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Submodule 8: Ecological Impacts
Change in Fire Regime
Plants are the predominate group of invasive species relating to fires. Their presence increases the incidence of fire in forests and rangelands, and the disturbances resulting from such fires often leads to increased opportunities for invasive plants to take hold. In fact, invasive plants that modify fire regimes cause some of the most extensive ecosystem changes known. Non-native plants can change the fire regime to such an extent that native plants switch from their roles as fire suppressors to fire promoters. Invasive plants that enhance fuel availability can initiate a vicious cycle, actually promoting their own invasiveness while further decreasing the presence of natives, especially those that are more fire-retardant.
Cheat grass (photo courtesy PLANTS
database)
Cheat grass or downy brome (Bromus tectorum) is an annual grass from Eurasia that is now found throughout the U.S.; it is most problematic in the western rangelands where it has greatly altered fire patterns. This grass grows during the cool months of winter and spring forming prominent hairy leaves. It quickly dries out leaving numerous seed-filled awned spikelets and providing a large amount of fuel. Once fire has swept through such an area, cheat grass can rapidly germinate and produce seeds while native perennial grasses struggle to become re-established. The presence of cheat grass increases the occurrence of fire from every 60-100 years in native rangelands to every 3-5 years. Once this regime has been established, native plants are not able to become re-established without extensive intervention using primarily chemical control methods to eradicate the cheatgrass.
Pre-fire fuel treatment techniques can be very effective for suppressing fires. Unfortunately, they can also serve as a means of introducing and establishing non-native plants to treated areas. To decrease the amount of fuel for wildfires, governmental agencies perform “controlled burns” and tree thinning to remove accumulated plant materials; this creates “disturbed land” which can provide an avenue for the subsequent introduction of invasive plants. This occurs because during any fire, wild or controlled, soils become disturbed, light levels increase in the under story, and nutrient and water cycles change.
Invasive plants can also affect other living organisms, including mammals, birds, fish and microbes. Modification of flora in a given habitat can lead to a decrease in native animal populations. Altered nutrient cycles can interfere with the function of bacteria and fungi, the principal “nutrient recyclers” of all ecosystems. The long-range effects of these and other ecosystem changes are not well understood.
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