EXPLOITATION COMPETITION IN MOBILE GRAZERS - TRADE-OFFS IN USE OF A LIMITED RESOURCE

Mechanisms underlying competitive effects between two species of marin e snails (Tegula aureotincta and Tegula eiseni) were examined; results indicated that the interaction involved exploitation of microalgae an d not interference. A behavioral experiment demonstrated that an indiv idual of either species was less likely to feed on a recently grazed p atch when the prior forager was T. aureotincta. Both snails responded only to the amount of food available on a patch, and, except at very l ow food levels, the likelihood that a snail would remain to feed was i dentical for the two species across microalgal densities. Where food w as scarce, T. eiseni was more likely than T. aureotincta to feed in th at patch. Density-dependent effects on microalgal standing stock diffe red between the snails; at low grazer densities, T. aureotincta depres sed food levels to a comparatively greater degree, whereas at high gra zer 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 densiti es, while T. eiseni grew comparatively better at low microalgal densit ies. Differences in effects on and growth responses to microalgae aros e from dissimilar foraging characteristics of the snails. Compared wit h T. eiseni, T. aureotincta moved more rapidly while feeding, harveste d twice as great an area per unit time, but was unable to crop microal gae as close to rock surfaces. While T. eiseni grazed a smaller area p er unit time, it removed relatively more microalgae per area grazed. T hese differing suites of foraging attributes may represent a trade-off to harvest food at different densities: cropping closer to the substr atum (area intensive grazing) yields more food when microalgae are sca rce, 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)-inte nsive (digs deeper) trade-off is a common feature of benthic grazer sy stems, knowledge of this aspect could provide great predictive insight into the patterns and causes of density dependence arising from explo itation.