Introduced blackbirds and song thrushes: useful substitutes for lost mid-sized native frugivores, or weed vectors?

The New Zealand avifauna has declined from human impacts, which might leave some larger-seeded native plants vulnerable to dispersal failure. We studied fruit dispersal in a lowland secondary forest near Kaikoura, where the only remaining native frugivores are relatively small (silvereye Zosterops lateralis, and bellbird Anthornis melanura). We tested whether two larger exotic frugivores (blackbird Turdus merula and song thrush T. philomelos) dispersed native plants with seeds too large for the two smaller native frugivores. Diet breadth was measured by identifying seeds in the faeces of 221 mist-netted birds, and by observations of birds foraging. We then compared the plant species dispersed to the range of locally available fruits. All four bird species had varied diets (6-9 plant species per bird species) that differed significantly, although Coprosma robusta was always the most-eaten fruit. As predicted, the maximum fruit size eaten was larger for exotic birds (11.3 mm diameter) than natives (7.4-7.7 mm diameter), but all birds ate mainly smaller fruits. However, 7/21 fruiting plant species were not seen to be dispersed by any species, and the chance of being undispersed was independent of fruit size. Blackbirds and song thrushes jointly dispersed all four woody weeds with fruits >7.5 mm diameter, but neither of the two similar-sized native plants. Although the two species of exotic birds dispersed some native plants, our study suggests that their net effect is negative through facilitating the spread of invasive weeds. Studies evaluating the contribution of exotic frugivores to novel plant communities need to distinguish potential effects (what the frugivores might be capable of doing) from actual effects (what the frugivores are observed doing).