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An Ounce of Prevention: Should TNC Do More About Invasives?

Gordon, Doria 10/26/2011

Once again, Mom was right: Avoiding an impact — whether you’re running with scissors or importing species without discrimination — is less painful than dealing with the consequences. The Conservancy is playing a leading role in trying to incorporate risk assessment into government decisions about species introduction. But do we need to do more on an organizational level?

Certainly, the science is clear: Research is demonstrating that preventing the import of species likely to become harmful invaders is more cost-effective in both economic and ecological terms than controlling those species once they have become established and identified as detrimental to human or natural systems (Leung et al 2002; Keller et al. 2007b).

The approach seems intuitive, particularly for intentional imports, if only we could identify (as Davis et al. 2011 caution) the high-priority species to prohibit. Risk assessment would allow exclusion of the small proportion of species that cause damage, while allowing commerce in valuable, non-invasive species (Keller & Lodge 2007). Unintentional imports of species are more difficult to prevent; in these cases, efforts generally focus on the pathway (shipping containers, seed contaminants, ballast water, etc.) rather than on the particular species (Lodge et al. 2006). In either case, pre-border prevention approaches are more effective than post-border inspection (Lodge et al. 2006).

For a century, the United States has incorporated prevention approaches into national regulatory policies to protect agricultural productivity. The U.S. Noxious Weed list contains roughly 100 species and some genera; the vast majority of those taxa are species that have never been recorded in the United States but are known to have such costly impacts to agricultural crops elsewhere that they have been prohibited for decades (Westbrooks 2002). Most other countries also regulate plant imports for potential impacts to agricultural productivity. Prohibited pests and pathogen lists are similarly dominated by taxa known to impact economically valuable species. Species
that carry diseases that impact human health (such as Gambian pouch rats (monkey pox; CDC 2003)) or livestock (such as ticks on imported African lizards (heartwater; Burridge et al. 2000)) are also readily prohibited. Governments have long and rightly prioritized
prevention of impacts to human health and livelihoods.

Only recently have regulated species lists started to include representation of species damaging to natural areas. The difference in the latter group is that the species considered are generally already present and spreading in the new range. In the United States, if those species are already considered too widespread for effective control (even if only present in one region) or have high economic values or a strong lobby, they are unlikely to be listed regardless of the current or potential damage incurred. If you doubt that statement, research the current battle to place nine constrictor species in the pet trade — all of which are demonstrated to pose a risk of invasion in the United States (Reed & Rodda 2009) — on the U.S. Fish and Wildlife Service’s Injurious Wildlife list in order to prohibit their import and interstate trade.

Countries such as Australia and New Zealand — which have unique flora and fauna and clear documentation of negative impacts by non-native invaders (e.g., Taylor 1984, Williams & West 2000) — use a more precautionary approach to potential invasives. These countries require the use of a predictive screening tool prior to import of any new species, be it plant, vertebrate or invertebrate (e.g., Bomford 2003, Roberts et al. 2010). If a species is predicted to have a high probability of becoming a harmful invader, the regulatory decision is made to restrict trade in that species. No assessment of potential economic value, stakeholder interest or other political considerations factor into the decision.

So how should preventative decisions be made? Precautionary listing has traditionally been based on evidence of harmful impacts of the species in other countries. Any species parasitic on a cereal crop in one location, for example, is considered too risky to import into other countries where that crop is cultivated, unless the environment would not support the parasite. After seeing the impacts of zebra mussels or fire ants or hydrilla in the United States, countries without these species are likely to prohibit them. And, in fact, the best predictor of whether a species is invasive in a new range is whether it has already been documented as invasive elsewhere outside its native range (Panetta 1993, Mack 1996). However, precautionary listing is insufficient for species that have no or only a brief history of translocation. This caveat is critical: species translocation rates have substantially increased in recent years (Levine & D’Antonio 2003, Lodge et al. 2006), and many species do not exhibit their full invasive potential for over a century (Kowarik 1995, Simberloff SC Sept. 2011).

Luckily, the more precautionary approach of countries like Australia means that predictive screening or risk assessment tools are available for testing in new locations (e.g., Gordon et al. 2008), and efforts are under way to develop such tools for multiple taxa
(e.g., Kohler & Lodge 2002, Keller et al. 2007a). The Nature Conservancy, while clearly investing in control efforts for the species identified to most significantly threaten our conservation targets on the ground, has been working for several years to incorporate
into the relevant U.S. national laws requirements for risk assessment prior to introduction of novel species. The Global Invasive Species Team was working on laws in South America and other countries as well; I don’t know if such efforts are still under way. However, different North America Conservation Region (NACR) operating units within the Conservancy have agreed to take the lead on different taxa. For example, the Conservancy’s Florida program is leading incorporation of risk assessments with known accuracy into various federal regulations — for plants, into the USDA Plant Protection and Quarantine (PPQ) rules, and for animals, into the U.S. Fish and Wildlife Service Lacey Act. The NACR Forest Pest and Pathogen Team is leading efforts to reduce the probability of accidental introduction of species impacting forests, and the Conservancy’s New York program is leading similar efforts to reduce the threat to aquatic systems through regulation of ballast water management.

In my experience, many other countries are very interested in implementing the tools that we are developing, increasing the leverage of this work and reducing the probability that harmful invaders will be translocated to new regions. I have been contacted by agency staff from every continent about the plant screening tools I have been testing, and am heartened that — despite the academic interest in novel ecosystems — many geographies are trying to prevent the homogenization and simplification of their biota.

So, is TNC still working on invasive species issues? Yes, at multiple scales that range from high-leverage prevention approaches that are having national and international impact, to more regional and local efforts to develop partnerships that (1) detect and respond rapidly to unintended and unanticipated imports and (2) manage those high priority species that are already here and are actively threatening the success of our conservation objectives. Greater coordination of all these efforts (see Serbesoff-King in this issue) would increase both our effectiveness and efficiency.

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Centers for Disease Control. 2003. Multistate outbreak of monkeypox — Illinois, Indiana, Kansas, Missouri, Ohio, and Wisconsin, 2003. Morbidity and Mortality Weekly Report 52:642-644.

Davis M.A. et al. 2011. Don’t judge species on their origins. Nature 474: 153–154.

Gordon D.R., D.A. Onderdonk, A.M. Fox, and R.K. Stocker. 2008. Consistent accuracy of the Australian Weed Risk Assessment system across varied geographies. Diversity and Distributions 14:234-242.

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Image: Calosoma sycophanta, imported into the United States in the early 1900s to control gypsy moths. Image credit: kqedquest/Flickr.