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Walker R.C.J.,Nautilus Ecology | Walker R.C.J.,Open University Milton Keynes | Whitmore N.,Nautilus Ecology | Rafeliarisoa T.H.,University of Antananarivo | And 2 more authors.
Biological Conservation | Year: 2012

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 8years 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 λ=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 170years. 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. © 2012 Elsevier Ltd. Source


Walker R.C.J.,Nautilus Ecology | Walker R.C.J.,Open University Milton Keynes | Rafeliarisoa T.H.,University of Antananarivo | Rafeliarisoa T.H.,Grewcocks Center for Conservation and Research
ORYX | Year: 2012

The Madagascar spider tortoise Pyxis arachnoides is endemic to the coastal dry forests of south-west Madagascar. In recent years its range has been reduced by c. 71%, with the species now confined to eight fragmented populations occupying a total of 2,464 km2. These remaining populations are facing a significant threat of extinction because of habitat destruction and other anthropogenic pressures. We developed methodology for a line transect distance sampling survey and applied it systematically across the species' range. The resulting distance model estimated a mean density of 226.9 tortoises km-2 (95% confidence interval, CI, 168.1-306.3) and a total population of 664,980 (95% CI 492,680-897,550). Fragmentation of the species' range suggests the current population could be <30% of the historical population. Of the remaining population 73.5% falls within protected areas. However, nine of these 12 protected areas are designated as IUCN category III, V or VI parks, allowing some extractive activities to be undertaken. The most effective strategy for the conservation of P. arachnoides would be to reduce the threats to this species and its habitat, and to develop and expand the current community-based conservation and poverty alleviation programmes in the region. © 2012 Fauna & Flora International. Source


Walker R.,Nautilus Ecology | Walker R.,Open University Milton Keynes | Luiselli L.,Centro di Studi Ambientali Demetra Srl | Rafeliarisoa T.,University of Antananarivo | Rafeliarisoa T.,Grewcocks Center for Conservation and Research
Amphibia Reptilia | Year: 2012

The spider tortoise (Pyxis arachnoides) is endemic to the coastal, dry forests of southwest Madagascar, one of the country's most threatened habitats. Very little is known of the biology of this Critically Endangered species. We devised a three year capture-mark-recapture study to assess mean annual survival for the following cohorts; juvenile, adult male, adult female and adult of both sexes, for a population of spider tortoises using a Cormack-Jolly-Seber model. Low recapture probabilities prevented the modelling of all but the adult of both sexes cohort. Mean annual survival was 0.823 (SE = 0.15; 95% CI = 0.565-1.0). We hypothesise that habitat loss could be impacting the survival of this cohort; however establishing control data to test for this is difficult because of the widespread habitat loss, even within protected areas. Therefore we suggest extending the study across a number of gradients of impacted habitat and increase the duration of the study to assess this risk to the population and improve model robustness. © 2012 Koninklijke Brill NV, Leiden. Source


Walker R.C.J.,Nautilus Ecology | Walker R.C.J.,Open University Milton Keynes | Rafeliarisoa T.H.,University of Antananarivo | Rafeliarisoa T.H.,Grewcocks Center for Conservation and Research
Chelonian Conservation and Biology | Year: 2012

The radiated tortoise (Astrochelys radiata) is facing a significant risk of extinction within the next few decades as a direct result of poaching due to the increasing popularity of the species as bush meat. Sampling across the species range revealed a mean carcass encounter rate of 2.2/km of transect, with highest levels of poaching within the Linta and Menarandra river regions. No poaching was recorded within the 2 protected areas within the core of the species range. Improved local law enforcement, in addition to community-based conservation initiatives, is critical to combating this crisis. © 2012 Chelonian Research Foundation. Source


Walker R.,Nautilus Ecology | Walker R.,Open University Milton Keynes | Rafeliarisoa T.,University of Antananarivo | Rafeliarisoa T.,Grewcocks Center for Conservation and Research | And 3 more authors.
Herpetological Journal | Year: 2014

Southern Madagascar supports two of the world's most threatened chelonians, Pyxis arachnoides and Astrochelys radiata, both thought to be rapidly declining as a result of habitat loss and poaching; however, to date quantitative data on this decline is lacking. We applied a conventional distance sampling procedure, monitoring populations twice over a 24-month period across the species' respective distributions. Population density for P. arachnoides dropped from 2.4 tortoises/ha (95% CI; 1.6-3.4) to 1.5/ha (95% CI; 0.8-2.8). Astrochelys radiata dropped from 2.1 tortoises/ha (95% CI 1.2-3.5) to 1.5/h. (95% CI 0.7-3.3). Local community-based conservation initiatives need to be expanded to combat this decline. Source

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