Individual study: The effect of grazing on the Texas tortoise Gopherus berlandieri, Chaparral Wildlife Management Area, Texas, USA
Kazmaier R.T., Hellgren E.C., , & Synatzske D.R. (2001) Effects of grazing on the demography and growth of the Texas tortoise. Conservation Biology, 15, 1091-1101
Overly intense, uncontrolled livestock grazing have been blamed for declines in Horsfield's Testudo horsfieldi, Mediterranean spur-thighed T. graeca and chaco Geochelone chilensis and desert Gopherus agassizii tortoises, as well as more general reductions in native species richness and diversity, increased dominance of alien species and degradation of the landscape though erosion and soil compaction. In the arid and semiarid shrublands of western USA, grazing has reduced the grass and herbaceous communities but enhanced invasion of woody plants. In this study, the effect of livestock grazing on the Texas tortoise Gopherus berlandieri, which is mainly restricted to the privately owned arid and semiarid shrublands of southern Texas and northeastern Mexico, is investigated.
Study site: The Chaparral Wildlife Management Area in the western Rio Grande plains of Texas, southeast USA, consists of 15 pastures of 260-750 ha. The area is dominated by mesquite-acacia Prosopis-Acacia thornscrub, with abundant prickly pear cactus Opuntia lindheimeri and a community of native and alien grasses, and forbs.
In 1983, 13 pastures were surrounded by a wire fence to prevent grazing by livestock. Cattle grazing was reintroduced from 1991, with a one-herd (400 steers), dormant season (early October to May), short-duration rotational (3-8 weeks) grazing system was initiated for each grazing pasture. In 1984, the remaining two pastures were also fenced and grazing was permanently prevented.
Experimental design: Two ungrazed and two grazed pastures were selected. Tortoises were censused by driving along tracks (recording the distance and time travelled) throughout the pastures between 7 April 1994 and 12 October 1997. The four sites had similar road densities (4.4-4.8 km/km²) and only interior tracks were censused. In total, 182 hours and 845 km of track were censused in the ungrazed pastures, and 224 hours and 2,168 km in the grazed pastures. The number of tortoises captured was recorded for each site. Each individual was sexed, aged, carapace length measured, and marked before release.
Tortoise captures: There were 132 captures of 106 individuals in the ungrazed pastures and 324 captures of 237 individuals in the grazed pastures.
Tortoise relative abundance: The relative abundance of tortoises did not differ significantly between grazed and ungrazed pastures. Based on distance travelled, there was an average of 4.7 (± 1.6 SE) tortoises/100 km travelled in grazed pastures and of 4.0 (± 1.1) tortoises/100 km travelled in ungrazed pasture. Based on time searching, there was an average of 4.3 (± 1.3) tortoises/10 hours travelled in grazed pastures and of 3.2 (± 1.1) tortoises/10 hours travelled in ungrazed pasture.
Tortoise life history traits: There was no difference in size, sex or age structure between grazed and ungrazed pastures.
Conclusions: These results suggest that Texas tortoises are tolerant to moderate levels of livestock grazing. However, the authors suggest that these results cannot be extrapolated to other American tortoise species.
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