Brisbane Rainforest Action & Information Network


Rainforest Remnants

How do we best preserve the richness and diversity of species in these unique places?

 - Paul Donatiu

For some time now, I have been pondering what might be some of the means available to us to preserve the richness and diversity of species, in rainforest remnants. As we know, today most preserved rainforest consists of fragmented remnants of once wide spread primary forest. These areas often exist as isolated forest patches amidst a variety of mixed land uses.

Rainforest remnants are characterised by core and edge features. Some of the factors that influence biodiversity within rainforest remnants include their size, and the degree of separation and amount of time since the remnant was cut off from continuous forest. As a result of these factors, there are distinctive mechanisms at work that contribute to the loss of species in rainforest remnants. This short discussion aims to highlight some of the planning tools that can be used to minimise the impact of these mechanisms.

Researchers have identified six broad mechanisms that contribute to the loss of species in remnants (see Table 1). To deal with these factors, several authors have reasoned that preserving large remnants that contain a core of undisturbed forest may be more beneficial to retaining species richness and diversity, than smaller remnants.

Scientists in North Queensland have examined the impact of edge effects in small areas of rainforest found within the Atherton Tableland. These patches ranged in size from 1.4ha to 590ha. They emphasised the importance of the perpendicular distance that edge conditions penetrated into a rainforest remnant, finding most evidence of disturbance within the first 200m of the remnant edge.

Together with shape and total area, two of these scientists constructed a Core-Area Model that predicted the area of undisturbed forest within a given remnant. On the basis of the penetration of edge effects described above, they argued that reserves of between 2000ha and 4000ha were of sufficient size to preserve more than 50% of original forest contained within the remnant. When I first read this assessment I felt dismayed! For while these figures can be applied to the tropical rainforests of the north, they are simply not transferable to Southeast Queensland where less than 10% of lowland rainforest remains. It seems that other ideas and planning tools are needed to help us preserve the richness and diversity of species in our local fragments of rainforest. I believe these tools fall into three general categories: -

Planning Tools

Buffers create intermediate zones between naturally occurring forest and cultivated or disturbed areas. They can play an important role in maintaining links between rainforest remnants and in assisting immigration between remnants. Buffers provide increased habitat areas, decrease the exposure of core areas to disturbance, and provide space for species populations to stabilise and get back on their feet after seasonal or naturally occurring losses.

The advantages that buffers offer are dependent on the quality, type and relative size of the buffer itself. For example a belt of secondary forest will provide substantially more habitat opportunities than a monoculture, such as a pine plantation. If not managed carefully, buffer zones may harbour exotics and pests in numbers sufficient to pose a threat to the core area within a remnant.

TABLE 1. Mechanisms of species loss in rainforest remnants (see Turner)

Mechanism
Primary Impact
Impact on the richness 
and diversity of species
1. Deforestation Altered landscape resulting in increased edge effects, disturbed catchments and other impacts. Habitat & subsequent species loss. If species distribution is uneven, increased susceptibility to extinction.
2. Restriction of population size Greater susceptibility to disease, natural population fluctuations, inbreeding and predation. Loss of species if population falls below viable levels for long term survival and reproduction.
3. Prevention or reduction of immigration Reduced movement amongst seed dispersing animals between remnants. Reduce seasonal migration of particular animal species. Lower plant species migration. Potential loss of rare plant species which naturally occur in low numbers. Reduced conservation value.
4. Edge effects Increased radiation.  
Increased air temperature. 
Decreased relative humidity. 
Increased wind exposure. 
Higher plant mortality at forest edge.  
Changes in micro-climates with proximity to edge. 
Edge effects increase as remnant size decreases.
Reduction in remnant size for species that cannot adapt to edge effects. 
Loss of trees to wind-throw. 
Reduced replacement of these species. 
However, evidence that some species unaffected or positively affected by edge effects. 
5. Cumulative effects Susceptibility to extinction for some animals & plants as a result of isolation in remnants. 

Loss of vertebrate fauna.

Changes in community structure.
6. Immigration effects Invasion of remnants by exotics. Altered habitats; loss of native plant species.


Corridors of native vegetation link rainforest remnants, and remnants with continuous forest. Like buffer zones, corridors enable individuals from large species populations to migrate between remnants. Corridors offer real pathways to sustain the genetic diversity within populations of rainforest species. Unlike buffers, corridors require their own undisturbed core area to operate effectively.

There is also evidence that corridors provide links between different vegetation zones, helping maintain certain species. For example, the Richmond Birdwing butterfly breeds on both montane and lowland rainforest varieties of the Aristolochia vine. Montane butterfly populations are occasionally wiped out as a result of climatic extremes, and new populations need to be built from lowland groups. The movement of species through corridors is dependent on corridor size, composition, intactness and locality. Corridors are rendered unfunctional by major divisions such as roads. In addition, corridors can facilitate the spread of fire and disease, and expand the number of sites that can be potentially invaded by exotics by increasing edge effects.

Mosaics are patchworks of forest remnants located within a given area. How useful mosaics are depends upon the number of remnants they contain, the distance between remnants, and how remnants are grouped or spread within the mosaic. Mosaics favour some species more than others – for example birds, and plants that produce wind-borne seed – highlighting the fact that slowly dispersing species are those most affected by the fragmentation of rainforests.

Mosaics do provide a basis for including small reserves that individually may not be able to sustain a diversity of species, but collectively may reduce species loss. Knowledge of the habitat requirements of particular rainforest species may indicate that a mosaic of small reserves is preferable for maintaining a diversity of species than one large reserve. Nonetheless, mosaics of rainforest remnants may not prevent some species from becoming extinct, for example – species that produce small amounts of seed, or are sedentary.

A Composite Model

An effective reserve system that can preserve the richness and diversity of species in rainforest remnants would ideally include the best aspects of the planning tools above.

Certain key considerations may be helpful in determining the application of these tools.

These are that:
The actual configuration of remnants is important.
Rainforest remnants should contain one large tract of undisturbed forest, or have readyaccess to continuous forest.
Buffers and corridors help maintain biodiversity and sustain forest productivity.
Grouping remnants, that demonstrate diversity in composition and species, offer
opportunities for healthy linkages between rainforest patches.
The integration of these factors into a composite model for the preservation of species richness and diversity in rainforest remnants is summarised in Figure 1.

 

 Essentially the model contains a core area with multipurpose buffer zones and corridors linking remnants. Alternate forest uses – such as cabinet timber plantations, bush foods, and areas of secondary succession – protect core areas of primary forest and promote seed dispersal. The diversity of land uses in buffer zones create opportunities to productively utilise land resources, and still retain some sensitivity to essential ecological processes.

Within this model it is possible to preserve smaller remnants by using buffers as an expression of the perpendicular distance that measures edge effects. In addition, supplementing core areas with buffer zones, corridors, and incorporating remnants into mosaics, may stem species loss due to chance events. In this way the model provides for a variety of successional stages within the rainforest community that can facilitate a range of alternate habitat sites.

Planning tools such as buffers, corridors and mosaics can perform well at a micro level. However, they require a broader context to focus their application. In order to protect biological communities and endangered species, Catterall and Kingston suggest a "set of nodes and core areas which form part of a regional area/open space system incorporating…. reserves, additional large core bushland areas and linkages established through careful considerations of spatial proximity and ecological relationships, including vegetation corridors and stepping stones". We have looked at some planning tools available to preserve species richness and diversity in rainforest remnants. Used collectively, these tools offer considerable flexibility to planners and environmentalists alike who wish to conserve rainforest biodiversity.

References & interesting reading:

Bierregaard, R.O., Lovejoy, T.E., Kapos, V., dos Santos, A.A., and Hutchings, R.W. (1992). The biological dynamics of tropical rainforest remnants. Bioscience 42, 859-866.

Catterall, C.P., and Kingston, M. (1993). Remnant Bushland of South East Queensland in the 1990’s: Its distribution, loss, ecological consequences and future prospects. Brisbane, Institute of Applied Environmental Research, Griffith University and Brisbane City Council.

Date, E.M., and Recher, H.F. (1988). The role of rainforest remnants in nature conservation. In Rainforest Remnants, (ed) S. Phillips, NSW National Parks and Wildlife Service, Hurstville. 26-30.

Diamond, J.M. (1975). The island dilemma: Lessons of modern biogeographic studies for the design of nature reserves. Biological Conservation 57, 205-219.

Laurance, W.F. (1991). Edge effects in tropical forest fragments: Application of a model for the design of nature reserves. Biological Conservation 55, 77-92.

Peart, D.R. (1996). The sustainable management of rainforest lands: An overview of research at a major tropical rainforest national park. Tropical Biodiversity 3, 145-155.

Simberloff, D.S. (1982). Big advantages of small refuges. Natural History 91, 6-14.

Simberloff, D.S., Farr, J.A., Cox, J., and Mehlman, D.W. (1992). Movement corridors: Conservation bargains or poor investments? Conservation Biology 6, 493-504.

Specht, A. (1988). The role of rainforest dynamics in conservation and management. In Rainforest Remnants, (ed) S. Phillips, NSW National Parks and Wildlife Service, Hurstville. 31-35.

Turner, I.M., Chua, K.S., Ong, J.S.Y., Soong, B.C., and Tan, H.T.W. (1996). A century of plant species loss from an isolated fragment of lowland tropical rainforest. Conservation Biology 10, 1229-1244.