Human activities and development

4WD vehicles, Hopkins/Te Awa Āruhe riverbed

Many activities associated with land development have adverse effects on the habitats and populations of native plants and wildlife:

• Water abstraction and damming for hydroelectric development alter river flows and developing braids and islands, reducing the amount and quality of braided river and wetland habitat.
• River engineering works, including willow plantings, also alter the way in
  which river channels form and develop
  over time.
• Wetlands are drained for pastoral development.
• Pasture grasses displace native plant communities and other introduced plants.
• Livestock graze or trample native plants in riverbeds and remaining nearby wetlands. Irrigation and fertilisation of pastoral land can result in nutrient run-off into waterways and reduced water quality.

Even recreational activities can have adverse impacts; for example:

• Uncontrolled dogs can scare birds away from their nests, causing eggs and chicks to die.
• Four-wheel drive vehicles used in riverbeds can squash fragile plants and crush the eggs and chicks of ground-nesting birds.

Many of these human activities and land use changes expose braided river ecosystems to invasion by exotic weeds.

Braided river weeds 

The number of weeds spreading in the wild is now similar to the number of New Zealand native plants. Weeds have a huge impact on native plants and animals in riverbeds:

• Many weeds grow bigger and faster than native plants, eventually replacing them.
• Weedy areas are no longer suitable for many native birds, lizards and invertebrates which need open gravelly riverbeds to survive.
• Weed roots bind and stabilise river gravels, creating fewer and deeper river channels which are unsuitable for wading birds to feed in.
• Aquatic weeds smother the bed of waterways or clog water channels, altering the habitat of native freshwater invertebrates, fish and wading birds.

Existing weeds that are of concern in upper Waitaki Basin braided rivers and wetlands include those listed below.  Most of the plants in the list link to the Weedbusters website.

Popular in gardens, Russell lupin is unwanted in river beds

• Broom
• Californian poppy
• Crack willow
• Didymo
• Gorse
• Grey willow
• Lagarosiphon
• Oxeye daisy
• Ribbed melilot
• Russell lupin
• Stonecrop
• Wild mignonette
• Yellow tree lupin

Other weeds that are not currently known to be in the upper Waitaki Basin but which would be of concern if they became established in its braided rivers or wetlands include:

• Egeria
• False tamarisk
• Hornwort
• Parrot's feather
• Purple loosestrife
• Wild thyme

Predators

Nest monitoring has shown that predation is the main cause of nest failure for birds that breed in braided rivers. An infra-red video study in the Tekapo, Ohau and Pukaki riverbeds found the main predators in these river systems to be cats, ferrets and hedgehogs. Stoats are important predators in more forested catchments. Australasian harriers/kāhu and Australasian magpies have also been filmed preying on eggs and chicks.

Introduced predators also eat native invertebrates and lizards.

Infra-red cameras captured these images of predators at banded dotterel/pohwera nests.