Disturbance and Invasive Plants: Introduction

Wild Horses on a Former Great Basin Shrubland Destroyed by Livestock Grazing, Invasive Plants, and Fire.

In the invasive plant literature, disturbance refers to an event that removes plants and alters the soil surface. “Disturbance is believed to be the major factor favoring plant introductions” (Radosevich et al. 2007: 58). Without disturbance, invasive plants would find no openings to become established and begin to spread and replace native species.

It is important to understand the nature and origin of disturbance that leads to plant invasions, because, as with global warming, it is often profitable to deny human responsibility for invasions so that a disturbance activity can continue.

Disturbance has natural causes and human causes. Here are some excellent discussions of the subject: Harper 1977, Kimmins 1997, Hobbs 1991, Humphries et al. 1991, Vitousek et al. 1996, Lounsdale 1999, Alpert et al. 2000, MacDougal and Turkington 2005, Milton 2003, Seabloom et al. 2003a b, Corbin and D’Antonio 2004.

Natural disturbance from floods, fires, weather events, and foraging animals are important, but most published research indicates human activities are responsible for most disturbance that open habitats to invasive plants. Livestock grazing, roads and trails, fire, and many other human-caused disturbance are the principal reasons that invasive plants become established (Radosevich et al. 2007).

Since floods, windstorms, landslides, and fires have always occurred, some plants have evolved physical and behavioral traits that allow them to cope with and even benefit from disturbance. We call these plants weeds. A problem in the garden, native weeds are beneficial for like band-aids, they protect injuries. Their rapid colonization of disturbed areas helps prevent soil erosion and preserve moisture.

Weeds, both native and introduced, are usually small, disperse rapidly, and must rebuild their above ground parts from seeds or roots every year. They are usually intolerant of shade. In time, the predisturbance residents that have long-lived above ground parts will replace the weeds (Grime 1979).

There aren’t many native weed species in North American deserts, and the number declines from the cool to the hot deserts. Prior to human introductions of foreign weeds, recolonizatiion in the hot deserts, the Sonoran and Chihuahuan, was by the shrubs and trees that were the predisturbance residents (Shreve 1964). Hot desert recolonization is slow, and can take more than a century (Rogers and Steele 1980).

Cheatgrass, the Scourge of the Great Basin Desert, was introduced from Eurasia in the late 19th Century by ranchers and travelers.

A few human-introduced weeds from Eurasian and African deserts are efficient colonizers of disturbed areas in all North American deserts. Some of the introduced weeds are able to invade surrounding areas of undisturbed natural vegetation. In most places, the native residents would eventually replace the introduced species except for one problem: The introduced weeds grow in such abundance that they provide a continuous cover of fine fuel. Thus, the introduced species add a major new factor to North American deserts—frequent fire.

Why are Introduced Weeds Better Colonizers?

Human conflicts with weeds probably began the first time humans began making clearings around homes or around desired plants. Primitive gardeners might have appreciated the valuable services performed by weeds, but they probably considered them pests just as we do today. As human populations grew and farming developed, conflicts with weeds must have increased. Some authors have argued that the longer history of human activities in Eurasia provided time for evolution of more and more weeds that could take advantage of the human disturbances (e.g., DeWet and Harlan 1975, Radosevich and Holt 1984, Di Castri 1989, Ellstrand 2003). It is true that most of the weeds around the world today originate in Eurasia.

I will review the principal types of disturbance in more detail in a future post.

References

Alpert, P., E. Bone, and C. Holzapfel. 2000. Invasiveness, invisibility and the role of environmental stress in the spread of non-native plants. Perspectives on Plant Ecology and Evolutionary Systematics 3:52-66.

Corbin, J. D., and C. M. D’Antonio. 2004. Competition between native perennial and exotic annual grasses: Implications for an historical invasion. Ecology 85:1273-1283.

De Wet, J. M. J., and J. R. Harlan. 1975. Weeds and domesticates: Evolution in the man-made habitat. Economic Botany 29:99-107.

Di Castri, F. 1989. History of biological invasions with special emphasis on the old world. Pages 1-30 in Drake et al., eds. SCOPE 37, biological invasions, a global perspective. John Wiley & Sons, New York. 506 p.

Ellstrand, N. C. 2003. Dangerous liaisons? When cultivated plants mate with their wild relatives. John Hopkins University Press, Baltimore, MD. 244 p.

Grime, J. P. 1979. Plant strategies and vegetation process. John Wiley and Sons, New York. 222 p.

Harper, J. L. 1977. Population biology of plants. Academic Press, New York.

Hobbs, R. J. 1991. Disturbance as a precursor to weed invasion to native vegetation. Plant Protection Quarterly 6:99-104.

Humphries, S. E., R. H. Groves, and D. S. Mitchell. 1991. Plant invasions of Australian ecosystems. A status review and management directions. Pages 1-127 in D. W. Walton, et al. eds. Plant invasions: the incidence of environmental weeds in Australia (Kowari). Australian National Parks and Wildlife Service, Canberra.

Kimmins, J. P. 1997. Forest ecology: a foundation for sustainable management. 2^nd ed. Prentice-Hall, Upper Saddle River, NJ.

Lonsdale, W. M. 1999. Global patterns of plant invasions and the concept of invasibility. Ecology 80:1522-1536.

MacDougal, A. S., and R. Turkington. 2005. Are invasive species the drivers or passengers of change in degraded ecosystems? Ecology 86:42-55.

Milton, S. J. 2003. Emerging ecosystems: A Washington-stone for ecologists, economists, and sociologists? South African Journal of Science 99:404-406.

Radosevich, S. R., and J. S. Holt. 1984. Weed ecology: Implications for vegetation management. Wiley, New York.

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

Rogers, G. F. and J. Steele. 1980. Sonoran desert fire ecology: Adaptive strategies of perennial plant species. U.S. Forest Service, General Technical Report RM‑81:15‑19.

Seabloom, E. W., E. T. Borer, V. L. Boucher, R. S. Burton, K. L. Cottingham, L. Goldwasser, W. K. Gram, B. E. Kendall, and F. Micheli. 2003a. Competition, seed limitation, disturbance, and reestablishment of California native annual forbs. Ecological Applications 13:575-592.

Seabloom, E. W., W. S. Harpole, O. J. Richman, and D. Tilman. 2003b. Invasion, competitive dominance, and resource use by exotic and native California grassland species. Proceedings of the National Academy of Science 100:13384-13389.

Shreve, F. 1964. Vegetation of the Sonoran Desert. Pages 6-186 in F. Shreve and I. L. Wiggins, Vegetation and flora of the Sonoran Desert. Stanford University Press, Stanford, CA. 1740 p. + 37 plates.

Vitousek, P.M., C.M. D’Antonio, L.L. Loope, and R. Westbrooks. 1996. Biological invasions as global environmental change. American Scientist 84:468-478.