Impact of Rats on Seabirds

An update from the Landcare Research Sustaining and Restoring Biodiversity Programme funded by the Foundation for Research, Science and Technology.

Birds transfer nutrients within and across our landscape, creating local nutrient-rich sites that support distinctive plant and animal communities. Prior to human settlement, the New Zealand mainland supported very high densities of nesting seabirds that functioned as major ecosystem drivers by transporting nutrients from the ocean to land and by cultivating soil with their burrowing.

One study under the Landcare Research collaborative research project Sustaining Critical Interactions between Functional Species is determining the relative importance of seabirds in restoring nutrient-rich coastal and island ecosystems. Using islands with seabirds present and others without seabirds due to rat predation, the research team is finding out how the birds affect linkages between above and below ground subsystems and is gaining an understanding of how rat eradication would help to restore the health of ecosystems.

 Peter Bellingham from Landcare Research says the study fits into global efforts on developing an understanding on how seabirds fit into ecosystems. ‘New Zealand is the world capital of seabirds,’ he says. ‘We have more species of breeding seabirds than anywhere else. Of the 350 species worldwide, there are 140 species here. Of these, 84 species breed in New Zealand with 35 species such as the Buller’s shearwater breeding nowhere else.

Tawhiti Rahi, a rat-free island in the Poor Knights.  The forest floor has dense burrows of seabirds such as Buller's shearwater (puffinus bulleri). Photo: David Wardle

Aiguilles, a rat-invaded island off Great Barrier is owned and managed by Ngati Rehua.  Seabird burrows are virtually non-existent on the forest floor owing to rat predation of the birds.  Photo: Tadashi Fukami

‘There has been a catastrophic loss of seabirds throughout the world. We still have some shearwaters and petrels but they used to breed throughout New Zealand including in the mountains. Currently restricted to offshore islands and some isolated headlands, their contributions to ecosystems are now very localised.

‘Our economic prosperity is based on what comes from the sea, for example, the use of rock phosphate as fertiliser. Nutrients that boost plant production such as phosphorous and nitrogen were once brought in by seabirds but these ecosystem engineers are now gone.
‘By looking at what the natural vegetation is like when there are no predators and what the impacts of rodents are, we are seeking to develop a practical approach to the restoration of ecosystems.’

How seabirds and rats affect ecosystem function.

The research was undertaken on islands offshore from Whangarei to the Bay of Plenty. Some have never been invaded by rats, while others currently have rats. The work was done with permission from and the support of tangata whenua, either in their role as owners of the islands or kaitiaki (guardians).

The multi-disciplinary team assessed how plant communities above the ground and invertebrates below are interlinked, how native ecosystem engineers (seabirds) affect ecosystem function and how alien predators (rats) disrupt processes. Sample plots in forest on each island were examined for burrow densities, vegetation structure, soil microbiota and invertebrates, and litter and wood decomposition.

The research showed the main effects of rat predation are that nutrients are no longer being brought in by seabirds, and trampling and burrowing which brings new soil to the surface has come to a shuddering halt. Plant and invertebrate species once present are gone and other species rushing in to fill the gaps.

‘The balance alters,’ explains Peter. ‘What was once forest turns to a thicket. Species such as karaka become more common. Above ground, the total carbon budget goes up by about a third, whereas there is not so much carbon below ground.

‘Soil communities change dramatically with numbers of animals from springtails to small beetles to moss larvae going into freefall. Land snails are also lower in number.

‘The results indicate that rats and seabirds act as major ecosystem drivers by exerting wide-ranging effects on both above and below ground systems. One example of the flow-on effects of rat predation is that the pohutukawa forest along our coastline which we think of as pristine, has developed and functions in a very different way from how it did before rats were introduced.’

QEII covenants help to protect the breeding sites of seabirds.

‘Seabirds such as petrels and shearwaters need advocates as they are an integral part of our landscape and economic prosperity,’ says Peter. ‘Farming and native forest both need marine phosphate. Our unique biodiversity, a key driver for tourism, is influenced by the action of seabirds.

‘Protecting the habitat and breeding sites of seabirds with QEII covenants helps to slow the disruption to our ecosystems. An example is the Puhi Peaks Nature Reserve in Kaikoura that protects one of our two remaining Hutton’s shearwater breeding colonies.

‘Now that we have an understanding of the basic flow-on effects of rat predation on seabirds, we are following through with a four year project on a bicultural approach to the restoration of coastal ecosystems. The outcomes of this will provide some practical strategies for those restoring our natural habitats including QEII covenanters.’

The Puhi Peaks Nature Reserve covenant in Kaikoura protects a breeding colony of the Hutton's shearwater (puffinus huttonii).
Photo: Dennis Buurman

A grey-faced petrel (pterodroma macroptera) after landing near its burrow, after dark, in the Rapanui covenant in Taranaki.
Photo: Barry Hartley