Variation in abundance of nectarivorous birds: Does a competitive despot interfere with flower tracking?

Abstract

Adaptive resource tracking in space and time may be disrupted by the modification of resources and competitors. Major global change drivers (e.g. land-use change) have induced declines in many native species, while facilitating only a few. Given that many resources are predicted to become increasingly scarce under the joint effects of climate and land-use change, disturbance-tolerant species that are able to defend high-value resources may further limit the persistence of disturbance-sensitive species. We sought to determine which nectarivorous birds track variation in flowering and if relationships between nectarivores and flowering are affected by on-transect vegetation structure or the occurrence of a native, hyper-aggressive species, the noisy miner Manorina melanocephala, which has become more prevalent. We measured eucalypt flowering and abundances of nectarivorous birds over the course of a year; we measured vegetation structure on the same forest transects. Nectarivores tracked spatial and some temporal variation in flowering, but this relationship was disrupted by noisy miners. Where present in sufficient numbers, the noisy miner excluded small-bodied nectarivores (<63 g) from fragments, limiting the ability of this numerically dominant component of the avifauna to gain access to flowering resources. Altered patterns of interspecific competition due to vegetation fragmentation and climate-induced degradation may have led to changes in the distribution of small nectarivore species that is a departure from the 'ideal free distribution' model. Interactions between noisy miners and small-bodied nectarivores appear to be best described by the 'ideal despotic distribution' model in which noisy miners exclude smaller competitors and monopolize local resources. Increases in the severity and frequency of extreme climatic events (e.g. long droughts) predicted under climate change may create a boom-bust pattern of availabilities of resources. The apparent insensitivity of noisy miners to such variation in flowering resource availability and the miners' influence on the ability of small nectarivores to access resources may lead to disproportionate declines in smaller-bodied nectarivorous species. Reduced tracking of flowering by nectarivores has the potential to disrupt ecosystem services (e.g. pollination, seed dispersal) and may have long-term consequences for the persistence of fragmented vegetation, adding further pressure on forest-dependent biota.