Arid Lands Newsletter--link to home page No. 37, Spring/Summer 1995
Conserving Biodiversity

The threat of exotic grasses to the biodiversity of semiarid ecosystems

by Dan James


Is there such a thing as a good plant or a bad plant? On an Arizona Native Plant Society field trip a few years ago, I pointed out the abundance of red brome (Bromus rubens) to some new society members. I explained that it was an adventive introduction from Eurasia and had spread over most of the American Southwest within the last 200 years. In casual conversation I said that it was a bad plant, which provoked a new member to ask how a plant could be bad. Easy answer, I thought. I began to explain how red brome, among other exotic grasses and weeds, likely was responsible for the decline of native flora and the degradation of much of the Sonoran Desert, but as I explained I realized that plants are perceived differently by those not trained in or familiar with native ecosystems.

Most of our neighbors have little or no idea of the effect exotic plants have had on natural ecosystems. Thousands of plants have been introduced, both intentionally and unintentionally, within the last several hundred years. Many have seriously impacted native ecosystems worldwide, and the Sonoran Desert of northwestern Mexico and southwestern United States is no exception.

Many introduced plants are of little or no concern. Most of the common food and fiber crops and many ornamentals grown in the arid and semiarid Southwest have origins elsewhere than in the Sonoran Desert. Our lives and economy depend on them and alone these species, when confined to agricultural or horticultural contexts, have little impact on native ecosystems beyond the fields devoted to their cultivation. There are, however, a large number of introduced plants that have become serious weed problems and many of them are drastically altering the species composition of the Sonoran Desert.

Environmental weeds

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Many of the problematic exotic weeds have origins in Eurasia. These include red brome, wild barley (Hordeum spp.), wild oats (Avena fatua), Mediterranean grass (Schismus spp.), wild mustards (particularly Brassica spp.), tumbleweed (Salsola spp.), and a host of others. These Eurasian plants are predominantly annual, cool season species. They germinate with the fall and winter rains and mature, set seed, and die in the spring, summer, and fall. Their seeds persist in the soil for years in varying degrees of dormancy, and a portion of these germinate when conditions are right. These plants are fast growers, using quantities of nutrients and creating biomass at many times the rate of most native species. They are very competitive.

Another group of invasive exotic plants has origins in Africa. The main culprits are Lehmann lovegrass (Eragrostis lehmanniana), Cochise lovegrass (Eragrostis trichophora), buffel grass (Pennisetum ciliare), Fountain grass (Pennisetum setaceum), Johnson grass (Sorghum halepense), and common bermuda grass (Cynodon dactylon). These are all perennial, warm season species. They germinate and become established in the spring, summer, and fall. They grow and set seed during the warm period and go dormant in cold weather or under drought conditions. Most of these species have only recently (within the last 75 years or so) been introduced to the Sonoran Desert. In contrast to the Eurasian species, which are almost wholly accidental here, the African grasses have been intentionally introduced for range improvement, both as forage and for erosion control. Fountain grass is used as an ornamental in residential landscapes and has been seeded on some public works projects for erosion control. It has been seeded extensively on much of the highway system in Southern California.

Many people see these plants merely as more herbage in a wide array of unidentifiable plant material. Some see them as beneficial, as forage, ground cover, or ornamentals. But their long-term impact on native ecosystems should be of great concern to those interested in preserving drylands biodiversity.

Aggressive competitors

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These exotic plants typically grow at faster rates than do native species and can outcompete them for water, nutrients, and light. This is critical enough, but there is another factor that makes them an even more serious pest problem: what most of us consider to be the common climax botanical components of the Sonoran Desert, the paloverdes and various cacti, are not fire adapted, but the exotics are and so they thrive under fire cycles.

When the desert burns--often owing to an increased fuel supply courtesy of the extra biomass generated by exotic weeds--the fast-growing, fire-adapted exotic species not only survive but spread into niches created by the loss to fire of native species. The absence of perennial native species and the loss of the cryptogamic crust associated with many stabilized climax desert soils create an ideal habitat for many of the exotics. Thus a cycle of disturbance is created: as the biomass of exotic species increases, so does the probability of fire; the burning episodes continue the disturbance and favor the invaders because they are fire-adapted; the native species decline and the biomass of the invader species increases further, and so on.

Time to reconsider the definition of a weed

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Traditionally, "weed" has been an agricultural term and has been defined as those plants that are harmful to crops or that have a detrimental effect on cultivated areas. Graham's Basic Dictionary of Science defines a weed as "any wild plant of no use to man which makes its way into farm fields and gardens where it is not desired," and the Arizona Department of Agriculture's list of "weeds" consists entirely of plants that fit the narrow definition of an agricultural pest. (And why not? They are, after all, the Department of Agriculture.) To an ecologist, however, this narrow definition is unpalatable.

I've always liked the definition that says a weed is simply "a plant out of place." This definition broadens the concept. Given that many exotic plants are pests in the broader environmental sense--in that they certainly can be considered plants "out of place"--perhaps it is time to reconsider the traditional definitions.

Under our broader definition, the exotic grasses being seeded on public lands certainly should be considered environmental weeds. The agricultural weeds are restricted for import into the state because of their economic impact on agriculture. Isn't the loss of native flora and fauna an economic impact, too?

Many of us living, working, and paying taxes here value our natural surroundings highly and a good many tourists from the world over come to Arizona to see the unique flora and fauna of the Sonoran Desert--and along the way they deposit millions of dollars in local pockets in return for food, lodging, fuel, and souvenirs.

The downside is that so much of what makes this dry country attractive to taxpayers and tourists alike is being lost to exotic environmental weeds and to those policies that foster their spread.

For further reading

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An excellent paper on this subject is: D'Antonio and Vitousek. 1992. Biological invasions by exotic grasses: the grass/fire cycle and global change. Annual Review of Ecology and Systematics 23:63-87.

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Author information

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Dan James, Revegetation and Grass Specialist with Western Sod of Casa Grande, Arizona, wrote "Some Principles and Practices of Desert Revegetation Seeding" for The Arid Lands Newsletter No. 32 (Spring/Summer 1992): Bioremediation and Ecological Restoration. He is a board member of the Arizona Native Plant Society, from whose newsletter, The Plant Press (Vol. 18 No. 3), this article was adapted.

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