Evolutionary limits to invasive species dominance

Many ecologists theorize that exotic species successfully invade areas because they no longer face the evolutionary factors that keep their numbers in check within their native range (e.g. species that no longer face predators that have evolved specifically to consume them). But what happens to the evolutionary history of invasive species after they establish and attain dominance outside of their native range? Do they evolve in a way that furthers their dominance or do they face self-limiting evolutionary factors?

Garlic mustard growing in the understory.In a recent study, scientists addressed these questions with respect to the plant, garlic mustard (Alliaria petiolata) a Eurasian species that aggressively invades North American forest understories. The plant is such a successful invader in part due to its production of toxic phytochemicals that harm mycorrhizal fungi and thus inhibit the growth of native plants that depend on the mycorhirza to take up nutrients. The scientists looked at a 50-year chronosequence of the plant to better understand its evolutionary history post invasion.

Findings
The researchers found substantial evidence that the garlic mustard faces evolutionary limits on its dominance over time. They found that with increasing age of the source population, there was a decreasing investment in producing the toxic phytochemical compounds. The study controlled for other factors such as latitude and longitude and still found the same results. In addition, they grew native plants alongside A. petiolata plants and found that the natives grew much better when planted next to A. petiolata derived from older populations. The study also looked at field data from the Illinois Critical Trends Assessment Program, which performs repeated forest vegetation surveys at 5-year intervals in permanent plots at randomly selected sites throughout the state. They found that cover of native woody species decreased in recently invaded areas and increased in areas with a longer history of A. petiolata invasion. According to the researchers,

"The findings of this study indicate that reduced invasiveness may evolve during the invasion process, suggesting that evolutionary forces may provide limits to invasive species when ecological limits have been removed during introduction."

The reasons for why this occurs are unclear. The scientists speculate that there may be multiple factors. For example, it may be that as the plant forms dense monocultured patches selection pressures quickly switch to lower chemical levels in favor of other strategies that are better suite for intraspecific competition. Another explanation may be that soil microbes gain resistance to the chemical over time reducing the evolutionary benefit of producing the toxin.

Conservation Implications
According to study, while eradication may be most critical in newly established populations, which are poised to quickly spread and outcompete native species, restoration efforts that reintroduce native plants and soil microbes may be more critical, and more effective, where populations are older and have reduced investment to allelopathic traits. The results also indicate that the extreme dominance exhibited by some exotic species may not be sustainable over time, as shifting selection pressures may ultimately result in feedbacks that limit population growth. However, it's not known whether similar processes commonly occur with other invasive species.

Source:	Proceedings of the National Academy of Sciences
Title:	Evolutionary limits ameliorate the negative impact of an invasive plant
Authors:	a) Richard Lankau, b) Victoria Nuzzo, a) Greg Spyreas, and c) Adam Davis
	a) University of Illinois at Urbana-Champaign; b) Natural Area Consultants, Richford, NY; c) Invasive Weed Management Unit, USDA, Urbana, IL.