A full list of the damaging biological and economic effects of invasive plants and animals risks sounding like hyperbole. It includes loss of western rangelands to invasive plants, blockage of power plant intakes to invasive mussels, and major declines of birds and mammals to invasive snakes in the Everglades. So vast is this "bioinvasion" (as some have termed it) that only rough estimates can be made of the numbers of non-native species now in North America or in the rest of the world. Moreover, despite a few rare successes, eradication of these species, once they become established, is extremely improbable. But because of the number and variety of non-native species, it is unclear how best to manage or prevent these effects or what legislation would be needed to address these problems. Laws concerning wild plants and animals do not form a comprehensive body at the federal level. Under the U.S. system, inherited from English legal tradition, the government regulates the taking of native wild animals generally while landowners control the native (and other) plants growing on their lands. Thus, colonial governments regulated native wild animals (to the extent there was regulation of the take of wild animals at all) and, after the U.S. Constitution was ratified, states retained the rights they had as colonies to control the wildlife within their boundaries. With states retaining the regulation of wildlife, the majority of wild plant and animal species are not federal responsibilities under current law. Moreover, because the problem of invasive species presents itself as a series of seemingly disconnected crises, legislation has become a patchwork, enacted as each crisis was addressed. The absence of a general federal responsibility for wildlife affects the federal role in addressing invasive species. A local or state government, faced with the recent arrival of a new invasive speciesâwhether terrestrial, freshwater, or marine; plant or animal; agricultural pest or recreational nuisanceâhas no single source to query to begin its response or guide it through a maze of options. Moreover, federal help, especially any timely help in the weeks or months after initial discovery, is rare to nonexistent and, if forthcoming, focuses more on information and less on practical assistance. The National Invasive Species Council (NISC) was created by Executive Order 13112 in 1999, and has addressed some aspects of the invasive species problem. It has taken steps toward sharing more information across governments and with the public. NISC asks specific agencies to take the lead in developing policies within their existing legislative mandates. In its 2008-2012 report, NISC outlined a set of actions to address the bulk of existing problems. These actions include developing legislative proposals to fill gaps in current law. However, legislative efforts to date have tended to focus on well-established problems: the invasion of a single species, a specific pathway of introduction, or damage or risks to agriculture. Invasive species occur throughout the United States, but some ecosystems are more susceptible to invasion than others. Hawaii and Florida are perhaps the major examples of this phenomenon in the United States. Both states have many threatened and endangered species and, not coincidentally, many invasives, and both were long isolated biologically and have a number of native species found nowhere else. The mild climates of Hawaii and Florida make it easier for the rich flora and fauna from other tropical and semitropical regions to survive, and they also make the states attractive to businesses that import, maintain, or breed non-native animals and plants, such as tropical fishes and ornamental plants. Another factor putting some environments at risk is the plethora of opportunities for new introductions. Easy transportation access increases the likelihood of invasion in aquatic habitats such as the Great Lakes. Seaports, where many ships exchange ballast water, are at particular risk. Even if only a tiny fraction of newly arriving non-native species survive in San Francisco Bay or Chesapeake Bay, the actual number of successful invasive species may be very large. The areas around airports, with increased levels of international traffic and tourism, also are at risk. Between or within countries, some pathways for species invasions already are well-known. They include transportation corridors, intentionally imported non-native pets, landscaping plants, aquaculture, and non-native organisms deliberately released for propagation in the wild. To some extent, the pathways of invasion between countries can be predicted. For example, brown tree snakes have a propensity to hide in dark places, and the damage they have caused on Pacific islands such as Guam has done much to focus attention on air stowaways. The arrival of a number of beetle species has played a similar role in focusing attention on pallet wood, packing crates, live plants, and airport warehouses as pathways and centers of biotic invasion. Legally shipped live animals or plants may harbor microorganisms, parasites, or seeds that pose a danger to other species, even if the animal or plant itself does not survive in the wild. In general, any arrival of living or untreated material offers a possible pathway for biotic invasion. A comprehensive review of possible pathways, their risks, options for control, and research needs is, to the authors' knowledge, currently lacking. Federal laws have tended to focus on black lists (anything not on the list is allowed) in contrast to white lists (anything not on the list is excluded). Each requires some, or even considerable, knowledge of the species to be listed to predict the likelihood of invasion. A black list can be prepared in various ways, but usually it is made up of species already shown to cause serious damage to fisheries, endangered species, or (especially) agriculture. This evidence may be based on experience with the species domestically or in other countries. Preventing the spread of the species after it enters the United States may rely on public education, penalties for shippers, monitoring, and other means. In general, black lists require time to gather information on the damage created by the species and then proceed through a regulatory response. This approach allows more flexibility for industries that depend on the importation of new species of plants or animals. Black lists do not readily address introductions by persons who are unaware that they are bringing in non-native organisms. With the white list approach, there is an attempt to predict potential harm before a species' arrival. The prediction would be based on known characteristics of a species, such as how it reproduces, the number of seeds or offspring, etc. Any species not on the list would be excluded. The mongoose, for example, has a history of becoming a pest on islands where it has been introduced. It seems unlikely that the mongoose would ever be placed on a (white) list of allowable species. In contrast, new varieties of orchids, sheep, tulips, or any other species with a long track record in this country likely would gain admission. The existence of a list, whether white or black, implies that importers actually know they are importing living organisms. An effort to prevent unintentional introductions would be compatible with any shade of list or no list at all. Potential pathways for unintentional introductions continue to be discovered, and there are likely other pathways yet to be determined. Federal laws concerning invasive species form a patchwork, stronger in some areas, such as agriculture and ballast water, and weaker or absent in other areas. Current laws do not clearly address prevention of biological invasion across foreseeable pathways (with the exception of ship ballast water); or early detection and rapid response (EDRR) before the establishment of the new species, when the focus of effort shifts from less expensive prevention to more expensive and less efficient control. Coordination of current efforts alone means that gaps will remain if they are due to lack of coverage by existing laws or agency jurisdiction. Congress could choose to address these gaps either by explicitly delegating such authority to the President or by crafting legislation. NISC has become the focus for federal efforts to control and prevent invasive species affecting a broad range of industries or ecosystems. However, gaps in authorities or shortages of personnel that have hampered efforts to limit the entrance of and damage from invasive species continue. Four agencies have had major roles in addressing invasive species for many years: Animal and Plant Health Inspection Service (APHIS), U.S. Army Corps of Engineers, Fish and Wildlife Service (FWS), and National Marine Fisheries Service. Yet even for these four agencies, important gaps in their authorities remain. Legislation to address invasive species could take a species-by-species approach, a pathway approach, or a combination of the two. The species approach implicitly assumes knowledge about a species' risk (black list) or safety (white list). A central dilemma, however, is the difficulty in making this prediction. Moreover, it assumes knowledge that a particular species actually is being imported. A pathway approach does not assume knowledge about any particular species, only that a particular set of circumstances favors the arrival of unwanted organisms. Some agencies, including APHIS and FWS, analyze the risk presented by particular species that may become invasive. However, addressing the multitude of agricultural pests relies on scarce agency resources at APHIS. In addition, FWS has been criticized for its slow response to the blacklisting of species under the Lacey Act. Both agencies would benefit from faster assessments of either species or pathways so they could direct resources to the most critical areas. Regulation by pathway is an approach suited to unintentional or unknowing introductions, as no list needs to be created. Among the most comprehensive pathway approaches to date has been the Non-indigenous Aquatic Nuisance Prevention and Control Act. Its goals put prevention on an equal or higher footing compared with control of species that are already established. It requires the participation of several federal agencies, promotes research, and implements regulations on the mid-ocean exchange of ballast water and other measures to exclude invasives from U.S. ports. A review of writings by various specialists in this field suggests a number of areas that might be explored by policymakers.