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National Wildlife Research Center (NWRC)


Improving Management Strategies to Reduce Damage by Forest and Aquatic Mammals

PROJECT GOAL: Advance and disseminate knowledge that improves tools, techniques, and strategies to reduce damage inflicted in forested and aquatic areas by herbivorous and omnivorous mammals.

Project Accomplishments 2010

Forestry Protection
Wildlife impacts on forest resources can be extensive. Cutting and gnawing on seedlings by deer, elk, mice, mountain beavers, pocket gophers, rabbits, and voles during the first 5 years of tree growth greatly hinder reforestation efforts following harvests and wildfires. Other mammals such as bears and porcupines damage mature trees. North American beavers and nutria alter riparian vegetation, which limits streamside restoration efforts, erodes roads and railways, and can endanger human health and safety. National Wildlife Research Center (NWRC) scientists are developing nonlethal tools and methods (e.g., repellents and habitat and behavior modification) to manage wildlife damage to forest resources.

White-Tailed Deer Browsing and Rubbing Preferences for Trees and Shrubs—Non-timber forest products (e.g., foods, herbal medicines, photo of browsing damage to saplingwoody florals, and handicraft products) are important sources of income for some landowners. However, white-tailed deer ( Odocoileus virginianus) can reduce the quality, quantity, and profitability of forest products by browsing twigs and rubbing stems, resulting in direct and indirect losses. NWRC researchers collaborated with colleagues from the University of Nebraska in a study to (1) evaluate deer damage—in particular, the frequency and intensity of browsing and rubbing—sustained by 26 species of trees and shrubs; (2) relate morphological features of trees and shrubs to damage levels; and (3) estimate the economic impacts of deer damage on the production of non-timber forest products.

Results from the study showed levels of browsing to be quite high in most species of trees and shrubs, with the highest intensity occurring in Chinese chestnut ( Castanea spp.) and dogwoods ( Cornus spp.) and the lowest in ginkgo ( Ginkgo biloba), curly willow ( Salix matsudana), scarlet curls willow ( S. matsudana var. ‘Scarlet Curls'), smooth sumac ( Rhus glabra), and pussy willow ( S. caprea.). Sparse trees or shrubs with one or a few exposed stout stems unprotected by dense branching (e.g., American elderberry [Sambucus canadensis], smooth sumac, and curly willow) sustained the most damage from deer rubbing. Trees and shrubs with many small diameter stems or with dense tangled branching (e.g., yellow-twig dogwood [ C. sericea var. 'yellow-twig'], forsythia [ Forsythia suspense], flame willow [ S. alba var. ‘Flame'], and streamco willow [ S. purpurea]) were least damaged by rubbing. The economic costs of deer damage to producers of non-timber forest products ranged from $9 per acre for pussy willow to $1,428 per acre for curly willow.

Repellents To Prevent Ungulate Browsing—Chemical repellents are frequently used to deter deer browsing on trees and shrubs. Repellents promote an avoidance behavior using one or a combination of several mechanisms—neophobia (tendency of an animal to avoid/retreat from an unfamiliar object or situation), irritation, conditioned aversion, and/or flavor modification. The relative effectiveness of repellents often depends on the individual animal's motivation to consume the protected resource. For example, when alternative foods are available, shiny ribbons (a visual repellent with no consequence) may provide significant protection to plants in localized areas; however, when alternative foods are scarce, visual repellents may no longer be effective.

In captive deer studies, NWRC scientists evaluated how feeding motivation may impact repellent efficacy. The incentive to consume test diets was manipulated by allowing captive deer to learn about two test diets that differed in energy content. Scientists then conducted a series of experiments to compare the different mechanisms of deer repellency and evaluate repellent effectiveness when the incentive to consume the treated diet was varied. They tested four repellents representing different modes of action (neophobia, irritation, conditioned aversion, and flavor modification). When the high-energy diet was treated with repellents, only blood (flavor modification) and capsaicin (irritation) proved highly effective. Rapid habituation to the odor of meat and bone meal (neophobia) presented in a sachet limited its effectiveness as a repellent under conditions with a high feeding motivation. Thiram, a stimulus used to condition aversions, was not strongly avoided in these trials.

These data support previous studies indicating that habituation to odor limits the effectiveness of repellents that are not applied directly to food, while topically-applied irritants and animal-based products produce significant avoidance behavior in deer.

Mountain Beaver Genetics—Mountain beaver ( Aplodontia rufa rufa) are endemic to the Pacific Coast of North America and can be found in California, Nevada, Oregon, Washington, and British Columbia, Canada. There are seven subspecies of mountain beaver. The U.S. Department of the Interior's (DOI) Fish and Wildlife Service (FWS) has classified one of these subspecies, A. r. nigra, as endangered and several other mountain beaver subspecies as populations of concern under the Endangered Species Act. However, in some portions of its range, mountain beaver cause significant damage to forestry interests and are managed as a pest species. Studies of mountain beaverpopulations are critical for understanding their status and informing wildlife damage management practices. In particular, molecular genetics techniques are particularly useful for explaining population demographics.

To aid research efforts related to mountain beavers, NWRC scientists developed and characterized 10 microsatellite markers from the A. r. rufa genome. The addition of these 10 markers to previously published ones provides a powerful tool for studying A. r. rufa populations. For instance, NWRC researchers are using deoxyribonucleic acid (DNA) analyses to test whether mountain beaver populations are closed systems or if individuals move across forested landscapes to new areas. Researchers are also studying the relatedness and connectivity among the last two remaining populations of the endangered A. r. nigra.

Composition of Beaver Colonies—Natural resource managers faced with resolving beaver damage often make decisions based on classic scientific literature that suggests three basic factors: (1) beavers are monogamous breeders that live in colonies; (2) colonies are composed of a breeding pair of adults, their offspring, and occasionally, the offspring from the previous year; and (3) beavers typically breed during the winter and have their offspring in the spring. However, these assumptions are not always correct. NWRC scientists recently analyzed the composition of beaver colonies at 89 chronic damage sites in 7 southeastern States and found several deviations from this conventional knowledge.

In this study, colony size ranged from two to 18 beavers. Eleven colonies contained one male and one female only, and only five of these were breeding pairs. Colonies contained anywhere from one to 11 males, while the number of females among colonies ranged from zero to eight. The mean age of beavers across all states in which the study was conducted was three, with an overall age range of one to 20 years. Thirty percent of all beavers captured were in the 1-year age class. In addition, at least one breeding female was found in 78% of all colonies. Breeding females were also found in every age class older than one year, and more than one breeding female occurred in 17% of all colonies sampled. The youngest age of reproductively active females was two, and the oldest was 18 years of age. Lactating and/or pregnant females were captured in every month except September, October, and November, suggesting a flexible or extended breeding season in southeastern colonies.

These data show that the dynamics of beaver colonies are more complex than previously thought. Issues such as female site fidelity, the reproductive potential of beavers, generally larger colony sizes, and the tendencies of juveniles to disperse to less optimal habitats or remain in their natal colonies indicate that beaver colonies in the southeastern United States—if left unmanaged—could prove detrimental to resources being protected in high-density areas. For instance, managers working in areas of long-term damage may underestimate the number of beavers that need to be removed in order to protect an area's resources. In this regard, the results of the study have serious implications for wildlife managers when determining whether to use lethal or nonlethal approaches to control beaver populations.

Den Sharing Among Lactating Beavers—During a radio-tracking study of North American beavers (Castor canadensis) near Phoenix, AZ, APHIS Wildlife Services field operations specialists and NWRC researchers discovered three adult, lactating beavers using the same bank den at the same time (seven occasions during 68 days). Two adult females 5.2 kilometers downstream also used the same den at the same time (three occasions during 45 days). For North American beavers, the conventional family unit comprises a single adult breeding pair, young of the current year, and yearlings born the previous year. However, the social organization of beaver colonies in this study area appeared to consist of multiple reproductive females with an extended family unit.

To test the hypothesis that these communal females were closely related, researchers sampled and compared eight, autosomal DNA microsatellites. Except for a pair of females in the first den, all communal females who shared dens were unrelated to each other. The observed communal denning of adult, reproductively active females in the area may be due to the limited amount of available habitat (i.e., suitable sites for foraging and denning) in southwestern Phoenix. The scarcity of available dens and proximate high-quality food in the area could be delaying and, in some instances, eliminating the dispersal of sub-adults and adults. These results suggest that relocating beavers as part of an integrated management strategy will likely not harm the beaver population in this area.


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