Changing predator-prey dynamics

Abstract

Predators commonly structure natural communities, but the exact outcomes of predation can be highly variable and context-dependent. While high predator densities are assumed to deplete prey populations, prey may persist if foraging constraints or intraspecific predator interactions suppress predator foraging efficiency. Thus, evaluating the impacts of predators depends on understanding the factors that moderate predation rates. Along the Atlantic coast of Florida, outbreaks of the predatory crown conch (Melongena corona) have contributed to the recent degradation of oyster reefs. Despite predictions of oyster population collapse as a result of increased predation pressure, reefs have persisted in a state of reduced mean adult oyster size and living oyster biomass. To quantify conch predation rates and determine whether a prey size refuge may be driving oyster persistence, we conducted multi-year surveys and field experiments that evaluated whether conchs exhibit size-selective feeding on oysters, as well as how the relative densities of oysters and conchs affect the conch functional response. Our experiments demonstrated that conchs selectively prey on large oysters, indicating the absence of a size refuge, but that per capita predation rates were significantly suppressed at high conch densities, likely due to intraspecific interference. Conchs exhibited a strongly predator-dependent response to changes in oyster density, with a ratio-dependent model explaining almost 60 % of the variation in per capita prey consumed. Our experimental results of conch size-selective predation on larger oysters closely aligned with our observational findings that mean adult oyster size was significantly reduced at high conch densities. However, these larger-scale survey results indicated that prey density alone is the best predictor of predator-driven oyster mortality on natural reefs, contradicting the strong predator-dependence in our experimental conch functional response. This mismatch between our experimental and observational results indicates that conch interference may operate primarily when high conch densities coincide with low oyster densities, and that this predator-dependence may be overwhelmed by other factors at a larger scale. Our study suggests that oyster persistence is not driven by a size refuge, but that intraspecific predator interference that suppresses predation rates may be important at high conch densities. As the frequency and extent of predator outbreaks continue to increase with global environmental change, understanding the factors that moderate predator-prey dynamics will help resource managers predict and manage predation effects on ecologically and economically important species.