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In the desert southwest vast areas of desert tortoise habitat have been invaded by exotic annual grasses. Recent research has demonstrated that some exotic pest plants have lower nutritional value for desert tortoises than native plant species. Although it is tempting to speculate that exotic pest plants may have a negative impact on desert tortoise populations by virtue of their nutritional inferiority, other effects of exotic pest plants may be more important. Desert tortoise habitat is characterized by communities of widely-spaced, long-lived, fire-sensitive shrub species and in some areas cactuses. Although fire had a role in the evolution of the desert plant community, it was probably minor with long intervals between fires and limited impact. With the invasion of fine-fuel species like exotic annual grasses the fire cycle has been significantly shortened and the potential for fires to propagate has increased. The result has been conversion of desert scrub landscapes to "weedscapes" dominated by exotic pest plants. It is hard to imagine that this has not had a negative impact on tortoises and other species that coevolved with plants in the desert ecosystem.
Weeds have had an impact on other tortoises as well. In Florida, habitat for the gopher tortoise, a Species of Special Concern, is being degraded by exotic pest plants. Gopher tortoises prefer areas with relatively low canopy and shrub cover but high herbaceous cover. Exotics like Brazilian pepper cause the percentage of shrub and canopy cover to increase thus causing tortoise densities to decline. Unfortunately, areas cleared of Brazilian pepper are easily recolonized by the invader making management of tortoise populations that much more difficult (Stewart et al., 1993).
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