IMPACTS OF FIRE AND GRAZING ON NATIVE VERTEBRATES*

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1 IMPACTS OF FIRE AND GRAZING ON NATIVE VERTEBRATES* Grazing and fire have the potential to adversely affect the biodiversity of the VRD. A number of projects are underway to measure the impacts of various grazing and fire regimes on vertebrate animals such as birds and reptiles and to get a better picture of broader biodiversity impacts in the district. The large areas involved and the complexity of how different animals respond to impacts, however, make this a relatively difficult task. Different approaches are being used to look at responses on red and black soil country and in the rugged sandstone country such as that in Gregory National Park. One of the questions being addressed is whether there are specific animals that provide an accurate indicator for broader levels of impact. That is, animals whose response to impact may provide a surrogate measure for impacts over a broader area and wider range of animals. One of the difficulties is the time and labour required to sample vertebrate animals over such large areas. Sampling in small plots may not provide enough information about those more mobile animals like wallabies or large birds, or for rare species that are generally only found with more intensive sampling. There is a network of over 200 vertebrate survey sites across the VRD. The sites are either 0.25 or one hectare in size and have been established according to a standard methodology developed in the Top End. Generally, the sites are more suitable for inventory than monitoring. They do, however, provide a large data set that can be used to test for indicators or surrogates of broader biodiversity health. A grazing gradient approach has been used on both red and black soil country where increasing distance from water reflects decreasing grazing pressure. This work has yielded some interesting results that can be used to develop improved regional fire and grazing management strategies. Rangeland monitoring sites have also been used to see if there is a correlation between land condition and vertebrate impacts. Finally, projects in areas within the arid zone of the Northern Territory and in Townsville (Queensland) are providing clues to measuring biodiversity impacts that may be useful for the VRD. Fire Sampling on fire plots established on black and red soils at Kidman Springs has shown that fire regime does influence the abundance of many vertebrate species, particularly birds and reptiles. Most importantly, the work has shown that clay and loamy soil sites that have been recently burnt support a more similar range of vertebrate animals than those sites that have not been burnt for some time. This suggests that the widespread application of any single fire regime over a large area will lead to a decline in biodiversity, and that a variety of burning regimes across the landscape is desirable. Over 50 plots have also been sampled in the sandstone country of Gregory National Park. This is aimed at comparing what animals are found in areas that have been subject to different fire histories and will assist in the development of fire management strategies for this type of country. * Alaric Fisher, Parks and Wildlife Commission of the NT

2 Grazing - black & red soils Work in the VRD and Barkly region has shown that while the effects are relatively muted, grazing on black soils will affect the composition of vertebrate species found in these areas. These effects are most pronounced for reptiles, for which the total number of animals (abundance) and the number of species found (species richness), decreases as grazing pressure increases. Decreaser and increaser vertebrate species have been identified. Decreaser species are sensitive to grazing effects and are more abundant further away from watering points. The decreaser vertebrate species also tend to be largely confined to the black soil habitat, such as the lizard Ctenotus rimacola. Increaser species, such as the brown songlark, increase in abundance as grazing pressure increases or are more abundant close to watering points. The establishment of many watering points has allowed these species to spread widely across a landscape where they previously probably occurred only sparsely. The implications of this are that uniform heavy stocking across a large area of black soil country will reduce or even eliminate some endemic vertebrates. Maintaining lightly grazed patches of country with a high level of perennial cover will safeguard against this situation. It follows that retaining areas that are distant to water should be a priority. Comparisons of paddock size, the positioning of water points and the total population of decreaser (grazing sensitive) species suggest that the population of these species is higher in paddocks with two corner water points than in paddocks with one central water point (Figures 1, 2), because there is more water-remote country. Work done in red soil country, on the other hand, suggests that current management regimes are not adversely affecting biodiversity. No trends in the abundance or richness of vertebrates were found along the grazing gradient at Kidman Springs and there was little difference between the exclosures and grazed sites. It must be stressed, however, that sampling was undertaken within a series of very good seasons and that management during poorer seasons is more likely to be critical for biodiversity conservation, land condition and production. A land condition approach Satellite information is being used to assist in the monitoring of a number of rangeland sites across the VRD. The attraction of using satellite information is that it enables the extrapolation of land condition and trend information across large areas and reduces the need for intensive ground based sampling. Work was undertaken to determine is there is a correlation between land condition assessed with the aid of satellite information, and vertebrate biodiversity. The idea was to determine what species are more likely to be found in sites classified under land condition criteria ranging from poor to good. This would enable much broader scale assessments of vertebrate impacts to be made from satellite information, without the need for labour intensive field based sampling. While some differences were found in the type of vertebrates found at good and poor sites, the relationships were quite weak. Unfortunately, the effects of canopy cover and landscape variations complicated assessments of vertebrate impact. Much more work will need to be done to use this technology for assessment of vertebrate impact.

3 A long term approach In another CRC project, the changing patterns of abundance of granivorous birds across the tropical savanna were studied. Granivorous birds are seed-eaters and include species such as parrots, pigeons and finches. This study took advantage of the fact that many bird-watchers, naturalists and scientists have recorded lists of birds from many locations within the savanna, going back for about a century. By comparing 'recording rates', or how frequently each species was recorded within these bird lists, it was possible to test whether each species had changed in abundance over a long period of time, and also map the broad geographic patterns of declines or increases. The results showed that many species of granivorous birds had declined, with the largest effects in the southeast of the tropical savannas in Queensland. It was concluded that while there was no firm evidence, the most likely factors in this decline were the spread of pastoralism and changes in fire regime. Other regional studies Two projects undertaken in other regional areas are providing information that may be useful for measuring vertebrate impacts in the VRD. A project based near Townsville in grassy ironbark woodlands is examining the number and type of species of both vertebrate and invertebrate animals on grazed and ungrazed loamy soils. It was found that species richness (the number of species) was not necessarily a useful indicator of grazing impact. This was particularly the case for birds, which were found to be the most resilient. Seventy percent of bird species found in ungrazed sites were also found in grazed sites. The richness of reptile and ant species showed more potential as an indicator, however, showing the greatest loss of species from ungrazed to grazed sites. Only 36% of reptile species and 43% of ant species found in ungrazed sites were also found in grazed sites. Other measures considered included species composition (the type of species present) and the presence or absence of individual species. Species composition was most likely to be different for reptile and ant species between grazed and ungrazed sites. Eighty-six percent of reptile species found in five or more sites showed significant effects from grazing. A CSIRO arid zone project has found that the most efficient indicators of impact across a range of habitat are understorey plants, birds and ants, although the latter tend to be less cost-effective. Potential indicators were assessed in terms of sensitivity to impact, the extent to which they were likely to be useful as surrogates for broader biodiversity, and ease of sampling and identification. Conclusions and further questions Each of these projects contributes to a much better understanding of the impacts of grazing and fire on vertebrate biodiversity. There are still a number of questions that need to be answered, however. It can be concluded that measures of species richness alone are not particularly useful. Reptiles and birds are the most likely to show detectable impacts, but the latter are generally much easier to sample. Potentially useful indicators for one

4 environment or region, however, may not be particularly useful for others and may also not give good indications of broader landscape health. Approaches that work beyond standard sample plot sizes may have some merit, as well as those that incorporate other elements of the landscape as surrogate measures of health. In terms of fire, biodiversity impacts can be reduced by employing a variety of regimes across the landscape and by limiting the widespread application of one single regime. Similarly, widespread heavy stocking across black soil country will lead to either the reduction or elimination of some unique vertebrate animals. The maintenance of lightly grazed patches of country with high levels of perennial cover is therefore more desirable. The positioning of central water points which provides for areas of relatively ungrazed portions of country is also important to maintaining habitats for a variety of vertebrate animals water point in center water points in 2 corners total population paddock size (km) Figure 1 The total population of a decreaser (grazing sensitive) species will decline as paddock sizes decrease, because there is a smaller area of water-remote country. Similarly, there will be a smaller population of decreaser species in a paddock with one central water point, than in a paddock with two waterpoints in opposite corners.

5 Figure 2 Water point distribution in the mitchell grass areas of the VRD. The dark blue dots are waterpoints. The coloured rings are circles of increasing distance from water, with each ring being 2.5 km wide. The red areas are more than 7.5 km from water.