In an age where the rate of new species invasions continues to increase and conservation resources have become scarcer, Davis et al. (2011) are correct that conservationists must continue to strive to refine and improve the way we manage pest species. That means we need to be clear about what we are trying to protect and what the key threatening processes are. There are well-established criteria and reasons for when and why we should engage in efforts to control or eradicate an invasive species (Bomford and O’Brien 1995, Owen 1998, Myer et al. 2000, Mack and Forster 2004, Panetta and Tsimmons 2004). There are also some pests and situations where the lack of resources or appropriate and effective control tools, monitoring methods or support from key decision-makers will require that we walk away or adapt.

But we need to be careful that, while acknowledging the criteria for engagement and the occasions when we do walk away or adapt, we are not infected by fatalism or a lack of vision (Simberloff 2002) regarding established invasive species — because we must control the impacts of these species to halt the ongoing decline in global biodiversity.

There are sound reasons for optimism that this task is possible. An increasing number of control and eradication success stories from around the globe (see, for example, Mack and Foster 2009) that are transforming how we think about plant, invertebrate and vertebrate pest management. The Nature Conservancy has an important role to play in disseminating these ideas and in ensuring development of invasive species control and eradication strategies at ecological meaningful scales.

Success Stories: Eradication on Islands, Freshwater Fish and More

Small to medium-sized islands have been particularly fertile locations for developing new techniques in invasive species control. Indeed, lessons learned from successful (and unsuccessful) eradication attempts have enabled techniques to be refined and applied to larger and larger islands, culminating in recent successes like the eradication of feral pigs from California’s Santa Cruz Island (Ramsey et al. 2009). Successful island eradications have resulted in well-documented recoveries of numerous bird, reptile, plant and invertebrate species (Towns and Broome 2003). Skeptics are quick to point out that islands offer a unique set of conditions that can’t necessarily be replicated on the mainland. However, success has not been limited to offshore islands. The eradication of African mosquito from Brazil, screwworm from the southern United States and Mexico, smallpox across the globe (Simberloff 2002) and barberry from five western European
countries (Stakman 1923) are just a few examples of successful continental and regional scale eradication programs.

Additionally, eradication methods for vertebrate pests developed on islands are now being refined and applied to successfully control the same species on mainland habitats in New Zealand and Australia. Predator-proof fences have been built to create “mainland islands,” where introduced mammalian pests are then eradicated from within the fenced area, and missing native elements reintroduced. One of the earliest examples is the Karori Sanctuary, a 550-acre preserve in the heart of Wellington, New Zealand’s capital city that has transformed public perception about these kinds of efforts, creating a demand that has been replicated around the country by local communities with public and private funding. The size of these initiatives varies, but some are starting to reach ecologically meaningful scales, such as the mountaintop preserve at Maungatautari, New Zealand that was the vision of a group of local farmers. This 8,400-acre preserve is enclosed by a 29-mile-long fence, which allowed both eradication of all major mammalian invasive species (e.g. rats, stoats, cats, brushtail possum, deer and goats) and successful reintroduction of various threatened species.

Where geographies, climate or the scale of the operation preclude fencing, management sites can be selected to take advantage of natural barriers to dispersal (e.g. large rivers, glaciers, mountain ridges) that slow recolonization. Large-scale suppression is more cost-effective and sustainable in these situations, especially if source populations can be targeted (Robertson and Gemmell 2004). For example, in South Georgia (a British territory in the South Atlantic), rats are being systematically eradicated from large swaths of the island, on a scale that will eventually dwarf all previous individual eradication efforts (South Georgia ~1000 km2, previous largest ~110 km2) (Towns and Broome 2003, Robertson and Gemmell 2004). Glaciers are being used as natural fences that partition the island into smaller, more manageable treatment blocks. Genetic analyses show the glaciers to be effective barriers to rat dispersal (Robertson and Gemmell 2004). Fences were also used on Santa Cruz Island to break it into manageable treatment units. But large-scale, multi-species control programs are also being undertaken in valleys where natural features like large rivers or mountain ridges constrain or slow reinvasion, making it cost-effective to maintain long-term invasive species control programs provided these programs produce measurable benefits for target species such as threatened taxa (Cullen et al. 2005, Caruson 2006).

Success is not limited to terrestrial environments: the United States has led the world in development of freshwater fish eradication efforts. Successful removal of invasive fish like brown and rainbow trout from headwater streams to protect localized endemic fish and amphibians have been occurring for over 70 years (Finlayson et al. 2010). Fish are typically eradicated by multiple treatments using the fish piscicides rotenone or antimycin, and recolonization prevented by natural (waterfalls) or purpose-built fish barriers. These methods have been exported around the globe and used to restore populations of threatened fish (e.g., Lintermans 2000), and prevent establishment and spread of new introductions (e.g., Brazier and Britton 2006). Equally, the Great Lakes sea lamprey control program has successful reduced impacts to valued sports fisheries for the last 50 years and formed the model for a large-scale international effort to develop effective control tools for common carp in Australia and the United States (Bajer and Sorensen 2010).

Eradication is the most cost-effective strategy for species that have been introduced, but it is not always possible, and maintenance control (sensu Myers et al. 2000, Simberloff 2002, 2009) is a standard approach used to safeguard and maintain conservation targets. However, whether the goal is control or eradication, successful operations share a common set of characteristics (Bomford and O’Brien 1995, Myers et al. 2000, Simberloff 2002, 2009, Mack and Forster 2009):

• An ability to access and target all individuals in a population;
• For plants, seed bank should be short-lived;
• A means to detect target species at low densities;
• Reinvasion is prevented;
• Clear lines of authority and an ability to compel action;
• Support from local communities and key decision-makers;
• Adequate resources for the life of the project (including sufficient duration to detect and remove propagules).

And eradication or control efforts must be underpinned by an understanding of the invasive species biology, particularly dispersal and home-range characteristics (Robertson and Gemmell 2004). It is important to critically evaluate control or eradication efforts against these criteria (Mack and Foster 2009, Owen 1998), as failed operations can usually be attributed to the failure to meet one or more of these
conditions (Myers et al 2002).

While the scale of the invasive species problem can seem daunting, we should remember that globally we are often dealing with a common set of invasive species. By sharing resources and knowledge, we can stem the tide. Exciting new tools are becoming available, like a humane feral swine bait developed in Australia (Cowled et al. 2008), a species specific toxin for quagga and zebra mussels derived from locally occurring soil bacteria (http://marronebioinnovations.com/pdf/zequanox_bg2.pdf), or sensitive DNA detection methods (Ficetola et al. 2008, Jerde et al 2011) that will allow managers to go where few have dared to go before. The Nature Conservancy’s science and stewardship staff have an important role to play in ensuring that these tools are used to their fullest potential and do not fall victim to unwarranted fatalism (Simberloff 2009).

Acknowledgements
Thanks to D. Gordon and R. Lalasz for useful comments on this article and suggestions of additional references.

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Image: Predator-proof fence warning at Karori Sanctuary, Wellington, New Zealand. Image credit: Teacher Traveler/ Flickr.