Literaturdatenbank
Literaturdatenbank |
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O'Brien, S. H. (2002). Population dynamics and exploitation of the radiated tortoise geochelone radiata in madagascar. Unpublished thesis , University of Cambridge.
Added by: Admin (06 Jan 2014 18:23:02 UTC) |
| Resource type: Thesis/Dissertation BibTeX citation key: anon2002 View all bibliographic details  |
Categories: General Keywords: Astrochelys radiata, Habitat - habitat, Madagaskar - Madagascar, Schildkröten - turtles + tortoises, Testudinidae Creators: O'Brien Publisher: University of Cambridge |
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| Abstract |
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Testudinidae Geochelone Astrochelys radiata Many species are threatened with extinction by overexploitation, yet sustainably exploiting populations can help to conserve them. The sustainability of a harvest depends on many factors, including a species’ biology. The radiated tortoise is predicted to be easily overexploited, and is illegally harvested for food and the pet trade, but the actual threat of extinction that this harvest poses is unknown. The aim of this thesis is to establish whether the radiated tortoise is currently being overexploited. The sustainability of the current harvest, and the potential to sustainably harvest the radiated tortoise, were assessed by (i) looking for a change in range size and abundance of the radiated tortoise in harvested areas, (ii) comparing demography under differing levels of harvest intensity, (iii) using population models to estimate persistence times under current rates of harvesting, (iv) comparing the size of current and theoretically sustainable harvests, (v) assessing recovery time following cessation of harvesting and (vi) considering whether legalising trade in radiated tortoises would assist conservation efforts. The current harvest, of at least 50,000 adult tortoises annually, is unsustainable and sustainably exploiting radiated tortoises will be difficult. (i) The radiated tortoise’s geographic range has contracted by one-fifth over the last 25 years; tortoise abundance has significantly declined close to centres of high demand; and commercial harvesters are travelling increasingly far to find sufficient densities of tortoises. (ii) Tortoise density was three times lower, and adult tortoises were smaller, in harvested populations, compared with an unharvested population. (iii) Adult tortoise annual survival was consistently high (0.947), juvenile survival was low and variable (0.152-0.703). Captive tortoises lay an average of five eggs in three clutches per year, which agrees with theoretical predictions for a species following a bet-hedging life history strategy. Models predicted extinction for a population of 10,000 tortoises within 2-15 years, under current rates of harvesting. (iv) The current harvest is at least 25 times larger than a predicted sustainable harvest. A juvenile-biased harvest would be much more sustainable than the current adult-biased harvest. (v) Recovery time following commercial extinction was estimated to be at least 42 years. (vi) Legalising trade, under current socio-economic and political conditions in southern Madagascar, is predicted to increase extinction risk for the radiated tortoise.
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