Invasive Species in Sonoran Desert Ecosystems
What is an invasive species, and why worry about it?


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Invasive.  Exotic.  Non-native.  Alien.  Introduced.  Non-indigenous.  Noxious.  Naturalized.  Several terms have been applied to the concept of organisms "out of place," and the definitions associated with these terms reflect both the standpoints (social, political, and economic) of those employing them and the perceived or documented threats posed to natives and ecosystems by the organisms to which these terms refer.  There are issues of provenance, severity, and scale embedded in each of these terms, as well.  Does an invasive species have to come from across the ocean, or simply from another part of the same continent, or merely from another river system or mountain range?  In many cases, it is not as important where the organism originates as what it does once in its new environment.  This issue forms the basis for defining an "invasive" species, though other subtleties of classifying "out of place" organisms are no less important to consider.

What is an invasive species?

tumbleweed (Salsola sp.)An invasive species is an organism "that has the potential to substantially alter the structure and function of natural ecosystems at a landscape spatial scale" (Peine & Lancia, 1999:268). Others have offered expanded definitions: colonizers are species that are introduced and spread rapidly, causing ecosystem-scale changes; invaders, more strictly, can enter intact vegetation with the potential to dominate or displace it (Bazzaz, 1986:107).  The process by which this takes place has been given the name "bioinvasion," or the movement of organisms among ecosystems resulting in newly established populations where none previously existed.  An invasive species can be a micro-organism which causes disease in native organisms; a parasite that diminishes the health (and, ultimately, fitness) of native organisms; a plant or animal that competes with a native counterpart for nutrients, space, and other resources, potentially displacing the native; and/or an organism that alters the composition or function of an ecosystem, potentially such that native organisms can no longer sustain their populations.

How does a species become invasive?

Demographic change among populations of plants and animals is nothing new, nor is interspecific competition or predation, nor is extirpation or extinction that can result from these processes.  The distinction made for modern-day invasive species issues hinges on "the human-caused breakdown of biogeographic barriers to species dispersal," wherein deliberate and accidental movement of species into new environments, by humans, occurs "at rates that are without precedent in the last tens of millions of years.  As a result, taxa that evolved in isolation from each other are being forced into contact in an instant of evolutionary time" (D'Antonio & Vitousek, 1992:63).

Human relocation of species (also known as "species transfers", "taxonomic redistribution", or even "biotic enrichment") is only the first step, though, in a species' becoming invasive.  It is not enough for a species to be able to exist in its new environment, although a close match between environmental conditions in the species' home environment and the environment to which it is introduced is fundamental to its survival there.  Beyond this, the organism must be able to establish a viable and growing population.  To do so, the new species must be capable of outcompeting and/or displacing native organisms (Herbold & Moyle, 1986; see Parker & Reichard, 1998, for discussion).  This ability lies in part with the organism's particular ecology and life history, including but not limited to its reproductive biology and its tendency to associate with humans (see Ehrlich, 1986).

Successful invasion is, as well, often facilitated by an important aspect of the new environment: absence of natural controls that regulated the invasive species' population in its homeland.  Without these diseases, parasites, and predators, the new species can spread rapidly.  On the other side of the same coin, native species may lack adaptations that allow them to resist competition from or predation by the invasive species, affording success to the invader through attrition.  If the invasion (particularly with plant species) is coupled with other disturbances to the new ecosystem (e.g., earth moving, plowing, fire, livestock grazing, changes to surface and groundwater hydrology), native populations may be weakened from the start and the ecosystem rendered more susceptible to an invasion (just why disturbances foster invasion, or how invasion occurs in the absence of disturbance, are issues that remain under debate, however; see Orians, 1986; Parker & Reichard, 1998).

What are the consequences of invasive species?

Invasive species have ecological as well as economic consequences. Agricultural "weeds" - considered to be any plants other than the crop plant, bearing in mind that many crop plants are also introduced species - can contaminate seed crops and reduce their value, pose health threats to livestock, or necessitate costly repairs to harvesting machinery. Invasive insects can harm crop plants and reduce their productivity, contaminate harvest surpluses, and act as vectors of disease among livestock or in human populations. Control of these organisms chemically, biologically, and mechanically exacts great expenditures of time and money in addition to the losses mentioned above; in the United States alone, these expenses may total many tens of billion (or, by some estimates, well over 100 billion) dollars annually (Pimentel, 1986; see Bright, 1998:174-194, for more discussion).

The ecological changes that result from invasion are several and interconnected, ranging from local to global in scale, although "the fraction of successful invasions that alter ecosystem not known" (D'Antonio & Vitousek, 1992:64).  The weakening or loss of a species in an ecosystem can leave room for other species to take its place, potentially reverberating up the food chain or even directly impacting other native species that were once dependent on the one that was lost (Schlarbaum, et al., 1999).  Genetic diversity within a species as well as biodiversity at the level of ecosystem may be diminished.  At a landscape scale, demographic changes, particularly in plant cover, can lead to loss of soil and increased erosion, affecting the physical structure of the habitat itself.  At a continental or global scale, these processes result in diminished "regional distinctiveness of Earth's flora and fauna" (Vitousek, et al., 1997) both through redistribution of species between and among continents - homogenization - and through losses of native taxa to deleterious impacts (predation, competition, disease) imposed by introduced organisms - what might be called depauperization.

The impact of any particular invasive species depends on many factors, including the number of individuals that constituted the original introduction, the number of locations in which introductions occurred, the mobility and fecundity of the invasive species, and the species' habitat specificity, or what conditions it requires to become abundant and widespread.  The narrower, or more specific, this set of conditions, the more restricted the consequences will be in scope, though effects may be locally more pronounced; the broader, or less specific, the species' habitat requirements, the greater the magnitude of disruption in both severity and geographic extent.  In turn, the degree to which a species becomes invasive will be proportional to the difficulty of reversing consequential environmental degradation and controlling further spread.

What can be done about invasive species?

"Control of exotic species is an extremely complex matter that involves societal values, policy decisions, economic analyses, ecological interrelationships, population dynamics, and technological capabilities" (Peine & Lancia, 1999:268).  Use of chemicals to control invasive plants, trapping or killing of invasive animals, or introduction of biocontrol agents (natural predators of an invasive species, usually from the species' homeland) are effective to varying degrees and at different geographical scales, and carry with them their own sets of consequences: incidental killing of native species, pollution of soil or water, selection for resistant individuals, proliferation of new invaders, for example (see Schlarbaum, et al., 1999; Dahlsten, 1986).

Relative to the ease with which inadvertent extirpations and even extinctions of natives occur due to bioinvasions, elimination of the invaders once they have become established in their new environment is difficult.  Accomplishing this would require location and elimination of all reproductively viable individuals of the invasive species in the invaded area.  This might be possible in some cases, such as with large animals, or on islands (physical or ecological) where biogeographic barriers have limited the spread of the invader and serve to prevent reinvasion during and after the eradication process.  In cases of other organisms such as microbes, insects, small and/or aquatic animals, and plants that grow rapidly, root deeply, and/or produce numerous seeds (which can lie dormant for years), "it is unlikely that an exotic can be exterminated and reinvasion prevented, except perhaps in very localized areas, because costs can be prohibitively high.  The alternative is a control effort that requires long-term commitment to constrain populations to acceptable levels and spatial distribution" (Peine & Lancia, 1999:269).
Stabilization of an environment in its invaded state is cited as a first step in the control process, including preventing both further introductions of non-natives and additional losses of native species.  Also advocated are reintroductions of natives whose populations have been diminished or eliminated by invasive species; if an ecosystem has been altered significantly, however, this may not always be an option nor be guaranteed success (Schlarbaum, et al., 1999).  A third approach involves redirecting patterns of disturbance so as to favor recolonization by native species over that by invasive species, potentially entailing large-scale manipulations of landscape processes such as fire and flooding. Any effort aimed at controlling invasive species and/or restoring native species and ecosystems entails decisions about what the end product should look like. Because invasions and changes to Sonoran Desert ecosystems have histories extending back a century or more, it is difficult to know exactly to what appearance, structure, and function - or to what point in time - an area in question should be restored; alternatively, deciding what aspects of the altered ecosystem to accept and maintain in the interest of what remains may be the only viable approach to managing an advanced invasive species problem (Patten & Halvorson, 1995).

What is the status of invasive species in the Sonoran Desert region?

In a survey of scientists working in the Sonoran Desert bioregion, "planting of exotic grasses" and "biological invasions" ranked 7th and 10th in the top ten threats to biodiversity in this region. Most of the other eight threats - including urbanization, population growth, water diversion, livestock grazing, groundwater depletion, conversion to farmland, and recreation - have indirect consequences either or both for placing stress on native biota and encouraging the spread of non-native organisms. In an assessment of native species and habitats at risk in the Sonoran Desert bioregion, 6 of 12 classes of species have invasive species as their major threat; for 9 of 14 subregions and habitat types, invasive or introduced species are cited among the major, worsening threats to native species in these areas. Across all classes of biota, it is estimated that several hundred non-native organisms are now present, and in many cases well established, in the Sonoran Desert bioregion; many are extensively distributed, and the most severe are causing ecosystem-level changes that are or are potentially irreversible (for more information, see Nabhan & Holdsworth, 1998).


  • "tumbleweed" (Salsola sp.) near an alfalfa field (Medicago sativa) edged by barley (Hordeum murinum) and Johnson grass (Sorghum halepense) near Maricopa, AZ (©1999,K.Mauz)



~ Links ~

Southwest Exotic Plant Information Clearinghouse
The Southwest Exotic Plant Information Clearinghouse is a cooperative effort among the U.S. Geological Survey, the National Park Service and Northern Arizona University to organize comprehensive information on exotic plant species in the southwest on one web location.

California Environmental Resources Evaluation System (CERES)
Environmental Information by Theme: Invasive Species
Reports and Publications, Data, Imagery and Spatial Data (Photos), Organizations, More Links

Plant Conservation Alliance
The PCA is a consortium of federal agencies and over non-federal cooperators representing various disciplines within the conservation field. PCA members and cooperators work collectively to solve the problems of native plant extinction and native habitat restoration, ensuring the preservation of our ecosystem.

Weeds Gone Wild: Alien Plant Invaders of Natural Areas is a web-based, public education project of the PCA's Alien Plant Working Group, providing information about the serious threat and impact of invasive alien (exotic) plants to the native flora, fauna, and natural ecosystems of the United States.

America's Least Wanted: Alien Species Invasions of U.S. Ecosystems
A publication of The Nature Conservancy
Harmful Non-Indigenous Species in the United States
Office of Technology Assessment

An Illustrated Guide to Arizona Weeds
by Kittie F. Parker, The University of Arizona Press, Tucson (A Complete Online Edition of the Printed Book)

Noxious Weeds Home Page
USDA Animal and Plant Health Inspection Service (APHIS)



~ References ~

Bazzaz, F. A. (1986) Life History of Colonizing Plants: Some Demographic, Genetic, and Physiological Features. In Ecology of Biological Invasions of North America and Hawaii, edited by H. A. Mooney and J. A. Drake, pp. 96-110. Springer-Verlag, New York.

Bright, C.  (1998)  Life Out of Bounds: Bioinvasion in a Borderless World.  W. W. Norton and Company, New York.

Dahlsten, D. L. (1986) Control of Invaders. In Ecology of Biological Invasions of North America and Hawaii, edited by H. A. Mooney and J. A. Drake, pp. 275-302. Springer-Verlag, New York.

D’Antonio, C. M. and P. M. Vitousek.  (1992)  Biological invasions by exotic grasses, the grass-fire cycle, and global change.  Annual Review of Ecology and Systematics  23: 63-87.

Devine, R.  (1998)  Alien Invasion: America’s Battle with Non-Native Animals and Plants.  National Geographic Society, Washington, D.C.

Drake, J. A., et al., eds.  (1989)  Biological Invasions: A Global Perspective.  John Wiley and Sons, New York.

Ehrlich, P. R. (1986) Which Animal Will Invade? In Ecology of Biological Invasions of North America and Hawaii, edited by H. A. Mooney and J. A. Drake, pp. 79-95. Springer-Verlag, New York.
Groves, R. H. and J. J. Burdon, eds.  (1986)  Ecology of Biological Invasions.  Cambridge University Press, New York.

Herbold, B. and P. B. Moyle. (1986) Introduced Species and Vacant Niches. The American Naturalist 128 (5): 751-760.

Hobbs, R. F. and L. F. Huenneke.  (1992)  Disturbance, Diversity, and Invasion: Implications for Conservation.  Conservation Biology  6: 324-337.

Martin, S. C. and R. M. Turner. (1977) Vegetation Change in the Sonoran Desert Region, Arizona and Sonora. Journal of the Arizona Academy of Science 12: 59-69.

Nabhan, G. P. and A. R. Holdsworth.  (1998)  State of the Desert Biome: Uniqueness, Threats and the Adequacy of Protection in the Sonoran Desert Region.  The Wildlands Project, Tucson.

Orians, G. H. (1986) Site Characteristics Favoring Invasions. In Ecology of Biological Invasions of North America and Hawaii, edited by H. A. Mooney and J. A. Drake, pp. 133-148. Springer-Verlag, New York.

Parker, I. M. and S. H. Reichard. (1998) Critical Issues in Invasion Biology for Conservation Science. In Conservation Biology for the Coming Decade, edited by P. L. Fiedler and P. M. Kareiva, pp.283-305. Chapman & Hall, New York.

Patten, D. T. and W. L. Halvorson. (1995) 2.2 - Impacts of Stressors on Hot Desert Scrublands. Arizona Comparative Environmental Risk Project (ACERP) Ecosystems Technical Committee Reports, Arizona Department of Environmental Quality, Phoenix.

Peine, J. D. and R. Lancia.  (1999)  Control of Exotic Species: European Wild Boar.  In Ecosystem Management for Sustainability: Principles and Practices Illustrated by a Regional Biosphere Reserve Cooperative, edited by J. D. Peine, pp. 267-290.  Lewis Publishers, Washington, D. C.

Pimentel, D. (1986) Biological Invasions of Plants and Animals in Agriculture and Forestry. In Ecology of Biological Invasions of North America and Hawaii, edited by H. A. Mooney and J. A. Drake, pp. 149-162. Springer-Verlag, New York.

Schlarbaum, S. E., R. L. Anderson, and F. Thompson Campbell.  (1999)  Control of Pests and Pathogens.  In Ecosystem Management for Sustainability: Principles and Practices Illustrated by a Regional Biosphere Reserve Cooperative, edited by J. D. Peine, pp. 291-306.  Lewis Publishers, Washington, D. C.

Vitousek, P. M., C. M. D’Antonio, L. L. Loope, M. Rejmanek, and R. Westbrooks.  (1997)  Introduced Species: A Significant Component of Human-Caused Global Change.  New Zealand Journal of Ecology  21 (1): 1-16.

Wilcove, D. S., D. Rothstein, J. Dubow, A. Philips, and E. Losos.  (1998)  Quantifying Threats to Imperiled Species in the United States – Assessing the relative importance of habitat destruction, alien species, pollution, overexploitation, and disease.  BioScience  48 (8): 607-615.


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