E-RAT-icate: Cost-effectiveness of Eradication vs. Control

by Christopher Wada, UHERO, September 24, 2013

Effectively and efficiently allocating now-limited conservation dollars is an important but difficult task. Ecological non-use benefits* and the costs of eradication and control programs are often difficult to quantify in dollar terms. However, such analyses are becoming necessary for justifying costly projects in the face of tightening budgets. In fact, without appropriate justification, many would likely choose not to protect endangered species if doing so would divert too many resources from other societal goals - such as those related to education, health care, and maintaining a standard of living.

Since most of the species listed under the Endangered Species Act end up on the list due to habitat loss, predation and competition from invasive species, protecting and restoring remaining native species often requires preventing the arrival of, and controlling, invasive mammals, birds, and plants. The problem is especially salient in Hawai'i, which is home to 317 taxa of endangered plants and animals - more than 30% of the nation's endangered species.

The Polynesian rat (Rattus exulans), brought by the first voyagers to the Hawaiian Islands, was the first invasive mammal species in Hawai‘i. It spread quickly and colonized the islands faster and farther than people. Native plant and bird species which evolved in the absence of rodents were especially vulnerable to the new threat. The Norwegian rat (Rattus norvegicus) and house mouse (Mus musculus) arrived soon after aboard European and American ships in the late 1700s and 1800s. Today, rats continue to disrupt native ecosystems by consuming native plant seeds and attacking nesting seabirds and their eggs.

The UHERO Project Environment team has been working with the US Fish and Wildlife Service to survey the costs of various rodent eradication and control projects across the Pacific. An important question that often arises is whether to control the invasive population indefinitely or attempt total eradication. The former incurs smaller costs indefinitely, while the latter generally involves a large cost up front but minimal maintenance (occasional surveillance) thereafter. Based on cost data from 15 eradication and 9 control studies collected from local rodent control project managers and documentation of projects completed outside Hawai‘i, investing in eradication appears to be more cost-effective for target areas in the range of 0-100 ha. The difference tends to be larger for bigger treatment areas.

Another important question to consider when drawing up an invasive species removal program is the method of removal, in this case, delivery of pesticides or traps. Based on data from 21 ground-based and 22 aerial eradication projects, it appears that ground-based methods are generally cost-effect for relatively small islands (<550 ha). For larger areas (>550 ha), however, the average cost for aerial eradication dropped to $359/ha, making it competitive or possibly even preferred to ground-based approaches.

Although the data is suggestive, there are many underlying factors affecting the results. For example, data from large aerial projects tended to be more recent and more detailed, which might suggest that earlier projects (many of which were ground-based) underestimated the true costs. One thing is for certain: the preferred method will depend on a variety of factors including the size and type of terrain, labor and equipment costs in the area, and any associated mitigation (e.g. to avoid harming non-target species). For more on economic approaches to managing invasive species, visit UHERO’s Project Environment.

*Non-use benefit: benefit associated with a good or resource that is not derived from use or consumption, e.g. the benefit of knowing that an endangered species exists even if that species is never directly used or consumed by people.