Bird Species Impoverishment, Niche Expansion, and Density Inflation in Mediterranean Island Habitats

Abstract

The causes and consequences of three classical features of insular faunas, namely species depauperization, increased population density, and increased niche breadth, were investigated by comparing bird communities in relation to habitat characteristics along ecological successions in southern France and the island of Corsica. Each succession comprised six habitats, which were selected on the basis of vegetation structure. There were nearly as many bird species in Corsica as on the mainland in the entire successional sequence, but fewer species in the late successional forested stages and more species in shrubby stages on the island, because of the shift of many typically forest—dwelling species into matorrals (evergreen Mediterranean shrub communities). Species utilized more habitats and subdivided them less in Corsica than on the mainland. The Corsican forest bird community was composed of a few small—sized resident species with very high populations. Thus insular faunas are very different in early and late stages of succession. These results differ in several respects from those of many earlier mainland—island comparisons, and emphasize the need for comparing a series of habitats between the island and the mainland, rather than a single habitat, as has generally been the case. These differences in species allocation to habitats between the island and the mainland are better explained by two non—mutually exclusive hypotheses than by a release in interspecific competition. The first is the so—called "habitat appropriateness" hypothesis, which predicts that the more closely a habitat resembles that which is optimal for a species on the mainland, the more species will compensate for the population densities of absent competitors. This is the case for the bird communities of the Corsican forests, which exhibit the main features of an insular syndrome, i.e., species impoverishment, high population densities, and intraspecific spillover. It is suggested that given an equitable climate and a stable level of resources, survival on the island is higher than on the mainland, dispersal is restricted, and populations expand into shrubby habitats as a result of spillover from optimal habitats. This explanation is further supported by the second hypothesis, named the "defense hypothesis" (Stamps and Buechner 1985), which stipulates that a stable allocation of resources all the year around in the benign insular environment results in a decrease in the vigor of territorial defense, a decrease in territory size, and an increase in territory overlap, which in turn allows for more individuals to coexist in the same amount of space. Long—term comparative studies on the population biology of tits of the genus Parus have shown that, compared to their mainland counterparts, Corsican populations of tits exhibit social structures supporting this hypothesis. Finally, a clear distinction between communities of early and late stages of succession is discussed in the light of history and speciation patterns in the Mediterranean area. The historical approach helps to explain the convergence of bird community composition on the mainland and the island at late successional stages.