Bird Species Distribution in the Galapagos and Other Archipelagoes: Competition or Chance?

Connor and Simberloff (1979) have argued that in many archipelagoes bird species are randomly distributed subject only to constraints of a minimum island size and a give number of occupied islands for each species and a given number of species per island. Thus, interspecific competition would not seem to be important in determining the distribution of birds. However, the Galapagos finches (Geospiza) are not distributed as predicted by a random model with contraints of minimum island size and each species inhabiting a given number of islands. Interspecific competition is a likely explanation for the dissimilarity of distributions in this genus, as availability of suitable habitat or isolation alone are not satisfactory as alternative explanations. For the birds and bats of the West Indies many cases of competitive exclusion may be included in the seemingly small difference that Connor and Simberloff found between the numbers of exclusive distribution and those expected under the constraints of their random model. However, factors other than interspecific competition may be responsible for the dissimilarity of these distributions. Random models are a weak method for studying the role of interspecific competition in animal distribution.