Invasion of Exotic Grasses/Changes in Land Use

Invasion of Exotic Grasses/Changes in Land Use

Invasive exotic grasses have occurred on every continent and can alter ecosystems by replacing native species with a dominant single species.  They compete by altering environmental conditions or resource availability.  Many of these species tolerate and enhance fires following which they are able to extend dominance by rapid growth.  While grasses can invade undisturbed native vegetation areas as well, exploiting disturbed areas is their strength.

In North America, invasions are most common in the arid west and Great Basin.  Introduction of European annuals may not have been planned but occurred with introduction of sheep and cattle.  One of the worst grasses is the European annual, Bromus tectorum, or cheatgrass that has widely replaced native bunchgrasses (Wolfe 2005). It is a winter annual that germinates in fall, grows in winter and spring, produces lots of seeds for its soil seed bank, and dies in early summer.

                                                    Bromus tectorum

Perennial grasses may have been introduced as vegetation for erosion control or for grazing land because the grasses tolerated the defoliation by ungulates better than native grasses.  Now that grazing no longer occurs over much of this land, the grasses have spread widely.  Crested wheat grass (Agropyron desertorm) and bufflegrass (Cenchrus ciliarus) are examples (Rogler 1983).

      

                                                         Agropyron desertorm

Invasive grasses have effects at multiple levels of ecological organization from population to ecosystem.  At the population level, they absorb light in their canopy and reduce water and nutrient availability to other species (Vasquez 2008).  At the ecosystem level, they alter the boundary humidity, reduce nutrient mineralization, and alter the fire regime.  Invasion of woody perennials in dense grasslands is unusual unless there is a disturbance.  By drawing down soil moisture with their dense shallow roots, invasive grasses are more adept at suppressing water-sensitive oak seedlings and preventing successional change (D’Antonio 1992).  However, saplings and adult woody species with larger roots can reach and obtain water and nutrients.  Another difference favoring the invasive grass A. desertorum over native grasses is its high seed output, lower seed predation, and large seedbank store.  Loss of plant diversity by grass invasions is accompanied by a loss of animal population diversity due to alterations of food supply or habitat.

                  Ammophila arenaria                                                 Eragrostis lehmanniana

European beachgrass (Ammophila arenaria) was introduced for erosion control but has eliminated native plant species along the coast of California and Oregon; furthermore, a few rare insect species that depended on these native plants have also been eliminated (Slobodchikoff 1977).  Lehmann lovegrass (Eragrostis lehmanniana) from South Africa has replaced native shrubs and herbs in the Sonoran desert, and reduced the population of native birds (scaled quail) and insects in the ecosystem (Medina 1987).  In Idaho and Wyoming, cheatgrass and other invasive plants have replaced sagebrush with the resulting loss of the sage grouse, jackrabbit, prairie ground squirrel, and their predators, the prairie falcon and golden eagle (Knick 2003).  Alteration of ecosystem effects (both increases and decreases) such as nitrogen fixation, litter quality, and decomposition rate, as well as allelopathic suppression of nitrifying bacteria, have been documented with invasive alien grasses.   Invasive grasses can affect the microclimate of an ecosystem by their production of dense litter that holds onto moisture and allows germination of seeds and sapling growth.  On the other hand, their shallow canopy and smooth aerodynamics, compared to forests or woodlands, leads to higher canopy and surface temperatures and lower relative humidity.  These conditions favor the growth of plants with C₄ photosynthesis (usually grasses) and fires to be discussed in the next blog (D’Antonio 1992). The invasive grasses result in loss of diversity and habitat, and cause regression of successional changes. 

References

1. D’Antonio, Carla M., Vitousek, Peter M., (1992). Biological Invasions by Exotic Grasses, the Grass/Fire Cycle, and Global Change. Annual Review of Ecology and Systematics.

2.  Steven T. Knick, et al. (2003). Teetering on the Edge or Too Late? Conservation and Research Issues for Avifauna ofSagebrush Habitats.  The Condor. 105:611-634

3. Medina, A. L., Diets of Scaled Quail in Southern Arizona. The Journal of Wildlife Management. 52:753-757.

4.  Rogler, G. A., Lorenz, R.J., (1983). Crested Wheatgrass– Early History in the United States. Journal of Range Management. Vol. 36:91-93.

5.  Slobodchikoff. C. N. and Doyen, John T. (1977). Effects of Ammophila Arenaria on Sand Dune Arthropod Communities. Ecology. 58:1171-1175. 

6. Vasquez, Edward, Sheley, Roger, Svejcar, Tony, (2008). Nitrogen Enhances the Competitive Ability of Cheatgrass (Bromus tectorum) Relative to Native Grasses. Invasive Plant Science and Management.

7. Wolfe, Benjamin E., Klironomos, John N., (2005). Breaking New Ground: Soil Communities and Exotic Plant Invasion. BioScience. 55:477-487.