By Richard Storey
Each summer many of us don a backpack and head into the mountains to immerse ourselves in spectacular scenery relatively untouched by human activities. Undoubtedly the tracks we follow will take us across dozens of clear, tumbling streams where we might refill our water bottles or splash cool water on our faces. But how often do we think about what lives in these streams?
We might think that being close to popular walks, the animal communities inhabiting these streams are well known to science. But despite over 150 years of freshwater research in New Zealand, little is actually known about the stream invertebrates (insects, crustaceans, worms, etc.) living at high altitudes, or how the animal community differs from one alpine stream to another. This is largely because most freshwater research focuses on areas where human activities could have an impact on stream-dwelling animals. High in the mountains, we assume that invertebrate communities are safe from human impact. And so there are still significant gaps in our knowledge of New Zealand’s freshwater biodiversity. This makes it hard to classify different types of streams, which is an important step for managing and protecting them. Also, climate change means that now no place is free of human impact, and alpine ecosystems may be specially vulnerable to temperature rises. Alpine communities might start changing before we have even described them.
For these reasons, in March 2012 I made a field trip through several sites in the Southern Alps. With field assistants from Department of Conservation, University of Canterbury and Environment Southland, I collected benthic (bottom-dwelling) invertebrates from 48 different streams, focusing on areas where a new stream classification system showed there was a wide diversity of stream types. We began at the Cascade Saddle near Wanaka where we followed the Dart River from its source beneath the Dart Glacier. Then we took samples along the Routeburn Track and from a number of steep, glacier-fed streams along the Milford Sound Road. Finally we helicoptered into several small headwater streams in the Eyre, Garvie and Takitimu Mountains in Southland.
We found a number of very interesting species living in these streams. One species of stonefly that turned up in several places had only been found previously in one spring in Canterbury. Other species of stoneflies and caddisflies were alpine specialists that are never found in lowland sites. Stoneflies and caddisflies are common groups of stream insects that eat fragments of decaying plant material or other stream invertebrates. Overall, all of the alpine streams we visited had an invertebrate community distinctly different from lowland streams. In particular, streams emerging from glacier mouths had a very specialised fauna, consisting of only one hardy species of mayfly, two stoneflies, two caddisflies and a number of midges. Glacier mouths are a very harsh environment, not just because of the extremely cold temperatures (less than 1 °C!) but also because the flow and river bed are always shifting in response to freeze-thaw cycles in the ice.
The results of this study will be very helpful in adding to our knowledge of New Zealand’s biodiversity, improving classification of New Zealand streams, and providing baseline data for detecting effects of climate change on alpine ecosystems. We hope they will also stimulate research in the ecology of this extreme environment.
Dr Richard Storey is an aquatic ecologist at NIWA.