Appendix S5 – Case study application
1. Action portfolios
Every combination of invasive species on each island was considered as a potential
action, except in the case of Macquarie Island. In consultation with experts, we
determined that on Macquarie Island, eradication of mice but not rats would be
impossible (as rats would consume the bait, depleting available bait to below a critical
density require for mouse eradication). We therefore did not allow any package that
contained mouse eradication in the absence of rat eradication.
2. Ecological benefit
We estimated the island population of each species of concern using a combination
of a literature review and expert elicitation. Two of the invasive-species states we
consider have been observed on these case study islands: all invasive species present,
and no invasives present (because all islands in our case study have had all invasives
successfully eradicated). The other potential states of the islands (all possible
combinations of invasive species) is more complex to estimate. Here we estimated
abundances of species of concern in these alternative invasive-species states using
expert estimation and observed invasive-threatened species interaction effects from
the literature (see Appendix 1).
3. Feasibility
Gregory et al. (2014) found a number of univariate relationships between island
attributes and the probability of successful eradication on islands for various types of
invasive species. We used their formula relating island size to probability of success
to calculate the feasibilities of eradicating each invasive species from each island (see
Appendix 2). Future applications of this framework could incorporate managers’
confidence in the feasibilities of the proposed eradication.
4. Costs of action packages
We used the relationship found by Martins, Brooke et al. (2006) relating the size of
an island with the cost of eradicating various invasive vertebrate species (see
Appendix 3). We recognize that this statistical estimator will underestimate the costs
of eradication from Macquarie Island due to its isolation, however since this is an
illustrative prioritization, we accept this difference. Unfortunately, these models do
not consider how the cost of actions will increase when multiple actions (an action
package) are performed on one island. We used expert advice to determine how the
costs of individual actions combine on each island. For example, since rat and mouse
eradication on Macquarie Island can share costs of both baits and helicopter time,
eradicating both at once costs less than the sum of the costs of the two individual
actions.
Gregory, S. D., Henderson, W., Smee, E., Cassey, P. (2014). Eradications of vertebrate
pests in Australia: A review and guidelines for future best practice. PestSmart Toolkit
publication. Canberra, Australia, Invasivse Animals Cooperative Research Centre: 76.
Martins, T. L. F., et al. (2006). "Costing eradications of alien mammals from islands."
Animal Conservation 9(4): 439-444.