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

Joanna K Carpenter,Colin F J O'Donnell,Elena Moltchanova,Dave Kelly

doi:10.1002/ece3.4157

Joanna K Carpenter, Colin F J O'Donnell + Show 2 more

Open Access

https://doi.org/10.1002/ece3.4157

Copy DOI

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.

Highlights

We found that hinau had significantly less dispersal from the ground at mainland sites where native frugivores have declined or gone locally extinct compared to island sanctuaries that more closely approximate prehuman frugivore communities
Our findings strongly suggest that hinau fruits were primarily dispersed by flightless birds in prehuman New Zealand
We found that ground-based dispersal of hinau is impaired on the New Zealand mainland compared to sanctuary islands, due to low frugivore numbers

Summary

| METHODS

Seed traps were established beneath the canopies of ten hinau trees per site on Kapiti Island, Blumine, and Catchpool Valley to obtain fruit handling and seed predation indices from the canopy. Between April and June, eight trail cameras were each trained on a fruiting branch in the canopy of a hinau tree across three of the sites (one on Kapiti Island, three at Catchpool Valley, and four on Blumine Island) As these trees needed to have suitable low-hanging branches, different trees were selected from the trees that had seed traps and ground cameras below them. Frugivore species were only included as present at a site if they were detected on the ground by a camera and had removed a fruit at one of the sites Using these criteria, Blumine had weka and kereru, Kapiti had only weka, Essons had weka, rats, mice, and pigs, and Catchpool had rats and mice. For the purpose of this summary, we assumed that fruits that were dispersed from the canopy were not vulnerable to ground predation (as the flesh from the fruit is removed, making the fruits unattractive to seed predators)

| DISCUSSION

Findings

| CONCLUSION

CONFLICT OF INTEREST