Invasive Exotic Plants Helped By Natural Enemies

Opposing Effects of Native and Exotic Herbivores on Plant Invasions
John D. Parker,* Deron E. Burkepile, Mark E. Hay

Although conventional wisdom suggests that invasive exotic plants thrive
because they escape the natural enemies that kept them in check in their
native ranges, a new study in the journal Science suggests the opposite.
Exotic plants that are in the presence of their natural enemies actually
do better in their introduced ranges. The research from the Georgia
Institute of Technology appears in the March 10, 2006 issue of the journal
Science, published by the AAAS, the science society, the world's largest
general scientific organization.

Each year, invasive exotic species cause an estimated $120 billion in
damage in the United States, not to mention the untold amount of harm they
do to the structure and function of native ecosystems. In this latest
study, researchers found that exotic herbivores, including cattle, rabbits
and goats introduced by Old World explorers, can encourage the spread of
invasive exotic plants -- increasing their relative abundance by nearly 70
percent over native plants.

"Exotic herbivores may facilitate the growth of exotic plants by
selectively consuming native plants, potentially freeing resources for
exotic plants that can resist these herbivores," said John Parker,
graduate researcher in the School of Biology at the Georgia Institute of
Technology.

Parker, along with Professor Mark Hay and fellow graduate student Deron
Burkepile, analyzed 63 published studies of more than 100 exotic and 400
native plant species. In addition to finding that exotic plant eaters
increased the percentage of exotic plants in a community, they found that
exotic plant eaters also increased the richness and variety of exotic
plants.

They also found that native herbivores, once thought to have little effect
on exotic plants, are far more effective in reducing their number. They
decreased the relative abundance of exotics by 28 percent and the absolute
abundance by 15 percent.

"These findings were interesting to us because, on most continents, many
of the resident herbivores have been hunted to extinction by early
settlers, often times to make room for their own domesticated and feral
herbivores from the old world," said Parker.

He also noted that this radical shift in herbivore composition may favor
exotic plants over natives.

Recent research, including a paper authored by Parker in Ecology Letters
last year, suggests that native herbivores actually prefer to eat exotic
plants over native plants. This research proposes that since the exotic
plants haven't yet adapted to the threats posed by native plant eaters,
they may not have the right defenses and are often easier prey than the
native herbivores' usual meal.

Moreover, most previous assessments of this "natural enemies hypothesis,"
have focused on the effect that specialized insects have on plants.
However, Parker notes that insects commonly reduce plant growth and
biomass, but vertebrate herbivores are often larger and thus more commonly
kill plants outright. Because of this, vertebrate herbivores often have a
stronger impact on plant communities.

The study's findings have serious implications for conserving ecosystems
and reducing the economic damage that invasive exotic species cause.

"Restoring native vertebrate herbivores to their natural ranges, while
reducing the number of exotic herbivores, could be an effective tool in
reducing invasive exotic plants," said Parker.

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The Science article referred to can be accessed online if you are a
subscriber or in the magazine itself at a library: Opposing Effects of
Native and Exotic Herbivores on Plant Invasions; John D. Parker, Deron E. Burkepile, and Mark E. Hay; Science 10 March 2006: 1459-1461. Vol 311.