‘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
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
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.’