Everyone can benefit from a little help from their friends, including invasive grasses. But that is bad news for arid landscapes in the West—and those who depend on them—when it comes to fire.
A recent study report, published Nov. 4 in the scientific journal, Ecology and Evolution, found that invasive grass species can essentially work together to out-compete native species and drive the fire cycle to create more frequent, more damaging fires.
Though it has long been known that invasive grasses help spark more fires and help them spread faster than native grasses and bounce back better after fire—thereby shifting the grass/fire cycle of an ecosystem—this study hoped to look at the impact of multiple species.
Among other things, the researchers found that, even when an invasive species does not directly drive changes in the fire cycle, its presence can benefit those that do. Similarly, they found that even when invasive species are in less-than-ideal settings or are individually a minority population in a landscape, the combination of multiple invasive species can still drive a landscape’s transformation into a more fire-prone place.
A trio of authors—Drs. Emma Underwood of University of California-Davis, and Robert Klinger and Matthew Brooks of the U.S. Geological Survey—investigated the relationship of four different invasive plants with themselves and fire in the Mojave Desert. The four invasive species were red brome (also called foxtail brome), cheatgrass, Mediterranean grass (also called split grass), and red stemmed filaree (also called cranesbill or storksbill).
The researchers examined the presence and distribution of the four species across the 807 sample plots in the Mojave Desert, 578 plots which had burned at least once between 1972-2010 and 229 plots which had not. They found the presence of the invasive plants was almost three times higher in plots that had burned once 10-20 years prior than in plots that had never burned. Similarly, they found the presence of invasive plants was three to five times higher in plots that had burned multiple times in the past 10-20 years compared to those that had never burned.
Some of the plots examined had almost nothing but one or more of the four invasive species.
“The invasive species we assessed are known to promote recurrent fire and/or reduce the resilience in postfire vegetation communities in the North American deserts, so an increase in their cover greatly increases the potential for long-term change in postfire communities and fire regimes,” the researchers noted.
“One implication of the overlapping distributions of the four invasive [species] is that the effects of one species could substitute for those of another species that is either not present or present at low abundance,” they added.
The researchers offered the example of the red stemmed filaree, which does not directly act as fire fuel. But it is competition for native plant species. In that way it effectively aids other invasive grasses like red brome and cheatgrass which do directly impact and drive an increased fire cycle.
“A perhaps even more important implication of the overlapping niches is the combined influence of the species (greater invasive cover due to joint positive responses to fire by two or more of the species) which could drive ecosystem changes even if those conditions are suboptimal for each.”
They gave the examples of when cheatgrass and Mediterranean grass coexist in mid-level elevations which are not their optimal habitat. If just one or the other species existed in a less-than-ideal habitat, they might simply be a “passenger species”; something that benefits from fire but does not drive change in the fire cycle.
“If they co-occurred in this situation, the species could potentially contribute enough biomass to promote changes in the probability of ignition, extent of fire, or other components of the fire regime,” the researchers explained.
“In effect, their co-occurrence could drive changes that might not otherwise happen if one or the other species was not present.”
In their implications section, the researchers suggested the practical application of these findings include helping resource managers “better predict the vulnerability of sites to wildfire.”
“Understanding the potential and actual impacts of co-occurring invasive species is particularly important in arid and semiarid ecosystems, given the often irreversibly outcomes associated with invasive plants and large-scale conversion of one type of desert plant community to another.” — Kerry Halladay,WLJ editor
