Abstract
The predation of young tortoise is considered a major cause of mortality for many tortoise species. The predation by common ravens has been identified as being responsible for significant decreases in tortoise populations. Mediterranean spur-thighed tortoise hatchlings and juveniles in Maamora forest (Morocco) were studied in order to describe the size/age class predation of common ravens on young tortoises and infer the drivers of predation risk and population abundance. The results showed a high level of predation on young tortoises (<75 mm carapace length) attributed to ravens in areas with low vegetation cover, representing 100% of the cases of mortality (n = 147), but it was moderate in covered areas (n = 19), representing 12–27%. The population structure of living juveniles differed significantly between covered and uncovered areas, thus suggesting that raven predation might modify juvenile population structure. Finally, N-mixture models showed a positive relationship between (i) bare cover and tortoise detectability that is only evidenced when the plot is far from a perch and (ii) population abundance and shrub species-richness, being higher in uncovered areas. Our results improve the knowledge on predation and survival on this critical stage in life, which is crucial for the conservation of the Mediterranean spur-thighed tortoise.
Highlights
The predation of young tortoise is considered a major cause of mortality for many tortoise species
The predation of young tortoises –their shell is soft or not sufficiently solid to protect them from predators– by mammals[7,10,11] and birds[12,13,14] is considered a major cause of mortality in many tortoise species, which was studied in North America[7,10,12,15,16]
The cause of mortality in those areas was associated with predation by the common raven (100%, n = 147), where only < 75 mm CL tortoise carcasses were found with signs of raven predation under perch and nest trees
Summary
The predation of young tortoise is considered a major cause of mortality for many tortoise species. The common raven Corvus corax is considered to be one of the most relevant predators of tortoises and is responsible for 70–91% of the mortality of desert tortoises Gopherus agassizii[4,13,17] Both single individuals and breeding pairs have been identified as responsible for significant decreases in tortoise populations, in addition to modifying population structure by affecting juvenile size/age classes.[12,18,19] its high numbers and opportunistic feeding habits have severe impacts on its target prey, whose population size may be reduced without affecting the condition of the predator, which will switch to another prey when this resource becomes scarce (e.g.4). Vegetation Bare ground cover Shrub cover Shrub height Shrub richness Predation risk Perch distance Nest distance Raven occurrence Single individuals Breeding pairs Groups Number of live tortoises Number of dead tortoises Raven predated tortoises
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