Faunal modifications on islands

Island mammals often differ drastically in their body size from their mainland conspecifics. On islands, larger size, which can be vital in mainland settings, can turn detrimental. The lack of large predators but most of all the low availability of resources sets insular populations under the strict control of both genetic and ecological constraints. Population densities are therefore confined between a critical minimum number of individuals needed to avoid extinction and a maximum number determined by the carrying capacity of the environment. Dwarfism and gigantism seem to be the only alternatives large-sized and small-sized animals have, respectively, to lower selective pressure when they move into insular settings. All the modifications mammals go through in island settings are imposed by the need to attain new ecological balances. The pioneer faunas are initially in a state of disequilibrium and are subjected to r-selection. Body size and population density then change in the geologically short lapse of only few thousand years reducing inter- and intraspecific competition and establishing new ecological balances which eventually lead to K-selective conditions. Elephants offer some of the most spectacular examples of insular dwarfism. Dwarf specimens are reported from numerous Mediterranean islands. Sicily and Malta, in particular, provided remains of at least three dwarf elephants, i.e. Elephas (Palaeoloxodon) falconeri, E. (Palaeoloxodon) antiquus leonardii, E. (Palaeoloxodon) mnaidriensis, although the picture is actually far more complicated and still needs to be cleared. Different mainland elephant species seem to have reached Sicily through time. The situation of the Indonesian small-sized elephantoids is not much clearer either. Various pigmy or even dwarf stegodonts developed repeatedly on the different Indonesian islands through time. The discovery of the dwarf Homo floresiensis, in particular, attracted the attention of researchers from all over the world. The remains of this hominid were found associated with a very high concentration of stone tools, suitable for hunting and preparing big game (which led some to believe that Homo floresiensis preferential hunted young Stegodon, although such a conclusion has raised considerable controversy), as well as with numerous juvenile Stegodon bones, which were originally thought to belong to a dwarf form. Flores was inhabited by two successive Stegodon species, the dwarf S. sondaari and the medium- to large-size S. florensis. For stratigraphic reasons, some believe that the still undesignated Stegodon associated with H. floresiensis might actually be S. florensis. Most Indonesian islands stegodonts, Stegodon sondaari included, increased their hypsodonty and developed relatively broad molar wear surfaces. This is explained supposing that these animals had to process larger amounts of tougher and more abrasive food than on mainland settings. Insular modifications seem almost codified. The regularity of the processes through time and space prove the channelized nature of insular edemizations.