Rangatira Island, part of the Chatham Islands group situated off the east coast of New Zealand, is a conservation priority in New Zealand due to its importance as a seabird breeding colony and a haven for threatened species. This includes birds endemic to the Chatham Islands and most importantly, the largest of two populations in the world of the critically endangered black robin (Petroica traversii). Black robins are especially vulnerable due to their tiny population size of only around 200 individuals.
Vegetation is vitally important for Rangatira Island to maintain its status as a wildlife sanctuary. It provides protection against the bitter and frequent winds blowing northwards from the Antarctic, locally known as the ‘roaring forties’. The island has a population of around 3 million burrowing seabirds, which equates to 14,000 birds per hectare. At this density, birds have the potential to greatly affect the terrestrial ecosystem through sea to land nutrient transfer in the form of bird poo. Vegetation regeneration is also disrupted by bird trampling and mechanical disturbance from the digging of nesting burrows.
A similar situation has occurred on Grassholm Island, off the coast of Wales. Grassholm Island supported half a million puffins (Fratercula arctica) in 1890, but trampling and burrowing by birds led to almost complete vegetation loss and extensive erosion. Eventually the island could no longer support a large seabird population. The paper ‘Burrowing seabirds affect forest regeneration, Rangatira Island, Chatham Islands, New Zealand’ by Cynthia Roberts, Richard Duncan and Kerry-Jayne Wilson aims to understand the relationship between burrowing seabirds and vegetation so that Rangatira may avoid the fate suffered by Grassholm Island. It was published in a 2007 issue of the New Zealand Journal of Ecology.
Clearing of vegetation for sheep and cattle farming started in the early 1940’s and anthropogenic affects on the Island’s vegetation continued until the removal of the remaining stock in 1959. The clearing of vegetation and undergrowth on the southern side of the island has resulted in the death of forest margins from salt and wind exposure. This barren and exposed area is called ‘the clears’ and is void of bird habitat and forest cannot recolonise the salty eroded soil.
Despite the potential for seabirds to cause a problem, prior to this study little was known about forest dynamics on Rangatira Island and the effect of burrowing seabirds. The observational field study carried out by Roberts, Duncan and Wilson investigated the dynamics of the relationship between burrowing seabirds and vegetation on Rangatira Island.
The forest was found to be dominated by Chatham Island ribbonwood (Plagianthus chathamicus) and has mostly regenerated since the removal of stock. With increasing burrow density, soil phosphorous increases and pH and seedling density decrease. There are also more burrows found in areas with lower altitude and lower tree density. Seabird exclosure plots under the canopy showed increased seedling density compared to when seabirds were not excluded. Plots comparing regeneration between canopy and canopy gaps showed that canopy gaps are characterised by significantly higher seedling densities and fewer burrows. It is clear from these results that both treefall gaps and seabirds strongly influence the regeneration of forest on Rangatira.
The study found that the main source of forest gaps seems to be the death of a species of tree daisy called the Chatham Island akeake (Olearia traversii). This species is slowly disappearing from the island, possibly reducing the ability of the forest to regenerate. There is a possibility that ribbonwood could progressively die and collapse, producing small canopy gaps resulting in a forest with a healthy age structure. Conversely, the young, even regrowth of ribbonwood could simultaneously succumb to the effects of extensive seabird burrowing resulting in catastrophic forest collapse. This would create an uninhabitable ecosystem similar to Grassholm Island and ‘the clears’ with catastrophic consequences for the fauna supported by the refuge that is Rangatira Island.