Effects of body size and resource availability on dispersal in a native and a non-native estuarine snail

I manipulated snail densities of two coexisting species of salt marsh snail , Cerithidea californica Haldeman (native) and Batillaria attramentaria Sow erby (non-indigenous) to investigate how resource levels set by intraspecif ic competition may influence dispersal rates. I used two distinct size clas ses of the snails (mature and immature) to determine if the effects of comp etition on dispersal differed between developmental stages of a consumer. D ispersal attempts were measured within enclosure pens by counting snails cl imbing the sides of the enclosure. The influence of snail density per se an d resource levels (which were set by snail densities) on dispersal rates we re separated by comparing responses of snails to density before and after r esources became depleted. For large snails of both species, dispersal incre ased as resource levels decreased, supporting the hypothesis that competiti on influences dispersal rates. Small snails of both species, in contrast, a lways dispersed at relatively higher rates than larger individuals, but wer e not influenced by variation in resource levels. This result corroborates other studies that have shown reduced competition in these species at small er size, and suggests that another mechanism, such as genetically coded beh avior to disperse when young, influences their behavior. Previous experimen ts demonstrated Batillaria's superior resource conversion efficiency; there fore, I had hypothesized that for any given resource level, Cerithidea woul d disperse more, because it was more affected by resource availability. Adu lt Batillaria, however, responded more sensitively to resource levels (i.e. , dispersed more at any given resource level) than Cerithidea. This counter intuitive result illustrates the potential importance of genetic limitation s on behavioral responses available to a species. Constraints on behavioral responses may have been accentuated since Batillaria is a non-indigenous s pecies whose evolved behavioral responses are not necessarily adapted to it s present, non-native environment. (C) 2000 Elsevier Science B.V. All right s reserved.