"Generalities about the effect of cattle grazing on North American tortoises should be avoided," say of Richard Kazmaier Oklahoma State University in Stillwater, Oklahoma, and his co-authors. This research is in the August issue of Conservation Biology.
Most tortoises live in dry areas that are ideal for rangeland, and livestock grazing has been implicated in the decline of tortoises in Argentina, southern Morocco and the Turkmen Republic. Grazing is also thought to threaten the desert tortoise, one of four tortoise species in North America. However, there is little direct evidence to support this belief.
Kazmaier and his colleagues studied the effects of moderate grazing on the Texas tortoise, which lives primarily on privately-owned rangelands in southern Texas and northeastern Mexico. The researchers compared tortoise populations in grazed and ungrazed pastures in the 6150-ha Chaparral Wildlife Management Area in southern Texas. Each grazed pasture had up to 23 steers/acre for a three-eight week rotation between October and May, when the tortoises typically hibernate.
To compare the tortoise populations, Kazmaier and his colleagues evaluated a variety of factors, including abundance, size, growth rate and adult survival. They found no differences in any of these factors between tortoises in grazed and ungrazed pastures, and so concluded that this moderate grazing regime has little if any effect on them.
However, the researchers stressed that there are some caveats. For instance, although the study area had been grazed previously, the current grazing regime began only six years ago. Thus, this work would not reveal any long-term effects of moderate grazing on the Texas tortoise and its habitat. Second, they caution that their work should not be applied to the desert tortoise, in part because the Texas tortoise evolved in presence of large grazers such as bison but the desert tortoise did not.
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Kazmaier's co-authors are: Eric Hellgren of Oklahoma State University in Stillwater, Oklahoma; and Donald Ruthven and David Synatzske of the Texas Parks and Wildlife Department's Chaparral Wildlife Management Area in Artesia Wells, Texas.
*Richard Kazmaier (830-676-3413, firstname.lastname@example.org)
*Eric Hellgren (405-744-9671, email@example.com)
*Donald Ruthven (830-676-3413, firstname.lastname@example.org)
*David Synatzske (830-676-3413, email@example.com)