Your Enemy’s Enemy May Not Be Your Friend

When a non-native plant appears on the scene, resource managers sometimes suggest introducing one of the non-native plant’s non-native predators to control it. New research by John D. Parker of the Georgia Institute of Technology, however, suggests this may not be the wisest course of action. In a review of 63 studies of the effects of native and non-native vertebrate herbivores on exotic plant invasions, Parker and his colleagues discovered that native herbivores are much more likely to suppress exotics, while introduced herbivores may even facilitate exotic plant invasions.

Exotic plants and animals, which, according to a recent Cornell University study, cost the U.S. over $137 billion every year (not to mention their effects on our native ecosystems), are thought to prosper in their new environments at least partly because they leave their old predators in the proverbial dust. This is known as the enemy-release hypothesis.

However, evidence is emerging through Parker’s analysis that new environments also carry new predators, ones the exotics haven’t evolved defenses against, and these natives are more than willing to dine on fresh fare. This idea, called the biotic-resistance hypothesis, suggests that native predators (in this case, herbivores) that don’t specialize on one type of native plant sometimes do a fine job of limiting the spread of many exotic and potentially invasive plants.

Indeed, according to Parker, bringing in exotic predators to control exotic plants may actually facilitate the establishment and spread of even more exotic plants, because exotic predators, too, are interested in fresh fare: they indulge in the novel taste of native plants that have no natural defenses, thus opening habitat for even more exotic opportunists. Parker calls this an “invasional meltdown,” and he believes that humans may inadvertently facilitate the process when they bring exotics into an ecosystem while at the same time removing natives.

For example, when colonizing North and South America, Europeans extirpated native herbivores, including bison and elk, and replaced them with exotic herbivores, such as sheep and cows. Consequently, says Parker, exotic generalist herbivores decimated naïve native plants and paved the way for invasions of exotic plants that were adapted to these herbivores. Thus, exotic plants may thrive not by escaping their native enemies, but by following them.

Parker’s study focused on vertebrate herbivores, but his study suggests that an examination of invertebrates is warranted, especially since they are the control measure of choice when dealing with many invasive species, including purple loosestrife. In either case, though, looking for a solution to the spread of invasive species right in our own backyards may prove to be the best solution to a growing problem.

 
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