Functional group-dependent responses of forest bird communities to invasive predator control and habitat fragmentation

Aim: Mounting global pressure on bird populations from invasive predators and habitat loss has driven a rapid growth in restorative and protective conservation action around the world, yet the efficacy of such actions is still not well understood. We investigated the relative effects of invasive predator control and habitat fragmentation on the abundance of native birds and invasive mammalian predators in native forest fragments. Location: Waikato region, New Zealand. Methods: We sampled invasive mammalian predator and native bird abundances using camera traps and bird counts at 26 sites in 15 forest fragments across New Zealand9s Waikato region. Fragment area, shape complexity, and surrounding land cover of exotic and native forest were determined in ArcMap. We further created two composite gradients reflecting predator control intensity and temporal distribution of control based on seven quantitative variables recorded in each of the five years preceding native bird data collection. Finally, we estimated the relative influence of these drivers on invasive mammals and functional groups of native birds using model averaging. Results: Of the two invasive predator control variables, only control intensity significantly affected invasive predator abundance and was also a more important driver than landscape or fragment structure, but responses varied among invasive mammal species. In contrast, both invasive predator control intensity and fragment structure were similarly important drivers of native bird abundance, though bird community responses varied markedly between functional groups. Main conclusions: Our findings suggest that spatial extent of invasive mammal control is important for controlling mammal numbers and enhancing bird abundance, especially for small insectivorous species, and that habitat fragmentation is less important for invasive mammals but at least as important for native bird communities. Consequently, both drivers should be given strong consideration when undertaking landscape-scale conservation and restoration of bird communities in human-altered landscapes threatened by invasive predators.