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What are the effects of glyphosate-based herbicides on wildlife species such as small mammals or birds?

Category: Environment and Wildlife

Studies show that the use of glyphosate products in accordance with product labels, does not pose a significant direct toxicological risk to small mammals or birds. Field studies have shown that the abundance of these animals and birds may decrease in treated areas depending upon their food and habitat preferences. However, these are indirect effects resulting from changes in the plant community that also occur in response to natural plant dynamics or which would occur in response to any effective vegetation management treatment.

The application of glyphosate-based herbicides in forest vegetation management is not considered to pose a significant risk of direct toxicity to small mammals or birds. Indirect effects resulting from alteration of vegetative habitat or food availability do occur, however these are transient effects and depend on individual species preferences. In general terms, those species preferring more open habitats are temporarily favoured over those preferring brushy deciduous cover. Overall, studies indicate that richness and diversity of songbirds or small mammals resulting from glyphosate induced habitat alterations remain within the range of natural variations. It is important to note that only a small portion of the area harvested by clear cutting is treated in any year (national long term average of 19%, based on the National Forestry Database) and that forest management plans are specifically designed to allow for a dynamic mosaic of stands across the landscape through time. Both of these factors mitigate the potential for widespread population level effects on these wildlife groups.

Numerous scientific and regulatory reviews have examined the potential direct effects of glyphosate on a wide variety of wildlife species including birds and small mammals Such reviews consistently conclude that the use of glyphosate products in accordance with product labels do not pose a significant risk of either direct acute or chronic toxicity to terrestrial wildlife species (PMRA 2015; USEPA 1993; Durkin 2003; Giesy et al. 2000; Tatum 2004; Couture et al. 1995; Thompson 2011). The detailed risk assessment conducted by Durkin 2003, calculated the risk to small mammals and birds based on the relationship between estimated exposure (e.g. via direct overspray or through consumption of contaminated vegetation, water, insects or fish) following application of glyphosate –based herbicides at a rate equivalent to 2.24 Kg a.e./ha, as compared to no observable effect levels in laboratory animals, considering both acute and chronic (longer term exposures). In summary the author stated that congruent with the USEPA 1993 assessment “none of the hazard quotients for acute or chronic scenarios reach a level of concern even at the upper ranges of exposure”.

While direct toxic effects on these wildlife groups are thus considered not to be of concern, potential indirect effects through habitat change must also be considered. There have been numerous studies examining this aspect and as noted by Guynn et al. 2004, wildlife response to herbicide-induced habitat alteration is highly variable ranging from studies that demonstrate no effect, short-term negative effects and for some species or communities positive effects. Species responses are reflective of their individual habitat preferences. In general terms, those species preferring more open habitats are temporarily favoured over those preferring brushy deciduous cover. A review by Sullivan and Sullivan (2003) involving 60 different published studies, noted that there were either no or very little significant reductions in richness and diversity of songbirds or small mammals resulting from glyphosate induced habitat alterations. Lautenschlager (1993), summarizing 14 studies conducted in northern conifer forests concluded that total songbird populations are seldom reduced during the growing season after treatment and also made the interesting observation that only studies that use kill or removal trapping show density reductions in small mammals. Gagne et al. (1999) reported no significant effects on species richness of small mammals following herbicide treatment in balsam fir forests of Quebec, but observed a reduction in red-backed vole abundance for two-years following a glyphosate-based herbicide treatment. The negative effect was associated with reduced cover and the authors concluded that in the short term, herbicide-treated plantations constitute poorer red-backed vole habitats than brush saw plantations, noting that this difference was likely due to the rapid recovery of vegetation by resprouting in the brush saw treatments and possibly also by residues of the cutting operation providing cover. Woodcock et al. (1997) assessed the effects on songbird densities as determined by territory mapping, mist netting, and banding and observed 20-38 species breeding within various treatment blocks. First year post-treatment assessments revealed that mean densities of the 11 most common species increased by 0.35/ha on the control plots. In contrast, densities on treated plots decreased by 1.1/ha (brush saw), 1.6/ha (Silvana Selective), 0.14/ha (Release) and 0.72/ha (Vision). A point of emphasis here is that essentially any effective vegetation management technique will alter available habitat to some degree. In at least this one study, songbird densities were relatively less impacted by herbicide treatments as compared to mechanical treatments. Response to habitat changes will vary with species, favouring certain species while resulting in out-migration of other species at least for some period of time depending upon their individual preferences and the temporal dynamics of the vegetation change post-treatment. An example of differential species response is provided by the study of MacKinnon and Freedman (1993) who also demonstrated that natural dynamic in avifaunal response to vegetation dynamics under natural successional pathways on forest clearcuts.

In April 2015, the PMRA released their latest review of glyphosate and declared that the weight of evidence indicates that glyphosate does not present unacceptable risk to human health. The full PMRA glyphosate review can be found here or please visit here for a summary version of the full PMRA review.

PMRA (Pest Management Regulatory Agency). Proposed re-evaluation decision – Glyphosate. PRCD2015-01. 13 April 2015. Available electronically at:

United States Environmental Protection Agency. Re-registration Eligibility Decision (RED): Glyphosate. In: Agency EP, editor. Washington, D.C.: Office of Prevention, Pesticides and Toxic Substances (7508W) 1993. p. 291.

Durkin PR. Glyphosate - Human health and ecological risk assessment report. Syracuse Environmental Research Associates Inc, Fayetteville NY 2003.

Giesy JP, Dobson S, Solomon KR. Ecotoxicological risk assessment for Roundup® herbicide. Rev. Environ. Contam.Toxicol. 2000; (167):35-120.

Tatum V. The toxicity of silvicutural herbicides to wildlife - volume two: Glyphosate and Imazapyr. NCASI Technical Bulletin. 2004(886):81.

Couture G, Legris J, Langevin R, Laberge L. Evaluation of the impacts of glyphosate as used in forests (English abstract, French text). Ministere des Ressources naturelles, Direction de l'environnement forestier, Publ No RN95-3082. 1995:187.

Thompson DG. Ecological impacts of major forest use pesticides. Chapter 5, In: Sanchez-Bayo, F., P. van den Brink and R.M. Mann (Eds.). Ecological impacts of toxic chemicals. Bentham Science Publishers Ltd. 2011; pp 88-110.

Guynn DC, Guynn ST, Wigley TB, Miller DA. Herbicides and forest biodiversity - what do we know and where do we go from here? Wildlife Society Bulletin. 2004; 32(4):1085-92.

Sullivan TP, Sullivan DS. Vegetation management and ecosystem disturbance: impact of glyphosate herbicide on plant and animal diversity in terrestrial systems. Environmental Review. 2003; 11:37-59.

Lautenschlager, RA.. Response of wildlife to forest herbicide applications in northern coniferous ecosystems. Can. J. For. Res. 1993a, 23: 2286–2299.

Gagné N, Belanger L, Hout J. Comparative response of small mammals, vegetation and food sources to natural regeneration and conifer release in boreal balsam fir stands of Quebec. Can. J. For. Res. 1999; 29:1128-1140.

Woodcock J, Lautenschlager RA, Bell FW, Ryder JP. Indirect effects of conifer release alternatives on songbird populations in northwestern Ontario. For. Chron. 1997; 73(1):107-12.

MacKinnon, DS, Freedman, B. Effects of silvicultural use of the herbicide glyphosate on breeding birds of regenerating clearcuts in Nova Scotia, Canada. Journal of Applied Ecology. 1993, 30: 395-406.

National Forestry Database (NFD). Pest control products use and silviculture sections; accessed summer 2015. Available electronically at: