Overstory #176 - Trees for nature conservation
If you have decided to use trees to maintain nature conservation values in your landscape, it is important to be clear about what you are trying to achieve. The actions that you take will vary considerably, depending on your objective. There are two broad approaches that you can take:
- the 'easy' option, which aims to 'make the landscape better' for nature conservation. This approach, sometimes called 'general enhancement', provides a wide range of general ecological principles that can be applied to your agroforestry enterprise; or
- a 'species retention' strategy. This has the more ambitious goal of keeping all of the species that currently occur in the landscape. General guidelines will not be sufficient for this task. Instead you will have to manage your landscape so that it meets the needs of the species that are there. This means knowing something about those species and any additional management which may be required to meet this type of objective.
The design principles for these two approaches differ considerably. This chapter focuses on design principles for general enhancement, as these require less knowledge about the plants and animals and are easier to apply within the context of an agroforestry enterprise.
It is not possible, within the scope of this article, to provide procedures for the more ambitious goal of retaining all of the species in your landscape. A brief outline of the species-retention approach is presented below.
It is important to remember that guidelines for general enhancement will not necessarily prevent species from disappearing from your area. These principles will simply reduce the probability of species disappearing and may reduce the rate at which they decline.
DESIGN PRINCIPLES FOR GENERAL ENHANCEMENT
Broadly speaking, agroforestry can help to protect plants and animals in two ways
- it can provide habitat for native species to occupy; and
- it can protect existing habitat that is occupied by the species that you want to keep.
As a general rule, agroforestry is more likely to provide habitat for animals than plants, and even then only a limited suite of animals with fairly general habitat requirements will be likely to use it. The main benefits for plants and more specialised animals will be the protection of existing habitat that agroforestry trees can provide.
The following principles will assist with both the provision of new habitat and the protection of that which already exists.
In general, more layers of vegetation will support more species by providing a greater range of habitats.
Try to incorporate a number of structural layers into your planting.
Ideally, these would include understorey shrubs, taller middle-storey shrubs or small trees, and an upper canopy of taller tree species.
Patchiness can be achieved by planting clumps or strips of different tree species next to each other or, where a single species is being used, by having trees of different ages in adjacent patches.
Patchiness in a landscape provides a greater range of habitats which can be occupied by more species.
As well as having a range of patch types, it is also useful to provide a number of patches of each type. This means that after a disturbance such as fire or harvesting, the biota in the less affected patches can recolonise the more affected patches.
Two important aspects of configuration include the distance between patches and the characteristics of the connecting vegetation between patches. Both of these factors affect the ability of animals to move around a landscape.
Patches that are close together and which are connected by suitable habitat are more likely to sustain populations of the species that live in them than are isolated patches.
The careful planting of trees in an agroforestry system can reduce the isolation of existing remnant vegetation or can create new habitat patches, provided that they are linked to each other or to existing vegetation. Whenever possible, avoid creating corridors that lead nowhere unless they are sufficiently wide and diverse that they act as habitat for the species that occupy them.
In general, there tend to be more species in bigger areas of habitat than there are in smaller areas. However, the number of species also depends upon the patchiness of those areas. Smaller areas may support more species than larger areas if the smaller ones are more diverse.
Aim for bigger rather than smaller areas but also incorporate patchiness into the design where possible.
Long, thin remnants have much more edge relative to their area and hence are more vulnerable to degrading edge effects such as increased nest predation and weed invasion. Riparian vegetation and corridors will be particularly susceptible to these edge effects because of their linear characteristics.
Compact areas will be better than long, thin areas for providing habitat and for protection against impacts from adjoining land uses.
Patches should have the least possible edge, and linking vegetation should be as wide as practically feasible.
Many of the recommendations listed above refer specifically to providing habitat for animals. Opportunities to conserve native plants are more limited. One obvious role is to use agroforestry to protect existing native vegetation from edge effects.
Native vegetation can be protected by planting buffers of trees around the margins of existing remnant vegetation or along the edges of riparian vegetation and corridors.
You need to be careful that the species used are not potential weeds which could threaten the plant communities they were intended to protect.
A general enhancement strategy tells us little about the tree species to use. The important thing is to provide the habitat attributes described above. Selecting a range of different plant species will contribute to greater habitat diversity and using local species will maximise the chances of meeting the needs of local fauna.
In general, native plants will be of more value to a wider range of animals than will exotics.
A diversity of plant species which produce fruits or flowers throughout the year will increase the diversity of animals that will be supported. If you choose to adopt a species retention strategy then the choice of species will be much more important, as they will have to meet the resource needs of the plants and animals that you are trying to retain.
Many of the processes which threaten biological diversity in remnant vegetation are also likely to have an impact in an agroforestry system. For example, grazing by stock can prevent regeneration of many plant species and can modify understorey, litter and soil characteristics. These changes alter the habitat quality for numerous mammal, bird, reptile and invertebrate species.
Stock should be excluded from any areas that are considered to have a nature conservation function.
Invasion by exotic weeds can restrict germination of a range of plant species and can alter microhabitats for small ground-dwelling animals. Feral predators such as cats and foxes also impact on the native fauna, particularly small to medium sized mammals and ground-dwelling birds. Predator control is hard to achieve if your neighbours are not also controlling their pests. It may be possible to coordinate predator control with neighbours or other members of your catchment group.
Fire management may present a conflict between agroforestry and nature conservation objectives.
Fire is an important part of natural ecosystems, with many plant communities reliant on occasional fires to maintain plant populations.
Obviously fire at the wrong time will present a problem for your agroforestry trees. Consequently, if you plan to use fire as a tool for managing your natural remnant vegetation it will be important to ensure that your design enables you to protect your agroforestry products. For example, you may be able to time the fire after harvesting the adjoining trees. Fire, however, is not a conservation tool to be used haphazardly and should only be used if there is a clear need. This may be the case if there are plants which are known to rely on fire for recruitment.
Harvesting plantation trees will obviously remove the habitat that you have previously created. If large areas are all cleared at once there is the potential to cause declines in populations of species that have occupied those areas.
Population decline can be avoided by planting trees in a pattern and at time intervals which will ensure that any harvested patch is adjacent to another patch that is not yet ready to be harvested.
Where to start
Agroforestry is likely to make the biggest contribution to nature conservation in a landscape if it is added to areas of existing conservation value.
- First identify the biggest and healthiest blocks of remnant vegetation on your property. These should form the core of your conservation network.
- Plant your trees in a way that increases the size of these blocks, buffers them from adjoining land uses and links them to other patches in the landscape.
- Consider coordinating your planning with neighbours or other members of your catchment group to link vegetation remnants on different properties.
How do you know if your nature conservation efforts are working?
Success for nature conservation means either that the number of species in your landscape increases, or that there is an increase in the number of individuals of the species that are currently there. In the latter case, your success would be greater if you had more of the species that were uncommon before you started than if you simply had a lot more of things that were already abundant.
Some species are easier to monitor than others. For example, it is easier to detect changes in the number of birds and plants than it is for reptiles and invertebrates. To monitor changes in bird numbers, all you need is a pair of binoculars, a field guide and a notebook. Select a few different locations which you would expect to benefit from your conservation actions and visit them early in the morning for three or four days over the course of a week or two.
At each site, record the number of species that you see and, if possible, count or estimate the number of individuals of each species.
If it is a small site, you can search the whole area for birds. If it is a big area, you can walk slowly along a defined route and record what you see along the way.
It's important to visit each site a few times because bird numbers tend to fluctuate from day to day. You can then calculate an average for your counts over the three days. This process should be repeated at least once a year to detect changes from year to year or, if you want to detect seasonal changes, four surveys per year would be appropriate.
The best indicator of plants benefiting from your actions is the presence of individuals of different height classes in the area you are protecting.
If all of the trees are old then there may be a problem. Often some form of disturbance is necessary for new seedlings to establish. Unfortunately, plant species in different areas don't all respond in the same way to disturbance so there are no general rules to apply. It will be necessary to seek advice from local conservation experts if you have this problem.
Monitoring other species is more difficult and may require trapping or searching under fallen rocks and litter. You may need permits from the state conservation agencies if you want to trap. It is best to consult with local conservation agencies if you are interested in more detailed monitoring of this sort.
Requirements for a species retention strategy
The main steps in a species retention strategy are:
- determine which species will be lost from your landscape if you do nothing
- identify the processes that threaten their persistence. These will include habitat loss, isolation, predation, grazing by stock, fire, weeds, etc
- group species according to the process that threatens them
- for each threatening process identify those species which are most sensitive tothe threat
- determine the level at which that threat must be managed to protect the most sensitive species.
After going through this process, you end up with a set of species, each of which is the most demanding for different landscape attributes. By meeting the needs of these species, you should also meet the needs of all other species in the landscape. Area-limited species are used to define the minimum patch size for each patch type; dispersal-limited species define the minimum acceptable distance between patches; and 'process-limited' species define the appropriate rates or intensities of fire, grazing, weeds etc. Such an approach will obviously require a knowledge of the species in your landscape and their habitat requirements. Consequently, it will generally be necessary to seek expert advice if such an approach is to be adopted.
Benefits and costs
The economic benefits of nature conservation may come directly from the provision of products that can be bought or sold in the marketplace, such as cut flowers or honey, or they may arise indirectly from the contribution that conservation makes to the value of other products. Because any marketable components of our natural biodiversity can simply be viewed as a new product and assessed by conventional marketplace economics, the market value of individual species will not be addressed further here.
It is worth recognising, however, that nature conservation itself can have market value if it can be 'sold' in the form of ecotourism. This requires either the availability of extensive areas of relatively pristine natural habitat, the presence of charismatic species which are of interest to tourists, or the combination of production and native elements in a configuration that is aesthetically pleasing. Agroforestry itself is unlikely to produce these values but may help to protect them where they exist.
Indirect benefits from nature conservation may arise where increased levels of biodiversity provide a greater number of species which participate in important ecosystem processes. Increased diversity may generate more pathways through which water, nutrients and energy can flow and thereby enhance ecosystem stability under conditions of environmental variability. More diverse systems may also have a wider array of pollinators and predators which can enhance the productivity of agroecosystems.
The major benefit that accrues from increased diversity, be it biological or productive, is an increase in future options. The continued loss of species from our agricultural landscapes contributes to a reduction in these options for future generations.
Costs associated with protecting or enhancing biodiversity will include management costs, costs associated with foregone production, or costs attributable to reduced returns from other farm products.
Increasing the diversity of plants used in an agroforestry system to enhance conservation value will result in a range of agroforestry products that will have different market values at any given time. This diversity means that a proportion of the land will be allocated to some uses or species that generate lower financial returns than others. At any point in time, the total returns from a more diverse business may be less than that which could be obtained from an enterprise specialising in a single product which has a higher market value. Consequently, strategies based on a wider range of plant species may cause a decline in short-term economic returns.
However, if this increased diversity results in more sustained levels of production under fluctuating environmental and market conditions, the associated costs may decline over time. In fact, a diversity of products can provide insurance against market collapse or pest outbreaks and may return a significant economic benefit in the longer term.
If conservation actions require plant species that do not produce maximum market returns or, in some cases, may not have any direct market value, the costs of these actions must be estimated in terms of the income foregone as a result of not selecting the most productive landuse.
Paradoxically, these costs and benefits may not be clear cut. Actions which may reduce economic returns in the short-term by reducing the area allocated to other productive uses may generate benefits in the medium to longer term by creating more sustainable production systems. The benefits of protecting biodiversity therefore lie in the contribution that a greater array of plants and animals makes to achieving more ecologically sustainable agricultural systems.
Hobbs, R.J., Saunders, D.D. and Arnold, G.W. (1993). Integrated landscape ecology; a Western Australian perspective. Biol. Conservation. 64231-8.
Lambeck, R.J. (1997). Focal species can they provide a multi-species umbrella for nature conservation. Conservation Biology. In press.
Lefroy, E.C., Salerian, J. and Hobbs, R.J. (1993). Integrating ecological and economic considerations a theoretical framework. Pp 209-244 in Hobbs, R. J. and Saunders, D.A. (Eds) Reintegrating Fragmented Landscapes Towards Sustainable Production and Nature Conservation. Springer-Verlag, New York.
Wilson, A. M. and Lindenmayer, D.B. (1996). Wildlife Corridors – pros and cons for wildlife conservation. Australian Journal of Soil and Water Conservation, 9.
This article was excerpted with permission of the publisher from
Lambeck, Robert. 1997. Trees for nature conservation. In Abel, Nick, Jenny Baxter, Alex Campbell, Helen Cleugh, John Fargher, Robert Lambeck, Roslyn Prinsley, Miles Prosser, Rowan Reid, Grant Revell, Carmel Schmidt, Richard Stirzaker, and Peter Thorburn. Design Principles for Farm Forestry A Guide to Assist Farmers to Decide Where to Place Trees and Farm Plantations on Farms, Rural Industries Research and Development Corporation (RIRDC), Kingston, Australia. .
To order this publication or for further details, contact
Rural Industries Research and Development Corporation (RIRDC) Street Address Level 2, 15 National Circuit, BARTON, ACT 2600, Australia
Postal Address PO Box 4776, KINGSTON ACT 2604, Australia
About the author
Dr Robert Lambeck is CEO of Greening Australia in Western Australia. Prior to joining as Greening Australia in 2000, Robert was a scientist with CSIRO Wildlife and Ecology (now CSIRO Sustainable Ecosystems) with a strong research interest in developing landscape designs for maintaining biodiversity in agricultural landscapes. Since then, Rob has since been involved in a number of initiatives that have implemented conservation plans developed as part of his own research. His experience in conservation plans and projects is based on science, agency and community-based delivery mechanisms.
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