ISLAND RODENT MANAGEMENT

Transcription

1 ISLAND RODENT MANAGEMENT Options for dealing with invasive roof rats on Bald Head Island, NC Roof rats (Rattus rattus) are an invasive species that have established a population on Bald Head Island, NC. These rats have caused thousands of dollars of damage to homes and businesses. Residents fear that the non-native species will continue to cause damage to buildings and homes. Further, they have concerns about human health, deleterious impacts to tourism, and impacts to wildlife. This paper provides an overview of the current practice being used to eradicate and presents other solutions to control or manage the rat population. CURRENT STATUS As of November 2015, bait stations containing poisoned bait are located around private residences on Bald Head Island. The poison currently being used on the island is Bromadiolone, an anticoagulant that blocks Vitamin K recycling and prevents blood from clotting, thereby causing death by massive and extensive hemorrhaging (Valchev, Binev, Yordanova, & Nikolov, 2008). Anticoagulant rat poisons are generally classified as either first-generation or secondgeneration. First-generation anticoagulants were developed in the 1950s, with secondgeneration anticoagulants following in the 1970s and 1980s (Daniels, 2013). Firstgeneration poisons are generally less toxic than their second-generation counterparts, and require several doses to cause death. Second-generation anticoagulants, such bromadiolone, typically require only one dose to be lethal and are more persistent in the liver and the environment (Gabriel, et al., 2012).

2 CURRENT STATUS The bait stations on Bald Head Island are placed at the homeowner s discretion by various pest control companies. Other than the extermination industry s recommendation to use second-generation rat poison on every property on BHI, there has been no coordinated effort on the island to eradicate roof rats. It is common belief that these bait stations will poison enough rats to eradicate the population within 5 years; however, the truth of this statement is unknown. Placement of bait on BHI is too widespread to make a sizeable impact on the population of roof rats. Bait stations must be placed within the home range of the rats in order to guarantee that individuals will intercept the bait. The home ranges of rats can vary based on a number of factors, including terrain and food availability. One study by Whisson et al. suggests that the home ranges can be as large as approximately 0.5 acres to 4.5 acres in a forest habitat (Whisson, et al., 2007*). However, ranges can be much smaller in an enclosed area with ample availability to food (Leung & Clark, 2005). Rats have been known to eat plant matter, insects, arthropods, bird eggs, and even sea turtle hatchlings (Caut, et al., 2008), all of which are generally available on Bald Head Island. Easily accessible trash is also a food opportunity for rats. When in a situation where garbage was present, bait stations were not effective in eradicating the rat population due to the presence of a readily available trash food source (Coddou, et al., 2014). The use of poison on Bald Head Island could impact the island s wildlife, specifically songbirds, foxes, and raptors. Second-generation anticoagulants have been shown to affect and subsequently contribute to the death of many species, including raptors and small and large mammals (Daniels, 2013; Gabriel, et al., 2012; Murray, 2011; Hosea, 2000; Brakes & Smith, 2005; Berny, 1997; Stone, Okoniewski, & Stedelin, 2003). Before death, second-generation poisons alter the behavior of rats, causing them to spend more time in open areas, thus making them more susceptible to predators (Cox, 1991). If eaten by predators such as fox or raptors, rodenticides can bioaccumulate and negatively affect the health of predators (Daniels, 2013). Non-target small mammals, such as BHI s native Hispid cotton rat, could also be affected by poisons by directly ingesting them. Pets such as cats and dogs have also been shown to ingest poisons (Valchev, et al., 2008). Table 1 shows a compilation of studies that found adverse effects in species caused by anticoagulant baits.

3 TABLE 1

4 CURRENT STATUS While there have not yet been any empirical studies on the effect of poison use on wildlife on Bald Head Island, the Bald Head Island Conservancy has been monitoring the populations of foxes, coyotes, and raptors on Bald Head Island. A census of known fox den sites in Spring 2015 yielded no indication of any fox reproduction this year. Anecdotally, this year was the first in over a decade when red-tail hawk nesting was not observed. BHI Conservancy will continue to follow predator populations on BHI in collaboration with the Village of BHI. Along with concerns over damage to buildings, homeowners are also concerned that rats may carry transferable diseases. While roof rats can directly cause leptospirosis, ratbite fever, and salmonellosis, these diseases are easily preventable. Leptospirosis is caused by contact with infected animal urine, and is a bacterial infection (CDC, 2011). Very rare in the United States, it can prevented by washing hands regularly and avoiding contact with stagnant water (MedlinePlus, 2014). Salmonellosis is also a bacterial infection that can be caused by coming into contact with rat feces, among many other things. Rat-bite fever is caused by a bite or scratch from an infected rat, or by handling a dead rat (CDC, 2011). It can be prevented by taking precautions when handling dead rats, such as wearing thick gloves and washing hands (CDC, 2011*). Washing food cans before opening them in also important in case rats have been in your cabinets. Washing hands regularly and cleaning up signs of rodent droppings is important and is an easy way to prevent these diseases. OPTIONS MOVING FORWARD OPTION 1 THE STATUS QUO Despite two years of baiting, the rat population does not seem to be in decline (preliminary observations using camera and live traps, BHI Conservancy, 2015). The current position of exterminators is to continue the use of poison outside of homes, although this has proven to be ineffective as of yet and has required significant financial investments from homeowners. In addition to the expense of continued use of bait outside homes, leaving poison outside also has the potential to negatively affect wildlife. The opportunity to observe wildlife on BHI is an important amenity for residents and visitors alike. It is important to note that to date the BHI Conservancy has not tested non-target animals, such as foxes and birds, for the presence of low levels of rat poison. Moving forward, the BHI Conservancy will explore options to test animals for the presence of poison and the levels of those poisons. Table 2 outlines the advantages, costs, and unknowns associated with the Status Quo Option, as well as the other options presented by the BHIC.

5 OPTIONS MOVING FORWARD OPTION 2 KEEP THEM OUT The second option for Bald Head Island is to eliminate the use of poison outside of homes entirely. Emphasis would be placed on sealing homes and trashcans to keep rats outside of homes and businesses. As with Option 1, there is little probability that rats will be completely eradicated from the island with this option. Poison outside of homes presents a risk to native wildlife on Bald Head Island, as explained above. Once rats are inside a building, they are more likely to stay there. With garbage and human food available as a food source, protection from weather and predators, and an area for nesting, rats have little incentive to leave a home once they have invaded (Leung & Clark, 2005). Putting rodenticide outside of homes has not been effective in reducing the rat population, and poses an unnecessary risk to wildlife. Buildings will be sealed and inspected to prevent rats from entering homes. This includes making sure doorways and windows are tightly closed, crawl spaces are sealed, and any holes in roofs or walls are blocked. Trees should be trimmed away from homes at least 12 inches. Most importantly, trash and food items should be kept out of reach of rats. Garbage cans both inside and outside should have lids and be sealed, crumbs should be swept, and food should not be left on counters. If homes are vacant, regular inspections should be conducted to look for potential entry points and any evidence of damage from rodents. For rental homes, stickers such as the one below will be provided by the BHI Conservancy to encourage guests to seal trash. While the benefit of this option is keeping poison away from wildlife, there are certain downfalls to consider. As with the status quo option, this option is unlikely to eliminate roof rats from Bald Head Island. With rats entrenched on the island, wildlife could suffer.

6 OPTIONS MOVING FORWARD OPTION 2 KEEP THEM OUT Invasive rat populations have been known to negatively impact native species of wildlife, and in some cases have even caused extinctions. Rats have had a hand in the extinctions (local or global) of numerous species of terrestrial birds and bats, and have affected over 170 taxa of plants and animals worldwide (Towns, et al., 2006). Rats have also been documented feeding on the eggs of shore-laying seabirds and sea turtle hatchlings (Caut, et al., 2008). If the rats on Bald Head Island cause a decline in local bird and sea turtle populations, tourism in the area could be impacted. With rat predation impacts on turtles, songbirds, and shorebirds at risk, it is important to note that the BHI Conservancy already has projects and funding to measure impacts to those species. Our internationally recognized Sea Turtle Protection Program will continue and has ample pre-invasion information to quantify future rat predation on sea turtle nests. Our collaboration with the Painted Bunting Observational Team, who annually measures painted bunting nesting and population abundance on BHI, will provide invaluable information. The painted bunting can serve as an indicator species, one that represents the environmental threat of rats to all songbirds. Third, with the construction of the terminal groin along west beach, the Village of BHI has a permit requirement to continue shorebird monitoring until In short, the BHI Conservancy is well positioned to assess the impact of rat predation on these sensitive wildlife. Table 2 presents an overview of the costs and benefits of this approach. OPTION 3 COMPLETE ERADICATION (MAYBE ) The final option for Bald Head Island is to broadcast poison at high levels across the entirety of the island. This approach will likely kill all wildlife vulnerable to bait and the predators of that wildlife. On BHI, this would likely include: roof rats, raccoons, cotton rats, squirrels, opossums, river otters, mice, bobcat, coyote, foxes, raptors, snakes, and others. This method has been used successfully in over 332 locations, 284 of which were islands (Howald, et al., 2007). As of 2005, less than 10 of these eradications took place in North America (Howald, et al., 2005). Brodifacoum was used in the majority of the eradications. This poison acts similarly to other second-generation anticoagulants; it has a high persistence and potency (Eason & Spurr, 1995). Poison is generally broadcast by planes or by hand, and must cover the vast majority of the island in order to intercept every rat and ensure a successful eradication (Howald, et al., 2005). In most cases where this approach has been taken, non-target wildlife, such as fox and birds, has either been relocated or was eliminated in the approach. When the approach was taken on islands, native wildlife species were held captive and purposefully reintroduced back to the island after the eradication was complete (Howald, et al., 2007; Howald, et al., 2005).

7 OPTIONS MOVING FORWARD OPTION 3 COMPLETE ERADICATION (MAYBE ) Modifications to the bait can be made to make the bait less appealing to non-target species such as birds; however, with poison being broadcast at such high levels, it is very likely that there will be significant declines in native populations. Secondary poisoning of non-target species is almost guaranteed, as brodifacoum is toxic to all vertebrates to some degree (Donlan, Gregg, Tershy, & Donald, 2003). In many cases, non-target native species have been observed to recover quickly after eradications are complete if proper precautions are taken beforehand. This is due to the limited time that the poison is available and present in the system (Howald, et al., 2007). A literature review of rodent eradications by Howald and others (2007) gives several recommendations for islands similar to Bald Head undergoing rodent eradications. Most eradications have taken place on islands smaller than 100 hectares; however, BHI is approximately 500 hectares (equivalent to 1,235 acres). Worldwide there have been less than 20 rodent eradications documented in areas larger than 500 hectares (Howald, et al., 2007). Firstly, Howald et al. (2007) recommends that land managers seek consultation with experienced practitioners before beginning any eradication. Also, there should be additional planning focused on the type, timing, and delivery method of the rodenticide and on mitigating the impacts to non-target species. Additionally, they recommend documenting the negative effects of the eradication process and the benefits of the island being rodent-free in a monitoring program (Howald, et al., 2007). Certain special considerations would have to be taken into account for Bald Head Island. As Bald Head Island is not a true island, there is additional concern that this approach may not be effective. Although this approach has been used in sensitive island habitats, it requires collaboration and a coordinated effort between multiple different agencies, including the Village of Bald Head Island, the Bald Head Island Conservancy, the Coastal Reserve, the United States Environmental Protection Agency, and the North Carolina Wildlife Resources Commission. Certain permitting will be required (Howald, et al., 2005). Without a measured negative impact of rats on rare or endangered species, it would likely be difficult to receive the proper permitting required to take this approach. The cost of an intensive eradication would be dependent on many factors, including the type of poison used, the amount of poison needed, the delivery methods of the poison (aerial vs. hand broadcasting), and on the precautions taken for non-target wildlife. Capturing and maintaining wildlife can be costly and could cause stress and physical harm to animals. Costs would mostly likely be short-term, as broadcast campaigns are generally shorter than bait station campaigns. The most ideal time for a broadcast eradication would be in the winter, when food availability is scarce and dependence on bait is increased (Howald, et al., 2007).

8 OPTIONS MOVING FORWARD OPTION 3 COMPLETE ERADICATION (MAYBE ) If this approach is taken, a plan must be developed and put into place to ensure that there is not a reinvasion of roof rats. Since it is still commonly debated how rats established themselves on Bald Head in the first place, this could be difficult. Rats could invade from Fort Fisher, could stow away in trucks coming over on the contractor barge, or could even have come over on the passenger ferry. A strict plan would have to be in place to ensure rats did not reach the island again, and even with such, it could still be difficult to keep rats out. Table 2 Table 2 is continued on next page

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10 CONCLUSION As this time, Bald Head Island faces three options for dealing with invasive roof rats on Bald Head Island. Option 1 (The Status Quo) is to continue with placing bait stations with second-generation anticoagulant bait outside of individual homes and businesses. This approach does not appear to be have been effective as of yet, and poses threats to wildlife. Option 2 (Keep them Out) focuses on sealing homes to keep rats outside, and eliminates the use of poison outside. While this option would protect wildlife species from the adverse effects of anticoagulant bait, there is potential that the rat population itself could begin to negatively impact native species. The third option (The Nuclear Option) requires intensive poisoning of the entire island. Costs for this option are potentially high and permitting could be difficult to secure. Native species will undoubtedly be negatively impacted by the high levels of poison; however, rats will most likely be eliminated from Bald Head Island, at least temporarily. REFERENCES (2011). Diseases directly transmitted by rodents. Center for Disease Control and Prevention. *(2011). Rat-bite Fever (RBF). Center for Disease Control and Prevention. (2014). Leptospirosis. MedlinePlus. Berny, P.J., Buronfosse, T., Buronfosse, F., Lamarque, F., Lorgue, G. (1997). Field evidence of secondary poisoning of foxes (Vulpes vulpes) and buzzards (Buteo buteo) by bromadiolone, a 4-year survey. Chemosphere, 35(8): Brakes, C.R. & Smith, R.H. (2005). Exposure of non-target small mammals to rodenticides: short-term effects, recovery and implications for secondary poisoning. Journal of Applied Ecology, 42: Caut, S., Angulo, E., Courchamp, F. (2008). Dietary shift of an invasive predator: rats, seabirds and sea turtles. Journal of Applied Ecology, 45: Coddou, A., Mills, H., Hamilton, N., Algar, D. (2014). Baiting effectiveness for introduced rats (Rattus sp.) on Christmas Island. Raffles Bulletin of Zoology, 30: Cox, P.R. (1991). Environmental effects of rodenticide use [Abstract]. PhD Thesis. University of Reading, Reading, UK.

11 REFERENCES Donlan, J. C., Gregg, H. R., Tershy, B. R., & Donald, C. A. (2003). Evaluating alternative rodenticides for island conservation: roof rat eradication from the San Jorge Islands, Mexico. Biological Conservation: Elsevier, 114: Eason, C. T., & Spurr, E. B. (1995). Review of the toxicity and impacts of brodifacoum on non-target wildlife in New Zealand. New Zealand Journal of Zoology: The Royal Society of New Zealand, 22: Gabriel, M.W., Woods, L.W., Poppenga, R., Sweitzer, R.A., Thompson, C., Matthews, S.M., Higley, J.M., Keller, S.M., Purcell, K., Barrett, R.H., Wengert, G.M., Sacks, B.N., Clifford, D.L. (2012). Anticoagulant Rodenticides on our Public and Community Lands: Spatial Distribution of Exposure and Poisoning of a Rare Forest Carnivore. PLoS ONE, 7(7). Hosea, R.C. (2000). Exposure of non-target wildlife to anticoagulant rodenticides in California. Proc. 19th Vertebr. Pest Conf. Howald, G., Donlan, C. J., Galvan, J. P., Russel, J. C., Parkes, J., Samaniego, A.,... Tershy, B. (2007). Invasive Rodent Eradication on Islands. Conservation Biology: Society for Conservation Biology, 21: Howald, G.R., Faulkner, K.R., Tershy, B., Keitt, B., Gellerman, H., Creel, E.M., Croll, D.A. (2005). Eradication of black rats from Anacapa Island: biological and social considerations. In Proceedings of the sixth California Island symposium (pp ). Institute for Wildlife Studies, Arcata. Leung, L.K.P, & Clark, N.M. (2005). Bait avoidance and habitat use by the roof rat, Rattus rattus, in a piggery. International Biodeterioration & Biodegradation, 55: Murray, M. (2011). Anticoagulant Rodenticide Exposure and Toxicosis in Four Species of Birds of Prey Presented to a Wildlife Clinic in Massachusetts, [Abstract]. Journal of Zoo and Wildlife Medicine, 42.1: Towns, D.R., Atkinson, I.A.E., Daugherty, C.H. (2006). Have the harmful effects of introduced rats on islands been exaggerated? Biological Invasions, 8: Valchev, I., Binev, R., Yordanova, V., Nikolov, Y. (2008). Anticoagulant Rodenticide Intoxication in Animals A Review. Turkish Journal of Veterinary and Animal Science, 32(4):