Theresearch runs counter to the enemy release hypothesis, first proposedby Charles Darwin in 1859, which holds that exotic species becomeinvasive because they are free from the pressures of being eaten bytheir natural enemies in their native environment. Left without thesecontrols, exotic plant species can run amuck and crowd out the nativeflora.
“What enemy release doesn’t take into account is thatwhile exotic plants may be free from their so-called natural enemiesfrom their home range, they gain novel enemies in their new range,”said John Parker, graduate student at Georgia Tech. “Because they’venever had to adapt to being eaten by these consumers, they may lack theappropriate defenses to ward them off, essentially going from thefrying pan into the fire.”
Parker and biology professor Mark Haytested the enemy release hypothesis with two species of crayfish foundin the southeastern United States and one species of grass carp thathad previously been introduced from Asia to control aquatic weed growth.
Theypaired 10 exotic plants with related native plants in the lab to testhow the crayfish would respond when given a choice between similarplants. They found the crayfish preferred the exotic plants by a ratioof three to one.
In another part of the study, they took abroader view of feeding preferences by feeding 57 native and 15 exoticplant species collected from 11 sites throughout the southeasternUnited States to both species of crayfish and to the Asian grass carp.Again they found native crayfishes preferred the exotic meals. But theexotic grass carp had no preferences. It shares little evolutionaryhistory with either native or exotic plants, so essentially all theplants were exotic to it– a finding that further suggests that theevolutionary history between plants and their consumers is an importantpredictor of plant edibility.
Parker and Hay were so surprised bytheir findings that they re-examined data from the scientificliterature on the feeding preferences of terrestrial herbivores,including three native grasshoppers and one native and four exoticslugs. The studies they looked at had never analyzed their data for thepalatability of native vs. exotic plants.
“We really wanted tochallenge our findings for aquatic systems,” said Parker. “We wanted toknow if perhaps aquatic and terrestrial systems work differently andour unusual results were the result of working in a system that nobodyhad looked at before.”
They were even more surprised when theirnew results looked exactly like their findings for aquatic herbivores.In these three studies, one conducted in the mountains of the PacificNorthwest, another in the plains of Texas and another in the forests ofupstate New York, all four native herbivores again preferred exoticplants over natives. Three of the four exotic consumers again had nopreference.
“Now we had essentially four separate studies with 11herbivores and over 300 plant species collected from all around thecontinental United States all saying essentially the same thing: nativeherbivores prefer to consume exotic over native plants,” said Parker.
Whilethe results of these studies run counter to the widely accepted enemyrelease hypothesis, they do support the ‘new associations’ hypothesisof biological control. This hypothesis holds that since native plantshave evolved alongside their native consumers, they’ve developeddefenses to them. Since the newly introduced plants haven’t evolvedwith the native consumers, they may lack appropriate defenses and maybe more prone to being eaten in their new environment.
“This isanalogous to disease theory in that you might be highly susceptible tonew diseases or enemies that you haven’t built up resistance against,”said Parker.
In addition to its biological importance, theresearch may help point the way to better strategies for controllingthe damage caused by exotic species – estimated by noted Cornellecologist David Pimentel to be more than $137 billion per year in theUnited States alone.
“Currently, most exotic plant control isdone with herbicides, mechanical removal or by importing the plants’exotic enemies,” said Parker. “Each of these methods has seriousdrawbacks, including high costs and the potential for harmful effectson native species. Our results imply that restoring native herbivorecommunities may be a viable option to help control exotic plantinvasions.”
Parker is now working on determining whether nativeherbivores do in fact control exotic plant growth in field settings, animportant step in determining whether biological control with nativeherbivores is feasible.
“Hopefully our results will also lead tobetter hypotheses about why some exotic species fare so well in theirnew environments,” he said.
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