The Composition of Landbird Faunas of Islands Round South-Western Australia: Is There Evidence for Competitive Exclusion?

Abstract

Three species of passerine bird, Zosterops lateralis (Latham), Lichenostomus virescens (Vieillot) and Sericornis frontalis (Vigors & Horsfield), have a widespread occurrence on islands round south-western Australia. The problem examined was whether the distribution patterns of the three species on these islands support hypotheses involving random colonization/persistence, unavailable suitable habitat, or competitive exclusion. The distribution pattern found was non-random. Evidence from the distri- bution of birds on sixty islands of different area and isolation suggested that Zosterops is the superior competitor on islands with area less than 10ha, but that Lichenostomus is superior to Zosterops on islands of area 10-100ha. Zosterops is also well represented on the more remote islands, whereas Licheno- stomus is not. Evidence from the availability of habitat and proportion of plant cover on twenty islands studied in detail suggested that Zosterops excludes Lichenostomus from islands as the proportion of cover available at 1.5 m above ground increases, and that Lichenostomus excludes Zosterops from islands on which both the area of suitable habitat is less than about 30 ha and the propor- tion of cover at 1.5 m above ground is very low. Sericornis was absent from all islands with area of suitable habitat less than about 60ha, irrespective of the amount of cover at 1.5 m above ground. However, it could not be established if this resulted from exclusion by Zosterops or Lichenostomus. Dispersal ability of these three species may be ranked as follows: Zosterops > Lichenostomus > Sericornis. The minimum habitat requirements seem to be: Zosterops, small area of relatively tall vegetation; Lichenostomus, small area of low vegetation; Sericornis, large area of low vegetation.