Three rat species, the Norway rat, black rat or ship rat, Pacific or Polynesian rat, and the house mouse are among the most widespread and destructive invasive mammals affecting islands. Through mostly unintentional introductions by humans, these rodents occupy >80% of the major islands worldwide. As a consequence of their omnivorous diet and large incisor teeth, introduced rats are probably the invasive animals responsible for the greatest number of plant and animal extinctions on islands. The effects of house mice on island ecosystems are less well known when compared with rats. We have reviewed past diet studies of these 4 omnivorous rodent species. Our review suggests that due to the high variability in diet, as well as direct and indirect effects of predation, all 4 species pose potential threats to many plants and animals on Pacific islands. Although rodent diets greatly differ among sites, diets appear to roughly follow this pattern: Norway rats and house mice generally consume proportionally more animal than plant (Norway rats consume many vertebrates and house mice mostly consumes arthropods), whereas black and Pacific rats generally consume proportionally more plant than animal. Studies specifically linking rodent diets with the population status of surrounding biota are needed in order to clarify the effective impacts of these rodent species. Much could be learned from rodent removal experiments, which could further expand our knowledge of invasive rodent species effects, and native species conservation, on islands.

Invasive vertebrates have had significant effects on native species, ecosystem processes, human health and economies around the world for more than 200 years. However, in the last 30 years, the breadth and impact of invasive species has accelerated with the growth and integration of high speed transportation networks. The effects of many non-native species are often difficult to predict prior to establishment. Current efforts to manage invasive species can be broken into the areas of prevention of introduction and detection of incipient populations, documentation of impacts, and development and implementation of management tools. We address various aspects of these areas under the broad headings of 1) invasiveness and pathways, and 2) impacts, managements, and challenges. One of the biggest challenges of invasive species management is the lack of interest and resources to manage species before they are established. Hence, few tools exist to manage populations once a population is established. As such, much of the focus of current research is dedicated to identifying the impact of species and the development of management tools. Further, these new tools must not only be effective but also adaptable to the ever changing dynamics of the economic, social, and ecological aspects of the locality invaded. The symposia at this conference on various invasive vertebrates highlight both successful and unsuccessful management efforts, investigations to determine the full impact of invasive species, and the development of novel control techniques.

The U.S. Department of Defense is in the process of restructuring military assets in the Pacific Basin that includes moving more troops to Guam. As a result of this process, the potential risk of vertebrate invasive species may increase across Micronesia. We identified the pathways through which goods and materials are moved throughout the Pacific basin and then developed a list of the most likely invasive vertebrates that could be moved in these pathways. We reviewed the available literature, interviewed experts, and evaluated pathways according to a fixed set of criteria to determine the risk of the pathway to transport invasive species. Some of the potentially high-risk pathways are military and commercial aircraft and vehicles, mail, shipping containers, and aquaculture. The following are species that may spread or become established in the Pacific without the implementation of measures to reduce risk: brown tree snake, habu, Asian beauty snake, common wolf snake, anole, gecko, coqui frog, cane toad, red-vented bulbul, Indian myna, and Indian mongoose.

Many invasive rodent species have become established in the United States and its territories, both on the mainland and on islands. While most were introduced accidently, some were introduced for food or fur. These rodents have caused serious impacts to native flora and fauna, agriculture, and other resources. They have caused the extinction of many species of birds in insular ecosystems. Although many methods are used to control or eradicate introduced rodents, rodenticides and traps are the main tools. Since the early 1990s, agencies have been eradicating rodents from various islands, primarily for conservation purposes. There have been numerous eradication attempts in the United States and most have been successful. We review introduced rodent impacts and eradications, both successful and unsuccessful, which have occurred, with an emphasis on the United States. Finally, we consider some research needs and some remaining challenges in invasive rodent management and eradication in the United States, including the use of toxicants, land access, public attitudes, resource availability, and monitoring difficulties.

We conducted an enclosure experiment at two low­land sites on the eastern side of the Island of Hawaii for 6 months (September 2004 to February 2005) to determine the effect of Eleutherodactylus coqui on 1) three invertebrate communi­ties (aerial, herbivorous, and leaf litter), 2) nutrient cycling rates and leaf litter decomposition rates, and 3) native and non-native plant growth rates. E. coqui was found to reduce all invertebrate communities at one study site, while the other site did not have any difference with treatment. At both sites, herbivory rates were lower in the presence of E. coqui than in their absence. At both sites, E. coqui was associated with increased NH4+ in leaf washes, increased K, Mg, N, and P concentrations in the leaf litter, and increased decomposition of leaf litter. P. cattleianum had more new leaves in presence of the E. coqui than in the absence of E. coqui across sites. Non-native plants had higher survivorship than native plants. In summary, invertebrate community responses to E. coqui differed by site but ecological processes responded similarly across sites. Path analyses suggest that the E. coqui increases leaf litter decomposition and non-native plant growth rates by making nutrients more available to microbes and plants. These findings are of concern in Hawaii, where E. coqui can potentially affect the 44 endangered invertebrates and potentially increase non-native plant growth.

Introduced black rats are among the most invasive species to islands worldwide. In addition to agricultural impacts, rats are vectors of disease, cause damage to native flora and fauna, and negatively impact threatened/endangered species. Eradication efforts have met with mixed success. Success or failure of an eradication effort can depend on the population density of the target species, which can influence rodenticide sowing rates. We used snap trapping grids to estimate black rat densities in two different forest types on Diego Garcia: coconut forest and mixed species forest. Individual snap traps baited with fresh coconut were placed every 10 m in a 100-m × 100-m (1 ha) grid in the mixed forest and every 20 m in a 220-m × 220-m grid (4.8 ha) in the coconut forest. Traps were checked twice daily for 7 and 11 days in the mixed and coconut forest, respectively. In total, 914 rats were captured on the coconut forest grid and 125 rats were captured on the mixed forest grid. Rat density in coconut forest was 187 rats/ha (95% CI: 176-201) and 88 rats/ha (95% CI: 82-104) in mixed forest. Stomach contents were examined in 121 rats trapped in the mixed forest: 81% contained coconut along with other vegetation or meat, and 67% contained coconut exclusively. It is likely that the high rat density is driven by an abundant coconut food source resulting in a variable distribution of rats among habitat types. Planning for eradication will need to consider the variability of rodent densities across different habitats, with management strategies developed to address this variability.

For centuries, coyotes have been controlled to protect livestock and/or enhance game populations. The intensity of control has varied widely and many types of control techniques have been used. The effects of these control techniques need to be evaluated to effectively resolve conflicts, to fulfill legal requirements, and to aid the development of new strategies. However, the influence of these techniques on coyote population size and structure is largely unknown. Furthermore, management decisions are often required before experimental tests can be developed, and conducting requisite experimental programs on meaningful scales are logistically prohibitive. Therefore, we developed an individual-based computer model to evaluate the effects of various control techniques on age structure including selective removal, random removal, and denning. This model is part of a larger effort to fully evaluate the effect of current management strategies on coyote populations and to eventually link this population model to a depredation model. Selective and random removal resulted in younger age structures, whereas denning produced population age structures similar to an unexploited population.

One of the goals of this Symposium was to bring together agricultural and conservation users of rodenticides to discuss the impacts of rodenticides on the environment, examine the current regulatory climate governing their use, and identify ways that users can reduce or eliminate these impacts. Some of the presentations in today’s symposium highlighted specific impacts, and the preceding talk described the scenario of what can happen if an issue related to pesticide impacts ends up in the courts. The three agencies that were represented on this panel (USDA, USFWS, and EPA) have regulatory oversight and enforcement authority for the use of rodenticides and/or the adverse effects resulting from the use of rodenticides. In addition, USDA and FWS are the primary federal users of rodenticides for agriculture and conservation. USDA holds the registrations for a number of rodenticide products for agricultural and conservation purposes. Panelists were asked to describe how Federal Insecticide, Fungicide and Rodenticide Act (FIFRA), Migratory Bird Treaty Act (MBTA), Endangered Species Act (ESA), National Environmental Policy Act (NEPA), the Animal Damage Control Act, and the administration of USDA’s pesticide labels apply to rodenticide adverse effects. Panelists were then asked to bring up an issue within the scope of their agency that they view as problematic for conducting eradication projects. Panelists were also asked to suggest proactive measures that the rodent eradication community can undertake to improve future rodent eradication efforts. Finally, the floor was opened to audience members for questions and comments.

The National Wildlife Research Center (NWRC) and its predecessor laboratories have a long history of developing materials and methods for managing rodents and the damage they cause. The NWRC has been influential in exploring, developing, and maintaining legal uses of many traditional field rodenticides such as strychnine and zinc phosphide. Products have been developed for managing rodents in a variety of locales, and for managing a variety of species, from commensal rodents in urban areas, to pocket gophers and mountain beaver in forests, prairie dogs and ground squirrels on rangelands, and nutria and beaver in wetlands. Considerable research has also been conducted on developing methods of managing rodents in underdeveloped countries. Recent efforts by NWRC have focused on development of tools for managing invasive rodents in conservation areas such as island ecosystems and development of alternative, nonlethal control methods.

Worldwide, it has been estimated that invasive species have negative economic impacts in the billions of dollars, with impacts to island ecosystems being among the most devastating. While it is estimated that the most costly and destabilizing impacts are upon ecosystem functions, such impacts are difficult to quantify monetarily, and exact mechanisms are poorly understood. In particular, the role invasive species play in altering energy flow through ecosystems, specifically regarding the recycling of nutrients associated with carrion, is poorly elucidated for most invasive vertebrates. How invasive amphibians and reptiles, which comprise the majority of the invasive species biomass in island ecosystems, may be affecting energy flow within the scavenging pathway is virtually unknown. By setting out camera traps associated with carcasses of 3 taxa (coqui frogs, geckos, cane toads), this study has identified the dominant scavenging vertebrates on the Big Island of Hawai’i, as well as the fate of sequestered energy that is available to be scavenged upon the death of invasive amphibians and reptiles. These data contribute to our understanding of the functional mechanisms through which invasive species alter energy flow and stability of insular ecosystems.