Habitat Destruction in Queensland – the Effect on our Native Wildlife
by Dr David Kault and Mr Steve McAlpin for the Queensland Conservation Council 5 June 1998
Introduction
Vegetation clearing and subsequent habitat destruction is occurring at an enormous rate in Queensland. An average of 30 hectares of forest and woodland was cleared every hour between 1991 and 1995, equivalent to 1 football field per minute, and this destruction is continuing (Slats 1997). Vegetation clearing is motivated largely by perceived short-term gains in pastoral productivity and to a lesser extent by other farming activities and urban expansion. There are a number of important adverse effects of vegetation clearing. These include:
– long term damage to land due to increased erosion and in some areas, potential destruction due to rising salt.
– The effects on rivers draining through cleared land, with increased situation and more rapid but shorter runoff after rain. This leads to floods, alternating with stagnant waters affected by blue green algae.
– Very large amounts of greenhouse gas emissions as felled trees rot or are burnt
– Probable effects on regional rainfall – trees bring rain
– The effects on native animals short of extinction of species
The last issue, particularly that of the welfare of native animals, has not received the attention it deserves in the public arena. This paper gives some estimates of the number of animals being killed as a direct result of current vegetation clearing activities in Queensland.
Methods
The estimates presented here are derived by combining information on the extent of vegetation clearing by locality with information on the distribution of a number of animals and point estimates of the density of the animals. Estimates of the exact amount of vegetation clearing by locality have been difficult to obtain. Until recently, the extent of vegetation clearing permits for leasehold land was the main source of information on vegetation clearing. However, since these permits exclude freehold and since not all permits on leasehold land are acted on, information gleaned from these permits is inaccurate. Preliminary information from satellite data is now more readily available (DNR 1997) although full details are yet to be published. The preliminary information includes reliable estimates on the total rate of vegetation clearing in Queensland together with the rate of vegetation clearing in blocks of size 30’ of latitude x 30’ of longitude (each block rough 2500 km2). The amount of vegetation clearing in each block is available in terms of 6 categories. These are 0-1 km2 per year, 1-2 km2 per year, 2-5 km2 per year, 5-15 km2 per year, 15-50 km2 per year, 50-100 km2 per year. If it is assumed that on average, the amount of vegetation clearing in each block of a given category is at the mid-point of the range for the category, the total amount of clearing in Queensland is overestimated in comparison to the available figure for the overall amount of clearing in Queensland (2620 km2/year).
A correct estimate is obtained by scaling the midpoints of the range for each category by a factor of 0.80. Accordingly, the average amount of clearing in each of the six categories listed above is taken to 0.4 km2/year, 1.2 km2/year, 2.8 km2/year, 8 km2/year, 26 km2/year and 60km2/year. Information on the range of various mammals is available in map form (Strahan 1995). Several species of mammals were selected on the basis that they were well known, widespread and unable to live on cleared land. These criteria reflect the aim of drawing attention to the issue of the consequence of vegetation clearing for the welfare of native animals, rather than emphasising other issues such as species extinctions which have already received public attention. The animals selected were the brushtail possum, the greater glider, the sugar glider and the long nosed bandicoot. The distribution maps of these species in Queensland was overlayed onto a map of the extent of vegetation clearing by categories. Counting the blocks in the range of each species and multiplying by the average amount of habitat destruction for each species in Queensland. The density of mammals within their range of distribution is not well known. However, an unpublished database and bibliography was kindly made available to the authors (Johnson, In Press). It was considered that the density estimates were available may tend to be overestimates of the true average density throughout the forested areas of the animals range, as it is likely that the researchers will tend to undertake studies in areas of particularly favourable habitat.
It is also noted that animal density is likely to vary considerably, particularly for short lived animals, due to seasonal variations. For several species a number of differing estimates of density were available from various parts of the range of distribution of these animals in Queensland. These estimates varied widely – by a factor of more than 30 in the case of both the brushtail possum and the greater glider. In order to compensate for the probable average density overestimates, the lowest density figure available was used for each animal. In addition, a range of values was available from the 1997 NSW State of the Environment Report giving an estimate of the number of birds (all species combined) destroyed with each hectare of bush cleared. The lowest figure in this range has been used as a conservative estimate of the number of birds killed by vegetation clearing in Queensland. The areas of habitat destruction for each species, together with the population density of each species was combined to give estimates of the total number of animals of each species killed by a vegetation clearing process.
Results
Area of habitat destroyed km2/yr, Animal density Number/km2, Animals killed per year, Animals killed per day
– Brushtail possum, 2490, 7, 17000, 48
– Greater Glider, 822, 9.2, 7500, 21
– Sugar Glider, 1120, 20, 22400, 61
– Long Nosed Bandicoot, 758, 90, 68000, 187
– Birds (all species), 2620, 1000, 2 620 000, 7200
Discussion
This paper draws attention to a strong argument against further vegetation clearing, an argument that has been given little public attention. One reason the issue of native animal welfare may have been underplayed, is uncertainty regarding the magnitude of the problem. It is clear that the estimates here are still subject to uncertainty. However, the estimates presented are conservatively based on the available evidence. The onus should be on those who dispute the magnitude of the problem to obtain more accurate estimates. It should be noted that habitat destruction really does equate to the killing of animals. Whilst some individual animals may flee vegetation clearing and escape to nearby suitable undisturbed habitat, this remaining adjacent habitat can be assumed to be at full carrying capacity. Any additional animals from a cleared area that does manage to establish itself in an adjacent area, will generally displace an existing individual, leading to the death of that individual. It should also be noted that the destruction of animals from clearing is in a sense more permanent than destruction due to causes such as car strikes. Of course all deaths are permanent, but animals killed by car strikes can be replaced from the local population whereas habitat destruction permanently removes the possibility of replacement or re-establishment of the local population in any given area.
References
– DNR (1997). SLATS – the Statewide Landcover and Trees Study – interim report. Resource Sciences Centre, Queensland Department of Natural Resources.
– Strahan, R. (1995). The Mammals of Australia. Reed Books, Sydney.
– Johnson, C. (In Press) Mammal abundance database and bibliography. Department of Biology and Tropical Ecology, James Cook University.