The Influence of Avian Biodiversity and a Weedy Understorey on Canopy Arthropod Assembly

Abstract

Identifying the factor causing species decline from a multitude of potential disturbances is essential for successful management. Invasive species are often drivers of decline, either through direct effects such as recruitment limitation, or through indirect effects such as habitat modification that facilitate biota changes in other taxa. In this study, we tested the importance of bird predation on arthropods in eucalypt canopies in wet sclerophyll forest that had been invaded by the understorey weed 'Lantana camara'. A strong top-down effect is in agreement with studies that show elevated numbers of insectivorous birds, such as the despotic bell miner, 'Manorina melanophrys', reduce damaging herbivorous insect numbers and their effect on tree health. Abundance, order and family composition were compared among sites, feeding guilds, among the presence and absence of a despotic bird, and among presence and absence within sites, using a three-way nested Multivariate Analysis of Variance (MANOVA), and tested our representation of insect feeding guilds with rarefaction curves. In total, we found arthropods from 20 families on eucalypt trees, 16 where bell miners were present and 18 where they were absent (a sampling efficiency of 78% and 92% of families based on the Chao-I index). Overall, there was a difference in the abundance of insects in areas where bell miners were present ('n' = 181), and where they were absent ('n' = 67). There was also a difference in the families present in areas with bell miners and without them. Under some conditions, despotic birds may not change the insect canopy community. Further studies should examine the effects of a despotic bird on the insect canopy community in a variety of conditions and throughout their range.