The role of introduced species in shaping the distribution and abundance of island reptiles

Summary Species interactions, as revealed by historical introductions of predators and competitors, affect population densities and sometimes result in extinctions of island reptiles. Mongoose introductions to Pacific islands have diminished the abundance of diurnal lizards and in some cases have led to extinctions. Through these population level effects, biogeographic patterns are produced, such as the reciprocal co-occurrence pattern seen with the tuatara and its predator, the Polynesian rat, and with the tropical gecko competitorsHemidactylus frenatus andLepidodactylus lugubris in urban habitats in the Pacific. Although competition has led to changes in abundance and has caused habitat displacement and reduced colonization success, extinctions of established reptile populations usually occur only as a result of predation.These introductions, along with many manipulative experiments, demonstrate that present day competition and predation are potent forces shaping community structure and geographic distributions. The human introduction of species to islands can be viewed as an acceleration of the natural processes of range expansion and colonization. The immediate biotic consequences of these natural processes should be of the same intensity as those of the human introductions. Coevolution may subsequently act to ameliorate these interactions and reduce the dynamical response of one species to the other. The role played by coevolution in mediating interactions between competitors and predator and prey is highlighted by the susceptibility of predator-naive endemic species to introduced predators and the invalidity of species-poor communities.     

On the urgency of conservation on Guadalupe Island, Mexico: is it a lost paradise?

Guadalupe is an oceanic island located in the Pacific Ocean off Mexico's northwest coast. Its flora is composed of many plant species with more northern affinities and disjunctions from the California Floristic Province. Almost 16% of the native plant species are endemic, including two monospecific genera. However, the activities of feral goats released in the early 19th century have devastated most of the island. At present, at least 26 native plant species have disappeared from Guadalupe and many more seem to be on the threshold. To add to the problem, 61 exotic plant species have been documented on the island, many of which are aggressive weeds. In this paper, we propose eight types of environmental conditions for the island where different species assemblages of the pristine flora probably existed before their demise. It is of obvious urgency that the island needs a recovery plan and the first step should include the eradication of the feral goats. Only after this process can subsequent conservation measures be applied to ensure any restoration of this natural heritage. The recovery plan will need to address both spontaneous and human-induced plant repopulation processes from the main island's three southern islets, which have never been impacted by goats. Also, it may be necessary to reintroduce non-endemic, native plant taxa from the nearest Californian islands and the Mexican coast in order to reestablish some of the island's original diversity of plant species and communities. However, it should be noted that the forested communities do not have great hopes of recovering in the short term, since the ground water and soil conditions have been significantly altered. Furthermore, the eradication of an estimated 4000 goats still living on the island (year 2000) depends upon the vacillating motivation and will of Mexican authorities.