Food Limitation and Interspecific Competition in Snail-Dominated Streams

Abstract

Food limitation and exploitative competition were implicated in two experiments on lotic grazers in eastern Tennessee. In a laboratory feeding experiment, grazing snails (Elimia clavaeformis) and caddisfly larvae (Neophylax etnieri) from upper White Oak Creek (WOC) grew 7 and 2 times faster, respectively, on a diet of high-biomass periphyton than they did on a diet of WOC periphyton. When fed on the high-biomass periphyton, both grazers accumulated disproportionately more neutral lipid, and snails increased their mean condition index 50% (ash-free dry mass per unit wet mass). Growth and condition of grazers fed WOC periphyton in the laboratory were quite similar to growth and condition in situ, indicating that laboratory conditions were realistic. Analysis of gut contents demonstrated considerable dietary overlap between the two taxa regardless of periphyton diet and suggested that quantity rather than quality of periphyton limited grazer growth and condition in WOC. In a natural experiment, periphyton and Neophylax from six streams containing Elimia were compared with periphyton and Neophylax from six streams that lacked Elimia. Periphyton biomass, Neophylax diapause mass, and Neophylax lipid content were substantially greater in streams lacking Elimia, implying that the snail created or exacerbated food-limiting conditions for Neophylax.