High densities of bell miners Manorina melanophrys associated with reduced diversity of other birds in wet eucalypt forest: Potential for adaptive management

Abstract

High densities of bell miners ( Manorina melanophrys) are known to accelerate dieback in eucalypt forests presumably through their negative impact on other avian insectivores and predatory insects. Some areas of moist eucalypt forest that are managed for sustained timber production also support very high densities of bell miners. In this study, we quantified the relative population density of bell miners in forests, and investigated the relationship between bell miner population density, the relative population density of other birds and species richness. A study site of 900 ha was selected in Olney State Forest on the Central Coast of New South Wales (33°7′ S, 150°22′ E), an area that has been managed for timber production since the 1920s. Monthly census counts were carried out over a 16-month period to include diurnal and seasonal variation. Bell miner population density ranged from 14–38 birds/ha and was found to be negatively correlated with diversity of other species. A linear empirical relationship was found between bird species richness ( y) and bell miner relative population density ( x) by the equation y = −0.12 x + 7.78 ( R = 0.9638, n = 8, P < 0.0001), and an increase in bell miner abundance was found to decrease the abundance of other species as given by the equation y = −0.32 x + 16.83 ( R = −0.9646, n = 8, P < 0.0001). Experiments elsewhere in which bell miners were removed from eucalypt woodland plots have shown a return of avian biodiversity and enhanced tree growth, and this study provides an empirical relationship describing the impact on avian diversity for a given forest. We recommend a test of the hypothesis that manipulation of the understorey during burning and logging, could be used to reduce bell miner population levels in wet eucalypt forests, with corresponding benefits to avian biodiversity and tree growth. Work toward these ends has been initiated by the bell miner Associated Dieback Working Group in northern NSW at present and our findings support their strategy.