It has taken only one thousand years of human occupation to fundamentally reshape New Zealand’s ecosystems and the pace does not appear to be slowing. Between 1997 and 2002 native land cover decreased by 16,500 hectares. 2498 Of New Zealand’s entire land area, 63 per cent has been converted for human occupation and use. 2499 It is estimated that nearly 90 per cent of wetlands have been modified for land development. 2500 Forests have been cleared, estuaries filled in and rivers drained, all degrading ecosystems in the process. This intensive transformation of landscape and habitats is one of the most significant contributors to New Zealand’s increasing loss of biodiversity.
As a result of the extensive clearance of native forests, New Zealand’s lowland indigenous forests have experienced high levels of loss. This is principally because of the development of pastoral agriculture and production forestry, which now cover between 80 and 90 per cent of low-elevation environments. Forest remnants are small patches of indigenous forest or shrub land which were once part of much larger forest systems. They are ecologically valuable, as they provide reservoirs for native plant seeds allowing natural regeneration.
They often occur in gullies and on steep slopes where they protect soils and streams. They also provide small examples of forest types that are now highly uncommon, such as the kahikatea/matai forest type of the Canterbury Plains.
In most regions, numerous forest fragments remain within the mainly pastoral farming matrix, but they are typically small and isolated. The historical threats of logging and clearance have been replaced by other threats which continue to cause decline in these forest fragments.
The fragmentation of lowland and coastal forests, especially on private land, is of particular biodiversity concern. Forest remnants require a greater level of management because they are highly vulnerable to a number of threats, particularly from invasive species. In addition, there is often little opportunity for species to interact between forest areas with the frequent absence of corridors linking these pockets. Forest remnants are also increasingly exposed to effects from surrounding land uses, such as land drainage and spraying.
Fragmentation leads to problematic edge effects. In undisturbed forests, there is normally a gradual transition between forest and grasslands. This transitional area can cover considerable distances. This transition area or ‘edge’ prevents wind entering the understory by ‘lifting’ it over the top. With protection from wind, the forest offers a humid microclimate that supports the germination of many native plant species. Forest remnants have a large proportion of their area exposed to the elements, which means the detrimental effects of wind and sun penetrating the understory are significant. These effects include increased wind-throw of trees, a drying out of the forest floor, and a greater susceptibility to weed invasions.
Since the late 19th century, fire has been used extensively for a range of agricultural purposes. It has been used to facilitate the movement of grazing animals, to promote new tussock growth which is palatable to stock, and to convert tussock grassland into improved pasture. Fire can cause major damage to tussock communities, because it kills the roots of the plants. The grasses can take decades to rebuild their above-ground biomass and energy stores following fire.
In addition, fire can completely remove bryophyte (moss) cover which ultimately alters the ability of tussock communities to store moisture. When fire is used as an agricultural method, it can cause the loss of tall tussock species, which are then replaced by short tussocks or invasive weeds such as Hieracium. Fire prevents regeneration of these plant communities, reduces plant diversity, and negatively impacts on the species that rely on grassland ecosystems for their survival.
Pastoral farming occurs on around 40 per cent (six million hectares) of the South Island high country, which generally encompasses the area between the Southern Alps and the eastern foothills extending from Southland to Marlborough. The traditional stock type is merino sheep, whose thick, fine wool allows the animals to flourish at high elevation. Most farmers also stock some cattle and deer. Average stocking rates have recently been estimated at 1.7 stock units per effective hectare.
Grazing has been well documented to drive a change in vegetation pattern and composition. It results in the alteration of tussock ecosystems, because palatable species are selectively grazed. Less preferred species can then dominate and invasive species are able to gain a foothold in the ecosystem. By reducing the vegetative cover, grazing alters the habitat and food webs of lizards and invertebrates. Overgrazing can significantly reduce biomass production and lead to declining food productivity. In addition, herbivores redistribute nutrients within the grasslands, trample soil and increase soil erosion by removing vegetation cover.
A demand for land to provide more productive farmland and space for urban development, has led to the reclamation and drainage of lakes, estuaries, wetlands and river margins, as well as the reduction of farm swamps. In many areas, this has resulted in the almost entire loss of wetland systems, and consequently the biodiversity they support. Land conversion in coastal areas threatens estuarine habitats, which are important to diadromous fish (fish that migrate to and from the sea) and to some commercial fisheries, such as snapper.
In urban catchments wetlands are often polluted by contaminated run-off including zinc from galvanised roofs, oil residues from roads, and also including polychlorinated biphyenyls (PCB’s), flame retardants and caustic cleaning products. Infrastructure development can also introduce barriers to fish migration.
Use of catchments for forest plantations can cause excess erosion and subsequent sedimentation at harvest, or modify run-off and ground water supply, ultimately damaging wetland ecology. In rural areas, grazing of wetlands damages vegetation, preventing regeneration of native species and exposing these communities to weed invasion. It also disturbs wildlife, damages their habitats and causes soil erosion and compaction. Poor farming practice can also increase the sediment and nutrient load in rivers, streams and wetlands. A loss of vegetation on wetland margins further increases the impacts of extra sediment and nutrients on wetland ecology.
Extraction of products such as peat, sand and gravel can alter water levels and encourage weed growth. In some situations this activity has led to greater biodiversity, such as through the valuable habitats now found in some of the gravel pits in the South Island. Whitebaiting and long-finned eel hunting further reduce populations of threatened native fish. Recreational practices such as jet-skiing, hunting or jet-boating can disturb plants and animals, and introduce pests like didymo, if users are careless.
Poor land management practices
Today, many farms are reliant upon the inputs of fertilisers, pesticides and herbicides to run a successful operation. The effects of agricultural chemical use can be long lasting. For example, the pesticide DDT was used by New Zealand farmers during the 1950s and 1960s, primarily to kill grass grub and porina caterpillars. Although its use on farmland was prohibited in 1970, high levels of DDT accumulated in the soils during the years of heavy application. Many New Zealand soils still contain high levels of DDT. The threat to biodiversity from the dependence on chemicals cannot be understated. The key issues arising from the use of toxic chemicals on productive land include:
- Impacts on the hydrological cycle as toxins run off into the waterways and end up accumulating in the groundwater and marine environment
- Pollution of the soil with the accumulation of heavy metals
- Eradication of insects and beneficial pollinating species which are integral to productive land systems
Other agricultural practices can also have a direct impact on biodiversity. Grazing stock can cause significant damage to remaining tracts of native vegetation. They browse on native plant species, disturb the ground and the native species living beneath the surface, and release large amounts of pollutants into the soil and waterways.
The activities which typically cause habitat loss and fragmentation in urban environments are:
- Earthworks and construction of new residential and commercial development, roads and other infrastructure
- Infilling and culverting of waterways
- Discharge of contaminants into waterways
- Destruction of riparian and coastal margins
Habitat loss affects all species in an area, not just one type. The loss of habitat, and subsequent loss of resources, food and shelter, is a significant threat to urban biodiversity. Habitat fragmentation produces isolated remnants that have exposed edges, making them particularly susceptible to degradation, especially by invasive pests and weeds. It is not uncommon for homeowners and urban developers to be unaware of the importance of retaining vegetation. Whilst removing a few native trees on one property may be seen as insignificant, when this is multiplied across an entire city, already fragmented areas of natural habitat become even more isolated. Research in the former Waitakere City showed that between 1993 and 2000 up to 30 per cent of vegetation across the city had been lost. This was from subdivision and development, and also from individual trees being felled on a large number of properties. 2501 Such trees often provide corridors for native birds trying to reach larger forests pockets. They also offer a food source and a supply of resources, such as nesting material. These corridors are important for maintaining genetic diversity.
Ministry for the Environment, 2007
Department of Conservation and Ministry for the Environment, 2000
Department of Conservation and Ministry for the Environment, 2000
Last updated at 1:55PM on February 25, 2015