Most landholders are now well aware of the benefits to agricultural production of strategically located "trees on farms". These benefits relate to agricultural productivity and have been well documented elsewhere. In summary, they include:
Windbreaks and shelterbelts: to increase productivity of crops, pastures and livestock;
Shade trees: to increase livestock productivity, particularly milk production and male fertility;
Tree planting to reclaim degraded land or prevent land degradation: (and it's tax deductible too!); Farm forestry: for diversified farm production;
Bushland industries: e.g. cut flowers and foliage, essential oils, tree seeds and a resource for ecotourism enterprises;
Aesthetics: well treed properties look better, are nicer places to live and may have higher re-sale value;
Carbon credits: an area yet to be fully explored.
This paper focuses on the benefits of remnant vegetation i.e. those patches of the original vegetation remaining in the landscape, and in particular, some of the less tangible "benefits" of native vegetation to agriculture. It also considers the wildlife – invertebrates, birds, mammals, reptiles and amphibians – supported by remnant native vegetation and the role that these organisms play in the agricultural ecosystem.
The following are some examples of the reasons why native vegetation should be retained (and re-established) in agricultural systems, and the functions that it performs.
Tree establishment is an expensive business. Even with recent developments in tree establishment technology, such as direct seeding, the expense in labour and materials is high. Also, it is very difficult to re-establish a complete vegetation community containing all of the essential elements; the trees, shrubs, ground flora, hollow logs and that most vital habitat component, old growth trees with limb hollows.
It is far more cost effective to maintain and manage existing vegetation than to try to re-establish it once it has been removed. A prime example is native grasslands. There is an increasing awareness of the value of many native perennial grass species in grazing systems, but, due to the difficulty of collecting seed, it is almost impossible re-establish them once they have been replaced by exotic pastures or crops.
Eighty five percent of the land area of New south Wales is occupied by agriculture. About 10% is occupied by National Parks and Nature Reserves.
As an example, in the Central Western Region of New South Wales, an area extending from Lithgow to Lake Cargelligo, 75% of the forests and woodlands have been cleared since European settlement. With them have gone 30% of the small mammal species that once lived there. Half of the remaining mammal species have lost more than 80% of their habitat and are likely to become extinct in the region. Half of the bird species in the Bathurst Basin are likely to become locally extinct in the next few decades under current land management practices. The situation is similar in other parts of the State.
Some vegetation communities, such as Grassy White Box Woodlands, are very poorly conserved within the National Parks and Wildlife Service estate, and exist only as tiny fragments in cemeteries, road and rail reserves and on private land.
Clearly, agricultural landholders have a critical role to play in maintaining the State's biodiversity.
Bushland remnants should be the basis for farm revegetation programs. They are a potential source of local seed and propagating material for revegetation projects. They are also habitat for many of the predators of insect pests that plague young tree plantations, e.g. Christmas beetles, chrysomelids and sawflies.
Bushland remnants also provide habitat for the wildlife that still exists in the agricultural landscape. Management regimes to ensure the long-term sustainability of remnant vegetation should be put in place before considering planting of wildlife corridors or other surrogate wildlife habitat.
Native vegetation provides habitat for a host of wildlife which eats insects. Birds are the most visible of these predators, but consider also the insectivorous bats, sugar gliders, echidnas, and predatory and parasitic insects that consume kilograms of insects every day.
Some examples include: Insectivorous bats eat half of their own body weight in insects per night;
Magpies eat thousands of pasture scarab larvae per hectare per year;
Ibis eat crickets and grasshoppers and their larvae;
Sugar Gliders eat massive numbers of insects, especially during summer;
Owls, hawks and eagles help to control mice, rats and rabbits.
These natural predators may prevent insect pest populations from reaching levels where they cause economic damage to crops and pastures, but they can only provided this essential ecological service if they have suitable habitat.
The role of vegetation in control of dryland salinity has been well documented. When deep rooted perennial vegetation is cleared from the upper slopes and ridges excess water accesses the groundwater via "recharge areas". The watertable rises, resulting in saline discharges and waterlogging on the more productive lower slopes and valley floors. Those tattered little bushland remnants up on the ridges may be all that prevents our more productive lower slopes from turning into a salty wasteland!
Much revegetation effort is expended on re-establishing trees on groundwater recharge areas. With The wisdom of hindsight, how much simpler it would have been to have retained the original native vegetation in these areas, which are often marginal for agricultural production once cleared.
All agricultural production relies on having a reliable supply of clean water. Yet most of our rivers and streams are degraded and unhealthy, usually as a result of uncontrolled access by livestock, and the consequent destruction of streambank vegetation. Watercourses are also critical for nature conservation. Not only do they provide a water supply for wildlife as well as livestock; but they are a nutrient rich part of the landscape that can support a greater diversity of organisms. They also form connecting corridors through the landscape.
Here, then, is a genuine "win-win" situation for nature conservation and agriculture. Healthy strips of native vegetation along watercourses help to provide a more reliable supply of better quality water for agricultural enterprises and provide habitat and movement corridors for wildlife at the same time.
The Brush-tailed Bettong is a small marsupial that was driven to extinction in NSW about 150 years ago by loss of habitat and predation by foxes. As part of their feeding behaviour, Bettongs utilise hypogeal fungi, spreading fungal spores as they feed. These fungi form mychorrizal associations with tree roots, and are important for healthy root function. What then, was the impact on tree health, and the ecosystem as a whole, due to the extinction of the Bettong?
This example is given, not for any singular significance that this species may have had, but to demonstrate the "wholeness" of the ecosystem, and that the loss of just one species can impact on the ecosystem, and that the loss of just one species can impact on the ecosystem in ways that we cannot predict.
These are just a few examples of the essential ecosystem functions provided by native vegetation and its associated fauna.
We can and must have healthy, productive agricultural lands. But within the agricultural landscape we need areas of healthy native vegetation; the existing remnants, linked together by connecting corridors and supplemented by areas of re-established "surrogate bushland". These areas of native vegetation support many of the essential ecological processes that allow us to practice agriculture. They must be regarded as an integral part of the agricultural ecosystem and not simply as an "optional extra".
Davidson, R. & Davidson, S. 1992, Bushland on Farms: do you have a choice? AGPS, Canberra. George, D. & Brouwer, D. 1996, Nature Conservation on Farms NSW Agriculture Continuing Education, Tocal. Partley, J. & Robertson, A. (eds.) 1998, Agriculture and the Environmental Imperative CSIRO Publishing, Melbourne.