Invasive species become junk food

Lake Erie water snake

Despite knowledge on invasive species’ predatory effects, we know little of their influence as prey. Non-native prey should have a neutral to positive effect on native predators by supplementing the prey base. However, if non-native prey displace native prey, then an invader's net influence should depend on both its abundance and value relative to native prey.

In order to  quantify the effect of non-native prey on native predator populations US researchers conducted a meta-analysis by reviewing 109 studies covering the interactions of 47 different prey species and 93 predator species.

The research shows that predators benefit most from eating invasive prey only if their traditional food sources remain intact - that is, if they are able to maintain their usual diet and eat invaders only as an occasional snack. Predator populations increased as much as 57 percent after an invasion of new prey but only when native prey remained abundant as well.

Eating non-native prey isn't as good for predators as eating native prey. It may be that the new prey isn't as nutritious, or that the predator hasn't evolved the ability to eat or digest it well. But in all these studies, whenever predators' diets were restricted to non-native prey, the predators did not perform as well as they did on native prey. We only saw a benefit to the predator when the non-native prey provided a supplemental food source.

There are also examples of success stories involving predators and invasive prey. For example, the Lake Erie water snake was endangered, but its status was recently upgraded in part because it adapted to feed on the round goby, an invasive bottom-feeding fish from Europe. Another case might be the common mud crabs  in the southeastern United States which are studied by the publication authors. the crabs might be developing a taste for the green porcelain crab, a relatively recently arrived invasive species. Such a switch to the super-abundant green porcelain crab could alter the mud crabs' overall community structure.

We want to caution that while we might see an increase in population abundance and density among some native predator species, that's not always a good thing, either. There could be unintended consequences for other native species.