1. Systems Analyst, CSIRO Sustainable Ecosystems Div., P.O. Box 310, North Ryde, New South Wales 1670, Australia (corresponding author).
2. Research Scientist, CSIRO Sustainable Ecosystems Div., P.O. Box 284, Canberra, Australian Capital Territory 2601, Australia.
3. Systems Modeller, CSIRO Sustainable Ecosystems Div., P.O. Box 284, Canberra, Australian Capital Territory 2601 Australia.