Optimal detection of invasive species: Comparing strategies when eradication is and is not possible

Background/Question/Methods

While there is a rich literature describing the optimal deployment of pesticides and natural enemies to control invasive species, little is known about how managers should sample to detect them. Past studies focus on finding equilibrium sampling protocols that are constant through time, despite the importance of transient dynamics in invasion biology. In addition, these models rely on the assumption that local populations of the invader are immediately eradicated after detection, even though eradication is often slow or impossible, and when it is possible there may be alternative controls to choose from. We consider a simple model that relaxes these assumptions, and use optimal control theory to describe how different control strategies: (1) eradicating local populations, (2) preventing outbreaks and (3) reducing spread, affect the sampling protocols which minimize the total cost of management and damages.

Results/Conclusions

We find the control action can greatly affect the optimal sampling strategy. In many scenarios, if the invasive is immediately eradicated following detection, a manager should sample less than in the case where detected patches are prevented from growing into local outbreaks.  However, when the spread of the invasive is much slower than it’s local growth, this pattern can be reversed. In addition, for invasive species that spread quickly, the best constant sampling strategy is up to 25% more costly than the optimal sampling strategy that varies through time. On the other hand, if the spread is slow, the best constant strategy can perform just as well as the optimal time varying one. Our results suggest that sampling guidelines from past studies may be effective for slowly spreading invaders, but potentially underpredict sampling effort for invasives that spread quickly.