Animals & Wildlife Magazine

Invasive Plants in North American Deserts

By Garry Rogers @Garry_Rogers

Impact of Invasive Plants

Invasive species, like a shock force preceding global warming,
are obliterating Earth’s ecosystems.

A diversity impaired firescape.

Once diverse landscape of small trees and tall Saguaro cactus was converted to impoverished shrubland after invasion by fire-prone plants.

Once they began crossing the oceans, Humans introduced thousands of plant species to new regions.  Freed of the diseases and competitors of their homeland, some of the introduced species began spreading into native habitats.  For instance, Cheatgrass (Bromus tectorum), introduced to the interior of the western U. S. during the 1800s began spreading, and now dominates many millions of acres.  Such invasions produce vegetation with low structural diversity and biotic communities with low genetic diversity (Tellman 2002).  In 2007, the BLM had this to say about the state of the invasion of the western U. S.

There has also been a fourfold increase in invasive weed populations since 1985.  Wildfires, drought, and invasive weeds are causing a steady degradation of soils, water quality and quantity, native plant communities, wildlife habitat, wilderness values, recreational opportunities, and livestock forage (BLM 2007:  30).

The BLM report agrees with many other conclusions.  Across the western U. S., invading plants are reducing fire-return intervals below the recovery time needed by native vegetation (Mack 1989, Billings 1990, Turner et al. 2010).  In many places, the change “has been so complete that only a sketchy picture of predisturbance conditions remains” (Radosevich et al. 2007:  26).  Broad expanses of native grasslands, shrublands, and woodlands have become weedlands (e.g., Burcham 1970, Heady 1977, Mack 1981, Babbitt 1998).

Great Basin landscape swept by fires spread by invasive plants.

A barren Great Basin landscape where native vegetation was removed by fire and replaced by Cheatgrass.

Combating the destructive impact of weeds on native ecosystems has become the central resource-conservation issue for managers of natural landscapes.  According to the Sonoran Institute:  “Invasive species are the second most significant threat to biological diversity after direct habitat loss”.  The U.S. National Park Service also ranks the threat posed by weeds as second only to the spread of urbanization (Mau-Crimmins et al. 2005).

Failed Land Management

The loss of the lower-valley and foothill shrublands and sagebrush steppe of the Great Basin and Colorado Plateau is an enormous catastrophe.  A similar loss of native vegetation is occurring across low-lying warmer Sonoran and Chihuahauan Deserts  (Turner et al. 2010).

Throughout the 20th Century, the goal of land managers across most of the western U. S. was to introduce species preferred by livestock, and eradicate unpalatable species whether native or not.  Some of the introduced species have invaded native habitats and displaced native species.  Despite the long history of the invasion, land-use managers still tend to overlook the destructive effects of the invasion.  They often express the view the natural landscape is of value only for production of usable plants.  They speak of sustainable harvest by livestock, but so far they have been content to take short-term views that do not stop the downward spiral of diversity and productivity.

For example, 20th Century range managers praised Crested Wheatgass (Agropyron cristatum), a Eurasian native, for its success on western ranges.  “Probably no other example exists on the dry western ranges of an introduced species that has met with unqualified success and that can be wholeheartedly recommended for increasing forage yield” (Stoddart and Smith 1943:  363).  Though Crested Wheatgrass can crowd out native species and form low-diversity monocultures (Marlette and Anderson 1986, Henderson and Naeth 2005), and can reduce soil fertility (Christian and Wilson 1999), public lands managers are still planting it (Davis et al. 2013).

The lost diversity and productivity of millions of acres of native rangeland makes it clear that past approaches to range management were inadequate to protect the land.  The BLM has finally admitted that there are negative consequences from use of introduced species (e.g., BLM 2007).  Current BLM management policies and practices include options to make necessary land use changes to stop the spreading deterioration resulting from the use of exotic species.  But since they insist on continuing livestock grazing, their efforts continue to fail.

BLM Management of Invasive Plants

BLM field technicians record a number of environmental conditions that serve as indicators of rangeland health (Pellant et al. 2005).  Important indicators of declining health—soil structural change and accelerated erosion—can appear after weeds have infiltrated or replaced native vegetation.  Pellant et al. (2005) designed the BLM rangeland health field manual to “. . .provide early warnings of potential problems and opportunities by helping land managers identify areas that are potentially at risk of degradation or where resource problems currently exist” (Pellant et al. 2005).

The field manual has serious limitations.  For example, it states that vertical vegetation structure is useful as an optional indicator of rangeland health when it affects the integrity of animal populations (page 41), but the manual includes no protocol for assessing animal population integrity.  The manual does not spell out procedures for actions needed in cases where invasive species have taken over a site.

Alien plant replacement of native shrublands should serve as an indicator of deteriorating rangeland health.  According to the BLM manual, such a change shows that a “relatively non-reversible” state transition has occurred (Pellant et al. 2005:  16).  By now, range managers should know that the appearance of invasive plants signals the potential loss of an ecosystem.  In my opinion, managers should elevate the significance of weed invasions and employ “controlled by management interventions” (Pellant et al. 2005:  72, Reference Sheet item 16).  Accumulated experience with weeds indicates that early detection and rapid response are essential to limit the cost of weed control and protect ecosystems.  Unfortunately, this simple idea has yet to be woven into the suite of standards for rangeland health.

It is interesting that every page in the BLM Rangeland Health manual includes a shadowy image of shrub skeletons in a matrix of cheatgrass.  The full-color photograph is on page 108 of the manual (Pellant et al. 2005).  Use of the image as a decoration bears silent witness to the need to elevate awareness of the grave threat to native ecosystems posed by weeds—even among the managers.

According to the BLM Arizona Standards for Rangeland Health (BLM 1997), the unifying goal for many rangeland health conditions is “significant progress.”  Despite many positive BLM annual reports, there has been significant deterioration due to the dramatic increase in invasive weeds.  The deterioration in diversity, water quality, soils, and other resource values that is caused by invasive species requires that land managers place weeds at the top of their list of critical issues of importance to the health of the public lands.

Invasive Plants Conclusion

Invasive non-native species are a central management concern because they “threaten biodiversity and other ecological functions and values” (Warner et al. 2003).  This statement represents a consensus by the scientists and land managers concerned with natural ecosystems.  Native vegetation is more diverse, resilient, and persistent than invasive plant vegetation; it provides food and cover for wildlife, absorbs precipitation, increases water storage, protects soil, reduces flooding and sedimentation, and helps maintain air and water quality.  Land management policies of the BLM and other agencies must be updated to respond quickly when invasive species appear.

My Opinion on Invasive Plants

Many people find esthetic and even moral reasons to protect the continued existence of wild plants and animals.  We need more of these people to help spread understanding of the effects of invasive plants.  Popular opinion can encourage our leaders to insist that our land mangers find long-term solutions to invasive plants.

Invasive Plants References

Babbitt B. 1998. Statement by Secretary of the Interior on invasive alien species. “Science in wildland weed management.” Symposium, April 8-10, 1998. Denver, CO. U. S. Department of the Interior.

Billings, W. D.  1990.  Bromus tectorum, a biotic cause of ecosystem impoverishment in the Great Basin.  Pages 301-322 in G. M. Woodell, ed.  The earth in transition:  Patterns and processes of biotic impoverishment.  Cambridge University Press, Cambridge, ENG.

BLM.  1997.  Arizona standards for rangeland health and guidelines for grazing administration.  USDI, Bureau of Land Management Arizona.  18 p.

BLM.  2007.  The Bureau of Land Management’s Performance and Accountability Report for Fiscal Year 2007.  Online:

Burcham, L. T.  1970.  Ecological significance of alien plants in California grassland.  Proceedings of the Association of American Geographers 2:36-9.

Christian, J. M., and S. D. Wilson.  1999.  Long-term ecosystem impact of an introduced grass in the Northern Great Plains.  Ecology 80:2397-2407.

Davis, K. W., C. S. Boyd, and A. M. Nafus.  2013.  Restoring the Sagebrush Component in Crested Wheatgrass-Dominated Communities.  Rangeland Ecology and Management 66:472-478.

Heady, H. F. 1977.  Valley grassland.  Pages 491-514 in M. G. Barbour and J. Major.  Terrestrial vegetation of California.  John Wiley and Sons, New York.  1002 p.

Henderson, D.C., and M.A. Naeth.  2005.  Multi-scale impacts of crested wheatgrass invasion in mixed-grass prairie.  Biological Invasions 7:639-650.

Mack, R. N.  1981.  Invasion of Bromus tectorum L. into western North America:  An ecological chronicle.  Agro-Ecosystems 7:145-165.

Mack, R. N.  1989.  Temperate grasslands vulnerable to plant invasions:  Characteristics and consequences.  Pages 155-179 in Drake et al., eds.  SCOPE 37, biological invasions, a global perspective.  John Wiley & Sons, New York.  506 p.

Marlette, G.M., and J.E. Anderson.  1986.  Seed banks and propagule dispersal in crested-wheatgrass stands.  Journal of Applied Ecology 23:161-175.

Mau-Crimmins, T., A. Hubbard, D. Angell, C. Filippone, N. Kline. 2005. Sonoran Desert Network Vital Signs Monitoring Plan. Technical Report NPS/IMR/SODN-003.  National Park  Service. Denver, CO.

Pellant, M., P. Shaver, D.A. Pyke, and J.E. Herrick. 2005. Interpreting indicators of rangeland health, version 4.  Technical Reference 1734-6. U.S. Department of the Interior, Bureau of Land Management, National Science and Technology Center, Denver, CO. BLM/WO/ST-00/001+1734/REV05.  122 p.

Radosevich, S. R., J. S. Holt, and C. Ghersa.   2007.  Weed ecology: implications for management.  589 p.

Stoddart, L. A., and A. D. Smith.  1943.  Range management.  McGraw-

Tellman, B. ed. 2002. Invasive exotic species in the Sonoran Desert region. University of Arizona Press and Arizona-Sonora Desert Museum, Tucson, AZ.   420 p.

Turner, R.M., R. H. Webb, T. C. Esque, and G. F. Rogers.  2010.  Repeat photography and low elevation fire responses in the southwestern United States.  Pages 223-244 in R. H. Webb, D. E. Boyer, and R. M. Turner, eds.  Repeat photography methods and applications in the natural sciences.  Island Press, Washington DC.  337 p.

Warner, P. J., C. C. Bossard, M. L. Brooks, J. M. DiTomaso, J. A. Hall, A. M. Howald, D. W. Johnson, J. M. Randall, C. L. Roye, M. M. Ryan, and A. E. Stanton.  2003. Criteria for categorizing invasive non-native plants that threaten wildlands.  Online:  and  California Exotic Pest Plant Council and SW Vegetation Management Association. 24 pp.


Garry Rogers
Garry Rogers

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