Differential responses of odor-mediated predators to density and distribution characteristics of prey patches

Background/Question/Methods: Predator foraging success within patchy prey distributions is dependent on the ability of the predator to locate prey patches as well as distinguish patch quality. The success of odor-mediated predators in locating prey patches in estuarine environments is dependent on their ability to detect and follow chemical plumes; the structure of which may be mediated by the density and distribution of those prey individuals within the patch. We examined the role of prey density and distribution in moderating predation rates by odor-mediated predators in the field. We compared predation rates on 1 m^2 patches of the hard clam Mercenaria mercenaria at combinations of two densities (15 clams/m^2 and 60 clams/m^2) and two distributions (random and aggregated) in a randomized block design. To distinguish between predator ability to locate patches and patterns of predation within patches, flume choice experiments were then performed challenging predators to distinguish between patches of differing densities (high = 10 clams vs. low = 3 clams) and distributions (aggregated vs. random; both containing 10 clams). Results/Conclusions: In areas where large mobile predators dominate (blue crabs – Callinectes sapidus), predation rates are significantly larger in patches of higher density and those with aggregated prey ( p = 0.042 and p = 0.02 respectively). In areas dominated by less mobile predators (gastropods – Busycon carica), predation rates are only greater in patches with aggregated prey (p = 0.06). In the laboratory flume both crabs and gastropods locate patches of high density and aggregated prey more frequently than patches of low density and randomly distributed prey, but when allowed to forage in flume prey patches, crabs consumed more prey than gastropods. Disparity between field and laboratory results for gastropods may be attributable to their slow consumption of prey; individuals may not necessarily benefit from high density prey patches in the field despite being able to find those patches more frequently in the lab. The patch characteristics that blue crabs and gastropods use to assess patch quality may be predator specific, and could result in the formation of a distinct sensory niche for each predator; reducing direct competition over patchy resources.