American Scientist

Aquatic invasive species present unique ecological threats. Pervasive habitat modification and the ease of global transportation have facilitated their spread, and they are now a leading threat to native species and to the functioning of entire ecosystems. Awareness of the consequences invasive species can have on freshwater habitats predates Darwin, but tools to fight them have been slow to develop. In fact, resource managers and conservation biologists still rely on methods outlined more than 75 years ago.

Around the time those methods were developed, research to treat another significant problem began in earnest: By the end of the 19th century, medical researchers were working to understand and treat cancer in humans. Since then, the medical community has created multiple tools that reduce cancer incidence and increase survival rates among people diagnosed with it. If we compare these advances to the development of tools to fight aquatic invasive species, 21st-century ecology has not progressed beyond the simple surgeries of the 19th century. The difference makes sense—the threat of cancer looms much larger, and its effects are more immediately apparent, than disruptions to complex interactions among species and ecosystems. Awareness of the useful functions of ecosystems has lagged behind awareness of human health problems—the term “ecosystem services,” which describes the sum of benefits an ecosystem provides to human and environmental health, was not coined until the late 1960s. But disruptions to those services have significant consequences: For 2006, for instance, the U.S. National Invasive Species Council (NISC) budgeted $1.26 billion for efforts to control invasive species. The time has come for scientists, managers and the many stakeholders affected by aquatic invasive species, including irrigation districts, public utilities, public health workers, and those who enjoy fishing, boating and spending time in natural habitats to develop better tools for controlling these organisms.

Although cancer research and invasive species biology may seem far removed from one another, we argue that invasion biologists can learn much from advances in cancer prevention and treatment. The diversity and complexity of cancer and individual cancer patients mirror the diversity and complexity of aquatic invasive species and of each invaded habitat. Just as invasive species have the potential to cause severe harm to both individual organisms and ecosystems, cancer harms the health of individuals and societies. To facilitate the development of tools that address cancer, the medical community uses a coordinated approach that consists of multiple steps: prevention, early detection, diagnosis, treatment options and rehabilitation. We propose using this framework to develop tools to prevent and treat aquatic invasive species.

Various researchers have suggested steps to take in treating aquatic invasive species, and in 2008 the NISC proposed a five-step plan that includes prevention and early detection. However, few groups have expanded the scope of their field research beyond documenting invasions. As a result, biologists know much about these organisms’ distribution and potential consequences, but the people tasked with treating the problem, such as state and federal resource managers, have few proven tools to prevent or abate invasions. A coordinated, research-based approach similar to the medical community’s response to cancer is needed to address these deficiencies.

In its recommendations to the U.S. government for 2012, the National Cancer Institute wrote that significant investment in screening and prevention should be prioritized in order to provide the best possible cancer care. The committee also noted that it is essential to fund research that improves diagnosis and treatment options. The quest to reduce cancer incidence is viewed with more urgency and is better funded, but the ecological, economic and social consequences of freshwater invasions are severe enough to warrant a similar call to action. The case studies offered here are from the Western United States, but many species in this region, including New Zealand mudsnails (Potamopyrgus antipodarum) and Eurasian watermilfoil (Myriophyllum spicatum), are global threats, and the American bullfrog (Lithobates catesbeianus) poses a challenge whose scale and severity offers useful lessons for dealing with similar species.

The Cancer Treatment Model

A basic understanding of the cancer treatment approach allows us to draw important parallels with the problem of invasive species. This approach comprises a series of five steps, which are repeated based on the patient’s needs (see the figure below). The first step is prevention. The costs of cancer are tremendous—Americans spent $228.1 billion treating cancer in 2008, yet we lost 1,500 people to the disease each day. Thus the medical community has invested heavily in preventive care, identifying factors that increase individuals’ cancer risk, such as age, medical history, family history and lifestyle, and working with patients to change what they can to reduce their risk. Outreach programs help people improve their awareness of these risks and make lifestyle changes (such as avoiding tobacco) to reduce them.

Outreach efforts can also increase participation rates in early detection programs. Early detection is a critical step: The earlier cancer is identified, the greater the patient’s chance of survival. Cancers that are found early are smaller and more limited to specific areas, whereas symptomatic cancers are more likely to have spread and are harder to treat. Researchers have spent great effort in developing accurate screening tools for people who have no symptoms. The best screening tools have high accuracy because they are disease-specific and redundant (multiple tests are used for each type of cancer). The decision to use these tools, which are often both invasive and expensive, is based on evaluation of an individual’s risk: People who are identified as high risk in the prevention stage are screened more frequently than people whose risk is deemed to be low.

Once cancer is detected, an accurate diagnosis must be made. Tumors range from benign to aggressive and life-threatening, and the need for treatment varies greatly between patients. Individuals with nonaggressive, localized cancer can live without therapy for many years, but they must be monitored to ensure that the cancer has not progressed. In contrast, aggressive cancers that have spread to lymph nodes require prompt treatment. Identifying the condition and stage of cancer is essential to prescribing treatment, because treatment methods themselves can have harmful side effects, and they are often expensive and emotionally difficult to undergo.

Development and selection of the most effective cancer treatment options requires a comprehensive understanding of each treatment’s intended and unintended impacts. Therefore, treatments undergo rigorous tests and clinical trials before they are made available for widespread use. Tests include assessing the medication’s effectiveness for its intended purpose, determining the most effective dosage and identifying contraindications or side effects that may limit the use of the treatment. The extensive array of treatments available allows care providers to choose a course of treatment that matches the characteristics of each person and type of cancer. Integrated approaches, which combine traditional and alternative treatment methods, can be more effective than a single treatment; each method seldom encompasses the cancer’s full complexity. For example, an integrated approach of chemotherapy followed by surgery and adjuvant therapy can be more effective at fighting breast cancer than surgery alone.

In rehabilitation, doctors and cancer survivors create a follow-up care plan that is organized around frequent checkups, during which doctors assess the effectiveness of treatment, monitor side effects, check for recurrence or spread and screen for new cancers. If the doctor finds that the cancer is still present or has recurred, the patient returns to the diagnosis step. A good care plan also helps survivors reduce future cancer risk by making lifestyle changes, as outlined in the prevention step. Each facet of this process is linked to the others, and the decisions made at one step allow better implementation of the next.

Aquatic Invaders

The medical community’s response to cancer is based on the idea that multiple tools are needed for each type, because the same cancer is expressed differently in individuals, and individuals vary in how easily their cancer can be detected and in their response to treatment methods. The interaction of invasive species with physical habitat and biotic community is similar—the impacts and the effectiveness of detection and treatment methods are context dependent. In our work for the United States Geological Survey (USGS), we encounter many examples of the harmful consequences brought on by aquatic invasive species. Like the medical community, our principal focus is on prevention and early detection in high-risk areas; but implementing all five steps of the cancer-treatment model is vital to the success of biodiversity conservation programs.

Prevention: The consequences of the introduction of invasive species are often irreversible and unpredictable, permanently compromising native ecosystems. Prevention strategies are the most effective and economical way of reducing invasive species’ spread. The invasion biology analog to cancer prevention is to identify factors that make an ecosystem vulnerable to invasion and implement strategies that minimize those factors. One risk factor is the probability that an invasive species will be introduced to an uninvaded site. For many aquatic invasive species, dispersal pathways are numerous and difficult to manage. The cancer treatment approach suggests that managers should focus efforts on the primary factor that can be reduced: human transport of aquatic invasives. Humans are responsible, intentionally or otherwise, for the introduction of many invasive species. For example, New Zealand mudsnails can attach to angling gear in one location and disperse into another when the boat or gear is moved to a new spot. Simple procedural changes, such as inspecting and cleaning boats and gear, can be implemented and enforced to minimize this problem.

Data from the Idaho Invasive Species Program to prevent the introduction of dreissenid mussels (Dreissena spp.) demonstrate that such changes can be effective. Dreissenids, which include the notorious zebra (Dreissena polymorpha) and quagga (D. rostriformis bugensis) mussels, have had severe ecological and economic impacts because they colonize all types of living and nonliving surfaces, including boats, docks and water-intake pipes, as well as native clams. In 2010, Idaho established 20 mandatory summer check stations on interstates and at major entry points into the state. Inspectors examined more than 43,000 boats from 49 states and found 11 boats contaminated with dreissenid mussels. These boats were disinfected prior to launching. Inspections also revealed that more than 100 boats were carrying aquatic plants, including invasive Eurasian watermilfoil. The boat inspections undoubtedly prevented transport of other invasive species as well. In addition to creating inspection stations, the state also engaged in outreach efforts through radio addresses and billboard campaigns; mandated that all watercraft display the Idaho Invasive Species sticker; and surveyed boaters about their knowledge of invasive species. These efforts help prevent the introduction of aquatic invasives by raising awareness about the problem and engaging the public in the prevention strategy.

Early detection : Prevention will not stop all invasions, but early detection of populations, when they are still small and localized, increases the probability we will be able to suppress them. For example, nonnative brook trout (Salvelinus fontinalis) were first detected in Yellowstone National Park in May 1985. A survey of 40 additional tributaries found that the brook trout were restricted to one stream. In August 1985, park managers used piscicides to eradicate brook trout in this stream. Unfortunately, successful early detection is rare. The tools currently available have low rates of detection, and the effort required to use them is not feasible in areas as vast and remote as the American West. Montana, for instance, has more than 6 million acres of roadless land. As a result, aquatic invasive species often go undetected for years, making it necessary to launch costly suppression efforts when they are finally found. In Yellowstone Lake, lake trout (S. namaycush) were detected a decade after their introduction. Since that time, more than $3 million has been spent on gillnetting efforts to remove the trout; even so, they remain abundant. Applying the cancer treatment approach to this problem suggests that greater funding should go to two research areas: creating more effective detection tools, and making spatial models that weight monitoring frequency according to invasion risk.

One class of detection tools that shows promise is molecular techniques. Aquatic organisms naturally release DNA into the environment. The extraction of this environmental DNA (eDNA) from water samples has been used to identify American bullfrogs, Asian carp and stream amphibians. In one study, it took a single technician just four minutes to detect an amphibian through eDNA sampling; it took two technicians more than 81 minutes to find this same species using traditional stream sampling techniques. Rapid field collection protocols, relatively simple equipment and low cost (less than $10 per sample) make eDNA sampling widely applicable to broad-scale monitoring efforts.

Weighting monitoring efforts according to risk assessment requires an understanding of each species’ biology, life history and transport vectors. For example, the risk of dreissenid mussel establishment after introduction is influenced by water chemistry. Based on this insight, managers have prioritized a list of dreissenid mussel monitoring sites in the Columbia River Basin so that areas with favorable water chemistry are sampled more frequently. With any monitoring plan, however, we must continually update and improve our understanding of invasive species’ colonization potential. For example, lake trout were considered only a threat to the lentic, or lake, systems where they were introduced. The trout have since moved into adjacent stream networks and are now a large threat to bull trout (S. confluentus) in and around Glacier National Park.

Diagnosis : The invasive species analog of the cancer treatment approach to diagnosis is twofold: Identify species’ impacts on native biodiversity, and monitor for any spread and new effects of the species. A primary focus of invasion biology has been assessing the consequences of invaders. These consequences manifest at multiple levels, affecting individual organisms, population genetics, community composition, ecosystem processes and economics. For example, in the Flathead River system in Montana and British Columbia, a leading threat to the success of native westslope cutthroat trout (Oncorhynchus clarkii lewisi) is hybridization with nonnative rainbow trout (O. mykiss). These irreversible changes have altered the spawning behavior of the migratory westslope cutthroat trout and led to reduced fitness in native populations. The severity of the invasion is likely to increase if hybrid populations that contain large ratios of rainbow trout are not reduced. If westslope cutthroat trout are deemed the highest priority for conservation, aggressive suppression treatments, such as piscicides and movement barriers, may be warranted—even though they may have detrimental consequences for other native taxa.

However, just as many tumors are benign, many aquatic invasives have negligible effects on native biodiversity. Suppression efforts in such systems may do more harm than good. For example, New Zealand mudsnails invaded Western North America in the 1980s and have reached population densities of greater than 100,000 individuals per square meter in many locations. Yet empirical evidence suggests that the mudsnails have had few detectable effects on native biodiversity, even though they can dominate nitrogen and carbon processing in streams where they invade. Aggressive and resource-intensive tactics to remove them, such as stream dewatering, may not be appropriate: The impacts of those tactics on species with high conservation priority are likely to be greater than the impacts of the mudsnails themselves on those populations. Managers need a decision-making support system to identify and prioritize which species to control. They also need resources for continued monitoring. Invasion effects may not be homogeneous across ecosystems, and current conditions do not predict future impacts.

Treatment options : Compared with the broad spectrum of cancer treatments available, few options exist for fighting aquatic invasions. Effective suppression requires development of taxon-specific treatments, rigorous testing to identify habitats where those treatments will be effective and an understanding of how new and traditional options can be integrated. Although such testing is common in the medical community, it is rare in invasion biology. There have been thousands of applications of piscicides to control invasive fishes, for example, but few studies have followed an application’s long-term success or examined its side effects. Uncertainty about whether piscicides’ deleterious effects are worth their benefits has caused some suppression projects to be litigated against and halted. In one instance, the Center for Biodiversity sued the U.S. Forest Service to stop a piscicide treatment, expressing concern that there is limited information available about how the treatment affects invertebrates. We need a broader spectrum of treatment options, but new technologies must undergo comprehensive testing to reveal the full range of effects onnontarget taxa and ecosystem services, to minimize side effects and thereby ease public resistance to their use.

Thorough testing and a deep understanding of the invader’s natural history will assist in identifying the limitations of treatment options. With electrofishing, the practice of sending electric current through the water to stun fish, efficiency is reduced in complex habitats. It is biased toward larger fish, and can thus promote higher survival and reproduction rates in smaller fish of the invasive species that survive the treatment. Because of such limitations, suppression tools are seldom effective when used alone.

Research on Eurasian watermilfoil in Lake Pend Oreille, Idaho, suggests that using well-tested suppression strategies in concert may yield success. This aquatic plant blocks sunlight needed for native plant growth and inhibits recreational uses of waterways. Testing found that the herbicide fluridone was effective against watermilfoil at depths of less than 4.5 meters, whereas triclopyr was more effective at depths of greater than 4.5 meters. Trials also showed that the herbicides suppressed native species as well. In both cases, the treatment reduced the amount of watermilfoil present but did not eradicate the species from treated areas. Preliminary studies have tested the combined use of herbicides and native fungal pathogens and shown that the two treatments together are far more effective at controlling the watermilfoil than either treatment alone. Other options that can complement herbicides include diver-operated dredging and hand pulling, which can further reduce localized or isolated infestations with little harm to native aquatic plants. Finally, winter water-level drawdowns may be effective, because the watermilfoil lacks overwintering structures to protect it in subfreezing temperatures. No single approach has proven successful in treating the range of habitats where watermilfoil is found, but integrated treatment strategies to suppress this and other aquatic invasive species are still uncommon. Such efforts should be given more attention.

Rehabilitation : The effects of efforts to suppress aquatic invasives can be short-lived. Even relatively mild treatments can have side effects that increase future susceptibility to infestation. For example, removal of invasive shrubs, such as Tamarix species, which have become established along the Colorado River among other places, can result in regrowth or reestablishment, reduce critical habitat for birds and facilitate new invaders. The medical community responds to comparable treatment trade-offs by returning to the steps of prevention and early detection—providing cancer survivors with frequent checkups and strategies to reduce future cancer risk.

Rehabilitation tools can be grouped into two categories: those that increase resilience to the impacts of invasions and those that increase resistance to reinvasion. Resilience tools restore habitat and natural processes (a river’s natural flow regime, for instance) that favor native species over invasives, and they manipulate food web interactions, which can limit invaders to low levels. These tools are used in degraded habitats where eradication is not possible and reinvasion is inevitable. Resistance tools are used in less disturbed habitats where invasives are the primary stress on native species. These efforts focus on active replacement of native species to saturate niche space, and on constructing barriers to reinvasion.

Strawberry Reservoir in Utah, a popular fishing and boating destination created in 1922, was treated with piscicides in 1961 and 1990 to remove nonnative fish and improve the cutthroat trout fishery. The 1961 removal resulted in only a brief success. Nonnatives were reintroduced, and human land use had degraded the cutthroat spawning habitat in nearby tributaries. In 1990, the application of rotenone was paired with tributary habitat improvements to foster natural reproduction of the trout. Large cutthroat trout were subsequently introduced to serve as biological controls on nonnative fish that had survived the rotenone or were later reintroduced. As of 2009, cutthroat numbers were high and nonnative fish populations were present but low. The striking difference between the 1961 and 1990 treatments is that in the latter, resilience tools were used.

In the Labarge Creek watershed near Labarge, Wyoming, the Wyoming Game and Fish Department (WGF) used the technique of isolation management in combination with the broad-spectrum piscicide rotenone to suppress invasive brook trout that threatened the persistence of native cutthroat trout (O. c. pleuriticus). In the late 1980s, WGF installed movement barriers on the creek’s headwater tributaries, removed nonnatives upstream of these barriers and restocked the areas with cutthroat. However, follow-up monitoring revealed that the cutthroat population did not persist upstream of the barriers because the isolated reaches lacked critical habitat. Managers moved the barriers downstream so that enough critical habitat existed within them to enable long-term persistence of the native cutthroat trout.

These cases share two elements that allowed for successful rehabilitation: hypothesis-driven monitoring, and knowledge about the basic biology of native and invasive species. In Strawberry Reservoir, managers tracked cutthroat recruitment and predator-prey dynamics to assess restoration efforts and to determine the number of large trout required to suppress nonnative fish. In Labarge Creek, managers used before-after comparison studies to assess the effectiveness of brook trout removal and movement barriers on cutthroat demographics. Knowledge of the factors limiting cutthroat allowed managers to tailor tributary habitat restoration to favor spawning and recruitment in Strawberry Reservoir. After seeing the results of the initial barriers, knowledge about the critical habitat needs of cutthroat in Labarge Creek helped managers locate a single barrier that will remain in place, minimizing the trade-off between isolation and connectivity. Rehabilitation plans require detailed knowledge about the system they are designed to treat, just as a doctor must have knowledge of a patient’s past medical history and current lifestyle to design an effective cancer rehabilitation plan.

What’s Next

How do we implement our proposed approach to aid efforts on the ground? As the fight against cancer demonstrates, moving toward a coordinated research approach requires more than just a conceptual framework. Federal policy, ample funding and time for implementation have also been critical to the success of the program. In 1971, President Nixon signed the National Cancer Act, which guaranteed consistently high levels of federal funding for the effort. This money has been allocated to multiple research areas, but the funding of more than 60 National Cancer Institute Cancer Centers has been especially critical in moving research into action. Each center brings together interdisciplinary scientists, doctors and patients in order to unite novel research with clinical practice. As a result, cancer centers are a major source of new discoveries and an effective conduit for delivering these advances to patients, health-care professionals and the public. Finally, it took time for this coordinated research to become effective at the level of treating individuals. The National Cancer Act did not result in instant progress; cancer incidence and cancer-related deaths finally began to decline more than 20 years later, in the 1990s.

Components of the model we propose already exist, but they have not been adequate to address the prevention and management of the many aquatic invasive species in the United States. For example, many state and federal policies prohibit the importation of invasive species, but these policies lack funding for enforcement, early detection and rapid response. When policy and adequate funding are matched with a coordinated research effort and allowed adequate implementation, there is evidence that progress can be made. The Great Lakes Fishery Commission, a joint Canadian–U.S. body, has used an integrated pest management approach to control invasive sea lamprey (Petromyzon marinus) populations. The commission supports research that is designed to inform management, and then coordinates this research among managers, agencies and universities. Federal programs, such as Sea Grant, provide high levels of consistent funding for the development of multiple treatment options, including lampricides, traps, sterilized males and pheromones.

It’s clear that such advances and breakthroughs require high funding levels. However, the financial support behind the sea lamprey program is an anomaly, as most efforts to control invasive species are grossly underfunded—a problem that the current economic crisis will only exacerbate. Gaining broad support for greater funding is a top priority; this is not an area in which it’s possible to do more with less. Still, the strategic allocation of funds will enable forward momentum in tool development, testing and implementation to combat invasive species. One way to accomplish these goals with limited funding is to develop centers similar to the National Cancer Institute Cancer Centers. These centers could minimize redundancy and foster the exchange of ideas and equipment among researchers, managers and other stakeholders.

Few robust, empirically tested tools exist for preventing and fighting aquatic invasive species, and researchers have been slow to develop and test new tools. But managers need a full toolbox so they can select the appropriate tool given the specific invader, habitat and potential side effects to native species. An approach based on the medical community’s response to cancer could expedite tool development and implementation and, ultimately, speed the achievement of biodiversity conservation goals.

Sidebar: Case Study—The American Bullfrog

The American bullfrog (Lithobates catesbeianus) was detected at multiple locations in the Yellowstone River corridor near Billings, Montana, in 2009. In May 2011, an interagency team including two of the authors (Sepulveda and Ray) began studies to document the bullfrogs’ impacts, work to eradicate them from the region and coordinate efforts to prevent further spread. Bullfrogs are global invaders that have been linked to many native amphibian declines. Once established, bullfrogs are extremely difficult to eradicate because they are highly fecund (females can produce up to 40,000 eggs per clutch) and can move to new locations with ease. In addition, they have high density dependence: Failure to remove all individuals from an environment results in higher rates of survival and reproduction for those who escape capture. Here we apply the cancer-treatment model to create a series of steps that could be taken to deal with this problem. In some cases, the actions have already been initiated; others are possibilities for the future.

To craft a prevention strategy, we must consider factors that increase the risk of bullfrog invasion. These include aquaculture (raising bullfrogs for food); the use of bullfrog tadpoles as fishing bait and live bullfrogs for pets; landscape ponds; and the use of these animals in research and teaching. Once they have been introduced into an area, bullfrogs are capable of dispersing across long distances and colonizing new sites. Outreach campaigns that discuss concerns about this invader, along with state laws that prohibit the use and transport of bullfrogs outside of their native range, can be used to increase public awareness and thus slow the species’ spread. To this end, the interagency team has placed educational placards at many recreational fishing access points along the Yellowstone River, and the state has put bullfrogs on a list of prohibited species. Another step the team has discussed, which would help prevent further spread, is a campaign to encourage landowners to report bullfrogs found on private lands and to implement simple strategies to control the frogs, such as dewatering ornamental ponds used as overwintering habitats.

Designing a stratified monitoring approach to detect bullfrogs at low densities is challenging in some locales, but it is possible in high-latitude habitats like Montana, where bullfrog larvae are limited to permanent bodies of water with minimal flow and vegetation-choked shallows that warm to greater than 20 degrees Celsius in the summer. An obvious early detection strategy would be to search for the frogs in such places. However, many sites fall outside of these niche restrictions, so research is needed to identify additional correlates of bullfrog presence, such as water depth and proximity to human population centers. Once hotspots are identified, calling surveys can be used to confirm the frogs’ presence—adults give distinctive and frequent vocalizations during the breeding period. Methods such as environmental DNA analysis can be used to test for bullfrogs outside of the breeding season. Incorporating bullfrog monitoring into other programs, such as exotic plant management efforts, breeding bird surveys and water quality monitoring, will also aid in early detection efforts.

Multiple studies show that bullfrogs harm native biota through competition, predation, habitat displacement and the spread of disease. In the Yellowstone River, bullfrogs’ impacts have not yet been thoroughly studied but may be substantial: The frogs share habitat with the Northern leopard frog (Lithobates pipiens). To diagnose the severity of the invasion, it is crucial to establish baseline density estimates for native amphibians in invaded and uninvaded sites, and to determine bullfrogs’ potential to consume native taxa across a gradient of bullfrog densities. To map the spread of the invasion, surveys are needed that describe the movement of bullfrogs away from known invaded sites in the Yellowstone River corridor and to identify areas that overlap with leopard frogs and other aquatic species of conservation concern.

Research shows that management actions targeting postmetamorphic individuals are more effective than efforts that only target larvae and breeding adults: The latter two life stages have strong density dependence. In addition, postmetamorphic bullfrogs learn quickly and are able to avoid multiple methods of capture. Thus, a suite of techniques to treat the problem, including seining, electrofishing, live traps, hand netting and shooting, is needed. Removing the water from habitats is also effective, but this technique is seldom practical in the field. However, managers in regions like Montana may be able to use the long, cold winters to their advantage because bullfrogs cannot tolerate freezing or hypoxia, and all lifestages are therefore restricted in winter to deep waters that are unlikely to freeze solid. Techniques that make these overwintering habitats unsuitable, such as manipulating carbon dioxide or pH levels, may prove effective.

The survival of exotic bullfrogs can be enhanced by the presence of other exotic species. Studies have shown that exotic fish reduce the size and abundance of macroinvertebrates that can be major predators of bullfrog larvae. Thus, to rehabilitate ecosystems after treatment, managers could pursue strategies that limit or remove other aquatic invasive species. This would make uninvaded habitats less suitable for bullfrog colonization and might serve to maintain bullfrogs at lower densities and minimize their impact in invaded habitats.

Proceeding through a carefully planned sequence of steps such as this one can make evaluation of progress easier and yield better results. When the steps have been taken, researchers can evaluate their effectiveness and determine whether it’s appropriate to return to the diagnosis stage—if bullfrogs are still a significant presence—or to the prevention stage. We are optimistic that such a coordinated approach, which engages wildlife managers, policy makers and the public, will yield good results. What’s more, we will be able to evaluate the success of each strategy in light of its place within the overall plan, which should allow us to make better decisions about future action.

Acknowledgment

The authors thank the biologists and staff at the Northern Rocky Mountain Science Center for their comments and discussion on the topics presented in this article. Any use of trade, product or firm names is for descriptive purposes only and does not imply endorsement by the U.S. government.