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Life on braided rivers

Life on braided rivers

Braided river ecosystems

Because braided rivers are globally rare, their braidplain ecologies are equally rare biodiversity hotspots. The life in them is highly specialised, the ecological relationships complicated by the fact that braided rivers stretch from the highest peaks in the mountains to the coast. Some rivers, like the Tasman and Makarora, travel just a short distance to end in glacial or alpine lakes. 

Others travel through alpine tussock lands, forests, dairy and sheep farms, exotic tree plantations and vineyards, narrow gorges of ancient Torlesse greywacke and not quite as ancient limestone filled with a menagerie of prehistoric fossils, through coastal plains, beside roads, through towns and cities, coastal wetlands and dunes until finally reaching the sea. 

A huge variety of birds, fish, reptiles, invertebrates, plants, lichens, mosses, and fungi, have adapted to thrive in these challenging and incredibly dynamic environments defined by channels of water weaving between temporary islands of gravel. And each of them are dependent on one another (Video 1).

Video 1

In recognition of their importance and value, braided rivers are the only ecosystem to have its own set of targets in the Canterbury Water Management Strategy (CWMS). Yet in spite of the resilience of life in and around these rivers, many species, indeed entire braided river habitats are under threat (Video 2).

Video 2

A unique evolution

“New Zealand is as close as we will get to the opportunity to study life on another planet.” – Jared Diamond

Elsewhere in the world, most species are threatened by pollution and habitat loss. The situation in New Zealand is further complicated because of our unique and relatively isolated evolution, for here the ghosts of the supercontinent, Gondwana still roam.

New Zealand – Aotearoa sits on what has been described as the ‘seventh continent’ of Zealandia, a mass of continental crust that stretched and sank beneath the ocean after breaking away from Gondwana some 85-130 million years ago. Living fossils such as the tuatara and Archey’s frog, creatures that pre-date the dinosaurs, are evidence that at least some parts of Zealandia have remained above the surface of the ocean ever since.

Isolated from other landmasses, over millions of years these ghosts of Gondwana were joined by a few hardy creatures carried by waves and winds from Australia, South America, the Pacific Islands, and (once largely ice-free) Antarctica.

Elsewhere, the demise of the dinosaurs heralded the rise of mammals.  Not so in New Zealand. Here, ecological niches were instead filled with birds, three tiny insectivorous and fruit-eating bats, and invertebrates.

Their evolution was a product of turbulent and often violent change. Around 40 million years ago, the tectonic forces that once stretched Zealandia began to change direction. Parts of Zealandia were pushed together along the Alpine Fault. Since then, a staggering 20 kilometres-thick section of the Earth’s crust has been thrust upwards. In the warm wet world of Miocene, rainfall eroded the mountains, what we now call the Southern Alps – Kä Tiritiri o te Moana, almost as fast as they grew. Glaciers came and went; carving out valleys along which flowed braided rivers that deposited the eroded material to form wide plains.

When humans arrived, these globally rare ecosystems were disrupted for reasons explained here and extinctions soon followed.

Today, on the plains away from the coast, despite having received little protection, only the braided rivers still have what little remains of our extraordinary history.

While more than 80 species of birds are found along braided rivers, a rare handful have adapted to breed almost exclusively in this dynamic and ever-changing environment. Less obvious but equally integral to these ecosystems are the native reptiles, invertebrates, fish, and plants. But their populations are declining, fast. Several species are at now at risk of extinction for reasons outlined in threats.

Endemic or native: what's the difference?

Endemic: found ONLY in New Zealand, nowhere else in the world. If they become exitinct here, they’re gone forever.

Native: native of New Zealand but also found elsewhere. This applies to many migratory birds.

New Zealand threat classification

Video 2

Download as a PDF: Change in threat status of riverine birds 

The New Zealand threat classification system outlines what animals and plants are placed into these categories and why. Each section is updated every few years. The current edition is 2021.

To place it in context Nationally Endangered means two stops to extinction. Nationally critical means next stop – extinction. 

Black-fronted terns, for example, are Nationally Critical – just one step from extinction. They don’t receive the same attention as iconic species like kiwis, and yet they are far more likely to vanish forever unless measures are taken, and taken very soon, to reverse this trend. Declining is an early warning alarm, signalling something is wrong.

Many birds and other species such as the white-fronted tern and black-billed gull are long-lived. While the total number of birds might appear to be reasonably high, if too few chicks reach adulthood to be recruited into the future breeding population, once older adults reach the end of their lifespan, the population can drop catastrophically. This appears to be the happening with several bird species.

Wrybill The only bird in the world with a bill that's bent sideways

Wrybill: the only bird in the world with a bill that's bent sideways: always to the right
Wrybill: the only bird in the world with a bill that’s bent sideways: always to the right

Further information & references

Specific research and references for individual species are found on their respective pages. Below is a general reference list.

Black-billed gull: the most endangered gull in the world.
Black-billed gull: the most endangered gull in the world.

Black-billed gull The most endangered gull in the world