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.
Chersobius signatus is endemic to South Africa, occurring mainly along the West Coast region of the Western Cape and Northern Cape, from Piketberg and around Citrusdal in the south, northwards across the Olifants River into the Namaqualand Hardeveld to the Springbok-Steinkopf area. Eastwards, the distribution reaches as far inland as the Klipwerf-Loeriesfontein-Calvinia area of the Roggeveld-Bokkeveld region in the Northern Cape. The most northerly records are from the Richtersveld (Bauer and Branch 2001) and from just north of Pofadder (Branch et al. 2007). Griffin (2003) did not substantiate any C. signatus records from Namibia, including a record from the Fish River Canyon Park (Boycott and Bourquin 2000). Earlier records of this species from Namibia (Mertens 1955, 1971) are referable to C. solus (Branch 2007).
Chersobius signatus occurs predominantly in the winter rainfall region of the northwestern Succulent Karoo and Fynbos biomes along the West Coast and adjacent inland of South Africa. It is found from a few metres above sea level on the West Coast to elevations of around 1,000 m in the interior at Springbok, Loeriesfontein-Calvinia, and the Cederberg Range (Boycott 1989). The species shows a particular preference for rocky terrain (Loehr 2002a), which includes typical Namaqualand and Hardeveld granite koppies in the north, and typical Sandveld and Cederberg sandstone koppies and rocky ridges in the south. It occurs in low to medium-high Namaqualand succulent blomveld and heuweltjieveld, and fynbos and strandveld shrub vegetation, both in the Succulent Karoo and Fynbos biomes. The species prefers to shelter in rock crevices or under medium to large boulders and rock slabs (Loehr 2002a), a behavior that provides protection to these small tortoises against temperature extremes (Loehr 2017b) and predation. Chersobius signatus has very small home ranges (average 3,470 square meters = 0.35 ha) and short daily movement distances (30-50 m) compared to other tortoises (Loehr 2015).
Chersobius signatus is a very small tortoise species: straight carapace lengths are 52-110 mm and 52-96 mm for females and males, respectively. Growth rates are higher for juveniles and females than for males, but growth rates are low and affected by rainfall. Females need 11-12 years to mature, which is substantially longer than expected for such a small tortoise (Loehr et al. 2007a). Generation time is estimated as 25 years. During years of drought, individuals have low body conditions (Loehr et al. 2007b) and experience negative growth (they can shrink), which holds serious consequences for female fecundity if the occurrences of drought intensify in the future (Loehr et al. 2007a, 2009). Females produce one large egg (up to 11.9% of body volume) at a time and may lay more than one clutch during their nesting season in the spring. Rainfall influences egg production and fewer females are gravid in years of low rainfall (Loehr et al. 2011).
Chersobius signatus feeds on a broad range of plant species, which includes the flowers, leaves, and stems of forbs, grasses, succulents, and shrubs (Loehr 2002b, 2006). A 15-year study at a small site within a larger area with suitable habitat near Springbok indicated that the population consisted of transient and resident individuals, with a relatively high survival rate for adults, but slightly lower survival for females than for males (Loehr 2010, 2017a). It appeared that periodic droughts did not directly influence survival of juveniles and adults (Loehr 2010). Chersobius signatus basks for long periods to maintain relatively high body temperatures in the winter and spring, the time of the year when food is abundant. Females appear to have higher body temperature than males through most of the year, possibly to enhance follicle and egg development in winter and spring (Loehr 2012).
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.
The main threat to Chersobius signatus is habitat destruction and degradation. Observations and focused research throughout its known range indicate that this species is intolerant of habitat modification. Severe habitat fragmentation has resulted from extensive agricultural development throughout the range and especially in the Sandveld region. This includes the irreversible alteration of the inter-koppie (small hill) habitat—a zone that probably plays an important role in inter-population gene flow and recolonization of koppies that have lost their populations. Overgrazing by domestic stock, especially goats, further degrades and threatens remaining natural koppie and surrounding habitats, whereas an increase in mining activity has transformed sections of the coastland (see Bourne et al. 2012). Increased predation by Pied Crows poses a threat for which the extent remains to be determined (Loehr 2017a). There is illegal collection for export to the international pet trade, although this is infrequent. Climate change predictions are that most of the western Succulent Karoo would experience increasing temperatures and decreasing rainfall, 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 have a negative effect on growth rates and fecundity of individuals, and consequently the survival of the species.
Uses And Trade
Some illegal off-take has taken place during the past decades for the pet trade. The extent and trend of this is unknown.
Chersobius signatus is included in CITES Appendix II and is protected in South Africa at national level by the National Environmental Management Biodiversity Act, 2004 (Act No. 10 of 2004)(NEMBA) and at regional level by provincial nature conservation ordinances and biodiversity laws. The species occurs in low densities in two national parks (Richtersveld and Namaqua), and in the Oorlogskloof and Goegap nature reserves, and Cederberg wilderness area. Population numbers in Goegap are low with approximately one sighting every five years. There is a small-scale conservation breeding program in Europe under the auspices of the Homopus Research Foundation and the European Studbook Foundation. Chersobius signatus was listed as Restricted by Boycott (1988) and as Lower Risk/near threatened in the 1996 IUCN Red List. The status was changed to Vulnerable in 2014 (Baard and Hofmeyr 2014), mainly due to human-induced habitat degradation and destruction, and the international reptile pet trade. Further research is required to assess population status, habitat conditions, land transformation and threats throughout the species’ range and to ensure that it has sufficient representation within protected areas. A conservation stewardship arrangement should be encouraged to include more natural habitat in formal conservation arrangements, and measures to stop illegal collecting for the pet trade need to be strengthened.