We have heard about invasive animal species like Boas introduced into Florida from Africa and Asia. How about invading plant life? Ecologists at the University of Toronto and the Swiss Federal Institute of Technology Zurich (ETH Zurich) have found that, given time, invading exotic plants will likely eliminate native plants growing in the wild. A study published in Proceedings of the National Academy of Sciences (PNAS) reports that recent statements that invasive plants are not problematic are often based on incomplete information, with insufficient time having passed to observe the full effect of invasions on native biodiversity. Invasive plant life simply may take longer to "take over" than invasive animals.
Invasive species may be either plants or animals and may disrupt by dominating a region, wilderness areas, particular habitats, or wildland-urban interface land from loss of natural controls (such as predators or herbivores). This includes non-native invasive plant species labeled as exotic pest plants and invasive exotics growing in native plant communities.
"The impacts of exotic plant invasions often take much longer to become evident than previously thought," says Benjamin Gilbert of U of T’s Department of Ecology & Evolutionary Biology (EEB) and lead author of the study. "This delay can create an extinction debt in native plant species, meaning that these species are slowly going extinct but the actual extinction event occurs hundreds of years after the initial invasion."
Much of the debate surrounding the threat posed to biodiversity by the invasions of non-native species is fueled by recent findings that competition from introduced plants has driven remarkably few plant species to extinction. Instead, native plant species in invaded ecosystems are often relegated to patchy, marginal habitats unsuitable to their non-native competitors.
However, Gilbert and co-author Jonathan Levine of ETH Zurich say that it is uncertain whether the colonization and extinction dynamics of the plants in marginal habitats will allow long-term native persistence.
"Of particular concern is the possibility that short term persistence of native flora in invaded habitats masks eventual extinction," says Levine.
The researchers conducted their research in a California reserve where much of the remaining native plant diversity exists in marginal areas surrounded by invasive grasses. They performed experiments in the reserve and coupled their results with quantitative models to determine the long term impacts of invasive grasses on native plants.
"Invasion has created isolated islands of native plants in a sea of exotics," says Gilbert. "This has decreased the size of native habitats, which reduces seed production and increases local extinction. It also makes it much harder for native plants to recolonize following a local extinction."
"Our research also allows us to identify how new habitats for native flora could be created that would prevent extinction from happening. These habitats would still be too marginal for invaders, but placed in such a way as to create bridges to other habitat patches," says Gilbert.
One of the more famous cases of an invading plant is the Kudzu vine. Kudzu was introduced from Japan into the United States at the Japanese pavilion in the 1876 Centennial Exposition in Philadelphia. It is now common along roadsides and other undisturbed areas throughout most of the southeastern United States. It has been spreading at the rate of 150,000 acres (61,000 ha) annually.
Kudzu's environmental and ecological damage results from acting through interference competition, meaning it out competes other species for a resource. Kudzu competes with native flora for light, and acts to block their access to this vital resource by growing over them and shading them with their leaves. Plants may then die as a result, resulting in the soil or substrate nutrients previously used by the original plant to become more freely available to kudzu.
The findings are reported in the paper "Plant invasions and extinction debts" in PNAS’ Early Edition this week.
For further information see Invading Plants.
Kudzu image via Wikipedia.