Principal Investigator Ian Noble Project s58

Research School of Biological Sciences Machine VP

Co-Investigators Sandra Lavorel and Mark Stafford-Smith

C.N.R.S., Montpellier, France and CSIRO, Division of Wildlife and Ecology, Alice Springs

Dynamics of Mistletoe Invasion of Fragmented Woodlands

The model investigates the effects of the fragmentation of woodlands on the spread of mistletoe populations. The model specifically examines the interactions between the biology of mistletoe dispersal and woodland pattern in terms of total tree cover and degree of tree aggregation. Hierarchically patterned maps of tree pattern are generated. The population dynamics is simulated for each tree by following mistletoe cohorts. Dispersal by mistletoe birds is modelled as a function of distance between trees and attractiveness of trees depending on size and resident mistletoe fruit production. Parameter values are derived from field studies of the demography of mistletoe in New England and Central Australia. The results are used to design field investigation of demography and bird behaviour. They are also used to suggest ways in which mistletoe spread could be controlled by manipulating tree pattern and disinfecting appropriate trees.

What are the basic questions addressed?

The aims of the model are both theoretical and applied. Theoretical questions concern the interaction of landscape patterns and the dispersal biology of organisms. We investigate which aspects of landscape heterogeneity (total fraction of trees and scale of fragmentation) affect most dispersal. We also examine which parameters of the dispersal strategy are most important in determining population success in the case of a bird dispersed species.

Practical questions concern disinfection of mistletoe invaded woodlands. The aim of the model is to use theoretical results to generate and compare disinfection scenarios.

What are the results to date and the future of the work?

As expected from field observations, mistletoe infestation is increased for fragmented woodlands with fewer and more scattered trees. The sensitivity analysis shows that fraction of ornithochorous dispersal, bird behaviour and most importantly seed survival during dispersal, significantly affect the spread of mistletoe in the landscape. Significant interactions exist between seed survival and other parameters. The results show that accurate observations of bird behaviour are needed to estimate parameter values more accurately before predictions can be made about disinfection strategies.

Further investigations will take into account further refinements in dispersal behaviour (e.g. introduction of non-host perching trees) and the possibility of fires.

What computational techniques are used and why is a supercomputer required?

The model simulates the dynamics of mistletoes in a landscape consisting of host trees. First, a landscape pattern is generated using a hierarchical dichotomy for tree location. The landscape consists of a number of bird territories, each containing a certain number of trees. The procedure allows controlled experiments on landscape structure. The demography of mistletoes is modelled at the scale of each host tree using cohorts of mistletoes of equal age. The survival, growth and seed production are calculated for each cohort on each tree. Then seeds are dispersed between trees within a territory according to the fraction of bird dispersal, distance between trees and attractiveness of individual trees calculated in relation to size and mistletoe fruit crop. Dispersal has a stochastic dimension which requires that seeds be dispersed individually. This in particular, and the whole demographical dynamics which is organized in a spatially and demographically hierarchical manner; yields a number of convoluted loops which are particularly appropriate for vectorisation. The present version of the program achieves about 90% vectorisation.

Publications

Spread of mistletoes in fragmented eucalyptus woodlands, S Lavorel, M Stafford-Smith and N Reid, Annual meeting of the Ecological Society of America, Snowbird, USA (1995).