Tuesday, February 22, 2011

The Fire & the Tortoise

Fire maintained ecosystems are often found in geographic regions with Mediterranean climates - southern California, southwest Australia, and of course the Mediterranean. Burning vegetation is an inconvenience, and even prescribed burns are often objected to by local citizens. But, burning-off left-over agricultural biomass has increased in many places. Suppressing fires can lead to the build-up of fuel -dead, dry vegetation- in fire maintained ecosystems and result in increased hazards to both humans and wildlife. How wildlife populations deal with fire is of interest for understanding adaptations to fire and for conservation. Ana Sanz-Aguilar and colleagues have examined how the Iberian Spur-thighed Tortoise, Testudo gracea ibera, manges to coexist with fire at the Cumbres de la Galera Biological Reserve in the Sierra de la Carrasquilla, in Spain. Their study area supported a high density population (about 20 tortoises per hectare) of Spur-thighed Tortoise and they estimated tortoise populations in areas that had frequent fires and areas that had not been burned. The Spur-thighed Tortoise is a long-lived species of the Mediterranean shrublands and the tortoises spend much of their lives sheltered under the vegetation or underground in burrows. These tortoises also bury themselves for hibernate and aestivate, so that they are protected from extreme temperatures, predators and possibly from fires. They found fire caused direct and delayed reductions in local survival, with young individuals being the most affected. Fire-related mortality was highest in juveniles and subadults than adults; this seemed to be related to differences in burrowing behavior. Summer fires had a lesser impact on adults because they spend summer and winter underground in burrows or by burying themselves to avoid temperature extremes. Juveniles and subadults tend to use more superficial burrows or take cover under the vegetation only a few centimeters in depth, thus and are exposed to higher temperature and smoke. The study areas that had fire frequencies similar to those occurring in areas uncontrolled for burns (less than one fire every 20–30 years) tortoise populations were able buffer the effects of fires. But, when fire frequency increased the probability of extinction dramatically increased, except for the largest populations. Thus, T. graeca is able to cope with natural fire frequencies, but the effects of more recurrent fires may severely threaten the species.

Citation:Sanz-Aguilar, A., J. D. Anadon, A. Gimenez, R. Ballestar, E. Gracia, and D. Oro.. 2011. Coexisting with fire: The case of the terrestrial tortoise Testudo graeca in mediterranean shrublands. Biological Conservation (2011), doi:10.1016/j.biocon.2010.12.023

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