EXOTIC WOODY WEEDS Use of simulation models to predict future ...

the parameters were adjusted to reflect this. This adjustment was minor, and only necessary for the
southeastern Australian limit.
2.3.2 A population model of Acacia nilotica: A tool for exploring weed management
and the effects of climate change
The study of Acacia nilotica has been fragmented in time, space and purpose. Consequently, our
knowledge is also fragmented. A plethora of information has been published on prickly acacia over a
period of more than a century (Fagg and Greaves 1990). However, despite some good field ecological
research and monitoring programs (Bolton, Carter, and Dorney 1987);(Carter, Jones, and Cowan 1991),
and an attempt to draw management guidelines from a simple transition matrix model (Mooy, Scanlan,
Bolton, and Dorney 1992), a holistic understanding of this ecological system has proved elusive. One
reason for this is the lack of a tool suitable for integrating the disparate information, and then
representing the system behaviour under various scenarios. In a partial response to these challenges, a
computer-based simulation model of the population dynamics of prickly acacia was developed in a
collaborative effort involving the current RIRDC project, the University of Queensland, CSIRO
Entomology and CSIRO Tropical Agriculture.
2.3.2.1 Model description
The prickly acacia model was created using DYMEX (CSIRO Entomology), a generic populationmodelling
tool that uses climate variables and environmental parameters to drive lifecycles. It does this
by describing the life-history properties of the average individual in each cohort on a weekly time-step.
The model simulates a paddock through the use of two interconnected lifecycles representing bore drain
and upland plant populations (Figure 2.3). The connection between the two populations is via seed
dispersal by cattle or sheep. The two lifecycles were necessary because in most parts of Queensland
where prickly acacia occurs, paddocks contain open bore drains (artificial channels fed by artesian
bores). Trees growing alongside these drains have markedly different growth rates, seedling and juvenile
survival rates, fecundities, plant densities and canopy covers than trees growing in adjacent upland
habitat. The only difference between the modelled upland and bore drain lifecycles is the way their soil
moisture environments are described: the bore drain soil moisture module includes an irrigation
component.
The model identifies six discrete lifestages: seedbank, seedling, juvenile, adult, flowers and seeds-inpods
(Figure 2.4). The flowers and seeds-in-pods are endostages ie lifestages that are contained within
another lifestage – in this case within the adult lifestage. This novel approach allowed us to include
processes of selective abortion of flowers and pods in response to frost or drought, as well as
differential development rates for pods depending upon their cohort-specific experience of
environmental conditions.
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