Synergistic impacts of aggressive species on small birds in a fragmented landscape

1. Attempts to conserve threatened species in fragmented landscapes are often challenging because factors such as habitat loss, habitat degradation and dominant species interact to reduce threatened species' capacity to survive and reproduce. Understanding how threatening and mitigating processes interact is critical if conservation measures are to be effective.

2. We used data from long-term monitoring of bird populations and multivariate latent variable models to quantify how Australian woodland birds respond to the presence of the Noisy Miner, a despotic species known to exclude other bird species. We then investigated the extent to which the presence of other aggressive species exacerbates the impacts of the Noisy Miner, and to what extent these impacts can be mitigated by dense midstorey plantings.

3. We found strong synergies between the Noisy Miner and two other aggressive species (Grey Butcherbird and Pied Butcherbird), despite weak effects of butcherbirds in isolation.

4. The impacts of aggressive birds are most pronounced for small-bodied species; but these impacts are lessened in the presence of high midstorey cover.

5. Synthesis and applications. Accounting for interactions reveals that revegetation may be capable of improving conservation outcomes even when the proximate cause of species declines (i.e. exclusion or predation by aggressive bird species) cannot be managed directly.