The effect of habitat degradation on the long term survival of the critically endangered Madagascar spider tortoise (Pyxis arachnoides)
Pyxis arachnoides inhabits the dry, coastal forests of southwest Madagascar; a biologically unique ecoregion severely threatened by unsustainable small scale agricultural practices. Using remotely sensed data we established vegetation loss remains unabated within our study area at 1.2% year-1; consistent with ecoregion wide vegetation loss between 1990 and 2000. We monitored tortoise population density on four occasions over 8 years and developed a stage class projection matrix to model the finite growth rate, matrix sensitivities and elasticities of the population. Monitoring revealed an actual mean population decline of 10.8% between 2003 and 2011. Our projection matrix model suggested the finite rate of growth to be ʎ = 0.986, indicative of 1.4% year-1 decline for the duration of the existence of the population, with adult survival as the most sensitive parameter to overall survival of the population. Projection modeling suggests that the population would possibly become functionally non viable in approximately 170 years. As our study was heavily reliant on the use of surrogate data, more information is needed on pre-adult survival and wild reproductive rates. Our study site currently represents a population of exceptional density, therefore, other less dense populations, subjected to the further stresses of poaching will likely become functionally non viable much sooner. This study highlights the utility of population projection matrices in determining the vulnerability of chelonians; a globally threatened taxon. In the case of the spider tortoise, well coordinated development programs and tighter protected area management are required to address the poverty induced drivers forcing this species closer to extinction.
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