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Publication Number: FHWA-HRT-08-034
Date: August 2008

Wildlife-Vehicle Collision Reduction Study: Report To Congress

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Chapter 7. Mitigation Methods that Seek to Reduce Wildlife Population Size

A comprehensive review by Knapp, Putman and others suggests that a reduction of the population size across a relatively wide area can be effective in reducing DVCs. (See references 52, 53, 54, 58, 61, 62, and 311.) Nonetheless, actual data on the effectiveness of population reduction programs on WVCs are few. For example, a field test showed that a deer population reduction program in Minnesota reduced winter deer densities by 46 percent and DVCs by 30 percent.(52)

Deer population sizes, especially those of white-tailed deer, have grown substantially over the last century in the United States.(55) This population growth is especially apparent since the 1960s.(55,57) This increase in population size was triggered by better protection, a matrix of habitat providing cover (forests) and food (agriculture, silviculture), the loss or decline of their natural predators, and more recently, reduced hunting pressure by humans through an increase in refugia (private land, (sub) urban areas), and a decrease in hunters. (See references 55, 57, 312, 313, 314, and 315.) Currently, white-tailed deer numbers are believed to be higher than they have ever been in the past several hundred years, causing or contributing to a series of problems, including DVCs. (See references 55, 57, 314, and 316.) This situation has triggered efforts to eliminate, reduce, or control the size of the deer population, especially white-tailed deer.

The fertility of white-tailed deer depends on their population density; their reproduction is higher at relatively low densities than at higher densities. The fertility of white-tailed deer is weakly density dependent for adult does.(317) However, the fertility of first-year and yearling females is strongly density dependent, with very low fertility when population densities exceed 30 deer/km2 .(317) These observations suggest that as population density is reduced, increased effort is needed to keep the deer density at the lower level. This phenomenon needs to be addressed in potential population size reduction programs.

The relationship between deer population density and the number of DVCs seems intuitive but is often complex and variable.(59,60) Nonetheless, a comprehensive review by Knapp, Putman and others suggests that a reduction of the population size across a relatively wide area can be effective in reducing DVCs. (See references 52, 53, 54, 58, 61, 62, and 311.)

This chapter discusses the following measures to reduce wildlife population size along roads

  • Wildlife culling.

  • Wildlife relocation.

  • Anti-fertility treatment.

  • Habitat alteration away from the road.

Wildlife Culling

Wildlife culling involves a substantial reduction of the population size through eliminating a large number of individual animals over a short period of time. This measure is typically applied to or proposed for deer. The culling is sometimes done by recreational hunters through increased quota; sometimes it is done by professionals, especially if there are refugia for the deer (private land, (sub)urban areas). (See references 52, 312, 318, and 319.)

A field test showed that a deer population reduction program in Minnesota reduced winter deer densities by 46 percent and DVCs by 30 percent.(52) Sharpshooting by professionals over bait was deemed to be the most effective and adaptable culling method in an urban setting, as opposed to controlled hunts in large parks and refuges or opportunistic sharpshooting by professionals.(52)

The killing of does (females) is more effective for reducing the population size than the killing of bucks (males), not only because the reproductive potential of the herd is more effectively reduced, but also because does tend to stay in their existing home range while bucks have a greater tendency to disperse.(55) The does are less likely to migrate and establish new populations elsewhere.

A modeling project by Porter and others showed that if female dispersal (i.e., animals that leave the area) was 8 percent, culling would have to reduce annual survival to 58 percent to maintain a population just under ecological carrying capacity (the maximum sustainable population size).(320) A further reduction of the annual survival to 42 percent would keep the population at 0.5 of the carrying capacity.(320)

Culling efforts are more likely to result in a substantial reduction in deer population size if the herd size is relatively small to begin with and if it is a closed population that does not allow influx of animals from nearby places. The effort has to be repeated periodically as the deer population will grow back to the same levels if the habitat conditions remain similar (i.e., it is not a one-time-only measure). In addition, the effort involved for population size reduction programs increases disproportionately with higher population size reduction goals, and substantial reductions (for example ≥50 percent) may be hard to obtain, perhaps capping the potential reduction in DVCs at 50 percent. Finally, wildlife culling can meet with strong public opposition.

Case Studies and Contacts

For information about a field test of four population management methods to reduce a deer population in Bloomington, MN, contact Michelle Doerr, 2887 Ulm Avenue, Stewart, MN 55385,ewiggers@islc.net.

For more information about a field test of the effects of baiting and supplemental antlerless seasons on deer harvest, contact Timothy Van Deelen, Department of Wildlife Ecology, University of Wisconsin, Madison, WI 53706-1598,trvandeelen@wisc.edu.

Direct Benefits

The relationship between deer population size and DVCs can be highly variable (see earlier discussion). Nonetheless, based on Doerr, Knapp and others, a certain percentage in population size reduction may result in a similar percentage in DVC reduction.(52,53) However, reductions in population size of 50 percent or more may be hard to obtain, perhaps capping the potential reduction in DVCs at 50 percent or less.(52)

Indirect Benefits

Indirect benefits of reducing wildlife population size include reduced negative impacts from "overpopulation" on agricultural crops, silviculture, and natural vegetation.(55)

Undesirable Effects

Wildlife culling may not be favored or accepted by the public, especially in areas that have a high degree of ecological integrity ("hands-off" approach).(57,321,322) A public relations campaign should be considered along with a culling effort.

Culling may not be possible or effective on private lands, in remote locations, or in urban and suburban areas.(55,312) If refugia are present, more intensive effort will have to be undertaken at locations that are accessible to hunters or wildlife managers.(312)

The effort will have to be repeated periodically as the deer population will grow back to the same levels if the habitat conditions remain similar; it is not a one-time-only measure.

Recreational hunters tend to focus on mature bucks rather than young animals or does, which is the least effective way to reduce the population size.(55) A change in regulations may be required to allow for greater quota, specifically for younger animals and does.(312,323)

The number of hunters is declining, perhaps demanding a shift from recreational hunting to professional culling.(313,318)

Baiting in order to facilitate wildlife culling increases efficacy but is illegal in some areas, and it can lead to undesirable side effects such as increased risk of the spread of diseases, reduction in the consumption of natural foods and consequent changes in the ecosystem, population increase and consequent starvation, crowding, fighting and injuries of deer, deer domestication and habituation to unnatural foods and humans, decrease in hunter satisfaction, and increase in concerns of the nonhunting public.(323,324)

Costs

The costs for a controlled hunt were estimated at $117 per deer killed. The cost of using professional sharpshooters was $108–121–194 per deer for conservation officers, park rangers, and police officers, respectively.(52) Others estimated these costs at $91–310 per deer.(287)

Guidelines

No guidelines were identified in the literature review.

Wildlife Relocation

Wildlife relocation involves the capture, transport, and release of animals (mostly moving deer to another location). It is typically considered if population reduction is required but culling is not an option.

At the Sea Pines residential area on Hilton Head Island, SC, a deer relocation experiment was conducted.(325) The relocated deer experienced relatively high mortality from capture-related causes, and 50 percent of the relocated deer dispersed from their release site.(325)

Case Studies and Contacts

For more information on a field test of the live-capture and small-scale relocation of urban deer on Hilton Head Island, SC, contact Jennifer Cromwell, United States Department of Agriculture, Wildlife Services, Moseley, VA, 23120, (804) 734-7739,jennifer.s.cromwell@aphis.usda.gov.

Direct Benefits

The relationship between deer population size and DVCs can be highly variable (see earlier discussion). Nonetheless, based on Doerr, Knapp and others, a certain percentage reduction in population size may result in a similar percentage reduction in DVCs.(52,53) However, reductions in population size of 50 percent or more may be hard to obtain, perhaps capping a potential reduction in DVCs at 50 percent or less.(52) The effectiveness can be seriously diminished if it is an open population that allows the individuals from neighboring populations to fill the gaps or that allows the relocated individuals to return.(325)

Indirect Benefits

Benefits include reduced negative impacts from "overpopulation" on agricultural crops, silviculture, and natural vegetation.(55)

Undesirable Effects

With an open population or relocation over relatively short distances, individuals from neighboring populations may fill the gaps or a substantial portion of the relocated individuals may return to the original location, seriously limiting the effectiveness of this measure.(325) In one study, 50 percent of the relocated deer did not remain in their release area.(325)

The effort will have to be repeated periodically as the deer population will grow back to the same levels (growth, immigration, including of individuals that were relocated) if the habitat conditions remain similar; it is not a one-time-only measure.

Relocated individuals tend to experience a lower survival rate and increased human-induced mortality, including from the capturing effort.(325,326) Relocation of deer can result in the spread of infectious diseases.(327) Wildlife relocation is, in general, not recommended.(328)

Relocated individuals may compete with individuals that are already present at the release site, or they may contribute to the growth and overpopulation at the release site and the negative effects associated with overpopulation.(55)

The effort may not be favored or accepted by the public, especially in areas that have a high degree of ecological integrity ("hands-off" approach).(57)

Costs

The costs for relocation were estimated at $387 per relocated deer.(326) Others estimated these costs at $431 or $400–2,931 per deer.(287)

Guidelines

No guidelines were identified in the literature review.

Anti-Fertility Treatment

Anti-fertility treatment can reduce reproduction of deer. (See references 329, 330, 331, 332, and 333.). This measure is typically considered or applied where killing (through hunting) is illegal (private lands, legislation) or impractical ((sub) urban areas, public pressure), where recolonization possibilities from elsewhere are limited (closed population), and where a relatively small deer population exists (e.g., 2000 or fewer breeding females). (See references 330, 334, 335, 336, and 337.) Some drugs have shown to be effective for up to 1 or 2 years, but repeated application is often needed. The reversibility of anti-fertility treatment can be considered an advantage (if reproduction is necessary later) as well as a disadvantage (continuing treatment required).(330,338)

Modeling efforts have shown that sterilization in combination with hunting can control the population size of deer.(334,335) For the deer on Cumberland Island National Seashore, GA, (herd size 1,500 animals), the model predicted that the herd size could be controlled at 750 animals if 200 sterilizations are done per year for the first 3 years, followed by 42 sterilizations per year for the following years).(334) Should the existing levels of hunting and predation continue, these numbers can be reduced to 81 (initial 3 years), but they would be higher for the following years (58 per year). Modeling by Seagle and Close showed that with contraception rates of less than 50 percent of female deer, the population growth curve was less steep, but herd size was not reduced.(335) A minimal contraception rate of 50 percent was required to reduce the herd size. With contraception rates greater than 50 percent, substantial changes in population size were only observed after 5–10 years. Variability of the results was high, suggesting that it is difficult to detect population changes in the field.(335) Another modeling effort showed that a deer population could be reduced by 30–60 percent in 4–10 years if 25–50 percent of the fertile females were sterilized annually.(339) Models developed by Porter and others showed that with 8 percent female dispersal, contraception would need to be effective in 32 percent of the females if the population were at about carrying capacity, and 68 percent if the population were at one-half of carrying capacity.(320)

Results of a field test on a suburban white-tailed deer population in Irondequoit, NY, using immunocontraception suggest that the measure may be most practical when a population density between 30 and 70 percent of ecological carrying capacity is acceptable. However, it is only considered useful in relatively small and closed populations (≤ 200 females).(336) Walter and others captured and treated suburban female white-tailed deer in Connecticut with a contraceptive.(340) Baiting and capturing of the deer was more effective in spring than in fall. The study indicated that treatment of about 70 percent of the population was possible.(340) A capture and sterilization program for a white-tailed deer population in Cayuga Heights, NY showed that capture efforts had to be relatively high because the traps were not sex or age specific.(341) Furthermore, the efficacy of the sterilization program was dramatically reduced because it was not a closed population.(341)

Case Studies and Contacts

For information on a field test using immunocontraception in a suburban white-tailed deer in Irondequoit, NY, contact Brent Rudolph, Department of Fisheries and Wildlife, Michigan State University, East Lansing, MI 48824, (517) 432-4943,rudolp13@msu.edu.

For information on a field test using a contraceptive in a suburban white-tailed deer in Connecticut, contact David Walter, Oklahoma Cooperative Fish and Wildlife Research Unit, Oklahoma State University, 404 Life Sciences West, Stillwater, OK, 74078,wdwalte@okstate.edu.

For information on a field test using sterilization in a white-tailed deer population in Cayuga Heights, NY, contact Johnny Merrill, Cornell University, Department of Natural Resources, Fernow Hall, Ithaca, NY 14853,jam82@cornell.edu.

Direct Benefits

The relationship between deer population size and DVCs can be highly variable (see earlier discussion). Nonetheless, based on Doerr, Knapp and others, a certain percentage in reduction of population size may result in a similar percentage reduction in DVCs.(52,53) However, reductions in population size of 50 percent or more may be hard to obtain, perhaps capping the potential reduction in DVCs at 50 percent or less.(336) The effectiveness is seriously diminished if it is an open population that allows the individuals from neighboring populations to fill the gaps.(341)

Indirect Benefits

Benefits include reduced negative impacts from "overpopulation" on agricultural crops, silviculture, and natural vegetation.(55)

Undesirable Effects

Fertility control and immunocontraceptives by some drugs disrupt normal reproductive behavior and can cause physical problems with the reproductive system, abscesses and inflammations, weight gain, changes in general behavior, and changes in the sex ratio in the herd.(342,343) However, health problems and effects on social behavior may not always occur, or they may be minimal.(338)

The efforts will have to be repeated constantly in a population, and depending on the drug used, the same animals may have to be treated multiple times during their lives.(See references 330, 334, 338, and 344.)

The effort is less effective, or not effective, when recolonization from elsewhere can occur (open population).(335)

The method may only be feasible for relatively small populations (200 or fewer females).(336)

Costs

Walter and others calculated that $33,833 ($1,128 per treated deer) was required to treat 30 deer for 2 years (labor was 64 percent of the total budget).(340)

Guidelines

No guidelines were identified in the literature review.

Habitat Alteration Away from the Road

Deer population density depends on the quality of their habitat. An abundance of food and cover, in combination with an absence of predators and hunting, allows for relatively high population densities. (See references 55, 57, 312, 313, 314, and 315.) In general, good feeding habitat for deer may include young forests (e.g., in harvested areas that have been replanted or that have naturally regenerated), agricultural lands (hay or alfalfa meadows, especially if they are fertilized and irrigated, and crop lands), lawns and gardens (including golf courses), and riparian habitat.

Good cover is provided by forests or shrubland. When there is a matrix of good cover with good feeding habitat, deer population densities are typically relatively high. The size of the herd can be reduced through culling, relocation, or anti-fertility treatment, but if the habitat remains similar, deer densities will quickly return to their original levels, partly as the result of density-dependent fertility. Therefore habitat alterations that will limit the population density in certain areas can be considered. These measures may include reducing the amount of edge habitat by having larger patches of cover and feeding habitat or reducing the quality and quantity of the available food.(345)

Reducing the quality of the available food may be achieved by certain mowing or cutting practices, allowing for natural succession to more mature forests (where applicable) with different grass-herb and shrub vegetation on the forest floor, and reducing or stopping irrigation and the use of fertilizers.(346) Reducing the quantity of the available food can be achieved by allowing the natural succession to more mature forests (where applicable) with less grass-herb and shrub vegetation on the forest floor, or making prime feeding habitat unavailable to the deer, e.g., through the use of wildlife fencing.(346,347)

Reducing the quality of the available food may only limit populations in relatively poor years, and stimulating attractive browse to lure animals away from croplands may only work on a small scale and affect primarily females and has to take into account the relative attractiveness of the foods available in the area(345,348,349)

Some of the measures discussed above may take a long time to take effect, while other measures may require a change in land use practices. However, these types of measures would reduce the frequency and level of population culling, relocation, and anti-fertility treatment needed to reduce deer population density to an "accepted" level.

Case Studies and Contacts

For information regarding movements of female white-tailed deer in relation to timber harvests in the central Appalachians, contact Tyler Campbell, Warnell School of Forest Resources, University of Georgia, Athens, GA 30602, (706) 542-4280,tcampbell@smokey.forestry.uga.edu.

For information on the effects of food or cover removal on spacing patterns and habitat use in roe deer (Capreolus capreolus), contact Sandro Lovari, Section of Behavioral Ecology, Ethnology and Wildlife Management, Department of Environmental Sciences, University of Siena, Via Mattioli 4, 53100, Siena, Italy,lovari@unisi.it.

For information on changes in roe deer population density in response to forest habitat succession, contact Robin Gill, Forest Commission Res. Div., Alice Holt Lodge, Farnham, Surrey GU10 4LH, United Kingdom,robin.gill@forestry.gsi.gov.uk.

Direct Benefits

The relationship between deer population size and DVCs can be highly variable (see earlier discussion). Nonetheless, based on Doerr, and Knapp and others, a certain percentage in population size reduction may result in a similar percentage in DVC reduction.(52,53) However, reductions in population size of 50 percent or more may be hard to obtain, perhaps capping the potential reduction in DVCs at 50 percent or less.(336) The effectiveness of habitat alteration is less dependent on having a relatively small and closed population compared to population culling, relocation, and/or anti-fertility treatment.

Indirect Benefits

This WVC mitigation approach minimizes damage to agricultural crops and gardens and lawns.

Undesirable Effects

If the habitat is negatively affected on a large scale within a short time period, population control may be required to avoid potential starvation or dispersal in response to the reduction in habitat quality and availability.

Costs

Costs are expected to be highly variable depending on site-specific circumstances. No costs were identified in the literature review.

Guidelines

No guidelines were identified in the literature review.

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