Dunefields are formed above the beach as dry sand blows inland. Dunes occupy about 1,100 km of the New Zealand coastline. An impressive dunefield stretches 130 kilometres from Paekākāriki to Pātea on the west coast of the North Island, and up to 19 kilometres inland. Wide, gentle-sloping beaches are framed by fore dunes, formed when wind-blown beach sands become trapped by plants and driftwood on the beach. If there is a ready supply of sands on the beach, more dunes may develop in front of the original. The dunes further back may be stabilised by plants, or blown inland if they fail to retain a good plant cover.
Plants of the fore dune
Beyond the high-water mark three plants can colonise the damp sands – pīngao (Desmoschoenus spiralis) and spinifex (Spinifex sericeus), two native sand-binding plants, and the introduced marram grass (Ammophila arenaria). The shape of the dune is influenced by the vegetation: spinifex and pīngao produce dunes with a low, regular profile; clumps of marram produce higher, irregular dunes.
Marram was widely planted on New Zealand dunes in the early 1900s, in an effort to stabilise dunes that were moving inland. It is now the dominant coloniser, often backed by stabilised dunes planted with introduced lupins and pine plantations.
New Zealand’s venomous native spider – the katipō – is a sand-dune specialist. Two species are now recognised: red katipō (Latrodectus katipo), with a red-orange stripe on its back, and black katipō (Latrodectus atritus), a wholly black spider. They inhabit the landward side of fore dunes, where they spread webs in pīngao and marram, or under driftwood. The female can deliver a nasty bite, but by nature is a retiring animal and only attacks if frightened. They are in serious decline throughout New Zealand as dunes become covered by dense marram, lupin, pines or pasture grasses – the spider prefers more open clusters. A similar-looking South African spider (Steatoda capensis), which can inflict a painful bite, has moved into coastal sites and may be competing for resources.
The harmless shiny black sand scarab (Pericoptus truncatus) is one of New Zealand’s largest beetles – 4 centimetres long. Its larvae make numerous little tracks that lead to holes in the dunes.
Middens – ancient refuse heaps – are scattered along the dunes of the North Island and much of the South Island. They date from pre-European times, and reveal a wealth of environmental and cultural information about coastal conditions 200 to 700 years ago. The oldest shell middens contain bones of forest birds and fur seals. But within 200 years of human settlement, fur seals had disappeared from northern sites and fewer forest birds were eaten at coastal sites.
About 450 years ago coastal gardening by early Māori failed in the South Island and southern North Island. This coincided with an advance of coastal dunes inland. One trigger was probably Māori clearance of coastal forest, which removed shelter and exposed the soil to wind erosion. Climate cooling may also have contributed to crop failure.