Twenty-Third Annual Meeting and Symposium of the
Desert Tortoise Council, April 3-5, 1998
Abstracts

The Thicket Biome, in the Eastern Cape, South Africa, supports four species of tortoises: the leopard tortoise Geochelone pardalis, angulate tortoise Chersina angulata, tent tortoise Psammobates tentorius and parrot-beaked tortoise Homopus areolatus. This study focused on the most abundant and larger species, the leopard and angulate tortoises in order to gain an understanding of their ecology and impact in this biome. Tortoises were radiotracked over a year. Dietary information was collected by direct observation and the analysis of feces, and the potential for seed dispersal was estimated from the occurrence of intact seeds in the feces of leopard tortoises.

Although leopard tortoises had large home ranges (mean = 105.4 ha, range = 11.6-443.6 ha, n = 10), 70 % of their activity was concentrated in activity nodes comprising on average only 1.4 % of the area of their home range. These activity nodes had a higher frequency of geophytes than non node areas. Angulate tortoises had much smaller home ranges (mean = 2.66 ha, range = 0.12 - 12.18 ha, n = 7), but again 70 % of their time was concentrated in nodes averaging 2.37 % of the home range area. Angulate tortoise nodes were dominated by trees and dense foliage.

Leopard tortoises consumed 28 plant species, with 6 species comprising 75 % of the diet. The principal food items were the geophyte Albuca sp. (40 % of the diet) and the couch grass Cynodon sp. (11 % of the diet). The forb Abutilon sonnerotianum and the alien invasive cactus Opuntia ficus-indica were preferred food items. In terms of plant consumption in relation to availability, these tortoises may be considered intermediate between trophic generalists and specialists, as they consume a relatively wide range of plants, but concentrate on a few of these.

An average of 114 Opuntia seeds and 39 Cynodon seeds were recovered per fecal sample. Given that the gut passage time of leopard tortoises is on average two weeks, and the extensive daily travel (mean = 600 m/day) by these tortoises, there is great potential for long distance seed dispersal. This may play a significant role in the dynamics of these alien invasive plants.

Elephants and domestic goats have been blamed for the loss of some geophytes in the Thicket Biome (Moolman & Cowling 1994). In the light of the importance of these geophytes in the diet of leopard tortoises, we hypothesize that tortoises could be playing a role in the observed loss of these geophytes, facilitated by habitat modification brought about by elephants or goats. This will be the focus of the next phase of this study.

References

Moolman, H.J. & Cowling, R.M. 1994. The impact of elephant and goat grazing on the endemic flora of South African succulent thicket. Biol. Cons. 68:53-61.