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

Abstract

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.

Endozoochory by mallard in New Zealand: what seeds are dispersed and how far?

“…For instance, introduced birds in Hawaii have replaced the role of extinct frugivorous birds as the primary vectors of some native plants ( Cox, 1983 ; Foster & Robinson, 2007 ). In New Zealand, introduced blackbirds ( Turdus merula ) and song thrushes ( T. philomelos ) dispersed more seeds than native frugivorous avifauna within secondary forested areas ( Wyman & Kelly, 2017 ; but see Macfarlane, Kelly & Briskie, 2016 ). However, alien plants can also disperse by endozoochory and this could have negative consequences for native biodiversity and ecosystem functions ( Reynolds, Miranda & Cumming, 2015 ; Green, 2016 ; Van Leeuwen, 2018 ).…”

“…In New Zealand, studies of avian endozoochory have largely focused on frugivorous avifauna in forested and native habitats ( Wyman & Kelly, 2017 ; Clout & Hay, 1989 ). Nearly 60% of New Zealand is comprised of low-lying agricultural and pastoral land ( MacLeod et al, 2008 ), which contains remnant patches of native forests, wetlands, rivers, and swamp areas, with a vast network of drainage ditches and ponds (e.g., Miskell, 1993 ; Buckland, 2008 ).…”

Excluding mammalian predators increases bird densities and seed dispersal in fenced ecosanctuaries

“…Most invasive mammals are ineffective dispersers and negatively impact seed dispersal through seed predation (Williams et al 2001, Duron et al 2017). For example, possums Trichosurus vulpecula consume seeds of several native and non‐native fruiting trees (Williams et al 2001), but they move only trivial quantities of seed compared to birds (Wyman and Kelly 2017), and trees typically experience lower germination success when dispersed by possums (Williams et al 2001, Wyman and Kelly 2017). Thus, invasive mammal browsing on fruits and seeds may reduce seed dispersal and recruitment for native trees (Campbell and Atkinson 2002), and mammal eradication may release trees from these direct impacts.…”

“…Mammals may also indirectly affect seed dispersal by reducing the abundance and activities of avian pollinators, which may reduce seed production in sites where birds are unprotected from invasive mammals (Anderson et al 2011, Wyman and Kelly 2017). Future studies should consider how fenced ecosanctuaries affect pollination and seed dispersal simultaneously, to deepen our understanding of the mechanisms driving higher seed dispersal in ecosanctuaries.…”

Effects of changes in bird community composition and species abundance on plant reproduction, through pollination and seed dispersal

“…Wild Pigs Sus scrofa (O’Connor & Kelly 2012) and Brushtail Possums Trichosurus vulpecula (Dungan et al . 2002, Young 2012, Wyman & Kelly 2017)), often either destroy flowers or seeds, or disperse seeds to unsuitable habitat, so that a net positive contribution is unlikely.…”