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New Zealand’s streams are home to hundreds of tiny animals that live on and under rocks, water plants, wood or debris. These animals include insects, crustaceans such as freshwater crayfish (koura), molluscs such as snails, mussels, worms and leeches. They range in size from less than 1mm long to over 10cm long and can often be hard to see at first glance. Together these types of animals are called invertebrates.
One very special insect that lives on riverbeds and terraces in the Mackenzie Basin, is the Robust grasshopper (Brachaspis robustus), a highly threatened species that epitomises the vulnerability and beauty of these special habitats. Numbers have dropped in recent years as habitat quality and quantity have both decreased (see threats). This is our largest lowland grasshopper, growing to 44mm, and is typically cryptic (hard to see) amongst the stones and boulders of its specialised habitat. Near Twizel, the Department of Conservation has created habitat and introduced a population of the grasshopper in an effort to secure this local endemic species.
River birds feed on invertebrates, primarily insects and some worms, found in braided rivers. In the water, under the stones, are the larvae of insects such as mayflies and caddisflies, particularly in riffles where the bubbling water has a high level of oxygen to support large insect numbers. Mayfly larvae are fast-moving and hide under rocks. Some caddisfly larvae build a case of sand grains to hide in. Adult flies fly over the water. In a braided river the availability of food is always unpredictable. During lean times, the birds must range from the riverbed into stable side channels and pond areas to find food. Others will use the opportunity of ploughed fields to search for beetles and worms.
Each bird species has evolved to feed on insects in distinct ways. Specialisation minimises competition for food between them.
Wrybills feed in shallow channels, riffles and the edges of pools. Their bent bill is specially adapted to allow them to reach under stones for mayfly larvae.
Black-fronted terns and black-billed gulls feed on the wing over main channels, catching insects in the air or scooping insects and fish from the water’s surface. They sometimes feed on insects from surrounding farmland.
Black stilts, with their long legs, wade in deeper slow- moving water, reaching insects on the bottom with their long necks and bills. Sometimes they dart at insects and small fish in riffles or muddy areas.
The long bill of pied oystercatchers allows them to probe deep into mud, sand or under pebbles, to find worms and insects. As well as using riverbeds oystercatchers also probe for worms and small beetles on pasture and ploughed land. On coastal areas, they feed on shellfish (hence their name) small crustaceans and cnidarians (jellyfish).
Banded dotterels feed on moths, flies and beetles found among scattered low vegetation on the high parts of the riverbed and along the muddy edges of lakes and rivers. They have a distinctive run-stop-peck-run movement while they feed.
* ‘Invertebrates’ means they have no backbone. Sometimes you will see the term ‘macroinvertebrate’. This means they can generally be seen without a ‘micro’scope, although a magnifying glass is often helpful to see details.