Long-term demographic fluctuations of the spur-thighed tortoise Testudo graeca in SW Spain

Abstract

We assessed the dynamics of a population of spur-lhighcd tortoises, Testudo graeea. placed under long-term protection from human interference. We estimated demographic parameters and constructed vertical lite 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, probabiy maximising both their long-term reproductive potential and Ihc 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 tirst 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 graeea.

The uncertainty of Late Pleistocene range expansions in the western Mediterranean: a case study of the colonization of south‐eastern Spain by the spur‐thighed tortoise, Testudo graeca

“…Because we were testing founder and bottleneck processes, we ran the program under the exponential growth model, using different wide uninformative priors. The generation time of T. g. graeca was set at 17.72, the mean value taken from those estimated by Díaz-Paniagua et al (2001) from different years and scenarios. To evaluate the stability of the results we ran the program five times for one of the employed priors (with mean 100,000), with the lowest interval being 2 9 10 4 iterations.…”

Human-mediated secondary contact of two tortoise lineages results in sex-biased introgression

“…The upper bounds of the prior distributions for parameters were set at t  = 0.1; qa  = 10 (effective ancestral population size); q1 and q2  = 0.1 (effective sizes of the Doñana and southeastern Spain populations, respectively) and m1 and m2  = 0 (the migration rates between both populations). For scaling the demographic parameters to individuals and years, a generation time of 17.72 years was used, which corresponded to the mean estimates for the Doñana population 60 and the mutation rates of loci ( μ ) were between 3 × 10 −9 and 2 × 10 −7 . The mutation rates were calculated from the number of alleles at each locus and the divergence time of the T. g. graeca lineage 22 .…”

Exurban sprawl increases the extinction probability of a threatened tortoise due to pet collections

“…The inclusion of these variables may have an effect of the values of fecundity, reproduction, and survivor of tortoises, and would presumably accelerate the extinction process. On the other hand, the intensity of tortoise collections may change between years as tortoise detection levels change considerably depending on meteorological factors (Díaz-Paniagua et al, 2001;Freilich et al, 2000), and hence accelerate or decelerate the effects of collection. In addition, our model assumes the best initial situation, that is, a stable tortoise population in an optimal habitat and with no other factors affecting it (e.g.…”

“…Natural abundance values ranges from 0.05 to 20 indv/ha, and are mainly related to climate (Anadón et al, 2009(Anadón et al, , 2010. Late reproductive maturity, high and constant adult survival, low and variable juvenile survival and high fecundity determine its population dynamics (Hailey and Loumbourdis, 1990;Díaz-Paniagua et al, 2001).…”

“…The survival rate of HATCH, JUV and AD (Hsurv, Jsurv, and Asurv) was obtained from a study carried out in the same population from 1999 to 2009 by means of capture-recapture and multistate models (Sanz-Aguilar et al, 2011) (Table 4). We used values of egg survival (Esurv) and Rep from studies carried out in a population in southwest Spain (Díaz-Paniagua et al, 1997, 2001 (Table 4). The default model considers a maximum density of tortoises (Maxden) of 20 tortoises/ha (Anadón et al, 2009(Anadón et al, , 2010.…”