The geometric tortoise (Psammobates geometricus) in a fragmented habitat along a national highway : status and mitigation

[摘要] ENGLISH ABSTRACT:The geometric tortoise, Psammobates geometricus, found in the Western Cape, SouthAfrica, is one of the world's rarest tortoises. At present, it is only known from fiveisolated geographical areas, one being the Worcester-Tulbagh Valley. The primaryaims of my study were to determine the status of geometric tortoise subpopulationsalong the Nl highway in the Worcester Valley, to determine the population structureof these subpopulations and, if necessary, to provide recommendations for themitigation ofNI highway-induced impacts on geometric tortoise subpopulationsalong the road. All Renosterveld remnants along the Nl between Du Toitskloof andWorcester were surveyed for the presence of geometric tortoises, and tortoise densityand population structure were determined for each remnant that supports tortoises.Two geometric tortoise subpopulations on opposite sides of the Nl were identified. Inthe larger remnant of the two, females dominated both the subadult and adult classes,whereas in the other remnant male frequency was slightly higher than femalefrequency for the adult class only. In both remnants, adult females were significantlylarger than adult males in mean carapace length. Mean body mass for adult femalesand males also differed significantly in both remnants. Analysis of habitat qualityshowed that both remnants supporting the geometric tortoise are seriously infested byencroaching indigenous and alien plants. The barrier effect of the Nl highway can bemitigated by constructing underpasses, linking the two remnants on opposite sides ofthe road. Furthermore, road mortality of tortoises can be avoided by erecting tortoiseproof fencing along appropriate sections of the highway in the Worcester Valley. Bylinking the remnants, the largest single area available to the geometric tortoise in theWorcester Valley will also be formed. An additional aim of my study was toinvestigate the correspondence of geometric tortoise density in individual habitatremnants to the West-to-East change in vegetation composition/characteristicsoccurring along the Nl in the Worcester Valley. Vegetationcomposition/characteristics were thus determined in all remnants also surveyed fortortoises. The West-to-East change in vegetation composition/characteristics of thestudy area indicates that there is a transition from Renosterveld in the West to karroidveld in the East.

[发布机构] Stellenbosch University