Preference alters consumptive effects of predators: top-down effects of a native crab on a system of native and introduced prey

Abstract

Top-down effects of predators in systems depend on the rate at which predators consume prey, and on predator preferences among available prey. In invaded communities, these parameters might be difficult to predict because ecological relationships are typically evolutionarily novel. We examined feeding rates and preferences of a crab native to the Pacific Northwest, Cancer productus, among four prey items: two invasive species of oyster drill (the marine whelks Urosalpinx cinerea and Ocenebra inornata) and two species of oyster (Crassostrea gigas and Ostrea lurida) that are also consumed by U. cinerea and O. inornata. This system is also characterized by intraguild predation because crabs are predators of drills and compete with them for prey (oysters). When only the oysters were offered, crabs did not express a preference and consumed approximately 9 juvenile oysters crab(-1) day(-1). We then tested whether crabs preferred adult drills of either U. cinerea or O. inornata, or juvenile oysters (C. gigas). While crabs consumed drills and oysters at approximately the same rate when only one type of prey was offered, they expressed a strong preference for juvenile oysters over drills when they were allowed to choose among the three prey items. This preference for oysters might negate the positive indirect effects that crabs have on oysters by crabs consuming drills (trophic cascade) because crabs have a large negative direct effect on oysters when crabs, oysters, and drills co-occur.

Figure 1. Diagram of trophic interactions in our study system.

Diagram of trophic interactions among native red rock crabs, Cancer productus, two species of invasive oyster drill, Urosalpinx cinerea and Ocenebra inornata, and two oyster species, native Ostrea lurida and non-native Crassostrea gigas. Solid lines represent direct interactions, dashed lines indirect interactions with all arrows pointing in the direction of the trophic effect.

Figure 2. Crab feeding rates and preference on native and non-native oysters.

Number (mean ± SE) of oysters consumed by crabs over 24 hours in the oyster preference experiment. Gray bars are Crassostrea gigas, Pacific oysters, and white bars are Ostrea lurida, Olympia oysters. Crabs were randomly assigned to one of three treatments (n = 12): (1) 25 C. gigas only, (2) 25 O. lurida only, or (3) the treatment labeled “Choice”, 25 C. gigas and 25 O. lurida.

Figure 3. Crab feeding rates and preference on oyster drills and Crassostrea gigas.

Number (mean ± SE) of prey consumed by crabs over 24 hours in the drill/oyster preference experiment. Black bars are Crassostrea gigas, Pacific oysters, gray bars are Ocenebra inornata, Japanese drills, and white bars are Urosalpinx cinerea, Atlantic drills. Crabs were randomly assigned to one of four treatments (n = 14): (1) 25 C. gigas only, (2) 25 Ocenebra inornata only, (3) 25 Urosalpinx cinerea only, or (4) the treatment labeled “Choice”, 25 C. gigas, 25 O. inornata, and 25 U. cinerea.