The extinction or local decline of animal populations (defaunation) can lead to impoverished ecosystem processes, e.g. seed dispersal. However, few studies have assessed the effects of restoring species or increasing animal population densities (refaunation) on ecosystem processes. We used a unique large-scale experiment on the North Island of New Zealand to assess whether the recovery of native fruit-eating birds increases bird-mediated seed dispersal. Our study design included three fenced sanctuaries that are hypothesized to be undergoing refaunation after the removal of invasive mammal predators followed by native species reintroductions, and three paired reference sites (with no reintroductions and minimal mammal control). We compared the density and diversity of birds, foraging rates, and the densities of bird-dispersed seeds between fenced sanctuary sites and reference sites. From January through May 2016 and 2017, we conducted point counts to survey birds, set seed traps to measure dispersed-seed densities, and conducted focal tree observations to determine foraging rates for six shrub/tree species. We used distance sampling to estimate bird densities and used generalized linear mixed models to evaluate relationships between sanctuary status and foraging rates or dispersed seed densities, while accounting for repeated measures in each site.
Results/Conclusions
We found higher bird species richness (4-6 more species/site) and higher bird densities (0.22-3.36 more birds/ha) in fenced sanctuary sites across both years for ten of eleven native species, including all seven native frugivores in our region, suggesting that sanctuaries are undergoing refaunation relative to reference sites. In contrast, two non-native frugivorous bird species had similar densities between sanctuary and reference sites. Foraging rates and dispersed seed densities were higher (0.1-0.6 more fruits consumed/observation period; 2-90 more seeds dispersed/plot) in refaunated sanctuary sites for several native plant species. Furthermore, dispersed seed densities were at or near zero for six of eight plant species in reference sites but were considerably higher in refaunated sanctuary sites. Our work suggests that reintroducing birds and increasing bird population densities through a release from invasive predators can lead to higher seed dispersal potential for several native plants. Our study is one of the first to demonstrate that fenced mammal-free sanctuaries are meeting ecological objectives and may offer a promising tool for restoring island ecosystems impacted by invasive mammals.