The ecological context and consequences of colonisation of a site by bell miners (Manorina melanophrys)

Abstract

Bell miner (Manorina melanophrys) colonies are closely associated with decreased avian abundance and diversity and an apparently associated increase in psyllid abundance. However, a causative link between the presence of bell miners and increased psyllid abundance has yet to be established. We took advantage of the movement of bell miners into two new areas to investigate the ecological consequences of bell miner occupation on both avian and psyllid abundance. We monitored the number of bell miners and other avian species, using area searches, and psyllid abundance by monthly counts of lerp on leaves. Bell miner presence alone had limited effect on either bird diversity or abundance. However, when miners were also giving their distinctive ‘tink’ vocalisation, a significant decrease in avian abundance and diversity was observed. This evidence supports the hypothesis that ‘tink’ vocalisations are used by interspecific competitors to detect bell miner colonies. At the time of initial occupation, new sites did not have significantly elevated levels of psyllids compared with surrounding areas unoccupied by bell miners. Six months later one of the two newly occupied sites had significantly more Cardiaspina spp. than either the long-established colony or an unoccupied control site. In contrast, infestations of Glycaspis spp. remained significantly lower at both new sites when compared with the long-established colony, but equivalent to unoccupied areas. Given this, we conclude that bell miner occupation does not necessarily lead to an increase in psyllid abundance, characteristic of tree dieback in some colonies, and that a causative link between bell miner presence and declining tree health remains to be demonstrated.