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 where snails grew faster when exposed to enhanced periphyton levels. We then tested the magnitude and relative strengths of intraand interspecific competition in similar stream and laboratory mesocosm experiments. Treatments were maintained in Plexiglas enclosures over a 7-week period with 0, 1× , 2× and 4× ambient biomass of each species alone, as well as mixed species treatments at 2× and 4× 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 intraand interspecific competition among E. cahawbensis and E. carinifera. An analysis of the strengths of intraand 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 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.