by John Walsby
Most of New Zealand's
countryside has been changed by deliberate or accidental introductionsof plants and animals and we now have an unnatural, managed landscape
made up largely of farms, plantation forests and residential settlements.
A few pockets of original natural habitats survive but most bush
blocks have had their large trees logged out and wetlands beside
lakes, rivers and harbours have mostly been reduced to narrow
fringes where there were once broad sweeps of marsh.
Over the last decade
the values of forest cover on steep slopes in helping to control
land slippage and retain topsoil and of marshland plants on lowland
flats in preventing rivers and lakes from becoming silted up,
have been much more widely appreciated. Wetlands have also been
shown to be efficient cleansers of pollutants from waste and run-off
water and in some areas are starting to be encouraged or planted
specifically for water purification purposes.
These changing attitudes
have come about as responses to failures of past land management
practices and to accommodate further development and changing
land use. Over just the last 20 years there have been substantial
changes in the farmed land. The size of the national sheep flock
has decreased by about 20% and large areas of land that was once
grazed is now used for other purposes
Steep country that
was never highly productive in grass and prone to slips has seen
large scale conversion to pine plantations and this should improve
the quality of stormwater run off. In both natural and plantation
forest the leafy canopy delays the fall of rain to the ground
below where the blanket of rotting branches, leaf litter, mosses,
ferns and other forest floor plants soaks up the water from a
downpour and only releases it slowly. By preventing it rushing
off bare slopes, erosion of topsoil is substantially reduced and
therefore is much less silting up of waterways.
Close to towns and
cities, especially in northern New Zealand, a large amount of
farm land has been "developed" for residential and industrial
use to accommodate the growing population and to feed the locals,
the area of market gardens has increased.
These changes have
in turn led to an increased demand for water for drinking, washing,
irrigation and industrial purposes and this is matched by an equivalent
increase in waste water. In addition as land is built over or
kept open for horticulture, storm water management has become
more difficult. Heavy rainfall gushes straight off these areas
flooding drains and scouring away exposed soil surfaces.
To clean up these intermittent
silt laden torrents, and to cleanse partially treated water running
from small sewerage treatment schemes elsewhere, artificial wetlands
are being developed with beds of rushes and sedges. The dense
thickets of their upright stems slow the water flow allowing fine
suspended clay particles to settle out and they become consolidated
among the growing root systems of the plants.
The marshes also absorb
and process nutrients which elsewhere can be troublesome.
Flushed as excess fertiliser
from horticultural blocks and farm paddocks or running out with
industrial and domestic waste water, high concentrations of nutrient
can stimulate algal blooms in still or sluggish waterways. When
these blooms die and are decomposed by bacteria, oxygen depletion
results and fish, crustaceans and other natural freshwater animal
life can suffocate and die.
Not many plants can
flourish with their roots in standing water or waterlogged soil
because the oxygen required for vigorous root growth is quickly
used up but not easily replenished. However, the stems of rushes
and sedges are hollow and this allows oxygen that enters above
the water level to diffuse down these pipes to the fibrous root
region.
By absorbing and utilising
the nutrients, the large wetland plants and the abundant microscopic
life that grows over and amongst them, all flourish.Their growth
supports teeming populations of small crustaceans and insect larvae
and these in turn become food for larger insects like the beautiful
damsel flies and dragon flies and for a number of wetland birds.
Though pukekos and
herons are still common, draining natural wetlands has destroyed
essential feeding and nesting habitat for such birds as the banded
rail, spotless and marsh crakes, bittern and the fernbird. Creating
new wetlands for improved land and waterway management may also
provide more suitable habitat for some of these uncommon and rare
birds.
