Island ecosystems are highly diverse and valuable to conservation. But the same elements that make islands so ecologically valuable – their distance and isolation – also make conservation interventions on islands (such as the eradication of invasive pests) expensive and difficult.
Because of the high-risk, high-reward nature of island-based eradications, deciding how to allocate funds between such actions is incredibly important.
Until now, published prioritisation methods assume that decision makers only have one action available on each island (normally to eradicate all vertebrate pest species), and managers either perform the action on the island or do nothing. But new research out of CEED, published in the Journal of Applied Ecology, has shown that if we want the best conservation outcomes on islands then we should be focusing on prioritising eradication actions rather than attempting to clear whole islands of pests. In so doing we can achieve much better outcomes for threatened species for the same investment.
There can be no doubt that eradicating invasive vertebrate pests on islands can yield enormous conservation rewards. Seabirds on sub-Antarctic Macquarie Island, including the endangered New Zealand subspecies of Antarctic tern and vulnerable wandering albatross, have seen significant increases since eradicating invasive mammals. The rapid decline of the largest colony of fairy prions has been halted on Tasman Island after cat eradication. Successful reintroduction programs have been undertaken for small vulnerable and endangered native mammals on the Montebello Islands and Faure Island in Western Australia thanks largely to eradication of cats from those islands.
These success stories were used to illustrate the method proposed by the CEED researchers. By applying the general rule to prioritise actions rather than places broadly, many more threatened species can be aided globally with our limited conservation funds.
Prioritising eradication actions rather than whole islands may mean choosing to leave less harmful (or more difficult or expensive to eradicate) invasive species on some islands. Redirecting the funds saved to instead eradicate invasive species on other islands that are more harmful (or more cost-efficient to eradicate) will improve, for example, the state of a whole group of islands.
"It's still important to be careful about which species remain on islands," cautions Kate Helmstedt, the researcher who led the analysis and completed her PhD in 2014 at the University of Queensland as an affiliate of CEED.
"You need to make sure to include any potential negative outcomes in the prioritisation process. There can be downsides to only eradicating some invasive mammals from an island ecosystem and leaving others. Sometimes by eradicating one invasive species (particularly a predator), another will be released from pressure and will increase their impact."
However, as long as these potentially negative factors are considered, Helmstedt and colleagues were able to show the improved flexibility of the prioritisation process still allows for significantly better overall outcomes.
Helmstedt, K.J., Shaw, J., Bode, M., Terauds, A., Robinson, S., Springer, K., Possingham, H.P. (2016) Prioritizing eradication actions on islands: It's not all or nothing. Journal of Applied Ecology, early online (DOI 10.1111/1365-2664.12599)
Image: The remains of a fairy prion on remote Tasman Island, the victim of a cat. The photo was taken prior to the eradication. (Photo by Justine Shaw)