The enemy of my enemy is my friend: Cascading effects of biotic resistance and the creation of invasive predator free space

Background/Question/Methods

Biotic resistance is the ability of a native community to suppress non-native species establishment or proliferation. One approach to understanding biotic resistance is to study invader spread following initial colonization, in order to quantify the degree to which biotic interactions limit invasion success. Many studies have examined the direct effects of biotic resistance on non-native species, but few examine the cascading indirect effects of resistance on other species within the community. We used field experiments to quantify biotic resistance resulting from predation by native predators (rock crabs; Cancer antennarius and Cancer productus) on an invasive gastropod (eastern oyster drills; Urosalpinx cinerea). We also measure the predatory effects of this invasive gastropod on a native foundation species (Olympia oysters; Ostrea lurida), which is known to increase the diversity of native associated species by provisioning additional habitat. We combine these results with observational survey data for all species to evaluate the potential for positive indirect effects of biotic resistance on an important native foundation species across a gradient of environmental conditions in Tomales Bay, a central California estuary.

Results/Conclusions

We report that survival of the invasive gastropod is less than 50% at sites with the highest native predator densities. Similarly, survival of native oysters is affected by invasive gastropods, and at sites with highest invasive gastropod densities, native oyster survival is less than 25%. Taken together, these data support the hypothesis that native crabs have positive indirect effects on native oysters by consuming invasive oyster drills. Observational data are in agreement with results of field experimentation and show that native crabs appear to contain the distribution of invasive gastropods to the inner-most region of Tomales Bay, and thereby create predator-free space for native Olympia oysters in the middle region of the estuary. This suggests that the loss of native predators documented in marine, freshwater, and salt marsh ecosystems may reduce biotic resistance to invasion with indirect cascading effects on native species by allowing the proliferation of invaders that can encroach upon the distribution of natives.