Simple rules for complex invasive weeds

Designing practical rules for controlling invasive species is a challenging task for managers, particularly when species are long-lived, have complex life cycles and high dispersal capacities. Previous findings derived from plant matrix population analyses suggest that effective control of long-lived invaders may be achieved by focusing on killing adult plants. However, the cost-effectiveness of managing different life stages has not been evaluated.

This investigation illustrates the benefits of integrating matrix population models with decision theory in evaluating the largest infestation of mesquite (a leguminous weed) in Australia. Included in the model is the mesquite life cycle, different dispersal rates and control actions that target individuals at different life stages with varying costs, depending on the intensity of control effort. It then used stochastic dynamic programming to derive cost effective control strategies that minimize the cost of controlling the core infestation locally below a density threshold and the future cost of control arising from infestation of adjacent areas via seed dispersal.

The researchers then applied a sensitivity analysis to show that four robust management rules guide the allocation of resources between mesquite life stages for this infestation:

(i) When there is no seed dispersal, no action is required until density of adults exceeds the control threshold and then only control of adults is needed;
(ii) when there is seed dispersal, control strategy is dependent on knowledge of the density of adults and large juveniles and broad categories of dispersal rates only;
(iii) if density of adults is higher than density of large juveniles, controlling adults is most cost-effective;
(iv) if density of large juveniles is equal or higher than density of adults, management efforts should be spread between adults, large and to a lesser extent small juveniles, but never saplings.
The study show that simple rules can be found for managing invasive plants with complex life cycles and high dispersal rates when population models are combined with decision theory. In the case of our mesquite population, focussing effort on controlling adults is not always the most cost-effective way to meet our management objective.

More info:

Pichancourt J, I Chade’s, J Firn, RD van Klinken & TG Martin (2012). Simple rules to contain an invasive species with a complex life cycle and high dispersal capacity. Journal of Applied Ecology 49: 52–62.

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