Independent and combined effects of multiple predators across ontogeny of a dominant grazer

Abstract

Ecosystems host multiple coexisting predator species whose interactions may strengthen or weaken top-down control of grazers. Grazer populations often exhibit size-structure, but the nature of multiple predator effects on suppression of size-structured prey has seldom been explicitly considered. In a southeastern US salt-marsh, we used both field (additive design) and mesocosm (additive-substitutive design) experiments to test the independent and combined effects of two species of predatory crab on the survival and predator-avoidance behavior (i.e. a non-consumptive effect) of both juveniles and adults of a dominant grazing snail. Results showed: 1) juvenile snails were more vulnerable to predation; 2) consumptive impacts of predators were hierarchically nested, i.e. the larger predator consumed both juvenile and adult snails, while the smaller-bodied predator consumed only juvenile snails; 3) there were no emergent multiple predator effects on snail consumption; and 4) non-consumptive effects differed from consumptive effects, with only the large predator inducing predator-avoidance behavior of individuals within either snail ontogenetic class. The smaller predator therefore played a functionally redundant trophic role across the prey classes considered, augmenting and potentially stabilizing trophic regulation of juvenile snails. Meanwhile, the larger predator played a complementary and functionally unique role by both expanding the size-spectrum of prey trophic regulation and non-consumptively altering prey behavior. While our study suggests that nestedness of consumptive interactions determined by predator and prey body sizes may allow prediction of the functional redundancy of particular predator species, it also shows that traits beyond predator body size (e.g. habitat domain) may be required to predict potentially cascading non-consumptive effects. Future studies of multiple predators (and predator biodiversity) should continue to strive towards greater realism by incorporating not only size-structured prey, but also other aspects of resource and environmental heterogeneity typical of natural ecosystems.

Predators mediate above‐ vs. belowground herbivory in a salt marsh crab

“…The risk of predation from Eurytium and Panopeus is high at creek heads since these predatory crabs are generalist predators that readily capture and consume a variety of prey items, including other marsh crabs (Lee and Kneib , Silliman and Zieman , Soomdat et al. ). However, rigorously showing that the indirect effects of predators on Sesarma that we observed in the laboratory also occur in the field would require further work.…”

“…Sesarma densities are mediated by predation from birds and nekton ) and from resident marsh crabs (Soomdat et al 2014; H. D. Vu, personal observation) but we do not know whether Sesarma shifts between feeding above-and belowground in response to the threat of predation. We conducted mesocosm and laboratory experiments to examine the effects of predators on above-and belowground feeding by Sesarma and to test whether predator-induced changes in feeding affect burrowing rates and Spartina productivity.…”

Marine ecoregion and Deepwater Horizon oil spill affect recruitment and population structure of a salt marsh snail

“…Littoraria irrorata also serves as an important food source for a wide variety of consumers, including fish, crabs, turtles, birds, and mammals (Silliman and Bertness , Soomdat et al. ).…”

“…Size‐frequency distributions of L. irrorata are often bi‐ or tri‐modal (Hamilton , Soomdat et al. , Zengel et al. ).…”

“…Size-frequency distributions of L. irrorata are often bi-or tri-modal (Hamilton 1978, Soomdat et al 2014, Zengel et al 2014). Measured growth rates of L. irrorata, coupled with studies of other snails, suggest that these modes represent age classes.…”