The concept of using biotic resistance by natives to prevent invasives from making inroads into natural areas is nothing new.
One of the earliest examples of this is the hypothesis that diversity in either species or function is a way for natural communities to resist encroachment by invasive species. This so-called Elton Hypothesis postulates that invasion is favored in areas with lower resident diversity, and that the greater the diversity, the less probability there is for an invasive to gain traction in the community.
The reasoning is that this resistance is a product of the fact that a diverse assemblage of species or functional types can use the surrounding resources more completely, and thus prevent invasives from easily being able to exploit an empty niche. A diverse community also has a greater degree of interspecific competition, which again means that it will be more difficult for invasive species to gain a foothold.
Unfortunately, some studies have shown that this does not happen, at least in some cases. For example, cogon grass (Imperata cylindrica) can invade a diverse community more easily compared to one where the grass Andropogon viginicus is dominant (Daneshgar and Shibu, 2009). This discovery prompted researchers to find specific species that can be used to counter other particular invasives.
Common buckthorn (Rhamnus cathartica) is an aggressive invader of forest and woodland understories in eastern North America. It can produce lots of seedlings, and its life cycle is such that it can rapidly overwhelm a forested area, creating dense growth where other trees and vegetation cannot flourish.
The species can be temporarily removed from an area by hand cutting/pulling, forestry mowers, chemical herbicides, and grazing by animals like goats. But its competitive ability allows it to quickly regenerate (via sprouts and seedlings) and again displace other species, so researchers wanted to find a better way of managing the invasive.
Into this role stepped a genus of grasses that have also been touted as crucial to preventing the spread of the invasive Taeniatherum caput-medusae (medusahead grass). These are the Elymus spp, a C3 grass genus that is part of the subfamily Pooideae, and which are related to wheat, rye, and other cereal grains.
Dr. Mike Schuster and his team at the Cover It Up project in the University of Minnesota found out that Elymus spp can be used after management efforts to suppress the re-establishment of R. cathartica and enable other species to grow and flourish. In addition to using the grasses to effectively compete for resources against seedlings and sprouts of the invasive tree, limited fire regimes can be used to kill the latter without unduly harming the Elymus spp.
The Elymus spp that they used so effectively against the invasive were Elymus canadensis (Canada wild rye), Elymus virginicus (Virginia wild rye), and Elymus villosus (silky wild rye). Wildflowers can also be used, but their longer growing times, less rapid growth, and cost makes them relatively ineffective, and they are used mainly to supplement the Elymus spp. All that is necessary for the grasses and the associated wildflowers to survive is enough light to sustain their vigorous growth (at least 10% open sky).
The success in using these methods to combat the invasive tree is great news indeed, and one hopes the forests saved from being overwhelmed by R. cathartica will someday give thanks to the low statured (but mighty!) grasses growing amongst them.
NOTE: Thanks to the great podcast In Defense of Plants for pointing out this great research that is being done to combat R. cathartica.
Literature Cited
Daneshgar P and Shibu Jose (2009). Broomsedge Communities Are Resistant to Invasion by Cogongrass (Florida). Ecological Rest. December 2009 27:383-385
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