Rationale
We base our assessment of Chersobius signatus on documented population reductions in the past and projected reductions into the future (A4). We estimate a population reduction of at least 30-40% over the past 25-50 years (1-2 generations) due to anthropogenic land transformation and other threats, where the causes of destruction have not ceased. The reduction in population size is based on surveys that showed habitat destruction and degradation, fragmentation, and the extirpation of populations, a decline in area of occupancy (AOO) and habitat quality, as well as an increase in predation by invasive Pied Crows. These declines are expected to be amplified by climate change. Predictions of climate change are that the western Succulent Karoo would persist under all scenarios, but temperatures will increase and rainfall decrease, causing an increase in aridity (Bourne et al. 2012). Loehr et al. (2007, 2009, 2011) demonstrated that the expected changes in rainfall pattern and temperature across the range are likely to affect growth rates and fecundity of C. signatus females, and consequently the survival of the species. When considering the past and projected future changes together, the decline in population size is projected to be in excess of 50%, qualifying the species as Endangered under criteria A4ace. Another contributing factor to population declines is that local populations become increasingly fragmented due to the extirpation of many populations and destruction of connecting terrain between their preferred rocky outcrop habitats.
Population trend
Trend
Chersobius signatus has a relatively small distribution and was previously known to be common in some areas. However, a long-term study in an area where the species used to be common showed that the resident population declined by 66% from 2000 to 2015, possibly due, in part, to increased predation by Pied Crows (Loehr 2017a). Field surveys in the early 2000s demonstrated the decline and finally extirpation of the species in many localities of its southwestern range, where it used to be common (E. Baard and M.D. Hofmeyr pers. obs.). The latter decline can be ascribed to substantial degradation of the habitat (Rouget et al. 2004), which has continued (Schoeman et al. 2013) due to overgrazing of the koppie habitats and transformation of the plains between the koppies. By virtue of the rock-dwelling behavior and specific habitat requirements (Loehr 2002a, 2012), and the small home ranges and short daily movement distances of the species (Loehr 2015), populations are highly fragmented, particularly in regions where low-lying habitat has been transformed. Population densities in most areas over the range of the species are low, and local populations are subjected to intense Pied Crow predation at several localities (M.D. Hofmeyr pers. obs.). The population near Pofadder requires further study to assess its extent and status.
In 2008, CapeNature and associates (University of the Western Cape) launched extensive surveys to update the population status of C. signatus in the southern and western part of its distribution range in the Western Cape Province. After visiting 22 localities where the species was known or suspected to occur, its presence was confirmed at 12 sites, including four new sites. These surveys indicated that the species was extirpated in some localities that previously had high population numbers and that extensive habitat degradation and destruction was placing remaining local populations under great threat (E. Baard 2015, unpubl. report to Turtle Conservation Fund). A follow-up survey by M.D. Hofmeyr (unpubl. data) showed that at least two more subpopulations in this southwestern region were extirpated within a short time period; the sites were overgrazed by goats with clear evidence of predation by Pied Crows.
Intermittent surveys in the northern range of the species between 2000 and 2015 (32 days) discovered only one site with a viable population; only one shell was recovered at another site (V. Loehr pers. obs.). Similarly, surveys above the escarpment (2000 to 2016) showed that the species is highly habitat-specific and usually occurs in low numbers (Hofmeyr unpubl. data). Over the range of the species, the frequency of predation by Pied Crows is increasing (Cunningham et al. 2016). The increased level of predation likely explains why a high-density population near Springbok declined by 66% over a 15-year period.
