Consumer responses to habitat depletion: food-refuge interactions in periwinkles

All living organisms exploit resources for survival and reproduction. Food resources provide the energy, and refuge resources the opportunity, to survive, grow and reproduce. It is a central tenet of ecology that the abundances of organisms are influenced by the supply of their resources - the users of a patchy world are themselves patchily distributed wherever they compete to exploit limiting resources. This process of competition combines with extrinsic processes such as predation and disease to determine the match between patterns of consumer abundance and patterns of resource-rich habitat. The proposed study will experimentally deplete and redistribute habitat for populations of consumers. Habitat loss is one of the principal causes of species rarity and endangerment, and thus a major concern in the conservation of wildlife as well as a prime objective in the management of pest species. We will explore consumer responses to depletion and redistribution of food and refuge resources within the context of generic conceptual models for which new hypotheses have been developed recently but still await testing. Recent generic models predict (1) that the densities of vulnerable populations are increased by redistributing habitat unequally, even without improving overall habitat richness; (2) that viability of vulnerable populations depends more strongly on individual death and local extinction than on birth and colonisation. Both predictions remain as yet untested, though they have potentially wide applicability across many scales of community structure. Our broad objective is to test them by experimentally manipulating the distribution and amount of habitat available for two species of rocky shore periwinkles (Littorina saxatilis and Melarhaphe neritoides). These small marine snails have a number of characteristics that predispose them to experimental fieldwork. They live at high abundances on the upper sections of rocky shores, making them readily accessible for studying population processes and individual behaviours. Both rely on crevices for refuge and microbial film for food, and they are predictable in their foraging activities. They tend to move only a few metres in a lifetime, and can easily be marked to record fecundity and rates of immigration and emigration from rock slabs. The study will simultaneously manipulate the amounts of available refuge and food for the periwinkles in order to find out how food and refuge sites interact in their effects on population densities. A portable drill will be used to create refuges on natural rock slabs, and cement filler will be used to fill in existing refuges. An electric toothbrush will be used to clean algal films from foraging areas around the experimental refuge sites, and watering with nutrients will increase the available food in test areas. We will monitor the responses of the snails to replicated alterations to patterns and densities of refuge and food. We will record changes to snail densities at the population level, and to foraging behaviour at the individual level.