Tenacity-mediated selective pecdation by oystercatchers on intertidal limpets and its role in maintaining habitat partitioning by 'Collisella' scabra and Lottia digitalis

Abstract

Limpet homing behavior and habitat partitioning were studied near Monterey Bay (California, USA) from the standpoint of predation by oystercatchers. Lottia digitalis and 'Collisella' scabra tend to be segregated with respect to oystercatcher accessibility: L. digitalis are most abundant out of reach of oystercatchers on vertical surfaces, whereas 'C'. scabra predominate on horizontal surfaces and vertical rock faces within oystercatcher reach. This difference in distribution correlates well with preference exhibited by foraging oystercatchers in that 'C.' scabra is neglected relatives to its greater abundance within reach in foraging areas, while L. digitalis and other related limpets are preferred by hunting oystercatchers. Limpet tenacity was measured with a device which simulates oystercatcher attack (a horizontally directed peck); 'C.' scabra were significantly more difficult to remove by this means. Furthermore, on softer rocks (e.g. sandstone) where 'C.' scabra often forms an indented home depression in the rock surface, limpets inhabiting the deepest home depressions were the most difficult to dislodge. We propose that the homing behavior of 'C.' scabra confers superior resistance to oystercatcher attack through the intimate fit to the home site, thereby allowing this species to inhabit regions accessible to birds. Thus, oystercatcher predation may act as one selective force in maintaining the homing behavior of 'C.' scabra. L. digitalis are removed selectively by foraging oystercatchers, and consequently occur as large individuals only on vertical surfaces out of oystercatcher reach. We suggest that it is the interplay between (1) homing by 'C.' scabra, (2) the resultant preference shown by oystercatchers for other limpets such as L. digitalis, and (3) competitive interactions between L. digitalis and 'C.' scabra on vertical surfaces which produces observed patterns of habitat partitioning by these 2 limpet species in central California.