Resource limitation and intraspecific patterns of weight x length variation among spring detritivores

This study examined the intraspecific variability of a phenotypic trait, the body weight/body length ratio, and its adequacy to provide unbiased information about patterns of resource availability among conspecific individuals. Individual body weight and length were measured for the amphipod Gammarus minus Say (and other detritivores) in samples from freshwater springs differing in expected resource availability, and from sites in which detritus abundance had been manipulated. Mean individual weight per length was lower: (a) in populations from low-richness than from high-richness detritivore guilds; (b) in populations with size-abundance distributions similar to that of the entire guild, rather than statistically different; (c) in population samples from outside than from inside the areas of detritus addition. Small-sized individuals (< 3 mm) showed the largest variation among both populations and population samples. Similar differences were qualitatively observed for the other common detritivores co-occurring with G. minus in some springs. These observed patterns were in agreement with the variation of resource availability expected among field conditions, supporting the relevance of weight per length as a measure of food limitation.