Native Plant Eaters Have Gourmet Palates

Here's a test. Take a crayfish, offer it two meals - one the native plants that it eats everyday, the other a gourmet meal of a similar, but exotic species of plant. Conventional biological wisdom predicts it will stick with the tried and true. But new research at the Georgia Institute of Technology suggests that plant eaters may be more adventurous than previously thought and prefer to nosh on exotic meals by a ratio of three to one. The findings, which appear in the September issue of Ecology Letters, could point the way to better strategies for controlling the billions of dollars in damage that invasive species cause every year.

The research runs counter to the enemy release hypothesis, first proposed by Charles Darwin in 1859, which holds that exotic species become invasive because they are free from the pressures of being eaten by their natural enemies in their native environment. Left without these controls, exotic plant species can run amuck and crowd out the native flora.

"What enemy release doesn't take into account is that while exotic plants may be free from their so-called natural enemies from their home range, they gain novel enemies in their new range," said John Parker, graduate student at Georgia Tech. "Because they've never had to adapt to being eaten by these consumers, they may lack the appropriate defenses to ward them off, essentially going from the frying pan into the fire."

Parker and biology professor Mark Hay tested the enemy release hypothesis with two species of crayfish found in the southeastern United States and one species of grass carp that had previously been introduced from Asia to control aquatic weed growth.

They paired 10 exotic plants with related native plants in the lab to test how the crayfish would respond when given a choice between similar plants. They found the crayfish preferred the exotic plants by a ratio of three to one.

In another part of the study, they took a broader view of feeding preferences by feeding 57 native and 15 exotic plant species collected from 11 sites throughout the southeastern United States to both species of crayfish and to the Asian grass carp. Again they found native crayfishes preferred the exotic meals. But the exotic grass carp had no preferences. It shares little evolutionary history with either native or exotic plants, so essentially all the plants were exotic to it- a finding that further suggests that the evolutionary history between plants and their consumers is an important predictor of plant edibility.

Parker and Hay were so surprised by their findings that they re-examined data from the scientific literature on the feeding preferences of terrestrial herbivores, including three native grasshoppers and one native and four exotic slugs. The studies they looked at had never analyzed their data for the palatability of native vs. exotic plants.

"We really wanted to challenge our findings for aquatic systems," said Parker. "We wanted to know if perhaps aquatic and terrestrial systems work differently and our unusual results were the result of working in a system that nobody had looked at before."

They were even more surprised when their new results looked exactly like their findings for aquatic herbivores. In these three studies, one conducted in the mountains of the Pacific Northwest, another in the plains of Texas and another in the forests of upstate New York, all four native herbivores again preferred exotic plants over natives. Three of the four exotic consumers again had no preference.

"Now we had essentially four separate studies with 11 herbivores and over 300 plant species collected from all around the continental United States all saying essentially the same thing: native herbivores prefer to consume exotic over native plants," said Parker.

While the results of these studies run counter to the widely accepted enemy release hypothesis, they do support the 'new associations' hypothesis of biological control. This hypothesis holds that since native plants have evolved alongside their native consumers, they've developed defenses to them. Since the newly introduced plants haven't evolved with the native consumers, they may lack appropriate defenses and may be more prone to being eaten in their new environment.

"This is analogous to disease theory in that you might be highly susceptible to new diseases or enemies that you haven't built up resistance against," said Parker.

In addition to its biological importance, the research may help point the way to better strategies for controlling the damage caused by exotic species - estimated by noted Cornell ecologist David Pimentel to be more than $137 billion per year in the United States alone.

"Currently, most exotic plant control is done with herbicides, mechanical removal or by importing the plants' exotic enemies," said Parker. "Each of these methods has serious drawbacks, including high costs and the potential for harmful effects on native species. Our results imply that restoring native herbivore communities may be a viable option to help control exotic plant invasions."

Parker is now working on determining whether native herbivores do in fact control exotic plant growth in field settings, an important step in determining whether biological control with native herbivores is feasible.

"Hopefully our results will also lead to better hypotheses about why some exotic species fare so well in their new environments," he said.