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

  • Lavorel S
  • Smith M
  • Reid N
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Abstract

A simulation model was used to study the interaction between landscape
pattern and components of the dispersal strategy of the mistletoe
Amyema preissii by mistletoe birds (Dicaeum hirundinaceum). The landscape
was modelled as a map of host trees for the mistletoes, characterised
by the total density and clumpiness of trees. A landscape was considered
as a set of equal sized bird territories, with the majority of seeds
produced in such a territory dispersed within that area. Age-specific
birth and death rates of mistletoes were measured in the field. Seed
dispersal was characterised by four parameters: the fraction of within-tree
seed dispersal, the ratio of attractiveness to birds of tree canopy
volume over attractiveness of mistletoe fruit number, seed survival,
and the fraction of seeds leaving their original territory. A sensitivity
analysis was carried out using a factorial design on landscape type
and dispersal parameters. General linear modelling of mistletoe population
size after 100 years showed that, in a given landscape, seed survival
was the strongest determinant, Total mistletoe population also increased
exponentially with tree density, but the number of mistletoes per
tree decreased. Population size depended on tree clumping as well,
with larger mistletoe populations sustained by woodlands with clumped
trees. For a given level of seed survival, population size increased
when birds were more attracted by canopy volume than by fruit crop.
The strongest increase in population size occured for a combination
of low tree density with high relative attractiveness. The relative
effects of the fraction of within-tree dispersal and tree density
depended on seed survival. For lower survival, fraction of within-tree
dispersal determined population size more strongly while for higher
survival, tree density became the dominant factor Population size
was negatively correlated with the fraction of within-tree dispersal.
Finally, population size strongly increased only if dispersal out
of a bird's territory represented 10% of the seed crop, a high value
which seems unlikely in the field. The results support the hypothesis
that woodland fragmentation promotes invasion by mistletoes. Although
simulated mistletoe populations deviated from our natural population
in having an excess of young individuals, sensitivity analysis produced
several non-intuitive results and is thus valuable in focussing further
efforts on field data collection. This study also illustrates how
a simulation model of population dynamics can help in determining
control strategies for an invasive organism. A reduction in seed
survival and disinfection of larger trees would appear to be the
most efficient strategy.

Author-supplied keywords

  • Arid woodland
  • Fragmented landscape
  • Invasion
  • Mistletoe Amyema preissii
  • Ornithochory
  • Simulation model

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