An overview of recent native vegetation clearance in Australia and its implications for biodiversity
Biodiversity Series, Paper No. 6
Andreas Glanznig, Biodiversity Unit
Department of the Environment, Sport and Territories, June 1995
Contribution of native vegetation clearance to biodiversity decline
The relationships between native vegetation clearance, habitat loss and fragmentation, and biodiversity decline are being increasingly recognised. This is reflected in the view of two independent working groups, expressed in a paper to the Prime Minister's Science Council in 1992. The working groups inferred that the factors causing biodiversity loss centre primarily on habitat destruction or modification and that 'first and foremost, by far the major factor has been clearing of natural vegetation' (Working Groups on Biodiversity 1992, p.7).
This conclusion is reinforced by the New South Wales State of the Environment Report which states that broadscale clearance or the severe modification of native vegetation is the major human activity causing the loss and fragmentation of habitat (NSWEPA 1993, p.79), and supported by the Ecologically Sustainable Development Working Party on Biological Diversity which considered that habitat loss remains the greatest threat to biodiversity (ESDWPBD 1991, p.34).
Further research is required to better elucidate the quantitative relationships between native vegetation clearance, habitat loss and fragmentation, and biodiversity decline. A general correlate is that the impact of clearance on biodiversity depends on its biogeographic significance and conservation status. It will tend to be significant in areas where ecosystems contain a relatively high diversity of habitats and high numbers of endemic species with restricted ranges, especially those that are also considered to be threatened.
The impacts of native vegetation clearance on the extent of particular ecosystems and ecological communities are well known. For example, the estimated original extent of the Big Scrub (subtropical rainforest between Lismore and Bangalow in northern NSW) was over 75 000 ha. However, by 1900 it had been reduced to about 300 ha scattered over ten remnant patches (Floyd 1987, p.96). Another example is the reduction of the grassy whitebox woodlands (found within the western slopes region of New South Wales) from several million hectares to probably 50 to 100 hectares with an intact understorey, scattered widely over a number of sites (Prober and Thiele in press; 1993; Thiele 1995, pers comm.). Furthermore, a recent assessment of areas in Queensland where the dominant vegetation type is brigalow (Acacia harpophylla) shows that of the original estimated extent of more than six million hectares, only about five per cent remains (Sattler 1995, pers. comm.), and only about 30 260 ha or 0.5 per cent were reserved (Gasteen 1987, p.150; Sattler and Webster 1984, p.149). Most clearance occurred in the 1960s (Sattler and Webster 1984, p.149; GOQLD 1990, p.16) and by the 1970s a large portion of the brigalow had disappeared (Gasteen 1987, p.148).
The immediate effect of clearance on plant and animal species can be significant. For vertebrate animals, comparative estimates of woodland bird densities indicate that between 1000 to 2000 birds permanently lose their habitat for every 100 ha of woodland cleared (Bennett unpubl. data cited in Bennett 1993, p.17), while it has been estimated that the clearing of mallee for wheat kills more than 85 per cent of the resident reptiles – on average, more than 200 individual reptiles per hectare (Cogger 1991, p.7).
Longer term effects of native vegetation clearance on species result from habitat loss and fragmentation working in combination with other threats.
The clearance of native vegetation disrupts ecosystems and habitats and results in the creation of remnant 'islands' or fragmented patches. Few of these remnants are large enough to sustain ecological processes such as water and nutrient cycling at the rates that existed before disruption. Many continue to be disturbed by threatening processes such as invasion by weeds or feral animals coming from the surrounding cleared land.
Fragmentation has two primary effects. First, it creates new edges between remnants and cleared or disturbed land leading to 'edge' effects. These include physical changes to the remnant in the border region, such as different levels of exposure to the sun and wind and changes in water cycles and the local air temperature (Saunders et al 1991). Biotic changes include invasion by opportunistic species with good dispersal or colonising abilities such as weeds and feral animals.
Second, it isolates and creates barriers between remnants. In most cases, recently isolated remnants can be expected to continue losing species (Saunders 1989). For some species the loss of a population of a species made too small to be viable may take considerable time due to the relatively long life of individuals. For example, it may take several hundred years to lose some species such as long living trees, particularly since adult plants are often less sensitive to changed environmental conditions than plants in seedling and juvenile stages. This phenomenon also applies to fauna including some long lived invertebrates such as individuals of a species of trapdoor spider in the wheatbelt of Western Australia which may live for at least 23 years (Main 1987, p.35).
The consequences of habitat fragmentation on biodiversity depend on the interaction of these effects and influences (Saunders et al 1991) and almost certainly vary for different species and habitats (Margules and Meyers in press; Davies 1993).
To illustrate the implications of habitat fragmentation on a particular group of species, the effects of habitat fragmentation on birds is briefly discussed in the box below.
Studies have investigated the medium to long term effects of fragmentation on different groups of species, in particular, birds and mammals. In relation to birds, Table 1 below summarises the threats to the 150 taxa described in a Royal Australasian Ornithologists Union report on threatened and extinct birds of Australia (Garnett,1992). The threats are divided into those that are continuing and those that no longer occur, either because the taxa are extinct or because the process has ceased and is no longer affecting the surviving birds. For instance, the clearance of mallee in the Western Australian wheatbelt has now almost stopped but the effects on fragmented populations are continuing.
The table highlights the prevalent threat of clearance and fragmentation of habitat, especially resulting from the conversion of land supporting native vegetation to use for agricultural purposes.
|Current threats||Former threats||Total|
|Clearance and fragmentation of habitat|
|Other environmental modification|
|Altered fire regimes||16||35||-||4||55|
|Grazing and/or trampling||10||35||-||7||51|
|Changed hydrological regimes||1||3||2||1||6|
|Shortage of nesting hollows (variety of causes)||3||20||-||1||24|
Note: Numbers of taxa affected: totals do not necessarily equal the sums of current and former threats because the taxa can be subject to more than one threat.
Source: Garnett (1992, p.8)
Numerous studies in many States have concluded that bird abundances are directly related to the degree of habitat loss and fragmentation, and heavily fragmented areas will be accompanied by net losses of species which can continue long after the initial clearing (see Recher and Lim 1990, pp.292-93). In Western Australia, Saunders (1989) found that a rapid loss of species had occurred in wheatbelt reserves since the clearance of the original vegetation 30 to 50 years ago. This finding has been reinforced by a more recent study which showed that forty-nine per cent or 95 of the 195 species of birds recorded in the wheatbelt (excluding vagrants) have declined in range and/or abundance since the region was developed for agriculture. Most of these losses are due to loss of habitat and fragmentation of the remainder (Saunders and Ingram 1995). This general pattern of regional loss and decline of bird species has been repeated in other States such as New South Wales (Barrett et al 1994) and Victoria (Loyn 1987).
The degradation of habitat by removal of the understorey of forest and woodland ecosystems, through grazing for example, is also significant since it can simplify these ecosystems and result in the loss of species and genetic variability. For example, a study in the Latrobe Valley of southeastern Victoria found that small heavily grazed patches (less than 10 ha) supported fewer forest birds and an increased number of farmland birds, including noisy miners Manorina melanocephala which aggressively excluded other species. Few birds ate insects in the canopy of these patches which showed signs of dieback due to insect damage (Loyn 1987). This type of impact is not directly addressed in this paper.
The effects of a range of threatening processes, usually acting together, have had a substantial impact on Australia's biodiversity which has suffered severe declines and extinctions, especially in the last 200 years. The rates of biodiversity decline, particularly in the last 50 years have been very high (see for example, BDAC 1992, p.4). This pattern of decline has led to predictions by scientists that, as well as continued losses of mammal species, there will be an accelerated loss of bird species, paralleling the losses in the mammal fauna which occurred from the 1900s onwards (Recher and Lim 1990, p.287). It appears that many amphibians are also in decline (Tyler 1994, pp.161-65; Richards et al 1993).