Virtually all of the world’s habitats are disappearing at an alarming rate. Biodiversity is plummeting, due in large part to anthropocentric habitat destruction; but increasingly frequent wildfires, climate change and pollution take their toll on the world’s habitats as well.
Nevertheless, one of the most insidious, yet underappreciated, threats to natural areas is the introduction of alien species. This is especially important for California residents, as some invasive species increase the risk of wildfires.
The Problem and the Cause
Despite the fact that most invasive species are benign (and often important) in their native range, drastic damage can occur when worlds collide. Some invaders arrive in new habitats as accidental stowaways or hitchhikers, inadvertently swept up in the machinery of global trade. Others arrive via the deliberate hand of Homo sapiens, who transplant these species for a variety of reasons.
Because alien animals often take the form of giant pythons prowling the everglades or exotic beetles destroying acres of forestland, they receive most of the media attention, while other invasive species fly under the radar. However, invasive plants often alter entire ecosystems; in some cases they threaten to strip the world of some of its most unique and amazing habitats.
California’s size and population have led to an abundance of invasive species. A few of these species are already causing serious damage by altering local fire regimes and exacerbating the extent, intensity and rate at which wildfires occur.
For example, the fire regime of chaparral habitat involves infrequent, high-intensity crown fires. However, invasive annual grasses are increasing the frequency of these fires. These changes in the fire regime have happened too quickly for the native plants to adapt, and many are beginning to disappear.
According to a 2012 study by the University of California, Riverside, non-native grasses were more abundant in areas that had been burned than native grasses were. (Allen, 2012) According to the study’s authors, Robert J. Steers and Edith B. Allen, many of these invasive species become more numerous after successive fires.
“We found that invasive annual grass cover was highest in the twice-burned stand and native annual plant cover was greatest in the unburned stand. Native annual species richness significantly decreased each time a stand burned resulting in low native annual plant diversity.” (Allen, 2012)
This pattern of alien species increasing the local fire risk is common in other Mediterranean climates around the world. (Sugihara, 2006) Unfortunately, a troubling pattern often emerges. Fires set the stage for invasive species to take hold, and invasive species set the stage for more fires. This pattern becomes a vicious cycle that rapidly accelerates the degradation of local habitats.
While some invasive species cause relatively little harm, others represent significant risks for California’s native habitats. Avoid planting these species whenever possible; instead, opt for native alternatives.
Most annual grasses in California are invasive species. Because they release their seeds (which often survive fires and remain viable) and die before the fire season begins, annual grasses drastically increase the available fuel for fires. After the fires pass, the invaders recolonize areas more quickly than native plants can.
- Wild oats (Avena sp.)
- Annual ryegrass (Lolium multiflorum)
- Bromes (Bromus sp.)
- Cheatgrass (Bromus tectorum)
- Spiltgrass (Schismus sp.)
Because they remain green for extended periods, perennial grasses do not represent the same type of threat that annuals do. However, many species produce and retain a considerable quantity of leaf litter. This litter dries quickly and serves as a significant source of tinder. Once these types of clumpy grasses ignite, they burn very intensely.
- Perennial ryegrass (Lolium perenne)
- Giant reed (Arundo donax)
- African fountain grass (Pennisetum setaceum)
- Pampasgrass (Cortaderia selloana)
Herbaceous plants often take over abandoned fields, roadsides and similar areas. Most species that are a problem in California produce seeds that survive fires. Because they are early germinators, many exotic herbs and forbs compete with native species following fires. Some of these plants produce allelopathic compounds, which inhibit the growth of nearby plants, giving them an additional competitive advantage.
- Black mustard (Brassica nigra)
- Fennel (Foeniculum vulgare)
- Blessed milk thistle (Silybum marianum)
- Artichoke thistle (Cynara cardunculus)
One of the greatest threats brought about by invasive trees is their ability to regenerate rapidly after fires. This is especially important when they occur alongside native species that are not particularly well adapted to fire. For example, willows (Salix sp.) and California sycamores (Platanus racemosa) – two riparian species that usually grow in close proximity to water – are very poorly adapted to fire. When invasive trees begin colonizing riparian habitats, they often form “fire corridors,” through which fire can travel and reach the native trees. After the fire has passed, the invasive species recover quickly, and outcompete the natives.
- Salt cedar (Tamarix ramosissima)
- Acacia (Acacia sp.)
- Eucalyptus (Eucalyptus sp.)
For more information about the connection between invasive plant species and wildfire in California, see Invasive Plants and Wildfire in Southern California, by the University of California, Division of Agriculture and Natural Resources.
Allen, R. J. (2012). Impact of Recurrent Fire on Annual Plants: a Case Study from the Western Edge of the Colorado Desert. Madroño.
Sugihara, N. G. (2006). Fire in California’s Ecosystems. University of California Press.[/vc_column_text][/vc_column][/vc_row]