Population dynamics of the Yellow-footed Antechinus in response to experimental manipulations of floodplain fallen timber loads.

A project undertaken at The School of Biological Sciences, Monash University, Victoria, and supervised by R Mac Nally

Together with habitat loss and habitat fragmentation through clearing of natural vegetation, change in habitat 'quality' has been identified as one of the principal factors affecting viability of populations and local biodiversity (Hanski 1999, Reed et al. 2002). Habitats have been changed in many ways, often by the removal of structural components that are required by certain species to survive. Large trees, with their hollows, shrubs and fallen timber (logs and large branches) all have been identified as critical resources for many animals in eucalypt forests in Australia, and forests all around the world generally.

Members of the Australian Centre for Biodiversity: Analysis, Policy and Management, at Monash University in Melbourne (Figure 1), have been studying fallen timber in floodplain forests of the Murray-Darling Basin since 1996 (Figure 2). The project has involved broad-scale mapping of the distribution and loads of fallen timber, and the relationship between biodiversity and wood loads. Our results for wood loads on 220,000 ha of floodplains of the Murray, the Darling and their major tributaries, showed that average loads were just 19 tonne per hectare. From sites where little disturbance has occurred, loads may reach 125 tonne per hectare, suggesting a depletion of perhaps 22,000,000 tonne of fallen timber in the areas we have studied. These results have been published late in 2002 in the journal Restoration Ecology (Mac Nally et al. 2000b).

Ultimately, however, we are most concerned about the impacts on biodiversity of depletion of fallen timber loads. Some general results based on correlative studies have been reported (Mac Nally et al. 2001), but with support of the Australian Research Council (ARC), we have been following the responses of animals to a medium-scale experiment on Gunbower Island, in northern Victoria (Mac Nally 2001). The experiment involved measurement of how birds, mammals and invertebrates have responded to the manipulation of fallen timber in 34 one-hectare plots. As with any experiment, it is important to compile information on animals' use of the plots for some time before the experiment because there will be natural spatial variation in numbers (e.g. due to variation in tree densities per hectare, or proximity to billabongs). We did this for one year (1999), manipulated the wood loads in March 2000 ( Figure 3), and followed the responses for two years with ARC funding. The treatments included a range of loads (0 to 80 tonne per hectare) (Figure 4).

Results from the first three years' surveys have shown that birds generally occur in greater diversity and richness in plots where wood loads exceed 40 tonne per hectare. The results for a near-endangered species, the brown tree-creeper, are particularly encouraging (Mac Nally et al. 2002a), with densities in plots with 40 tonne per hectare or more increasingly substantially after the wood load manipulation (Figure 5).

A deficiency in much of the ecological research is the short duration of most projects. With the generous support of the Hermon Slade Foundation, The Gunbower Island project has been extended for another three years (2002 - 2004), providing us with the opportunity to continue on with our studies o f the main species of ground-dwelling mammal, the Yellow-footed Antechinus, Antechinus flavipes (Figure 6). Three years of results have already been published (Mac Nally and Horrocks 2002), but the opportunity to have five years data following the manipulation is a significant addition to the work. The project now also has been supplemented by a new PhD project. Hania Lada, who will be co-supervised by Andrea Taylor and Ralph Mac Nally, will add value to the project by conducting studies of the genetics of the experimental population. the population goes through huge density cycles (from a s few as three individuals up to more than 80!), and we want to know whether this forces the population through 'genetic bottlenecks'. We are also interested in finding out whether the population has become genetically isolated by the Murray River now having water all year round. In some summers in the eighteenth century, the river broke up into a system of disconnected pools, which would have favoured dispersal and hence gene flow in the antechinus populations across the floodplain.

Interest in the ecological importance of fallen timber in forests and woodlands continues to grow at a fast pace throughout the world. Fallen timber is a much-depleted structural component of these habitats, and the current work ins beginning to provide a solid basis for the management recommendations and actions for restoring forest and woodland biodiversity and ecological function.

References

Hanski, I. 1999. Habitat connectivity, habitat continuity, and metapopulations in dynamic landscapes. Oikos November 87:209-219.

Mac Nally, R. 2001. ‘Mesoscale’ experimental investigation of the dependence of riparian fauna on floodplain coarse woody debris. Environmental Management and Restoration 2:147-149.

Mac Nally, R., and G. Horrocks. 2002. Habitat change and restoration: responses of a floodplain forest-floor mammal species to manipulations of fallen timber in forests. Animal Biodiversity and Conservation 1:41-52.

Mac Nally, R., G. Horrocks, and L. Pettifer. 2002a. Experimental evidence for beneficial effects of fallen timber in forests. Ecology 83:in press.

Mac Nally, R., A. Parkinson, G. Horrocks, L. Conole, and C. Tzaros. 2001. Relationships between terrestrial vertebrate diversity, abundance and availability of coarse woody debris on south-eastern Australian floodplains. Biological Conservation 99:191-205.

Mac Nally, R., A. Parkinson, G. Horrocks, and M. Young. 2002b. Current loads of coarse woody debris on south-eastern Australian floodplains: evaluation of change and implications for restoration. Restoration Ecology 10: 627-635.

Reed, J. M., L. S. Mills, J. B. Dunning, Jr., E. S. Menges, K. S. McKelvey, R. Frye, S. R. Beissinger, M.-C. Anstett, and P. Miller. 2002. Emerging Issues in Population Viability Analysis. Conservation Biology February 16:7-19.