Karoo dwarf tortoises (Chersobius boulengeri) prefer and disperse doll's roses (Hermannia spp.)

Abstract

Tortoises inhabit arid regions where they may represent a significant herbivorous component, but are among the most threatened of tetrapod groups. Studying tortoise diets can help predict potential trophic disruptions following extinctions of tortoise populations, and inform conservation. We studied the diet of Karoo dwarf tortoises (Chersobius boulengeri), an endangered endemic of the arid Nama and Succulent Karoo biomes in South Africa, using vegetation analysis, DNA metabarcoding, morphological evaluation of faeces, a germination trial, and focal feeding observations. DNA metabarcoding was highly successful in the identification of dietary items. Karoo dwarf tortoises fed on a broad diet, including approximately 10 taxa eaten frequently. Specifically, doll's roses (five species in the genus Hermannia) were eaten more frequently than would be expected based on their availability, whereas grasses were rejected, for a dietary Proportional Similarity Index of 0.31–0.38. Dietary selection was likely related to water and nutrient contents of plant taxa. At least four plant species (including doll's roses) germinated from faecal samples, suggesting mutualistic relationships with Karoo dwarf tortoises. The habitat preference of Karoo dwarf tortoises (i.e., well-structured rocky terrain) may provide germination microsites with ample shade and run-off water to benefit seedling recruitment in an arid environment.