clearly demonstrates that species impact is not necessarily reflected by measures of abundance or biomass. and that secondary production should be considered.