| Abstract: | 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 intra- and 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 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 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. |
