Assessing Alternative Strategies for Weed Management: Containment Versus Eradication?

Dr. Helen Murphy (CSIRO)

Containment is a frequently advocated strategic objective for countering plant invasions. The goal of containment is to prevent establishment and reproduction of a species beyond a predefined area, whereas eradication aims to remove all individuals of a species.  Containment is often perceived as a valid fallback option when eradication has failed or is deemed impossible with the available resources.  However, many infestations are likely to be no more amenable to containment than eradication, because the ecological drivers that determine containment success are the same as those that limit successful eradication, e.g. seed-bank persistence, dispersal mechanisms and capacity, and detectability. 

CSIRO Tropical Ecosystems Hub researchers in collaboration with QPWS (Project 7.2 Invasive species risks and responses) have been using modelling approaches to understand where, why and when land managers might shift their management focus from eradication to containment.  We have undertaken a net present value analysis of the costs of eradication and containment and derived rules to guide land managers in determining (1) the circumstances under which a containment strategy is likely to be more effective or efficient than an eradication strategy, (2) the effect of a ‘breach’ (i.e. establishment or reproduction outside predefined management areas) on each type of management strategy, and (3) the situations in which containment would form a valid fall-back strategy for a breach in an eradication program. 

While containment has one major advantage over eradication, in that a smaller area can be managed, this must be balanced against its disadvantage; that it must continue indefinitely. Our modelling results show how different invasions will be more effectively managed by either eradication or containment based on the soil seed longevity, the discount rate, and most importantly, the size of the infestation relative to the width of the buffer zone which is defined by dispersal capacity.  Thus, invasive species with long soil seed bank lifetimes in economic systems with high discount rates will tend to be better managed with containment than eradication. Crucially, there is a threshold invasion size below which it will be better to eradicate than contain, and above which the opposite is true.

Our research also clearly shows that the two management strategies incur very different additional costs if they experience an unexpected breach. This suggests that a consideration of the likely types of breach in a given system may provide further support for either eradication or containment, but also that automatically accepting containment as a default fall-back for a breached eradication program is not valid.

Figure 1. Zones of eradication and containment. When the invasion diameter is small relative to the distance over which seeds are dispersed, eradication is likely to be cheaper than eradication over the long term. When seeds are long-lived in the soil or the discount rate is high, containment is likely to be cheaper than eradication.

Figure 2. A simple model of weed management and possible breaches of containment. The radius of the occupied zone, r, is comparable to the size of the invasion. The width of the buffer zone, d, should be related to dispersal processes of the invader (Fletcher et al. 2013).

For more information, contact Dr. Helen Murphy (helen.murphy@csiro.au)