Grazing regime effects on bird biodiversity overwhelmed by an interference competitor

The majority of the world's mammal biomass is domestic livestock. Intensive livestock grazing can negatively impact some elements of biodiversity. We quantified the effects of grazing management on bird occupancy over a 10-year period in patches of Australian endangered temperate woodland within paddocks subjected to one of three grazing regimes: continuous set stocking, rotational, and total livestock exclusion. Grazing may affect birds through altering resource availability, influencing vegetation structure, or affecting the occurrence of noisy miners (Manorina melanocephala), a native honeyeater that engages in interference competition with other birds. Using Bayesian generalized linear mixed models and multispecies occupancy modelling, we quantified relative and interactive effects on bird biodiversity of grazing, vegetation structure, and noisy miner occupancy. We found only muted grazing regime effects on woodland birds. However, many species responded to vegetation structure. Small invertivorous woodland birds responded positively to sapling abundance, and some (larger) species responded positively to the abundance of trees >50 cm. Furthermore, species richness increased with increased sapling abundance and trees >50 cm. The largest effects were associated with noisy miner occurrence, which drove reductions in small-bodied, invertivorous birds, effectively “swamping” the positive effects of vegetation attributes on these species. The profound impacts of noisy miners present challenges for bird conservation in woodland ecosystems, especially those subject to grazing. Management to reduce grazing pressure by domestic livestock may eventually help limit the impacts of the noisy miner, in part by facilitating natural regeneration of temperate woodlands to which small-bodied woodland birds respond positively.