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

Frugivorous birds in the Northern Territory, Australia move among patches of monsoonal rainforest and require many patches to maintain their populations. 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 relationship between the occurrence of six frugivorous bird species, patch area, and various measures of patch isolation. Rose-crowned fruit-doves (Ptilinopus regina) and pied imperial-pigeons (Ducula bicolor) exhibited strong relationships with a particular isolation measure: the percentage of rainforest within 50 km of the study patch. This result was not a statistical artifact of correlations between the measure and floristic or geographical variation between patches. Yellow orioles (Oriolus flavicintus) and common koels (Eudynamys scolopacea) were also influenced by this isolation measure, but various other factors (e.g. patch size and proximity to coastal areas) were more significant. Figbirds (Sphecotheres viridis) and great bowerbirds (Chlamydera nuchalis) were not particularly affected by patch isolation, and are probably not strongly dependent on rainforest. These relationships can be used to predict the consequence of destroying patches: for example, if half of the rainforest patches in the Darwin region were destroyed, our models predict that 80% of the pigeons and 90% of the doves would disappear. A threshold value was apparent for the pigeon, dove, and koel, suggesting that a critical degree of connectedness between patches over a very large area (32 km2 of rainforest in a 7900 km2 circle) is necessary to maintain these species.