Exploitation Competition in Mobile Grazers: Trade‐offs in Use of a Limited Resource

Mechanisms underlying competitive effects between two species of marine snails (Tegula aureotincta and Tegula eiseni) were examined; results indicated that the interaction involved exploitation of microalgae and not interference. A behavioral experiment demonstrated that an individual of either species was less likely to feed on a recently grazed patch when the prior forager was T. aureotincta. Both snails responded only to the amount of food available on a patch, and, except at very low food levels, the likelihood that a snail would remain to feed was identical for the two species across microalgal densities. Where food was scarce, T. eiseni was more likely than T. aureotincta to feed in that patch. Density—dependent effects on microalgal standing stock differed between the snails; at low grazer densities, T. aureotincta depressed food levels to a comparatively greater degree, whereas at high grazer densities, T. eiseni had a relatively greater effect. The relative growth performance of the snails differed as a function of available food: T. aureotincta had superior body growth at high resource densities, while T. eiseni grew comparatively better at low microalgal densities. Differences in effects on and growth responses to microalgae arose from dissimilar foraging characteristics of the snails. Compared with T. eiseni, T. aureotincta moved more rapidly while feeding, harvested twice as great an area per unit time, but was unable to crop microalgae as close to rock surfaces. While T. eiseni grazed a smaller area per unit time, it removed relatively more microalgae per area grazed. These differing suites of foraging attributes may represent a trade—off to harvest food at different densities: cropping closer to the substratum (area intensive grazing) yields more food when microalgae are scarce, but precludes grazing a larger area per unit time (area extensive grazing) through fast movement, which provides a greater food intake when microalgae are abundant. If an area extensive (moves faster)—intensive (digs deeper) trade—off is a common feature of benthic grazer systems, knowledge of this aspect could provide great predictive insight into the patterns and causes of density dependence arising from exploitation.