Long-term demographic fluctuations of the spur-thighed tortoise Testudo graeca in SW Spain
We assessed the dynamics of a population of spur-thighed tortoises, Testudo graeca, placed under long-term protection from human interference. We estimated demographic parameters and constructed vertical life tables based on age structures for two different study periods separated by a 12-yr interval. Average age at maturity was 6.9 yr for males and 8.5 yr for females. Clutch size varied with female size, but tended to be constant among years. Average annual female fecundity over 9 yr was 8.4 eggs and decreased significantly in years of high rainfall. Females seemed to reproduce yearly, but with a constantly low output, probably maximising both their long-term reproductive potential and the probability of producing offspring during favourable years for hatchling survival. Adult survival was high and constant between periods. Juvenile survivorship from egg to 6 yr and recruitment into the 5–8-yr-old cohorts were significantly higher in the second period. Life tables indicated a predominantly negative demographic trend for the first period, with an average population increase rate of 0.977, and an overall positive tendency for the second period, with an average increase rate of 1.091. These figures suggest that the population experienced a variation that was closer to the maximum estimates for the first period, mediated mainly by an increase in juvenile recruitment. The data indicate that, besides high adult survival rates, occasional high juvenile recruitment bouts may have an important regulatory effect on the population dynamics of Testudo graeca.
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Document Type: Research Article
Publication date: December 1, 2001