Illinois Natural History Survey - University of Illinois

Cost and Controversy in Managing Urban Deer

Unlike deer in rural Illinois and other parts of the country, urban deer are not regulated by large predators or hunting, thus they often reach extreme population levels in remnant urban natural areas. Deer-vehicle collisions in urban areas are an index of population trends for urban deer. From 1981 to 1992 the number of deer-vehicle collisions increased from 266 to 1,300, respectively, in the three Chicago metro counties (Cook, DuPage, and Lake). Roads adjacent to natural areas also may show unacceptable levels of dangerous car-deer collisions. In addition, intense herbivory associated with high populations threatens the biodiversity of palatable native plants and causes costly damage to ornamentals. To address these problems, managers are faced with the unhappy task of reducing deer populations.

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White-tailed deer, a very common inhabitant of Illinois in urban and suburban areas.

In urban forest preserves, culling deer (using trained sharpshooters in highly controlled situations) is usually the only way to safely and efficiently reduce a deer population. Other methods (sterilization, contraception, trap and transfer) are being researched actively but will not work currently because of prohibitively high costs and severe logistical problems.

An additional complication in urban deer management is that per capita reproduction is density-dependent, meaning that the average number of fawns produced and reared by each doe increases as the total population decreases. This is analogous to having a bank account where the interest rate (reproduction) is a declining function of the account balance (population size). Maximum return (number of new recruits in the next breeding season) is thus achieved at some intermediate account balance where a declining balance and increasing interest rate is optimized. Similarly, deer reproduction increases as managers begin to remove deer, thus requiring managers to work ever harder to reduce the deer population to offset the population's increasing reproductive effort.

For several years, the DuPage County Forest Preserve District has had a progressive deer management program that is beginning to enhance conservation efforts on the native vegetation of preserves where culling is done. Culled deer are processed, inspected, and donated to Chicago area charities. We have been examining the reproductive tracts of culled deer and with other field data we have been identifying the life history mechanisms behind density-dependent reproduction.

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Too many deer close to urban areas can have negative impacts on humans as well as the environment.

Examination of over 550 reproductive tracts from a period of six years of intensive removal effort suggested that pregnancy rates for adults and yearlings remained high (89%). Pregnancy among fawns (indicating conception during the first breeding season) varied from 0 to 25% as population density decreased. Fawn mothers always gave birth to single young whereas older does tended to have twins. In addition, the fecundity of older does increased slightly with decreasing density, and we recorded several mothers with triplets. The most dramatic effect was seen in the number of female fawns that showed up six months later during the population's breeding season. This measure, known as recruitment, increased dramatically with decreasing density. Taken together, the reproduction and recruitment data suggest that the principal mechanism behind density dependence is fawn survival. In other words, the trade-off for maintaining high deer populations is high mortality among newborn fawns.

Fawn mortality is the topic of an ongoing research project, but probably is due primarily to predation by coyotes, dogs, and small carnivores. Secondary causes may include disruption of the mother-fawn bond due to crowding and a limited number of secure fawning sites.

Finally, as if the problems caused by density dependence weren't bad enough, the effort and cost required to cull an individual deer probably increases exponentially as population density declines. Therefore, the need to work ever harder, coupled with the fact that cost increases rapidly as deer density declines, may determine the real-world limits to maintaining deer at reduced population levels. In remnant urban natural areas, conservationists concerned with the negative effects of herbivory may need to accept sustained intense management of resident deer populations.

Timothy R. Van Deelen and Dwayne R. Etter, Center for Wildlife Ecology

Charlie Warwick, editor



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