Introduction of mammalian seed predators and the loss of an endemic flightless bird impair seed dispersal of the New Zealand tree Elaeocarpus dentatus

Abstract

Understanding the mutualistic services provided by species is critical when considering both the consequences of their loss or the benefits of their reintroduction. Like many other Pacific islands, New Zealand seed dispersal networks have been changed by both significant losses of large frugivorous birds and the introduction of invasive mammals. These changes are particularly concerning when important dispersers remain unidentified. We tested the impact of frugivore declines and invasive seed predators on seed dispersal for an endemic tree, hinau Elaeocarpus dentatus, by comparing seed dispersal and predation rates on the mainland of New Zealand with offshore sanctuary islands with higher bird and lower mammal numbers. We used cameras and seed traps to measure predation and dispersal from the ground and canopy, respectively. We found that canopy fruit handling rates (an index of dispersal quantity) were poor even on island sanctuaries (only 14% of seeds captured below parent trees on islands had passed through a bird), which suggests that hinau may be adapted for ground‐based dispersal by flightless birds. Ground‐based dispersal of hinau was low on the New Zealand mainland compared to sanctuary islands (4% of seeds dispersed on the mainland vs. 76% dispersed on islands), due to low frugivore numbers. A flightless endemic rail (Gallirallus australis) conducted the majority of ground‐based fruit removal on islands. Despite being threatened, this rail is controversial in restoration projects because of its predatory impacts on native fauna. Our study demonstrates the importance of testing which species perform important mutualistic services, rather than simply relying on logical assumptions.

Long seed dispersal distances by an inquisitive flightless rail (Gallirallus australis) are reduced by interaction with humans

“…Weka are important, yet unappreciated, seed dispersers in New Zealand [20]. They have been recorded consuming the fruits of over 26 native fleshy-fruited plant species, including some of New Zealand's largest-seeded species [62], and the fruits of low-growing divaricating shrubs.…”

“…Their wild diet is dominated by fruit and invertebrates but also includes lizards, carrion, and the eggs and chicks of ground-nesting birds [24]. They swallow fruits whole and either defecate or regurgitate the seeds intact [20,25].…”

“…We modelled seed dispersal distances for two large-seeded plant species commonly eaten by weka—miro ( Prumnopitys ferruginea, Podocarpaceae) and hinau ( Elaeocarpus dentatus, Elaeocarpaceae) [20,21] . We were particularly interested in these two species, as weka have been shown to be the most important disperser for hinau in terms of fruit removal [20], and they are also likely to be an important disperser for miro where weka are present, as its large fruits are consumed by few other species [21]. In addition, both plant species feature some fruit traits (e.g.…”

“…Their charisma and cunning results in them often being fed by, and stealing food from, people [19]. Recently, weka have been shown to be important seed dispersers for some New Zealand plant species [20], but they have been studied much less than volant, frugivorous birds such as the kereru (New Zealand wood pigeon Hemiphaga novaeseelandiae [21]). As a result, no one has measured how long weka retain seeds in their guts, and how far they disperse seeds.…”

The forgotten fauna: Native vertebrate seed predators on islands

“…birds, mammals and invertebrates). However, with the exception of rats (in particular, black or ship rats Rattus rattus and Pacific rats Rattus exulans) and to some extent feral pigs Sus scrofa (Carpenter, Kelly, Moltchanova, & O'Donnell, 2018;Gawel, Rogers, Miller, & Kerr, 2018;O'Connor & Kelly, 2012), very little is known about their impacts, and how their impacts compare to those of lost 5. We highlight the need to understand how these altered seed predator communities are affecting native plant populations, particularly in the context of reduced pollination and seed dispersal.…”