Structure and projected decline of a Karoo dwarf tortoise population

Abstract

Inconspicuous, secretive, or sparsely distributed species receive relatively little research attention, potentially leading to uncertainty about their status and lack of efforts to conserve them. Karoo dwarf tortoises (Chersobius boulengeri) are endemic to South Africa, spend most of the time in retreats at remote arid locations, and are seldom seen. We conducted a 3‐year (2018–2020) mark‐recapture study to investigate the size and structure of the only Karoo dwarf tortoise population currently known to exist. The population in the 16‐ha core of our study site consisted primarily of adult males and females, at a density of 3.3 individuals/ha. Many individuals had severely worn shells and appeared old. Small individuals (straight carapace length <65 mm) represented just 8% of the population and were mostly recent hatchlings. Overall, tortoises had high estimated survival rates (0.77–0.95; lower 95% confidence limit for the smallest tortoises was 0.16), despite a 15‐month drought. The lack of small individuals may reflect low levels of recruitment and population decline. Predation by corvids was an obvious threat to all size classes. We estimated that the local population across the 250‐ha study area was 800–900 males and females, and recommend precautionary conservation measures focused on reducing human‐subsidized avian predation.

Habitat Use by the Rock-Dwelling Karoo Dwarf Tortoise, Chersobius boulengeri

“…Karoo Dwarf Tortoises are seldom seen (Loehr and Keswick, 2022) and only a few studies have been conducted. Previous workers have qualitatively described their habitat as ''rocky ridges and outcrops'' (Boycott and Bourquin, 2000) or ''dolerite ridges'' (Branch, 2008).…”

“…In contrast to sills, plateaus harbored few Karoo Dwarf Tortoises. This species (Loehr, 2022; Loehr and Keswick, 2022) and other tortoises (Kristan and Boarman, 2003; Loehr, 2017; Segura et al, 2020) are depredated by birds, and relatively open plateaus expose them to predators. The low frequency of tortoises that I observed on plateaus may reflect past predation or avoidance by the tortoises.…”

“…Less overlap among male Karoo Dwarf Tortoises, compared to females, may have resulted from the large physical spaces between males that were equipped with radio transmitters (Supplemental Fig. A; see Data Accessibility); 22% of the males and 66% of the females present at the core of the study site (Loehr and Keswick, 2022) were tracked, so several overlapping home ranges may have been missed. However, male Karoo Dwarf Tortoises vigorously attack, ram, and bite other males, and may chase subordinate males for >10 m (Supplemental Video A; see Data Accessibility).…”

“…Although recordings were made during the summer mating season, males did not seem to travel greater distances than females to find mates, as has been found in other tortoise species (Lagarde et al, 2003; Hofmeyr et al, 2012). Despite similar MDMDs in male and female Karoo Dwarf Tortoises, associated predation risks may be larger for males that represent the smaller sex, because avian predation appeared to be targeted at smaller individuals (Loehr and Keswick, 2022).…”

“…More recent surveys suggest that populations are being extirpated, resulting in the species being classified as Endangered (Hofmeyr et al, 2018). At present, only one population is known to contain males, females, and juveniles (Loehr and Keswick, 2022). The deteriorating conservation status of the species amplifies the need for information about habitat use, to help conservationists assess if nature reserves with incidental records (i.e., Gamkapoort and Anysberg, and Karoo National Park; Hofmeyr et al, 2018) and potential new reserves are likely to sustain populations, and to inform surveys for additional populations.…”