My final PhD paper is finally out! So, I think this is the perfect time to post an overview of what I did & lessons learnt. While all of the papers focus on invasive species control, they vary from being quite theoretical to applied.
In this paper, we introduced the population dynamic model that I used for most of my thesis. The basic idea of this paper was to determine the best way to allocate control effort spatially in order to minimise the abundance of an invasive species at a specific location. We showed that the optimal allocation is related to the ratio of spread rate to growth rate. If this ratio is small, then control effort should be focused in a small area, while if the ratio Is large, then control effort should be spread out more widely in the landscape.
In this paper, we considered the spatial problem, along with the associated problem of removing an invasive species from an island. In the spatial problem, we looked at how a buffer zone heuristic compared to the optimal solution, and we found that an appropriately chosen buffer zone performed extremely well. We also found an interesting trade-off for island eradications, between the species growth rate and the diminishing marginal returns of control effort. In particular, the faster the species growth rate, the faster eradication should be completed. Even though it’s intuitive to think that a high growth rate means that eradications will take longer, it actually means that it’s imperative to complete them quickly.
In this paper, I solved for optimal spatio-temporal resource allocation. These results really pointed toward a principle of targeting the source of invasions. That is, the part of the landscape which is responsible for the majority of spread. It’s important to note that the location of the ‘source’ can change through time. High-density regions close to low-density regions are prime examples, while a high-density region surrounded by other high-density regions would not be a source, as it would not be able to effectively cause spread into new regions.
In this paper, we focused on fire ant control at Ashmore Reef in the Timor Sea. There were two main aspects to this: understanding how different ant baits affect the population and modelling the detectability of fire ants. There are two classes of ant bait available: toxin and growth regulator. We used pilot data to quantify the effect of each bait and then used this to suggest that a combined use of both over two years would be required to complete eradication. Following any eradication attempt, it is vital to estimate the probability of success. We used detection experiments to quantify the probability of detection and then optimise the number of lures that should be used. Further, we considered the possibility of detector dogs, and we found that, provided sufficient use, canine detection would be a cost-effective option.