Invasive exotic plants helped by natural enemies

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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