Pushing s*** uphill
Seabirds, soil fertility and the survival of our species (10 minute read)
In a fracas of flapping, a flock of startled seagulls rises as one. A moment before, they were standing on the shore, all facing the same direction. Now they are airborne, and I warily watch to see which direction they take. I think that they might be heading towards me, but then they turn and head the other way. As much as I love them and their squawking, squabbling ways, I don’t want to be underneath as they fly over.
When I was at secondary school, we used to call gull flyovers “air raids”, because there was always the danger of them dropping something unpleasant. My school was beside Lake Pupuke, and the playing fields were often occupied by flocks of gulls, along with ducks, geese and occasionally swans. Some of them were confident to the point of pushy, and would demand food when we were eating our lunches. I usually found a crust or two for them, which only encouraged them to be more demanding. And they did make an awful mess.
New Zealand has three species of gull, although most people only encounter two. My favourite is the red-billed gull or tarāpunga, a white gull with pale-grey wings, black-marked tail and a vivid red bill, legs and eyelids. There’s also the much larger black-backed gull or karoro, with a white body, black wings and yellow bill with a red spot on the end. The third gull is the black-billed gull or tarāpuka, similar in appearance to tarāpunga, but smaller and with a black bill and legs. It’s more associated with rivers than the coast. Unlike the other two, it doesn’t have the habit of hanging out in towns and cities, scavenging food from humans.
While I think of gulls as seabirds, strictly speaking only the tarāpunga is a true seabird, that is a bird which gets most of its food from the sea. The tarāpuka feeds on small fish, insects and other small animals in or around rivers and the coast. The karoro will eat just about any animal it can catch or scavenge from land or sea, including insects, fish, other birds and eggs, small mammals – sometimes even newborn lambs – and dead seals. But the tarāpunga depends on krill to breed, even though it’s certainly partial to discarded chips and other scavenged delights.
As well as gulls, New Zealand has many other seabirds – so many, in fact, that we claim the title of seabird capital of the world. Globally, there are 360 species which get their food mainly from the sea, and 38 of them breed only in New Zealand (including the sub-Antarctic islands and the Kermadecs, but not Antarctica). A further 48 species breed in New Zealand but also breed elsewhere. Our seabirds belong to four different groups: gulls, skuas and terns; penguins; albatrosses and petrels; and a variable group which includes gannets and cormorants, among others. Wading birds, such as the ruddy turnstone, aren’t counted as seabirds, since they feed mainly on the boundary between land and sea.
New Zealand doesn’t just have a lot of seabird species. In sheer numbers, we have a lot of seabirds too. Exactly how many, though, is hard to say. Our most abundant species is the tītī or sooty shearwater, which belongs to the same group as albatrosses and petrels. It’s estimated that there were just under 23 million around New Zealand in 2005, which sounds like a lot until you realise that there were probably nearly 40 million in the mid-1970s. There are also estimated to be over a million breeding pairs of the takahikare or white-faced storm petrel, while a number of other species have numbers estimated to be over 100,000, such as the tākapu or Australasian gannet and the rako or Buller’s shearwater.
While such numbers might suggest these birds are doing well, it isn’t that simple. The huge decline in the number of tītī shows that these birds face serious problems. Unlike our land birds, which are mainly threated by introduced mammal predators, seabirds face a range of complex threats. The tītī decline is attributed to a combination of human harvesting, being accidentally caught by fishing vessels, predators and climate factors. Depletion of fish stocks by commercial fishing may also contribute. The extent to which these factors can be managed varies. Tītī are an important traditional food for Māori from Rakiura or Stewart Island, and harvest is traditionally managed, using information from a range of sources, including mathematical modelling. Predators can also be managed relatively easily, and accidental catch by fishing vessels has reduced considerably over the last few decades. However climatic factors, such as El Niño events and climate change, are harder to control.
The tītī isn’t the only common seabird which has declined enough to warrant inclusion on New Zealand’s list of threatened species. So too has the rako, the tarāpunga, and the tara or white-fronted tern, the most common tern around the New Zealand coast. Even many of those which aren’t classed as threatened species have seen precipitous declines since the arrival of mammal predators. Many, such as the takahikare, are now found mainly or entirely on predator-free islands, when they would once have been found all around the New Zealand coast.
Other species are in even more trouble. The tara iti or fairy tern is New Zealand’s most endangered bird, with fewer than a dozen breeding pairs. The tāiko or black petrel was once abundant around the North Island and parts of the South Island, but now breeds only on Aotea and Hauturu (Great and Little Barrier Islands). Its larger cousin, the Westland petrel, also known as tāiko, still breeds on the mainland on the South Island’s West Coast, but has suffered considerably from being caught by fishing vessels.
There are glimmers of hope, though. The Chatham Island tāiko was thought to be extinct until rediscovered in 1978 by local landowners Manuel and Evelyn Tuanui, working with ornithologist David Crockett. The Tuanui family later donated 1000 hectares of their land as a reserve to protect the tāiko and other endangered species on the Chatham Islands. The takahikare-raro or New Zealand storm petrel was also believed to be extinct – for more than 100 years – until birdwatchers photographed it near Whitianga in 2003. Ten years later, it was found to be breeding deep in the forest interior on Hauturu.
Despite these hopeful stories, there’s more bad news than good when it comes to New Zealand’s seabirds. At least half of the species which breed only in New Zealand are classed as threatened to some degree (this figure includes the Kermadec and sub-Antarctic islands). A number of those which are also found overseas are endangered throughout their range as well. Where once there were large colonies of seabirds from one end of New Zealand to the other, there are now relatively few large colonies on the mainland. As with the tītī, there are many interlinked factors involved, including introduced predators, human hunting, being accidentally caught by fishing vessels, disturbance, pollution, declining fish stocks and climate change.
For those passionate about New Zealand’s wildlife, this is a tragedy simply because we have lost so many species, and we don’t want to lose more. But the loss of our native seabirds, both in terms of absolute numbers and in terms of species, has much more significance than most of us realise. In fact, I had no idea until a few months ago, when a chance comment from an ecologist sent me looking for more information.
If you encounter a large colony of seabirds, whether penguins, petrels or gannets, one thing is unavoidable – these birds produce, to put it politely, a lot of waste. With a bird such as the tara or white-fronted tern, which breeds on the edge of estuaries and the mouths of rivers, much of that waste is likely to be washed back into the ocean when there are storms or floods. This isn’t the case for all birds, though. Even the kororā or little penguin, the smallest of the penguins, can walk a considerable distance inland – up to 1.5 kilometres, and up to 300 metres in altitude. The waste that they produce as they nest on land ends up in coastal forests, or it did when our coastline was mostly forested. Birds that fly may nest even further inland. Some species of tītī used to breed 20-30 kilometres inland on the North and South Islands of New Zealand.
Why does this matter?
Over the last couple of years, I’ve been looking at the state of New Zealand’s land and waterways. When we cut down most of our forests, I realised, we also lost a massive quantity of our soil. It was washed away from land too steep, wet and unstable to be held together by the roots of pasture grasses, and ended up in the sea. We can’t get that back. It’s carried by gravity, flowing one way.
However, seabirds are like biological pumps. They take krill or fish from the sea and turn it into fertiliser on land. They bring the fertility of the ocean to the land, carrying it uphill, in a form which plants can easily use. This can be seen with the Westland petrel, which breeds in coastal forest north of Greymouth and enriches the soil by carrying nutrients from the ocean to the forest. The colonies move around, meaning that, over long time scales, nutrients are added over a much wider area than that occupied by the birds at any one time. However, the full implications of these nutrients on forest growth, and the fertility of our land in general, are still not well-understood. It’s clear, though, that we have suffered a massive loss in fertility, as forest destruction led to our soils washing into the sea, and the catastrophic declines in mainland seabirds meant that losses were no longer replenished.
The most important nutrients carried by seabirds from ocean to land are nitrogen and phosphorus. These are limiting nutrients in many environments, which means that the growth of plants is limited by the availability of one or the other of these. It means that in some environments, the growth of plants, and therefore the growth of every species in the environment which depends on them, is dependent on the seabirds. Even environments some distance from seabird colonies can be enriched by them. If you’ve ever been close to a dense colony, you’ll be familiar with the smell of ammonia which emanates from the thick layer of waste which covers everything. Ammonia is a gaseous form of nitrogen which dissolves easily in water and is easily absorbed by plants. The gas is carried on the wind, dissolves in rainwater and then falls on land some distance from seabird colonies.
Even though seabirds have declined in many places, seabirds still make a significant contribution to the movement of nitrogen and phosphorus on a global scale. For example, the amount of nitrogen added to the land by seabirds is not much less than the amount added to the land by lightning. Despite this, traditional models showing the way these nutrients cycle through the environment don’t mention the input of seabirds, even though they do include lightning.
The unacknowledged contribution of seabirds to soil fertility on land is part of a larger problem in the human relationship with nature. The air we breathe, the water we drink, the soil which grows the food we eat, the climate we depend on to prevent us either overheating or freezing – all of these exist because of nature. All depend on the complex interactions of plants, animals and microbes which make up our living environments, interactions which we barely understand.
We might live in cities, soar in the stratosphere in gravity-defying aircraft and communicate in an instant with people on the far side of the planet with barely a thought about how remarkable that really is. We might have created computer software which can identify plants, translate languages, write articles of questionable accuracy on all manner of subjects, predict what will get us addicted to our phones and manipulate us into believing the lies of Donald Trump and Elon Musk. We might think that we can move fast and break things as Mark Zuckerberg said, without suffering the consequences. We might, like Shane Jones, think that we can sacrifice a species if it’s in the way of a mine.
But we don’t fully understand the mind-blowing complexity of the natural world. We don’t understand which species are crucial to our survival. We don’t understand what’s keeping us alive. We can’t keep trying to live apart from nature. We need to start noticing, and appreciating, all that the natural world does for us.
Your bird knowledge is amazing.
Many thanks for sharing. When species disappear, the losses always go much deeper than you might think at first.