An Environmental Approach
The map above shows sub-bioregions in the south-east of Australia from Melbourne to Sydney. Koalas at Phillip Island have been through a genetic ‘bottleneck’ and have little genetic diversity. Free from chlamydia, so there are no constraints on breeding, these koalas have been translocated all over Victoria, including Raymond Island, into NSW around Numerella. To the west of Sydney the Campbelltown koalas also have low genetic diversity and are free of chlamydia and their are concerns about the impact of the disease when they come into contact with koalas that do carry chlamydia.
Koalas in the Strzeleckis and those from Dignams to Wapengo are understood to be closely genetically related and, although details on their genetic make-up are yet to be released, are understood to differ from koalas around Nowra, the only other known endemic coastal koala population between Melbourne and Sydney.
In order to understand biodiversity it is necessary to consider land from an environmental perspective and Bioregions are the largest defined areas to provide that perspective. The ‘Bega Valley Rainshadow’ region provided the largest area of primary koala habitat in the NSW section of South East Corner Bioregion. Koalas could be found at low densities and linking high density populations to the south on the Gippsland plains and to the north toward Sydney.
The formation of the ‘Bega Valley Rainshadow’ region has been attributed to ‘differential erosion’ (Beams 1980) due to the ‘mafic’ mineralogy of Bega Batholith granitics making them more erodible than the adjoining Bemboka granidorite (felsic and adamellitic) that occupies the escarpment. However, another influence on near coastal areas is ‘sodium’ from the ocean that increases the soil erosion and dispersion.
Prior to the European invasion and occupation, Aboriginal management was focused on the more fertile and productive areas. While there is debate about the impacts the original human invaders had, there seems little doubt that the loss of the ‘mega-fauna’ and their replacement with fire as a management tool, was less beneficial for the environment. Forests outside of agricultural areas were used mainly as walkways to access the coast and smaller fertile areas adjacent to it.
Nowadays production forests and ‘protected’ forests in National Parks and reserves are ‘managed’ in the first instance, at a State scale.
In this case ‘management’ of the South East Corner Bioregion (SECB) is split between the Victorian and New South Wales Governments. The area ‘managed’ by the Southern Rivers Catchment Management Authority (SRCMA) is based on administrative, Local Government Areas, as opposed to environmental criteria.
Prior to the Federal and State Regional Forest Agreements there was ‘Total Catchment Management’ but the SRCMA only does private land.
As indicated in the graphic the water catchments from Dignams Creek to Wapengo are mostly located in the South East Coastal Ranges sub- Bioregion with a small area in the Bateman sub- Bioregion.
Current management is not based on environmental criteria, does not take a holistic approach, is not adaptive and cannot acknowledge that extensive canopy die-back is part of a deforestation process.
Catchments and landscapes
The Dignams Creek catchment is largely within the Eurobodalla Shire and catchments to the south in the Bega Valley Shire.
” . . . So commonplace and seemingly abundant is soil that urban Australians treat soil degradation with indifference. Soils are also viewed as lifeless, inert and therefore beyond the principles of conservation. In reality, however, the soils dynamic cycle of biological and chemical processes make it a living entity. Environmental scientists contemplate soil as a place where life and death meet and exchange vital energies (Hillel,1991). Soils, therefore, in reality could be and ought to be projected through an imagery involving the ‘crucible of life’ and soil degradation as the crack in the crucible, allowing the elixir of life to escape.”
Media Representation of Forestry and Soil Issues in the Australian Urban Press, 1990-1991. Dr. Gordon Waitt, Australian Geographical Studies, 1995 – 33(2)299-307
Forests growing on the Murrah (yellow) soil landscape (1), the only ones in the SECB known to be still supporting breeding koalas, dominate water catchments from Dignams Creek to Wapengo.
The Cobargo (brown) soil landscape (2) was one of the more fertile areas that supported Aboriginals and koalas at high densities because it was “primary koala habitat”. Like most agricultural land in the Bega Shire these areas had largely been cleared by the beginning of last century. Sometime between 1900 and 1910 all of the koalas that remained on agricultural land were found dead and dying at the bases of their trees.
A plausible explanation for this outcome comes from the negative impacts on soils as a result of losing a functioning ecosystem. One of these impacts would be an increase in soil acidity that, as most gardeners are aware, has a negative impact on the growth of vegetation. For koalas this change would reduce both the growth and the available nutrients in eucalyptus leaves.
For soils in the SECB this increase in soil acidity would have been accompanied with be an increase in soil sodicity (Little,1994 pt 1). Sodic soils are more erodible and dispersible in water and as indicated in the following chart ( Little 1994. pt2) the Exchangeable Sodium Percentage (ESP) is greatest in the deepest soils .
Evidence for this change in forested areas comes from sediment studies in the Murrah River Catchment, which is one third cleared, mostly in the upper reaches and two thirds forested. These studies found that most of the sediment had been deposited in the past 30-40 years, about a third of the sediment had been deposited during a flood in 1971.
By this stage integrated logging had commenced, in the belief that cutting down more trees would make them grow quicker.
The NSW government’s koala recovery plan focuses on planting trees on agricultural land. However, soils on these lands have a low level of fertility, similar to the Murrah soil landscape.
(1) Tulau, M. (1997) Soil Landscapes of the Bega-Goalen Point 1:100,000 Sheet, Department of Land and Water Conservation, GPO Box 39, Sydney, NSW 2001.
(2) Tulau, M.J. 2002, Soil Landscapes of the Narooma 1:100 000 Sheet, Department of Land and Water Conservation, Sydney.
The forests remaining after agricultural lands were cleared could only support koalas at low densities. By the 1990’s most of the remaining koalas were constrained to forests adjacent to the largest areas of cleared land. As well as forests from Dignams to Wapengo, koalas could also be found on the southern edge of the Bega Valley, in the forests of Yurammie and Tantawanglo.
Also in the early 1990’s Bell-Miner Associated Dieback (BMAD) began to affect forests in gullies and along streams. In early 1998 the first extensive canopy die-back occurred during a spell of dry weather. Apart from unsustainable logging, BMAD that degrades higher quality habitat and Dieback Associated with Dry weather and Drought (DADD) are the greatest threat to forests, koalas and many other species.
Flora at a local scale
The graphic below shows the modelled pre 1770 forest ecosystems (3) in a sub-catchment of the Murrah River catchment. Although only a couple of hectares of the 180 ha sub catchment was cleared on private land, logging and burning undertaken 30 years ago drastically altered the original tree species composition.
The changes to tree species composition are more evident on upper slopes and along ridges where heavily logged “Coastal Foothills Dry Shrub Forest” is mostly dominated by the mid-storey tree Black Forest Oak (Allocasuarina littoralis) and Silver Top Ash ( E. sieberi).
Although Black Forest Oak is not subject to DADD, Silver Top Ash is badly affected and many of these as well as other Eucalyptus species are either dead or severely defoliated.
At lower topographic areas “Coastal Gully Shrub Forest” and “Hinterland Wet Shrub Forest” and “Hinterland Wet Fern Forest” are subject to BMAD. Due to the reduction of soil water holding capacity most of what was “Hinterland Wet Fern Forest’ at top centre of the graphic is now swamp that does not support the Eucalyptus trees.
(3) Keith, D. and Bedward, M. (1999), Native Vegetation of the South East Forests region, Eden, New South Wales: Cunninghamia, Volume 6(1) 1999, National Herbarium of New South Wales, Royal Botanic Gardens Sydney, Mrs Macquarie Road, Sydney, NSW 2000.
The goanna (Varanus varius) is a formidable predator occupying the top of the food chain but their numbers, like the large forest owls, have declined. The decline of large native carnivores is due to a reduction in large old trees with hollows and a reduction in prey.
Goannas require trees with hollows for hibernation and owls need them for nesting. However, goannas and owls also have an indirect dependence on termites as the former use termite mounds to lay their eggs and termites frequently play a role in the formation tree hollows.
To understand why prey species have declined and in addition to reduced habitat loss, it is necessary to consider the factors that have led to the extinction and general reduction of herbivores.
Forest managers blame feral predators almost exclusively for the loss of native species but fail to consider the degree to which their management has led to the reduction or extinction of many species.
A good example of species decline that is difficult to link to feral predators can be found in Forests NSW’s 2009-10 Research and Development Report (3.6Mb).
According to this report numbers of the Eastern Horseshoe-bat (Rhinolophus megaphyllius), monitored in an old mine shaft in Mumbulla State Forest, had reduced by 50% but according to FNSW the reasons for the decline are unknown.
A possible reason for the reduction of insectivorous bats is the reduction of various insect species, like Cicadas (family: Cicadidae), that spend their formative years sucking on tree roots. Decades ago the sound of the male Cicadas during summer was almost deafening, but nowadays there are very few Cicadas.
At the bottom of the food chain are many species of insects, all of which play a critical role in maintaining ecological processes. Along with the more numerous and perhaps better-understood insects, like ant and termite colonies, another important species is the forest or garden hopper (Talitroides topitotum).
These little creatures are crustaceans that are only found in moist areas where they eat decaying forest litter. There may be hundreds of forest hoppers per square metre in suitable habitat.
Apart from being an important food source for some terrestrial species, particularly Lyrebirds (Menura superba), the Eastern Whipbird (Psophodes olivaceus) and the Dusky Antechinus (Antechiuns swainsonii), the exoskeleton of a forest hopper consists largely of Calcium.
Calcium reduces soil acidity and counters the negative effects of Sodium in the soil, increasing the Cation Exchange Capacity of soils, making more nutrients available for plants and more palatable for herbivores.
Forest management should aim to have high densities of forest hoppers. Unfortunately, logging and ‘fuel/hazard reduction’ burning destroy this habitat and it may take decades for suitable conditions and the species, to return.
The Far South Coast Koala Management Framework proposes four categories of koala habitat namely –
Category A : Areas likely to be currently used by a resident/breeding population of koalas and/or have the potential to be used by a resident/breeding koala population in the next 50 years.
Category B : Areas suitable for a resident koala population, or important corridors or buffer areas, that are important to facilitate population expansion and increasing viability for the region’s koalas over the long term (greater than 50 years).
Category C : Areas that may have transient koalas, however are unlikely to become areas used by a resident population in the next 50 years.
Category D : Areas that are unlikely to be used by koalas in the foreseeable future (50 years).
Koalas are the only large arboreal marsupial that isn’t directly dependent on old trees with hollows. What they do need is fertile soils with adequate water-holding capacity, so eucalyptus trees can provide their food and water. Hence koalas are indirectly dependent on old trees with hollows to support all the other creatures necessary to maintain and improve the availability of nutrients and water to trees.
The following map of the Dry/Murrah River catchment, based on soil landscapes and showing land tenure, broadly defines the first three categories as Class A, B and C koala habitat.
Class C habitat includes riparian soil landscapes along the Dry/Murrah River, these areas have been separated in the following table providing land areas and the lengths of mapped watercourses.
|Land Ha.||Water Kl.||%Land||%Water|
As indicated Class A habitat has the largest area but the second longest length of watercourses. Class B habitat is generally cleared land and has the greatest length of watercourses per hectare. Even when riparian areas are included in Class C habitat the length of watercourses is per hectare is less than either Classes A or B.
Historically koalas were at there greatest density in Class B habitat, are now constrained to Class A habitat at much lower densities and apart from riparian areas along the river, Class C habitat has probably never supported breeding koalas.
Riparian areas are included in Class C habitat as trees in these areas are frequently subject to BMAD. In addition trees planted in these areas are unlikely to reach maturity due to the negative impacts of flood-waters and particularly the debris they carry.
Restoration of riparian areas requires an approach that begins at the headwaters of catchments in first order watercourses.
The capacity of trees to take up nutrients is associated with the Cation Exchange Capacity (CEC) of soils. Soils with a high CEC have the greatest capacity to hold ‘exchangeable cations’ by electrical attraction and supply nutrients to plants. There are no soil landscapes in the SECB that have a high CEC, most have low or very low CEC.
Increasing soil CEC requires management that is focused on this purpose. The outcomes of current forest management are a continuing reduction in CEC that increases soil acidity, erosion and dispersion.
To be continued/revised and added to . . .