Compensatory Scope and Resource Allocation in Two Species of Aquatic Snails

The resource allocation strategies of the freshwater snails Stagnicola elodes and Physella gyrina were examined using artificial diets diluted to various degrees with cellulose. Even when nutrients were diluted by 50 or 75%, both species showed remarkable maintenance of their rates of assimilation (94 to 294% of controls) and production (82 to 140% of controls). This was mainly achieved by varying rates of feeding (148 to 397% of controls) and respiration (76 to 839% of controls). S elodes was twice as large as P. gyrina and had slower rates. The hypothesis that this endows S. elodes with greater compensatory scope (i.e., a greater range of response in feeding or respiration rates) was strongly supported. This observation has broad implications for theories of evolution and niche breadth. Particular attention was devoted to trade—offs between somatic and reproductive support. With 75% dietary dilution, the two species showed exactly inverse strategies. S. elodes supported growth more than reproduction (90 and 59% of controls, respectively), whereas P. gyrina supported reproduction over growth (116 and 38% of controls, respectively). Their strong compensatory ability suggests that these snails normally operate at submaximal rates that are homeostatically maintained. Shifts in resource allocation may be engaged as "anticipatory" safety measures, even before absolute shortages arise.