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Providing evidence to improve practice

Action: Remove vegetation using herbicides Terrestrial Mammal Conservation

Key messages

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  • Six studies evaluated the effects on mammals of removing vegetation using herbicides. All six studies were in the USA.

COMMUNITY RESPONSE (0 STUDIES)

POPULATION RESPONSE (4 STUDIES)

  • Abundance (2 studies): Two controlled studies (one replicated) in the USA found that applying herbicide did not increase numbers of translocated Utah prairie dogs or alter mule deer densities in areas of tree clearance.
  • Survival (1 study): A replicated, site comparison study in the USA found that applying herbicide, along with mechanical disturbance and seeding, increased overwinter survival of mule deer fawns.
  • Condition (1 study): A replicated, controlled study in the USA found that applying herbicide did not reduce bot fly infestation rates of rodents and cottontail rabbits.

BEHAVIOUR (2 STUDIES)

  • Use (2 studies): Two replicated, controlled studies in the USA found that applying herbicide increased forest use by female, but not male, white-tailed deer and increased pasture use by cottontail rabbits in some, but not all, sampling seasons.

Supporting evidence from individual studies

1 

A controlled study in 1979–1981 at two grassland sites in a national park in Utah, USA (Player & Urness 1982) found that herbicide application did not increase establishment of translocated Utah prairie dogs Cynomys parvidens. In the first year of translocation, the average number of prairie dogs counted on the site sprayed with herbicide (1.7) was not significantly different to that on the unsprayed site (0.3). In the second and third year, no prairie dogs were counted on either site. One site was treated with the herbicide, 2,4-D, at a rate of 2.2 kg active ingredient/ha (date of treatment not given) and one site was not sprayed. Sites were 5 ha each. On each site, 200 artificial burrows were created. In early-summer 1979, two hundred prairie dogs were translocated and released across four sites (the sprayed and unsprayed sites and two further sites not detailed in this summary). Counts were conducted through summer and fall of 1979 and in summer 1980–1981.

2 

A replicated, controlled study in 1986–1988 of a woodland in Oklahoma, USA (Boggs et al. 1991, same experimental set-up as Lochmiller et al. 1991 and Leslie Jr. et al. 1996) found that applying herbicide did not reduce bot fly Cuterebra infestation rates of rodents and cottontail rabbits Sylvilagus floridanus. Prevalence of bot fly did not differ between plots treated with herbicide (present on 64 of 342 animals examined, 19%), or untreated plots (25 of 133 animals examined, 19%). Eight 32.4-ha plots were treated with the herbicides, tebuthiuron or triclopyr (at 2.2 kg/ha), in March or June 1983 and four plots were not sprayed with herbicide. Rodents were collected using snap traps in July–September and December–March during 1986–1988. Cottontail rabbits were collected by shooting in January and July of 1987–1988. Animals were examined for bot fly burden.

3 

A replicated, controlled study in 1986–1988 of forest and grassland at a site in Oklahoma, USA (Lochmiller et al. 1991, same experimental set-up as Boggs et al. 1991 and Leslie Jr. et al. 1996) found that herbicide-treated pastures hosted more cottontail rabbits Sylvilagus floridanus than did untreated pastures during some, but not all, sampling seasons. In three of 10 comparisons, cottontails were more abundant in herbicide-treated pastures than in untreated pastures (0.8–1.1 vs 0.1–0.2 rabbits/ha), in two cases they were less abundant on treated than untreated pastures (0.0 vs 1.9 rabbits/ha) and for the other five comparisons no difference was detected. Four 32.4-ha pastures were treated with the herbicides tebuthiuron or triclopyr at a rate of 2.2 kg/ha in March or June 1983 and two were untreated control pastures. Rabbit density was estimated by walking transects three times each July and February, from July 1986 to February 1988.

4 

A randomized, replicated, controlled study in 1988–1989 of an upland hardwood forest with tallgrass prairie in Oklahoma, USA (Leslie Jr. et al. 1996 same experimental set-up as Boggs et al. 1991 and Lochmiller et al. 1991) found that applying herbicide increased forest use by female, but not male, white-tailed deer Odocoileus virginianus. Female deer preferentially selected herbicide-treated plots over untreated plots in spring, summer and autumn, but there was no difference in winter. Males showed no preference between treated or untreated plots (see original paper for full results). Four blocks, each consisting of five 32-ha plots, were studied. In each block, the herbicides, tebuthiuron and triclopyr, were sprayed in 1983 in one plot each, as well as in two plots that were also burned each April, in 1985–1987. One plot was not burned or sprayed with herbicide. Two additional pastures that were burned but not sprayed along with adjacent areas that were not burned or sprayed were also monitored. Ten female and seven male deer were radio-tracked, in 1988–1989.

5 

A replicated, site comparison study in 2005–2008 of a pine-juniper forest in Colorado, USA (Bergman et al. 2014) found that herbicide application (combined with seeding and preceded by mechanical disturbance and initial seeding – referred to as advanced management) increased overwinter survival of mule deer Odocoileus hemionus fawns. Management actions were not carried out individually, so their relative effects cannot be determined. Average overwinter survival was highest under advanced management (77%), intermediate under mechanical disturbance and seeding without follow-up actions (69%) and lowest with no habitat management (67%). Mechanical management, commencing in 1998–2004, involved removing and mulching trees to create open areas. These were seeded with grasses and forbs. In advanced management plots, follow-up actions, two to four years later, involved controlling weeds with herbicide and further seeding with deer browse species. Fawns were radio-collared on eight study plots; two advanced management plots, four mechanical management plots and two unmanaged plots. Survival was assessed by monitoring fawns from capture (1 December to 1 January) until 15 June, in winters of 2004–2005 through to 2007–2008, three to six years after mechanical treatments.

6 

A replicated, site comparison study in 2006–2009 in six pine and juniper forest sites in Colorado, USA (Bergman et al. 2015) found that treatment with herbicide, alongside clearance of trees and sowing seed, did not alter mule deer Odocoileus hemionus densities compared to clearance of trees alone. The effects of herbicide and reseeding could not be separated in this study. In areas that were sprayed with herbicide, cleared, and sown with seeds, deer density was not higher (5–31 deer/km2) than in plots that were cleared but not treated with herbicide or sown with seed (6–37 deer/km2). Six sites were cleared of trees, two to eight years before deer surveys, using a bulldozer and by chopping vegetation into smaller pieces, or mulching individual trees to ground level by hydro-axing. On two of these sites, unpalatable grasses were controlled with herbicides and seeds of plant species eaten by mule deer were sown. The four remaining sites were not further managed after tree clearance. Deer numbers were estimated by sighting marked individuals during aerial surveys, in late winter each year of 2006–2009. Areas surveyed were 15–84 km2/site.

Referenced papers

Please cite as:

Littlewood, N.A., Rocha, R., Smith, R.K., Martin, P.A., Lockhart, S.L., Schoonover, R.F., Wilman, E., Bladon, A.J., Sainsbury, K.A., Pimm S. and Sutherland, W.J. (2020) Terrestrial Mammal Conservation: Global Evidence for the Effects of Interventions for terrestrial mammals excluding bats and primates. Synopses of Conservation Evidence Series. University of Cambridge, Cambridge, UK.