Threats to wetlands
Wetlands face many pressures. Drainage and infilling directly affect wetlands. Some land management practices have indirect effects, for example, nutrients and sediment can wash from the land into the wetland. Plant and animal pests can also cause problems. Careful management can decrease the effects of these threats on our wetlands.
Drainage and cultivation have greatly improved our ability to farm land profitably in the Waikato region. But it has also caused the loss of 75 percent of our region’s wetlands. We must maintain our remaining wetlands to protect their biodiversity and maintain the services that wetlands provide (for example, flood management and improving water quality).
Excessive drainage at the edge of peat bogs can cause a drop in the wetland’s water level. This can stop the formation of peat and changes the types of plants that grow there.
It is not only drainage of wetlands or near wetlands that can affect their water levels. Changes further upstream, such as damming or channelling waterways, can change how much and when water reaches a wetland.
Find out more about managing peat to protect surrounding wetlands.
For policy information on diverting and damming water check out our Regional Plan.
Game bird hunting, eeling and whitebait harvesting are important traditions for many New Zealanders. Hunting and harvesting these species puts pressure on their populations, but it also encourages people to conserve wetland habitats. Conserving wetland habitats for hunting and harvesting has spin-offs for many other native animals and plants.
Peat is harvested from some wetlands in the Waikato region (such as Torehape in the Hauraki Plains) for horticultural use. Drains are dug to lower the water level and dry out the top surface of the peat. The vegetation is removed and the dried out peat skimmed off the surface. At Torehape this is happening in sections, so that some natural vegetation is left to provide wildlife habitat. After the top layer of peat is removed, the water levels and native vegetation are restored to a functioning bog again.
Stock with uncontrolled access to wetland areas can:
- increase nutrient levels through their urine and dung
- compact or erode the soil
- disturb wildlife
- graze on and trample wetland plants
- open up sites for weed invasion
- carry weed seeds in their hooves, coat, or dung
- become bogged down and trapped.
Wetlands, and valuable stock, can be effectively protected from each other with standard fencing.
Fertiliser applied to farmland can affect nearby wetlands through nutrient enriched run-off, leaching and spray drift. Nutrients can also enter wetlands during storms or floods, when eroded soil washes down into wetlands. While swamps play an important role in naturally filtering out sediments and nutrients, excess nutrients in wetlands, particularly low nutrient bogs, can change the delicate balance of plants and animals in the wetlands. At the margins of peat bogs, increased nutrients aid the invasion of exotic plants, such as grey willow.
Research at Moanatuatua Reserve has found increased phosphorus levels in the peat bog, probably from top dressing (aerial application of fertiliser). Phosphorus has been show to inhibit the root growth of wire-rush, the main peat forming plant at Moanatuatua. Since 2002 the peat surface at Moanatuatua has shrunk by up to half a metre.
Plant pests threaten native biodiversity in many Waikato wetlands. They can grow very quickly and compete against native plants for space, nutrients and light. Grey willow, introduced in 1925, now covers over 10,000 ha of wetland in the region (one third of all the remaining area).
The exotic grass Glyceria maxima is a persistent problem for replanting programmes in some wetlands and requires ongoing management to stop it from smothering the young native plants. Other plant pests include various species of waterlily, parrot's feather and yellow flag iris.
Our Regional Pest Management Strategy sets out management programmes for plant and animal pests in our region that need to be managed in order to protect our environment.
Find out more about plant pests in the Waikato region.
Deer, rabbits, hares, and pigs have become established in some wetlands and pose a threat to native plants and animals. Rats, cats, possums, hedgehogs and mustelids are also found in Waikato wetlands. They kill native birds especially during the breeding season when incubating birds are often reluctant to leave the nest. Young chicks and eggs are also taken, as are other native animals such as bats, lizards and insects.
Exotic fish such as koi carp and rudd affect aquatic vegetation through grazing. They also affect water quality by stirring up sediments as they feed. Find out more about pest fish in the Waikato region.
Fires were a natural occurrence in wetlands, particularly large peat bogs. Many native wetland plants are adapted to fire. Some have underground rhizomes (runners) that can rapidly resprout after a fire. Others rely on fire for their existence as they grow in burn holes (small clearings created when fire removes the vegetation in localised areas).
But wetlands now are much smaller than they used to be, and a single uncontrolled fire can burn most or all of a wetland. When a whole wetland is burnt, few plants and animals survive to recolonise the burnt area. Animal survivors in particular will not live for long without the cover and food provided by nearby vegetation.
The lack of fires (because of necessary fire control) means that burn hole habitats are uncommon. This has caused some plants that depend on burn holes, such as the swamp helmet orchid Corybas carsei, to become threatened with extinction.
In the 1980s two fires in the Whangamarino Wetland destroyed 57 per cent of the vegetation. These fires left large areas vulnerable to introduced plants. In areas of high fertility these exotic plants could dominate the native plants and prevent natural regeneration1.
- Clarkson, B. 1997: Vegetation recovery following fire in two Waikato peatlands at Whangamarino and Moanatuatua, New Zealand. New Zealand Journal of Botany 35:167–179.
Updated August 2017