Does density-dependent predation on a non-native prey facilitate its escape from natural enemies?

Background/Question/Methods: Native predators have been shown to provide biotic resistance of invasive prey across ecosystems including coastal communities. However, early in the invasion process, native predators may initially ignore an invader when it’s rare and only increase consumption once it becomes more abundant. Here we examined whether a native predator’s consumption of the invasive filter-feeding crab, Petrolisthes armatus, was a function of its abundance relative to native prey and the similarity of invasive P. armatus to a native prey. We quantified and compared the consumption of P. armatus by the native mud crab predator, Panopeus herbstii when predators received varying ratios of invasive to native prey: abundant (1:3), equivalent (1:1) and rare (3:1). We also manipulated whether native predator crabs were given ecologically similar (Eurypanoepus depressus) or dissimilar (Geukensia demissa) native prey.

Results/Conclusions: We found that the consumption of invasive prey was higher in treatments where relative abundance of the invader was higher (ANOVA F_{2, 92} = 5.61, p = 0.005). However, the ecological similarity of native to invasive prey did not affect consumption of P. armatus (F_{1, 92} = 0.157, p = 0.69). Female P. herbstii tended to consume more P. armatus than male, however this was not statistically significant (F_{1, 92} = 1.30, p = 0.069). Using Bayesian analyses, we found that while consumption of invasive P. armatus did increase in response to increased abundance of P. armatus, the estimated preference for invasive prey was often lower than the expected ratio within each treatment. Furthermore, native predators consumed more native prey than expected even when native prey were rare, suggesting that predators still exhibit a strong preference for familiar native prey. These Bayesian analyses more clearly showed the amount of individual variation among P. herbstii, which could also contribute to an overall population level avoidance of this invasive prey. Together these results suggests that an invader’s escape from its natural enemies may be a result of density-dependent predation, along with native predators showing a preference for native prey.