Processes of island colonization by terrestrial organisms

Abstract

BIOGEOGRAPHIA — vol. XVIII - 1995 (Pubblicato il 31 ottobre 7996) Le piccole [sole Circumsarde e il loro significato Biogeografico Processes of island colonization by terrestrial organisms AUGUSTO AZZAROLI Um'verrz’tri diFz're71ze - Dipartz'772eIzt0 di Scienze dellrz Term Key words: Island Biogeography, Palaeogeography SUMMARY Colonization of islands by terrestrial organisms may have taken place through three main models of dispersal: a) vicariance, in case a part of a continent split off and became an island, with its endemic animals and plants; b) migration through land bridges, continuous or formed by discontinous, temporarily emerged islands and «stepping stones»; c) migration by swimming, or by passive transport on natural rafts. It is argued that in the majority of cases colonizations took place through models a) and b). Contrary to a current opinion, model c) may have occurrend only in restricted, favourable areas and in exceptional cases. Geological history and present conditions of several islands are analyzed in order to show how even far away oceanic islands may have been invaded through overland migrations, and cases are discussed in which migra- tions according to model c) are to be ruled out. Examples are brought forward from New Zealand, Madagascar, the Canary and Azores islands, the Western Mediterranean islands, the Antilles, the Galapagos and Indonesian islands. If correctly understood, island biogeography may become a highly useful tool for reconstruction of palaeogeography. crustal movements and geological processes of our planet. Colonization of islands by terrestrial organisms is a problem of primary interest not only for its biological implications on evolution of endemic animals and plants but also for the highly valuable information it may provide in the field of palaeogeography and geological processes in general. The problem is not always easy to solve. In fact, the dilemma of colonization by active, or pas- sive overseas transport versus migration through temporarily emerged land bridges may not always be decided on the basis of physical evidence: in most cases argument turns necessarily on speculations about the possiblity, or impossiblity of an assumed way of migration. There is geological evidence that land bridges were formed between Northern Africa, Southern Europe and several islands during the Messinian dessiccation of the Mediterranean, but there is no positive evidence that such bridges were actually used by organisms to migrate between continents and islands: this will necessarily remain a reasonable assumption and nomore. On the other hand, it is impossible to demonstrate, at least at the present state of knowledge, that no land connections were formed between, say, Corsica or Crete and the mainland during the Pleistocene, so that there was no possible way of migration except overseas transport. 33