Assessing the global conservation status of ecologically poorly undertood, threatened chelonian species : the case of the critically endangered Madagascar spider tortoise (Pyxis arachnoides)

Walker, Ryan Campbell James (2012). Assessing the global conservation status of ecologically poorly undertood, threatened chelonian species : the case of the critically endangered Madagascar spider tortoise (Pyxis arachnoides). PhD thesis The Open University.



The IUCN Red List for Threaten Species is widely regarded as the authority for assessing a particular species conservation risk. However, this process has come under some criticism for designating species as high conservation risk on a precautionary principle, using limited data, grey literature or anecdotal information. The Madagascar spider tortoise (Pyxis arachnoides) is one such species, up-listed from Vulnerable to Critically Endangered status, based on data of limited spatial resolution and anecdotal information, suggesting risk from poaching and habitat loss, resulting in significant reduction in population size, thus allowing the criteria of Critically Endangered to be met (A4cd; E). This study assesses these risks to the species at a range wide level, using presence/absence surveying and incorporating data into a geographical information system to describe distribution contraction within differing areas of land use. This was coupled with a line transect distance sampling procedure to assess real time population size within the current area of occupancy. A Mark-Capture-Recapture, Cormack-Jolly-Seber model and a stage class, projection matrix was developed to model mean annual survival, finite growth rate and population survival. Finally, remotely sensed habitat loss was monitored within the core of the species range. These data suggest that the species has suffered a reduction of 70. 8% from suspected historical extent of occurrence to the current area of occupancy, now inhabiting a total area of 2,463.8 km2, with 73.5% of the current distribution occurring within existing or proposed protected areas. Range wide population density equals 226.9 (95% Cl 168.1-306.3) tortoises km-2, with a total global population estimate of 664,980 (95% Cl. 492,680-897,550). Adult mean annual survival is 0.82 (SE± 0.15), however juvenile survival cannot be modelled due to low recapture. There is an actual mean population decline of 1 0.8%, coupled with mean forest loss of 1.2% yr-1 (2003-2011), within the core of the range. The projection matrix model reveals finite population growth to be λ=0.983, indicative of a c. 1.4% decline annually, with adult survival the most sensitive parmeter to population survival. 'Projection modelling suggests the species will probably become ecologically non-viable in approximately 170 years. Results confirm the species IUCN Critically Endangered status. Development programs 'and tighter protected area management are required to address the poverty induced drivers threatening this species. This assessment provides an ideal model for similar dry forest tortoise species suffering status data deficiency. The author proposes a temporary five year condition to be placed on the designation of species considered to be of high conservation concern, similar to this species, up-listed on the Red List using anecdotal information or grey literature, whilst funds are secured and empirical peer reviewed data is obtained to confirm the status in the wild.

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