Small and sensitive to drought: consequences of aridifiation to the conservation of Homopus signatus signatus

Abstract

Several climate models predict that the western Succulent Karoo in South Africa will aridi-fy. This region includes the range of the smallest tortoise, Homopus signatus signatus. Although the effects of rainfall on the physiology and ecology of H. s. signatus received attention in recent years, the results of these studies have not been integrated to facilitate conservation planning. Here we evaluate the importance of body size and the responses of H. s. signatus to rainfall variation to make recommendations for the taxon's conservation. The small body of H. s. signatus offers one solution to its habitat of low primary productivity and rocky slopes. Nevertheless, female fecundity and egg size increase with female size, and large eggs result in large hatchlings capable of surviving their harsh environment. Females accumulate nutrients in the rainfall season, winter, but also in the dry season, to enable the production of large eggs. Egg production decreases during drought, although some females continue to channel resources to reproduction, apparently at the cost of their own growth. Reduced fecundity and growth, a result of aridification, would likely lower the production of large eggs and hatchlings. Therefore, conservation measures that reduce the mortality of large females may aid population sus-tainability. Because egg and hatchling size might drop below a minimum viable size in an aridified environment, H. s. signatus conservation would benefit from the development of suitable habitat corridors to enable tortoise movements to regions that will receive sufficient rainfall in the future.