MethodsAllocating biosecurity resources between preventing, detecting, and eradicating island invasions
Highlights
► We use a simple model of island invasion to find how resources should be allocated between prevention, surveillance, and eradication. ► We illustrate its application with a case study of black rat (Rattus rattus) invasion on Barrow Island, Western Australia. ► Optimal investments depend on the effectiveness of actions and the impacts caused at different levels of invasion. ► Our model provides decision support by identifying the trade-offs inherent in each management strategy.
Introduction
Invasive species are a major threat to biodiversity (Primack, 2006). They also disrupt ecosystem services (Cook et al., 2007), reduce agricultural and forestry productivity (Julia et al., 2007, Yemshanov et al., 2009), and damage public infrastructure (Burnett et al., 2008). Given the scale of the problem, it is important to find smart and efficient ways to manage invasive species and their impact. This is reflected in the number of studies finding cost-effective control and eradication strategies (Epanchin-Niell and Hastings, 2010), and investigating the trade-offs involved in intervening at different stages of invasion (Bogich et al., 2008, Burnett et al., 2006, Burnett et al., 2008, Finnoff and Tschirhart, 2005, Finnoff et al., 2007, Leung et al., 2002, Mehta et al., 2007, Moore et al., 2010, Olson and Roy, 2005).
While removing individuals with a view to eradication can reduce or eliminate further impact of an invasion, investing in quarantine or inspection can prevent invasions at the outset, and surveillance increases the chance of early detection, which increases the chance of successful eradication (Rejmanek and Pitcairn, 2002, Timmins and Braithwaite, 2002). How should managers be allocating limited resources between these three different actions? So far, few studies have examined the relationship between all three facets of management (Carrasco et al., 2010, Polasky, 2010). In this study, we develop a simple model of insular invasion, and find the allocation of resources to prevention, surveillance, and removal that will minimise the total cost of management and impact. We use a case study of black rat (Rattus rattus) invasion on Barrow Island, Western Australia, to illustrate our model and consider the implications of the results.
Our applied conservation focus differentiates this study from previous economic analyses of this problem. Although the model developed here can be applied to any isolated site, a case study of island invasion is particularly relevant for conservation. Islands have more exotic plant species than mainland sites of the same area (Lonsdale, 1999), and invasive mammals such as rodents and cats are a major threat to island biodiversity (Donlan and Wilcox, 2008, Harris, 2009, Jones et al., 2008, Towns et al., 2006). Defending islands from the impact of invaders is, therefore, a key conservation objective.
The main focus of this study is to find optimal pre-emptive strategies that defend an island from the impact of a pest that is currently absent from that island. Prevention is often seen as better than a cure, but is it always more cost-effective? We ask, under what circumstances is each management action optimal? Along with these pre-emptive strategies, we find optimal strategies when the species has already invaded and become widespread. When is it best to eradicate, and when is it more cost-effective to give up and surrender to invasion?
Section snippets
Materials and Methods
Let us assume that the steward of an island is concerned about the possibility of a particular species invading the island. If the species invades it may be present as a localised population, or it can grow to be widespread across the island. We assume that localised populations may not be detected, whereas a widespread invasion will certainly be detected. We define four possible states of this system, based on the coverage of the invader and the manager's belief (Fig. 1): absent, localised and
Optimal Pre-emptive Strategies
We first report the optimal allocations to quarantine, surveillance, and removal when the pest is absent. For Barrow Island, it is optimal to invest $250,000 in surveillance only, given the estimated effort curves and costs of impact (using the lower bound of our estimate for the cost of impact of a widespread population). Surveillance is more effective than quarantine and the impact (and cost of eradication) of a localised population is relatively small, making it best to catch and eradicate
Discussion
In this study, we identified when we can expect each of three management approaches to most cost-effectively mitigate the impact of invasive species: prevention through quarantine, catching invasions early with surveillance, and preventing further impact through removal. From our results we can infer some general rules of thumb. When all management actions are equally effective, quarantine is preferred over other actions as it can prevent all impact costs. Quarantine is also preferable when it
Acknowledgements
Many thanks to Cindy Hauser, David Roberts, Andrew Solow, and four anonymous reviewers for helpful comments that improved this manuscript, and to Dorian Moro for discussion of the Barrow Island case study. This work was supported by an Australian Postgraduate Award, the AEDA Commonwealth Environment Research Facility, the Australian Centre of Excellence for Risk Analysis, and an Australian Research Council Discovery Grant (DP110101499). The Australian Research Council funded HPP as a Federation
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