Invasive species, like rats, cats and weeds, threaten many species on offshore islands.  Some of these threatened species occur nowhere else in the world.  Defending islands from the impact of invaders is therefore key to maintaining their biodiversity value.  Some islands do not have these invasive species, yet.  On other islands we have successfully removed one or more of the invaders.  For these pest free islands how much effort should we put into quarantine, reducing the risk of a species invading?  Alternatively we could spend money on surveillance, looking for the pest on the island with the view of eradicating it before it gets out of control.    How should we allocate our resources to these two activities?

Compared to the scale of the problem, funds available to manage invasive species are limited. Hence we use a cost-minimisation approach to prioritise investment in these invasive species management actions.  We develop a simple model of an island under threat of invasion by a pest (animal, plant or disease) to develop rules of thumb to address this question.  We find the allocation of management resources to quarantine and surveillance that will minimise the total cost of management and impact. 

Results/Conclusions

In the optimal allocations, joint investment in both quarantine and surveillance is uncommon.  Investment in quarantine is optimal if quarantine is more cost-effective than surveillance or if the cost of an incursion is similar to that of a full blown invasion (e.g. large losses are incurred even at low pest density). Surveillance is optimal if it is considerably more cost-effective than quarantine or if the cost of an incursion is a small fraction of the cost of an invasion (e.g. invasion impact is proportional to pest density).  Investment in quarantine is also favoured as our ability to eradicate a pest declines.

We apply the model to preventing black rat (Rattus rattus) invasion on Barrow Island, Western Australia.  For Barrow Island our model predicts that surveillance is the optimal strategy.  This result contrasts with the current management strategy of large investment in quarantine.  We suggest that this is due to a tendency for managers to act in a risk averse way and the difficulty of estimating costs that combine management, environmental and social factors.