Native versus exotics grasses: the race is on...

Native versus invasive plant growth: the race is on! Who's going to win?

The journal Restoration Ecology has published a study demonstrating the advantages of planting a native species early to help it withstand the competitive effects of an exotic species.

Sonoran desert ecosystem at-risk from bufflegrass invasion.Buffelgrass (Pennisetum ciliare), a perennial bunchgrass native to Africa and parts of Asia, has invaded much of the southwestern United States and northern Mexico. Bufflegrass is a particularly nasty exotic, arguably the worst in the Sonoran dessert. Where it establishes, it can increase fire frequency and intensity. As a fire tolerant species it expands its cover at the expense of fire sensitive natives creating a positive feedback loop.

Researchers, Jason Stevens and Jeffrey Fehmi from the University of Arizona, looked at how timing of establishment affects the competition between bufflegrass and a native bunchgrass, Arizona cottontop (Digitaria californica). The scientists planted bufflegrass side-by-side with Arizona cottontop, at different age intervals to test competition between the two species. They compared it against controls in which each grass was planted alone (side-by-side with itself). They found that when the exotic species was planted earlier or at the same time as the native grass, the exotic substantially reduced the growth of the native compared to the control. Whereas when the native grass was planted earlier, the exotic grass had no reduction on the native grasses' growth compared to the control.

So the local lesson is that if you're doing land stewardship in the Southwestern United States, an effective strategy in overcoming bufflegrasses' competitive advantage is to establish native perennial grasses early subsequent to a disturbance event (i.e. before bufflegrass establishes). This lesson likely has applicability to many native-exotic species relationships around the world. Other studies have found similar results.

But why does the invasive grass have a competitive advantage?

Perhaps most interesting about this study is that researchers found that bufflegrass reduced the biomass of the cottontop by the same amount whether the bufflegrass had been planted earlier or at the same time as the cottontop. This suggests that early emergence isn't a driving factor in giving the exotic grass an advantage in competing against the native (otherwise it would have reduced biomass even more when it was planted earlier). It must already have the advantage due to other reasons not clear from the study - perhaps from lower resource requirements or faster growth rates. Regardless, the study shows that early establishment of the native Arizona cottontop helps level the playing field.

Implications for conservation:

• Establishing native species (e.g., Arizona cottontop) so that they are actively growing before invaders (e.g., buffelgrass) emerge may improve the survival and vigor of natives.

• Pre-invasion management intervention can reduce the resources (space) available to invaders, and this management can be focused on the areas most likely to be invaded (e.g., roads, trails, proximity to invaded sites, etc.).

• Under natural conditions, buffelgrass’ competitive abilities appear to confer an advantage over natives, which allow it to invade when establishing at the same time as natives.

Source:	Restoration Ecology
Title:	Early establishment of a native grass reduces the competitive effect of a non-native grass
Authors:	a) Jason Stevens and Jeffrey Fehmi
	a) University of Arizona