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

Eberle, Dirk (2003-12)

Thesis (MSc)--Stellenbosch University, 2003.

Thesis

ENGLISH ABSTRACT: The geometric tortoise, Psammobates geometricus, found in the Western Cape, South Africa, is one of the world's rarest tortoises. At present, it is only known from five isolated geographical areas, one being the Worcester-Tulbagh Valley. The primary aims of my study were to determine the status of geometric tortoise subpopulations along the Nl highway in the Worcester Valley, to determine the population structure of these subpopulations and, if necessary, to provide recommendations for the mitigation ofNI highway-induced impacts on geometric tortoise subpopulations along the road. All Renosterveld remnants along the Nl between Du Toitskloof and Worcester were surveyed for the presence of geometric tortoises, and tortoise density and population structure were determined for each remnant that supports tortoises. Two geometric tortoise subpopulations on opposite sides of the Nl were identified. In the larger remnant of the two, females dominated both the subadult and adult classes, whereas in the other remnant male frequency was slightly higher than female frequency for the adult class only. In both remnants, adult females were significantly larger than adult males in mean carapace length. Mean body mass for adult females and males also differed significantly in both remnants. Analysis of habitat quality showed that both remnants supporting the geometric tortoise are seriously infested by encroaching indigenous and alien plants. The barrier effect of the Nl highway can be mitigated by constructing underpasses, linking the two remnants on opposite sides of the road. Furthermore, road mortality of tortoises can be avoided by erecting tortoise proof fencing along appropriate sections of the highway in the Worcester Valley. By linking the remnants, the largest single area available to the geometric tortoise in the Worcester Valley will also be formed. An additional aim of my study was to investigate the correspondence of geometric tortoise density in individual habitat remnants to the West-to-East change in vegetation composition/characteristics occurring along the Nl in the Worcester Valley. Vegetation composition/characteristics were thus determined in all remnants also surveyed for tortoises. The West-to-East change in vegetation composition/characteristics of the study area indicates that there is a transition from Renosterveld in the West to karroid veld in the East.

AFRIKAANSE OPSOMMING: Die geometriese skilpad, Psammobates geometrieus, wat in die Wes-Kaap, Suid- Afrika, voorkom, is een van die wêreld se skaarste skilpaaie. Vandag word hierdie skilpad net nog in vyf geïsoleerde geografiese streke gekry, een van hulle die Worcester-Tulbagh Vallei. Die primêre doel van my studie was om die status van geometriese skilpad subpopulasies langs die Nl snelweg in die Worcester Vallei te ondersoek en om die populasiestruktuur van hierdie subpopulasies te bepaal. Verder was die doelook, om aanbevelings te maak oor hoe die impakte van die pad op die geometriese skilpad subpopulasies langs die Nl verminder kan word. Al die Renosterveld oorblyfsels langs die Nl tussen Du Toitskloof en Worcester, en binne die bekende gebied van die geometriese skilpad, is deursoek vir geometriese skilpaaie. Populasiedigtheid en populasiestruktuur van die skilpaaie is bepaal vir elke oorblyfsel wat skilpaaie bevat. Twee geometriese skilpad subpopulasies is gekryaan teenoorgestelde kante van die Nl. In die groter oorblyfsel, het wyfies albei die onvolwasse en volwasse klasse gedomineer. In die ander oorblyfsel is gevind dat die frekwensie van mannetjies ietwat hoër was as die frekwensie van wyfies, maar vir slegs die volwasse kategorie. Binne albei oorblyfsels is gevind dat volwassse wyfies betekenisvol groter in gemiddelde doplengte is as volwasse mannetjies. Gemiddelde liggaamsgewig het ook betekenisvol verskil tussen volwasse wyfies en mannetjies in albei oorblyfsels. 'n Analise van habitatkwaliteit het gewys dat beide oorblyfsels wat die geometriese skilpad ondersteun grootliks ingeneem is deur inheemse indringers, sowel as uitheemse plante. Die versperrings effek van die Nl snelweg kan verminder word deur duikwege te skep wat die twee ooblyfsels, op teenoorgestelde kante van die pad, verbind. Bowendien, deur 'n ondeurdringbare heining langs passende dele van die snelweg in die Worcester Vallei op te rig, sal skilpadmortaliteit vermy word. Deur oorblyfsels te verbind, salook die grootste enkele area, beskibaar aan die geometriese skilpad in die Worcester Vallei, gevorm word. 'n Addisionele doel van hierdie studie was om veranderinge in vegetasie langs 'n wes-na-oos gradient binne die studie area te korreleer met die teenwoordigheid van skilpaaie langs hierdie gradient. Vegetasie samestelling/eienskappe is dus bepaal vir elke oorblyfsel wat deursoek is vir skilpaaie. Die wes-na-oos verandering in vegetasie samestelling/eienskappe binne die studie area wys dat daar 'n oorgang van Renosterveld in die weste na 'n karooagtige veld in die ooste is.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/53639
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