Quantifying seed dispersal by birds and possums in a lowland New Zealand forest

Declines in native birds in New Zealand have raised questions about whether seed dispersal limits plant regeneration and whether introduced mammals such as brushtail possums (Trichosurus vulpecula) can replace absent native birds. We determined the relative contribution to seed dispersal by birds and possums in native secondary forest at Kowhai Bush, Kaikoura. The number of seeds dispersed per hectare per day by each animal species was determined based on the number of seeds per faecal pellet, the number of faecal pellets per animal per day, and the density of animals per hectare. Five dispersers had many seeds in their faecal samples: bellbirds (Anthornis melanura , mean 11.5 seeds per sample), silvereyes (Zosterops lateralis , 7.0), song thrushes (Turdus philomelos , 8.3), blackbirds (Turdus merula , 15.1) and possums (8.4). However, possums produced half as many faeces per day as birds and were present at lower densities than birds (1 possum, 3 bellbirds, 2.7 silvereyes, 6.4 song thrushes, and 3.2 blackbirds per ha). Consequently, on a per hectare basis, possums dispersed <3% of the total seeds, much less than bellbirds (22%), silvereyes (12%), song thrushes (33%) and blackbirds (30%). Possums also destroyed approximately 15% of seeds found in faeces, reduced the germination of gutpassed Coprosma robusta  seed to half of that from bird faeces, and did not swallow fruit any larger (max 7 mm diameter) than those moved by birds. Consequently, possums provided little benefit from seed dispersal.