Very large area requirements for frugivorous birds in monsoon rainforests of the Northern Territory, Australia

Frugivorous birds in the Northern Territory, Australia move among patches o f monsoonal rainforest and require many patches to maintain their populatio ns. Using bird survey data from 149 rainforest patches and GIS coverage of all of the ca. 15,000 patches in the Northern Territory, we analyzed the re lationship between the occurrence of six frugivorous bird species, patch ar ea, and various measures of patch isolation. Rose-crowned fruit-doves (Ptil inopus regina) and pied imperial-pigeons (Ducula bicolor) exhibited strong relationships with a particular isolation measure: the percentage of rainfo rest within 50 km of the study patch. This result was not a statistical art ifact of correlations between the measure and floristic or geographical var iation between patches. Yellow orioles (Oriolus flavicintus) and common kee ls (Eudynamys scolopacea) were also influenced by this isolation measure, b ut various other factors (e.g. patch size and proximity to coastal areas) w ere more significant. Figbirds (Sphecotheres viridis) and great bowerbirds (Chlamydera nuchalis) were not particularly affected by patch isolation, an d are probably not strongly dependent on rainforest. These relationships ca n be used to predict the consequence of destroying patches: for example, if half of the rainforest patches in the Darwin region were destroyed, our mo dels predict that 80% of the pigeons and 90% of the doves would disappear. A threshold value was apparent for the pigeon. dove, and keel, suggesting t hat a critical degree of connectedness between patches over a very large ur ea (32 km(2) of rainforest in a 7900 km(2) circle) is necessary to maintain these species. (C) 1999 Elsevier Science Ltd. All rights reserved.