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The realisation damaging habitats from our actions needs mitigation not only to prevent habitat loss, but also to prevent the loss of the animals and plants which live in them, has by and large lead to widespread concern and action.
All over New Zealand there are many groups planting trees to restore native habitats. Currently there is a strong focus, and rightly so, on restoring our riparian and wetland areas because of their ongoing degradation as a result of human activity which includes farming. Several years ago there was a focus on planting eroding land to help control soil loss as a result of clearing vegetation and replacing it with pasture. There is an increasing awareness of the importance of planting our coastal dunes to prevent loss of coastal land.
What is a habitat and what is the result of habitat loss?
Basically habitats are where plants and animals live and there is a multitude of environmental factors which impinge on their inhabitants from the physical, for example soil, air, water and temperature, to the biological factors such as food, predators and disease. A habitat is specific to a particular plant or animal and what is not generally recognised is that a change in any one of these environmental factors, be it an increase or decrease, can impact on one population of organisms which will likely impact indirectly on another population in another habitat.
A simple example is the recent appearance of the Varroa mite which has decimated wild bee populations in New Zealand. The lack of bees has greatly reduced the
pollination of plants which depend on bees to produce fruit. It is estimated that one third of the food we eat depends on bee pollination. The presence of Varroa in the environment of native bees has impacted severely, first on bee pollinated plants and, secondly, on the ability of our habitat to provide our food.
Unfortunately similar scenes like this are being played out countless times in our native plant and animal habitats today; no less severe, but maybe not quite so obvious an impact. Any interruption to the food chain, be it a change in a population of plants, herbivores (plant-eating), carnivores (animal-eating) or decomposers (break down dead plants and animals) will impact on the habitat of organisms further up or lower down the chain, thereby upsetting the fine balance of nature.
What is being done to prevent habitat loss?
It may surprise but habitat loss is a natural phenomenon. The coming of an ice age or the eruption of a volcano at one end of the spectrum, or at the other end a storm, flood or fire will cause partial or complete habitat loss for living things. Over history plants and animals have survived or died out as a result of habitat loss. Those that survive have been able to adapt and evolve in their new environments.
However, human intervention has brought a new perspective to life on earth. As human populations have grown the impact on natural habitats has become greater.
Early New Zealand settlers had to clear forests for pasture, extract natural resources and alter landscapes in order to survive. They also introduced plants and animals which have become pests. These early settlers were generally unaware of the impact their actions had and were to have on the living environment.
Recently it has been recognised that pillaging the land, waterways and the seas cannot continue and we have adopted a more sustainable approach to preserve our natural and physical environment.
Hence attention is being given to protecting many of our endangered animals (eg our national icon the kiwi) and restoring and replacing lost or damaged habitats, by planting native trees and other native plants to prevent erosion and preserve our waterways.
How does planting trees restore natural habitats?
Plants (producers) are the food source (leaves, wood, seeds and fruits) for herbivores, which in turn sustain the carnivores and they all provide the dead material on which decomposers can act. Each of these levels is a link in a food chain which interacts with other food chains, creating finely balanced food webs necessary to maintain life. Among other things, native plants provide shelter for the native animals they support, retain substrate and often prevent invasion of plant pests.
Restoration of natural habitats takes time and involves a process of succession which involves several stages over time and plants may be adapted to just one of these stages. Their adaptations will suit either the first (pioneer) phase, or the mature phase, or somewhere in between. As a succession proceeds, plants will increase in height, soil builds up providing a seed bed for other plants which grow and overtop them. The pioneers, being shade intolerant die off and are replaced by shade tolerant species. The progress of succession may be reflected by the trees and shrubs of different stages of succession but each stage will have its own characteristic animals as well.
It is worth noting that unfortunately planting trees does not prevent the invasion of animal pests many of which predate our native animals. That is why restoration projects, to be successful, need to consider creating a pest free environment by installing fences prior to planting trees.
In restoration projects why do we plant some plants and not others?
Restoration first and foremost involves plants which provide the food and shelter for other living things which depend on them. But not all plants are suited for restoration plantings. Pioneer plants can best tolerate the conditions which prevail when bare ground is exposed as a result of loss of forest cover, or by erosion of hillsides, collapsed stream banks or drained wetlands.
In general, pioneer plants must be able to survive in and tolerate exposure to, among others, sunlight, wind, high and low temperatures and low levels of moisture.
Some pioneer plants may be specific to particular habitats or region. But there are several native plants which are able to survive in a wide variety of habitats.
Light-demanding pioneer species such as manuka and kanuka are ideal for first plantings. Kanuka grows best on drier or more fertile soils while manuka will grow in wet or dry conditions. Other hardy species include karamu, koromiko, mahoe, lacebark, ribbonwood, wineberry, cabbage tree and flax.
That is why students in the Trees for Survival programme grow and plant these trees and shrubs.
Once these plants are well established (three to five years), main canopy trees such as tawa, kokekohe, titoki, pukatea, hinau and kamahi can be planted in between. Totara and kahikatea can be planted at the outset but mostly these forest trees, along with rimu, matai and miro, prefer some shade and shelter.
Sometimes it is not necessary to underplant as the seeds of trees which grow over the pioneers are dispersed by birds and/or wind.
Natives or exotics?
Many exotic plants introduced to New Zealand have had significant ecological or economic impact. For example, Gorse (Ulex europaeus) is a major invasive plant species in New Zealand and millions of dollars are spent each year on its control. A redeeming feature is that gorse stabilises land prone to erosion. Poplar and willow have also been extensively used to control erosion.
Pinus radiata forests have provided us with wood for building and pulp for paper for domestic and export markets. A preferred option over further pillaging of native timber.
A quick look at a pine forest understory shows how different the
habitat is from say, a native beech forest understory. Native forest floor plants and animals simply cannot survive in the very different habitat created under the pine forest. This is because native forest floor plants and animals are adapted to have very specific roles to play not available on the floor of the pine forest. The role organisms play in their habitat is called the ecological niche.
This is why restoration projects can only involve native plants and not introduced exotics. Not all agree calling preference given to native plants ‘eco-racism’. However, the purpose of restoration is to restore native habitats in order to encourage the survival of our native plants and animal heritage. An understanding of the concept of ecological niche shows that exotic plantings provide different habitats and therefore limit the survival of native plants and animals.
The reality is that succession takes a long time. Isn’t it great that school students may be able to revisit their planting sites, maybe in 20-30 years’ time, to see the results of their handiwork? Maybe they will see some forest trees emerging through the canopy of the pioneers they planted when they were at school!
A useful resource:
While maybe not totally applicable to all regions of New Zealand, the Auckland Council has produced a series of booklets which are particularly useful resources relating to restoration projects.
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