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  Environment » Natural Resources » Biodiversity » Forest fragments » Threats to forest fragments

Threats to forest fragments

Most areas of native forest in the Waikato region (88 percent) are small forest fragments of less than 25 hectares. Pests and weeds threaten all areas of native forest in New Zealand. However, our small forest fragments face an additional range of problems that in larger sites are less important. Threats to forest fragments need to be identified and managed if our native plants and animals are to survive in pockets scattered across the region.
Photo of a forest with little undergrowth. On this page: Edge effects, Land use next door, Size and shape, Isolation, Other threats

Edge effects

Forest fragments that are small and uneven in shape have most of their vegetation exposed at the fragment’s edges. In small fragments (less than 25 ha) in the Waikato region, 73 percent of the forest vegetation is within the ‘edge zone’. These areas are exposed to damaging ‘edge effects’ from outside elements.

The edges of forest fragments face:

  • Exposure to wind and sun.
  • A change in the type of species living in the edge zone.
  • More weeds and pests invading from the edges.
  • Exposure to new threats from different land use next door.

Land use next door

Most Waikato forest fragments (99 percent) are next to pasture, but a small number are surrounded by pine forests, urban areas, or natural areas such as scrub or wetlands. The type of land use next to a fragment will affect how healthy it is. Land use types most similar to native forest (such as native scrub or plantation forest) are usually the best ‘next door neighbour’.

Find out more about types of land use in the Waikato Region. The Waikato Regional Council monitors regional land use changes through our indicator.

Size and shape

Most Waikato fragments (88 percent) are under 25 ha in size. Small fragments may not have the variety or number of plants needed for year-round food, or provide big enough territories in which native animals can forage and breed.

Smaller patches of native forest:

  • May be too small to support healthy populations of some species.
  • Support few species that need large territories, such as kaka or kiwi.
  • May lose important species that pollinate or spread the seeds of native plants.
  • Will probably lose genetic diversity.
  • Are more affected by ‘edge effects’.

Many fragments (especially in gullies and alongside streams) are long and skinny shapes, with a great deal of ‘edge’. This leaves them more at risk from the edge effects described above. In very large forest blocks only a small portion of their vegetation is in the edge zone, even if the shape is uneven.


Nearly all (99 per cent) of the forest fragments in the Waikato are separated from each other by pasture.

Isolation can cause problems for wildlife trying to find enough food or a breeding partner. Species that can't travel between fragments may die out in some areas. Isolation can also mean that fewer seeds of native plants are carried to sites where they can germinate and grow.

Find out more about forest fragment isolation in our indicator.

Other threats

Other threats to forest fragments include damage caused by pests and weeds, drainage or water shortages, putting in vehicle tracks, land slips, and in drier areas, fire risk.

Learn more about fragment edge effects and isolation, and find out how to manage and restore forest fragments.

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