Spread of mistletoes (Amyema preissii) in fragmented Australian woodlands:a simulation study

A simulation model was used to study the interaction between landscape patt ern and components of the dispersal strategy of the mistletoe Amyema preiss ii by mistletoe birds (Dicaeum hirundinaceum). The landscape was modelled a s a map of host trees for the mistletoes, characterised by the total densit y and clumpiness of trees. A landscape was considered as a set of equal siz ed bird territories, with the majority of seeds produced in such a territor y dispersed within that area. Age-specific birth and death rates of mistlet oes were measured in the field. Seed dispersal was characterised by four pa rameters: the fraction of within-tree seed dispersal, the ratio of attracti veness to birds of tree canopy volume over attractiveness of mistletoe frui t number, seed survival, and the fraction of seeds leaving their original t erritory. A sensitivity analysis was carried out using a factorial design on landscap e type and dispersal parameters. General linear modelling of mistletoe popu lation size after 100 years showed that, in a given landscape, seed surviva l was the strongest determinant, Total mistletoe population also increased exponentially with tree density, but the number of mistletoes per tree decr eased. Population size depended on tree clumping as well, with larger mistl etoe populations sustained by woodlands with clumped trees. For a given level of seed survival, population size increased when birds we re more attracted by canopy volume than by fruit crop. The strongest increa se in population size occured for a combination of low tree density with hi gh relative attractiveness. The relative effects of the fraction of within- tree dispersal and tree density depended on seed survival. For lower surviv al, fraction of within-tree dispersal determined population size more stron gly while for higher survival, tree density became the dominant factor Popu lation size was negatively correlated with the fraction of within-tree disp ersal. Finally, population size strongly increased only if dispersal out of a bird's territory represented 10% of the seed crop, a high value which se ems unlikely in the field. The results support the hypothesis that woodland fragmentation promotes inv asion by mistletoes. Although simulated mistletoe populations deviated from our natural population in having an excess of young individuals, sensitivi ty analysis produced several non-intuitive results and is thus valuable in focussing further efforts on field data collection. This study also illustr ates how a simulation model of population dynamics can help in determining control strategies for an invasive organism. A reduction in seed survival a nd disinfection of larger trees would appear to be the most efficient strat egy.