The impact of introduced predators on the Mallorcan midwife toad Alytes muletensis

The endemic midwife toad of Mallorca, Alytes muletensis, was once widespread across the island, but a dramatic decline has resulted in the persistence of only a small number of populations in the mountainous north-west. Predation and competition from introduced species such as the viperine snake (Natrix maura) and green frog (Rana perezi) are widely believed to have been imp0l1ant agents of decline. This thesis set out to test that theory by assessing the impact of introduced predators on the distribution of the toad, and on the population structure, morphology and behaviour of tadpoles. GIS analysis revealed the preferred habitat of the toad to be high elevation torrents surrounded by steep sides and containing many pools. This habitat is thought to favour the persistence of cool plunge pools throughout the year, providing optimal conditions for larval growth and development. Although steep sides surrounding a torrent did not prohibit the access of introduced snakes and frogs, the presence of these predators was negatively associated with elevation, resulting in reduced predation pressure at higher altitudes. These findings may be used to optimize the location and design of future reintroductions and to enhance existing sites. The presence of introduced predators was found to influence the growth, development and population structure of larval A. muletensis. Bimodal size-structures of larval popUlations in sites with snakes and frogs reflected size-selective predation or changes in recruitment, growth and development under the threat of predation. Differences in tadpole morphology between populations reflected rapid and reversible plastic responses to snakes and frogs, but no such responses were found towards a native invertebrate predator. It is likely that intense selection pressure has driven the evolution of inducible morphological and behavioural defences since the introduction of vertebrate predators onto Mallorca some 2000 years ago. Although captive-bred tadpoles showed similar responses to wild tadpoles, experiments showed that behavioural responses to predators were slightly weaker in the former. This may be a result of a captive-breeding bottleneck and supports the decision of the recovery group to stop releasing individuals from the original founder stock and to establish three new bloodlines.