Life in and around rivers
Before widespread human settlement, most lowland rivers were bordered by dense native forest with many ferns, mosses, liverworts and fungi. The forested river banks were home to birds, lizards, slugs, snails, flatworms, earthworms and insects. Overhanging vegetation provided shade and food for native fish, and kept water temperatures low so that the growth of algae was controlled.
Since the clearance of forest for pasture, these habitats and many native species have been lost because of several factors:
- With increased light, algae and introduced weeds have spread in rivers, while the warmer water has killed many organisms.
- Silt from eroding banks threatens water quality and the fish and insects that live in rivers.
- Hydroelectric developments have flooded or dried up parts of rivers, destroying habitats.
- Exotic species compete with native plants and fish.
- Farming leads to runoff of fertiliser and animal waste that has killed native species.
Untouched river environments can host a range of native plants. Wetland vegetation, including kahikatea, cabbage trees, mānuka, New Zealand flax and rushes, often grows on river margins. Conifer–broadleaf forest grows beside many wild rivers, and mosses, liverworts and ferns can grow both near and in the water.
Around South Island braided rivers, plants such as willowherb, matagouri, silver tussock and many sedge species are common.
Introduced plants such as pines, willows, lupins and hawkweeds are spreading through many river banks and beds. They smother native plants, and harbour pests such as the willow sawfly, and predators such as wild cats, ferrets and hedgehogs. Threatened plants include a forget-me-not (Mysotis uniflora) that forms mats near braided rivers, a river-dwelling moss (Fissidens berteroi) and a liverwort (Schistochila nitidissima).
Fish and crustaceans
Around 40 species of native freshwater fish, including lamprey, eels, smelts, southern graylings, galaxiids, torrentfish, bullies and flounder, are found in New Zealand rivers. They prey mainly on invertebrates including the native crayfish Paranephrops zealandicus and P. planifrons.
Aggressive introduced trout compete for food, and 10 species of native fish, including koaro and the dwarf īnanga, are now considered threatened.
New Zealand rivers are home to many species of wetland and wading birds. The paradise shelduck is found on wide gravel riverbeds, while the grey duck prefers slow streams. Wading birds such as the pied stilt, banded dotterel, black fronted dotterel and wrybill plover make their nests mostly on the beds of braided rivers in the eastern South Island. Black shags, little black shags and little shags are often seen fishing in rivers as well as coastal waters.
A pretty pest
Russell lupins were planted in front of the Hermitage hotel at Mt Cook in 1930 to cover a rock slide. They soon spread to the Mackenzie Basin, and although they blaze with colour in summer, they are a major threat to braided river environments. Lupins can quickly colonise islands in the riverbed, destroying the habitats of native species.
Some birds that live around braided rivers, notably the black stilt, wrybill and black fronted tern, are threatened by predators and loss of habitat. The whio (blue duck) has to compete with trout for insect species. It is now reduced to a few small populations on remote, fast-flowing rivers in the North and South islands. The brown teal, once common on swampy streams and tidal creeks shaded by overhanging trees, has also been dramatically reduced in numbers, and is now found mainly in Northland.
Insects such as caddisflies, dragonflies and mayflies are found on the banks of rivers in their adult stage, and in the water during their larval stage. Snails and worms live on the banks and in the water itself. The range of some insects has been limited because of changes to their habitat. For instance, the robust grasshopper (Brachaspis robustus) used to be found in dry, stony riverbeds in Otago and South Canterbury but is now confined to the Tekapo, Pūkaki and lower Ōhau rivers and a few sites further east.