NON-NATIVE PLANTS AND URBAN AREAS – SHOULD WE CARE?
by Wendy C. Hodgson
Non-native plant species have received considerable attention by scientists, and more recently, the press, politicians and land managers – and for good reason. Most people do not understand exotic species’ capability of altering the desert and urban landscape dramatically.
Why and how certain exotic plants become invasive is not well understood (there are some interesting hypotheses, including the role of increased C02 in the atmosphere due to fossil fuel burning and vehicle emissions). But, it is certain that certain exotic plants will become invasive over time. In our desert ecosystems, buffelgrass is one of a number of exotics that is of great concern.
This perennial warm-season grass is native to Africa and was imported in the 1930’s to Arizona as forage for cattle. It thrives in arid conditions, often out-competing native plants for water, sunlight, nutrients and space. In addition, during dry periods this grass provides fuel for fast-moving fires. The Sonoran Desert is an ecosystem not evolved with fire; native plants are widely spaced and the annuals that do fill in such spaces during and after periods of favorable rainfall are generally small. Long-living species including saguaros and ironwoods usually die after intense or repeated fires.
Overgrazing and other man-initiated impacts that are changing our diverse succulent desert to a woody shrub and/or grass monoculture ecosystem, have contributed to this problem. According to Dr. Mark Dimmitt, Arizona-Sonora Desert Museum, “just ask people to imagine their beautiful desert hillsides covered with grass without the saguaro, without the paloverdes, without the ironwoods…. These invasive weeds are threatening to turn our desert hillsides into grass, not a grassland, but a monoculture wasteland” (Arizona Daily Star 2005). Red brome is another exotic that has virtually taken over much of the Sonoran Desert, and any attempt to eradicate is generally
accepted as moot. This plant also contributes to extensive and hot fires such as Arizona’s second largest fire in recorded history, the Cave Creek Complex fire of 2005.
Tucson has “reached the threshold,” according to Travis Bean, a University of Arizona Senior Research Specialist and exotic plant expert at the School of Natural Resources, adding that “the desert as we know it could be destroyed” (Bean 2006).
Buffelgrass now exists in dense flammable stands along every major surface street and highway in Tucson, “with Phoenix only a few years behind.” Travis adds that Tucson and Phoenix have now become the source of buffelgrass spread into the surrounding deserts. With growth of these cities, there will be greater chances of human caused fires and establishment of this and other exotics. Our Sonoran Desert habitat, “the ecological backdrop to southern Arizona’s lucrative eco-tourism, resort and development industries, could become a thing of the past, leaving a monoculture of burning buffelgrass in its wake,” according to Travis.
Is all of this unsubstantiated overreactions and rabble-rousing rhetoric? No. A number of cases of destructive fires in Pima County have been analyzed and the common denominator for all has been the abundance of buffelgrass in and near these areas (Bean 2006). We can now predict when and where such events will occur with the presence of flammable stands: Saguaro National Park, Organ Pipe Cactus National Monument, Ironwood Forest National Monument, Coronado National Forest – and the majority of Phoenix’s system of Natural Parks and Preserves. Within Papago Park, buffelgrass populations appear to be increasing. In a number of our preserves, including South Mountain Park, there occur large populations of buffelgrass, Sahara mustard and fountain grass, all species that contribute to large, damaging fires. Still not convinced? Consider this: with a greater interface of urban areas and their inhabitants with the parks and other more “wild” areas, such fires will continue to impact homes, businesses, and human life (Bean 2006). Increased fire risk presents challenges to local fire departments, threatens utility infrastructures and eventually may lower property values and raise insurance rates.
As bleak as this sounds, there is hope. We must remember that we created the opportunities for their successful establishment following our unintentional or intentional transport of these plants. We can slow or reverse some of the problems that we have created.
There is much we do not know about introduced species and how they affect habitats. Some introduced species may serve a function in colonizing overgrazed or highly eroded areas where native plants may not survive, providing habitat for other plants or wildlife. It is clear, however, that certain species do become invasive after a period of time. We must persuade or support those whose mandate is to set and carry out policies addressing appropriate management of our public resources based on sound science. More of the public, politicians and land managers need to take the issue of non-native plants seriously.
The Pima County Board of Supervisors adopted a resolution to manage invasive species within the county (Bean 2006). The Arizona Department of Agriculture listed buffelgrass as a restricted noxious weed and Governor Napolitano created the Invasive Species Advisory Council in 2005. Public education is key to the successful recovery of desert habitats.
Those people living near the preserves have a responsibility to learn what plants are appropriate for their landscapes. Do not buy known or potentially invasive species such as fountain grass or African sumac, and discourage nurseries from selling these plants. Forbid landscapers from planting invasive species in your yard or at your place of business. Vehicles, including those used in construction, shoes, clothing, wind and animals often transport seeds – take the time to clean them off. Educate others – too often the media misses the chance to educate and inform, referring to desert fires as “brush fires,” with no reference to non-native plants as the primary fuel-providing source (Bean 2006).
Consider becoming involved in or helping create programs that are aimed at eradicating these species, such as The Sonoran Desert Weedwackers, a community network of volunteers coordinated by the Tucson chapter of the Arizona Native Plant Society (ANPS). This volunteer weed eradication program operates in cooperation with the county, national park service, botanical gardens, museums and other scientific institutions (Siegel 2002).
PMPC, The City of Phoenix Parks Department and the Arizona Native Plant Society, Phoenix Chapter, have initiated plans to develop a buffelgrass eradication program in Phoenix Mountains’ parks and preserves. Please call the Parks Department or go to www.phoenixmountains.org for information on an eradication effort near you.
Species of Extreme Concern in our Sonoran Desert
- Buffelgrass (Pennisetum ciliare)
- Fountaingrass (Pennisetum setaceum)
- Sahara mustard (Brassica tournefortii)
- Red Brome (Bromus rubens)
- Tamarisk (Tamarix ramosissima)
- Hawk’s-eye (Euryops multifidus)
- Arugula, rocket-salad (Eruca vesicaria ssp. sativa) Yes, the same plant you like in your salads.
Other Species of Concern
- African sumac (Rhus lancea)
- Stinknet (Oncosiphon piluliferum)
- African-daisy (Osteospermum spp.)
- Yellow-bird-of-paradise (Caesalpinia gilliesii)
- Tree tobacco (Nicotiana glauca)
- Corn-mustard (Sinapis arvensis)
- Johnson grass (Sorghum halapense)
- Natal grass (Melinis repens)
References:
Bean, T. 2006. Buffelgrass in the Sonoran Desert: can we prevent the unhinging of a unique American ecosystem? The Plant Press 30(1): 4-5.
Siegel, B. 2002. Community responds to invasive grass threat.
Arizona Native Plant Society. .
Reprinted from PMPC’s LOOKOUT Newsletter Spring 2006