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View Results Of Survey Taken This Past Year Dr. Kenneth Raposa Research Director Roger Greene Manager Narragansett Bay National Estuarine Research Reserve 55 South Reserve Drive Prudence Island, RI 02872
Size of the Prudence Island white-tailed deer herd The Rhode Island Division of Fish and Wildlife (RIDFW) began estimating the annual size of the white-tailed deer population on Prudence Island in 1977. Since then, the density of deer on Prudence Island has exceeded 41 deer mi-2 (16 km-2) according to RIDFW surveys (Figure 1). Mean density between 1977 and 2002 was 67 deer mi-2 (26 km-2), with a peak of 95 deer mi-2 (37 km-2) in 2001. In eight of the last 12 years, deer density did not drop below 87 deer mi-2 (33 km-2) (Gibson, unpublished data).
Effects of overabundant deer on flora and fauna A wide range of deer densities and carrying capacities has been reported in the literature. Many researchers present what are considered to be high deer densities, or densities above which habitats become impacted by the deer themselves. Some of the levels cited in the literature are 26-44 deer mi-2 (10-17 km-2) (Healy 1997), 18 deer mi-2 (7 km-2) (Tilghman 1989), and 5-16 deer mi-2 (2-6 km-2) (Rooney 1995). Clearly, the variability associated with the reported levels is high. However, while the carrying capacity for deer on Prudence Island is unknown, the Prudence Island deer herd has remained above most of these levels cited in the literature for over 20 years. High deer densities result in a variety of adverse environmental conditions. Common signs of too many deer include browse lines (horizontal lines on trees, often 5-6 feet in height, below which vegetation has been removed by deer browsing) and an altered or degraded vegetation understory (Tilghman 1989; Healy 1997). Further effects of overabundant deer include a shift in vegetation species composition, a reduction in species richness, and a reduced or eliminated ability for forests to regenerate themselves (Tilghman 1989; Strole and Anderson 1992; Rooney 1995; Alverson and Waller 1997; Healy 1997). It has also been shown that when deer densities are reduced to approximately 20 mi-2, the restoration of forest vegetation will begin and vegetation species diversity will increase, although it may take years to reverse the damage caused by high deer densities (Horsley et al. 2003). Browsing by overabundant deer adversely affects other wildlife by limiting food and cover provided by understory and other vegetation (McShea and Rappole 1997). It has also been shown that opportunistic nocturnal encounters leads to direct predation on juvenile ground bird nestlings by deer (Pietz and Granfors 2000). Logically, there should be a higher number of encounters, and thus higher predation on ground nesting birds, as deer density increases. A 10-year study conducted by the US Forest Service has shown that high deer densities can reduce the richness and abundance of intermediate canopy-nesting songbirds. This study showed that the eastern wood pewee, indigo bunting, least flycatcher, yellow-billed cuckoo, and the cerulean warbler were all negatively affected when deer density exceeded 20 mi-2, and that the eastern phoebe and American robin were affected when deer density exceeded 64 mi-2 (deCastela 1994). Although there are no long-term data on the abundance of these bird species on Prudence Island, the density of the deer herd on Prudence has greatly exceeded the 20 mi-2 density level cited in this report since at least 1977, and has exceeded 64 mi-2 in thirteen of the last sixteen years (Figure 1).
Effects of deer on tick abundance and incidence of tick-borne diseases The relationship between deer density, deer tick abundance, and the incidence of tick-borne diseases is very clear. As deer density increases, so does the abundance of deer ticks, as well as the incidence of Lyme disease, babesiosis, and ehrlichiosis, all of which are caused by tick bites (Krause et al. 2002). Deer ticks in all developmental stages use deer as hosts, and 95% of all adult deer ticks feed on deer, with the remaining 5% feeding on alternative hosts such as feral cats and raccoons (Wilson et al. 1990). Deer ticks are most abundant where deer density is highest and there is a direct correlation between deer tick abundance and deer scat (Wilson et al. 1985; Anderson et al. 1987). In residential settings, the risk of infection in humans is directly related to deer density (Lastivaca et al. 1989). Thus, it is clear that the risk of becoming infected with a tick-borne disease is directly related to the density of deer; the higher the density, the greater the risk of infection (Krause et al. 2002). It follows then that the most sensible way to reduce the risk of infection is to reduce the density of deer. Reducing the density of deer, or in extreme cases removing deer entirely, results in a reduction in deer tick abundance over time and as a result, a concurrent reduction in infection. When deer were reduced by approximately 90% on Great Island in Massachusetts, the rate of Lyme disease infection was reduced from over 3 cases per 100 people per year before deer removal to less than 0.2 cases per 100 per year after removal (Wilson and Childs 1997). More specifically, when deer density was reduced to 6-10 mi-2 at this site, the incidence of Lyme disease was reduced by 80% (Telford, personal communication), and ten years after deer removal only one additional case of Lyme disease had been reported (Ebel, personal communication). A similar study in the Crane Reservation in northern coastal Massachusetts examined the effects of a more gradual reduction in deer density on the abundance of deer ticks. At this 2.2-mi2 site, deer were reduced by 82% over a six-year period, from a density of 171 deer mi-2 to a density of 29 mi-2 (from 350 deer, down to 60). This reduction led to a decrease in the average number of larval ticks to about one-half of the level before deer were reduced. Abundance of nymphal ticks also decreased, but not by as much (Deblinger et al. 1993). Data from the Rhode Island Department of Health showed that in 1989, approximately 29% of Prudence Island’s full-time residents tested seropositive for Lyme disease. In addition, from 1999 to 2000, Prudence Island had a very high percentage of residents that seroconverted to Lyme disease (13%), babesiosis (9%), or both (5%); thus, during this time a total of 22% of Prudence Island residents seroconverted to either one of the tick-borne disease or both. (Krause et al. unpublished data). This is higher but comparable to other sites known to have high rates of exposure such as Block Island (20%) and Brimfield, Massachusetts (15%) (Krause et al. unpublished data). The high rates of exposure on Prudence Island are more alarming given the fact that concurrent infection with both Lyme disease and babesiosis can increase the severity of disease and complicate diagnosis (Krause et al. 2002). Both of these diseases are contracted to humans through the bite of the deer tick, the abundance of which is directly related to deer density.
By all accounts, the density of the white-tailed deer herd on Prudence Island has remained at a level where the herd itself is degrading the island’s habitats and is subjecting the residents of the island to a high level of risk to tick-borne diseases. The mean density of deer on Prudence Island between 1977 and 2002 was approximately 67 deer mi-2. This density exceeds any other density found in the published literature that was considered high enough to severely damage or degrade the habitats used by deer. There are no studies on Prudence Island that examine the effects of deer on habitat, vegetation, or birds or other animals. However, some of the effects of overgrazing by large numbers of deer are apparent on Prudence Island. Browse lines are clearly seen in many areas of the island (Figure 2), while other forested habitats lack a developed vegetative under-story (Figure 3). In addition, much of Prudence Island is heavily overgrown with the Asiatic bittersweet (Figure 4), a species that is not a preferred food for deer and which competes with and inhibits other native vegetation. If left at these high levels, deer will eventually inhibit the forests of Prudence Island from regenerating themselves, and may lead to declines in other wildlife species that depend on the vegetation that the deer are overbrowsing. In addition to the negative impacts on habitat, the high densities of deer are also increasing the risk of exposure to tick borne diseases to the residents and visitors of Prudence Island. It has been clearly shown that the risk of exposure to these diseases is directly related to the density of the deer that provide blood meals for the deer ticks that carry the diseases. Approximately one-third of all Prudence Island residents have tested positive for Lyme disease and as recently as the year 2000, 22% of residents of the island were newly exposed to either Lyme disease, babesiosis, or both within the last year. The abundance of deer ticks, and the risk of exposure to disease, is so high on Prudence Island, that residents remain at risk even in their own yards where vegetation is regularly mowed (Carroll et al. 1992). These high rates of disease can be directly attributed to the exceedingly high deer densities on Prudence Island. In other locations, studies have shown the abundance of deer ticks and the incidence of Lyme disease can be significantly reduced by reducing the density of deer (although it is not clear if this is a linear relationship). The Harvard School of Public Health has stated that deer reduction on Prudence Island would have a similar result (Ebel personal communication). In summary, the density of deer on Prudence Island is too high. The deer are degrading some of the island’s habitats, and are unnecessarily increasing the risk of tick-borne diseases to all who use the island and the units of the Narragansett Bay National Estuarine Research Reserve on Prudence. It is difficult to determine what the density of deer on Prudence Island should be, however, based on other studies it seems that the density should be reduced to levels of approximately 20 deer mi-2 in order to both reduce the risk of disease and to alleviate the damage to island habitats. It should be emphasized, however, that even if the deer herd were to be reduced to this level, it might take a long time to restore the island’s degraded natural habitats. For example, one study showed that the diversity of forest vegetation began to return 10 years after deer densities were reduced (Horsley et al. 2003). In addition, even at these low deer densities, deer ticks will remain on Prudence Island and there will still be a risk of contracting a tick-borne disease (albeit a much lower risk). More Info Available In A PDF Format Literature Cited Alverson, W. S. and D. M. Waller. 1997. Deer populations and the widespread failure of hemlock regeneration in northern forests. Pages 280-297 in W. J. McShea, H. B. Underwood, and J. H. Rappole, eds. The science of overabundance: deer ecology and population management. Smithsonian Institution Press. Anderson, J. F., R. C. Johnson, L. A. Magnarelli, et al. 1987. Prevalance of Borrelia burgdorferi and Babesia microti in mice on islands inhabited by white-tailed deer. Applied Environmental Microbiology 53:892-894. Deblinger, R. D., M. L. Wilson, D. W. Rimmer, and A. Spielman. 1993. Reduced abundance of immature Ixodes dammini (Acari: Ixodidae) following incremental removal of deer. Journal of Medical Entomology 30:144-150). DeCastela, D. S. 1994. Effect of white-tailed deer on songbirds within managed forests in Pennsylvania. Journal of Wildlife Management 58:711-718. Healy, W. M. 1997. Influence of deer on the structure and composition of oak forests in central Massachusetts. Pages 249-266 in W. J. McShea, H. B. Underwood, and J. H. Rappole, eds. The science of overabundance: deer ecology and population management. Smithsonian Institution Press. Horsley, S. B., S. L. Stout and D. S. deCastela. 2003. White-tailed deer impact on the vegetation dynamics of a northern hardwood forest. Ecological Applications 13:98-118. Krause, P. J., R. Pollack, L. Closter, D. Christianson, and A. Spielman. 2002. Lyme disease, babesiosis, and human granulocytic ehrlichiosis on Block Island: a review. Pages 209-216 in P. W. Paton, L. L. gould, P. V. August, and A. O. Frost, eds. The ecology of Block Island. The Rhode Island Natural History Survey, Kingston, RI. Lastivica, C. C., M. L. Wilson, V. P. Berardi, et al. 1989. Rapid emergence of a focal epidemic of Lyme disease in coastal Massachusetts. New England Journal of Medicine 320:133-137. McShea, W. J. and J. H. Rappole. 1997. Herbivores and the ecology of forest understory birds. Pages 298-309 in W. J. McShea, H. B. Underwood, and J. H. Rappole, eds. The science of overabundance: deer ecology and population management. Smithsonian Institution Press. Pietz, P. J. and D. A. Granfors. 2000. White-tailed deer (Odocoileus virginianus) predation on grassland songbird nestlings. American Midland Naturalist 144 (2):419-422. Rooney, T. P. 1995. Restoring landscape diversity and old growth to Pennsylvania’s northern hardwood forests. Natural Areas Journal 15:274-278. Strole, T. A. and R. C. Anderson. 1992. White-tailed deer browsing: species preferences and implications for central Illinois forests. Natural Areas Journal 12 (3)139-144. Tilghman, N. G. 1989. Impacts of white-tailed deer on forest regeneration in northwestern Pennsylvania. Journal of Wildlife Management 53 (3):524-532. Wilson, M. L. and J. E. Childs. 1997. Vertebrate abundance and the epidemiology of zoonotic diseases. Pages 224-248 W. J. McShea, H. B. Underwood, and J. H. Rappole, eds. The science of overabundance: deer ecology and population management. Smithsonian Institution Press. Wilson, M. L., G. H. Adler and A. Spielman. 1985. Correlation between abundance of deer and that of the deer tick, Ixodes dammini (Acari: Ixodidae). Wilson, M. L., T. S. Litwin, T. A. Gavin, et al. 1990. Host-dependent differences in feeding and reproduction of Ixodes dammini (Acari:Ixodidae). Journal of Medical Entomology 27:945-954. Figure 1. Density of white-tailed deer on Prudence Island, RI 1977-2002. Density data provided by the Rhode Island Division of Fish and Wildlife. Also shown for reference is a deer density level that is typically regarded as ecologically high (20 mi-2).
Figure 2. An example of deer browse lines on eastern red cedar trees towards the north end of Prudence Island.
Figure 3. An example of a degraded under-story in a forest toward the south end of Prudence Island.
Figure 4. Example of an area overgrown by the deer-resistant Asiatic bittersweet. High densities of deer often lead to shifts in vegetation species composition, with deer-resistant species becoming more prevalent. Arrow indicates significant areas of bittersweet invasion.
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