Resumen de Drivers and threats of mediterranean spur-thighed tortoise population dynamics in the core of the distribution range
In the border populations of southeastern and western Spain, intensive studies have been carried out looking at the demographic, ecological and genetic parameters of the Mediterranean spur-thighed tortoise, including long-term studies (>40 years). Nevertheless, in the core range, characterized by an optimal habitat, the number of such studies is limited, and none are long-term. Consequently, knowledge of the species is largely based on the detailed study of border populations, which could be under substantial selective pressure to adapt to local conditions and therefore might differ from core populations.
In this context, the study of the tortoise populations in Maamora forest (northern Morocco), located in the optimal habitat at the core of the distribution range, allows the examination of possible demographic differences as regards size and population structure, fecundity and juvenile survival, and their comparison with border populations.
On the one hand we studied the ecological drivers –reproductive success (comprising fecundity and recruitment), predation and diseases– which are relevant to forecasting future adaptations of the tortoise population dynamics to changes in climatic conditions (chapter 1). On the other, we analysed the socio-ecological aspects of the practice of collecting tortoises for pets, one of the main threats to the species across its distribution range (chapter 2).
Reproductive success is considered one of the principal drivers that determine the life history of the species and the evolutionary changes it may undergo. Both the body condition and the survival of hatchlings can be modulated by maternal size and/or age, as well as vary interannually due to climatic conditions (chapter 1a). Indeed, in our study, despite recruitment success being higher in the longer/older females (higher number and survival of hatchlings), it fell in years with low rainfall. The other two synergistic drivers that modulate population dynamics are predation and diseases, which, if there has been long-term interaction and coevolution between tortoises and the species involved, might mean a lower trade off for tortoises. Specifically, we studied common raven predation on hatchlings and juveniles, which has been previously documented in other tortoise species, although there is no evidence of coevolution of these two species (chapter 1b). Our results showed that common raven predation on young tortoises (<75 mm carapace length; CL) in low cover vegetation areas was high, and thus might modify juvenile population structure. In addition, we studied tick parasitisation (chapter 1c), and observed high levels of tick infestation, especially on adults and males. The latter showed poorer body condition, which could have reproductive consequences through influencing pairing behaviour in the breeding season.
Finally, collection of tortoises for pets might be considered to have an additive effect with respect to fecundity, predation and disease, which might play an important role in modifying demographic population parameters. Our results showed that in protected areas, where there is no such collection and habitat was not degraded, the tortoise populations were characterized by high and dense populations where individuals had better body condition and the population structure was more balanced (chapter 2a). Once the ecological consequences of collection for the pet trade had been evaluated, we studied the human dimension (local perception and knowledge of the species) and quantified the number of tortoises in captivity (chapter 2b). Our results showed that most of the residents (urban and rural) in the surroundings of Maamora forest had tortoises as pets and had got them from wild populations in the forest. Most of the residents did not consider the species to be wild, and their biological and ecological knowledge of the species was limited.
The results of this thesis highlight the need to implement long-term studies, which should include both individual demographic factors (sex, body condition, growth rate, fecundity) as well as those at the population level (size, age structure, sex ratio, predation, disease). This should be across the distribution range due to the population differences expected as distance from optimum habitat at the core increases, and especially within the context of climate change scenarios when more frequent droughts are expected. In addition, from a socio-ecological perspective the collection of tortoises for pets should be included in conservation strategies, as well as ascertaining the magnitude of local non-commercial collection for pets. As such, knowledge of the number of captive tortoises, the ecological effects of pet collection on wild populations, and changing the social perception of this species locally is crucial for the success of the conservation of the species.
