When you think of birds in New Zealand’s forests you might imagine a tui serenading you from the treetops, or perhaps the iconic kiwi scurrying through the undergrowth. What probably doesn’t spring to mind is the shrieking of hundreds of thousands of seabirds as they crash-land on the forest floor. However, this is what you might experience if you were lucky enough to set foot on Rangatira Island in the Chatham Island archipelago, 800 kilometres to the east of the New Zealand mainland.

New Zealand is often considered the seabird capital of the world, which would surely make Rangatira, and the rest of the Chatham Islands, the seabird capital’s busiest street. Although they spend most of their lives in or above the open ocean, seafaring species must return to land at some point to breed and raise their young. And when they come back, they return in force. One estimate has five million seabirds attempting to nest on Rangatira, which is only 218 hectares in size. These birds, which include the white-faced storm petrel pictured above, nest in burrows which are usually in forested areas. With so many birds nesting in such a small place, the burrows are packed incredibly close together. So close, in fact, that when walking over them you need to wear special shoes with a large wooden sole called “petrel boards” to avoid collapsing the forest floor.

So many burrows should have an impact of some sort on the forest. Rangatira’s seabirds are rather clumsy on land as they have evolved for life on wing and wave, so they can hardly be blamed for trampling growing plants in the stampede to reach their breeding burrows. The team of Cynthia Roberts, Richard Duncan and Kerry-Jayne Wilson packed their petrel boards and headed to Rangatira to investigate.

Their study encompassed several years and involved measuring different aspects of the island’s forest. Ten by ten metre plots were set up at random points in the forest, where the type and age of the trees was recorded. In these plots, seabird exclusion zones were created to measure the growth of seedlings on the forest floor in areas with and without seabirds. Finally, the number of seedlings growing under gaps in the forest canopy was compared with the number of seedlings growing under a closed canopy.

The research team found that, as expected, many more seedlings grew in the areas where seabirds had been excluded compared to the areas where the birds had free reign. Despite this trampling and a multitude of seabird burrows (1.19 per square metre!), the biggest influence on seedling growth was the amount of space in the canopy directly above the seedlings. When the forest canopy blocks out sunlight, the seabird stampede stops most seedlings from growing. However, the forest can still regenerate as long as there are gaps in the forest canopy to let light in.

But here we run into a problem with the type and age of the trees. Rangatira was extensively farmed up until 1959, with much of the island’s forest being cleared. Today, the forest on Rangatira is largely all the same age and species (Chatham Island ribbonwood) having regenerated from 1959 onwards. Larger, older trees (Chatham Island akeake) are few and far between and these are the ones that create most of the gaps in the forest. This doesn’t look likely to change either, with a clear majority of the new seedling species being the now dominant Chatham Island Ribbonwood, not the old-growth cover of Chatham Island akeake.

The trees being the same age and species, a lack of canopy gaps, and intense seabird burrowing destroying seedlings and loosening roots all mean the forest is more vulnerable to a big storm event flattening a large number of trees. Something like this would be a disaster, not only for the seabirds but for the many unique forest birds on Rangatira, the most famous of which is the critically endangered black robin.

While this forest collapse is a possibility, the more likely outcome is that individual trees die over a longer period of time, allowing the forest to regenerate in the gaps the dead trees create. However, the effects of the previous land clearance and millions of nesting seabirds means that the future of the forest is uncertain. In an undisturbed forest the high concentration of seabirds wouldn’t be a problem, but because of the previous clearing of the island this disturbance has the potential to affect the regeneration ability of the forest.

The findings from this research have been used to inform further studies on island restoration, something particularly important to the Chatham Islands and its birds, like the black robin. It has also been used in comparisons between burrowing seabirds and rats, again focused on New Zealand’s world leading island conservation efforts. The full study conducted by Cynthia Roberts, Richard Duncan and Kerry-Jayne Wilson can be found online at the New Zealand Journal of Ecology: Burrowing seabirds affect forest regeneration, Rangatira Island, Chatham Islands, New Zealand (2007)

Watch this space for the future of Rangatira’s forests, and maybe one day you could be lucky enough to strap on a pair of petrel boards and witness the Rangatira seabird stampede for yourself!

Fraser Gurney is a postgraduate student completing a Master of Science (Conservation and Ecology). He wrote this article as part of his assessment for ECOL 608 Research Methods in Ecology.