Meta-analysis reveals a positive effect of non-native prey on native predator populations

Background/Question/Methods

Despite vast knowledge on invasive species’ predatory effects, we know little of their influence as prey.  In the simplest case, the purely additive insertion of a non-native prey should have a neutral to positive effect on native predators because it simply adds another food item to the predators’ choices.  However, if a non-native prey is displacing native prey species, then the invader’s net influence should depend on both its abundance and value relative to native prey. We conducted a literature search and meta-analysis to quantify the effect of non-native prey on native predator populations.

Results/Conclusions

Our search identified 54 studies covering 46 native predator species and 17 non-native prey species.  Despite diverse representation at the species level, the existing literature is taxonomically skewed at higher levels of organization and the majority of the studies were observational. Our meta-analysis revealed relatively negative effects of non-native prey on predators, but only if studies employed a substitutive design that examined effects of each prey species in isolation. Studies that simultaneously incorporated alternative prey always showed a net positive effect of the non-native prey.  These findings suggest that the positive influence of non-native prey does not likely stem from a higher per capita value as food, but rather from the complementary role of non-native prey as an additional, supplemental food source that results in little perceived reduction in the (preferred) native prey base. Whether these elevated predator populations have indirect, negative consequences on the native community is key to understanding the full effect of non-native prey as subsidies.