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How are large and small mammal populations affected by vegetation management and glyphosate use?

Category: Environment and Wildlife

Numerous independent scientific and regulatory reviews consistently conclude that glyphosate-based herbicides, when used in accordance with product labels, do not pose a significant risk of direct toxicity to wildlife, including moose, deer and small mammals. Therefore, direct toxicity is considered an exceedingly low risk. Indirect effects through reduction of deciduous woody brush species are known to at least temporarily limit the utilization of herbicide treated sites by moose. However, given that annually only 1/3rd of forest cutover areas are treated with glyphosate-based herbicides, small and large mammals will typically adapt to this temporary change on a small portion of the landscape by moving to, or feeding in, alternative areas including the 2/3rds of cutover areas annually that are regenerated without herbicide treatment.


Risk assessments and independent scientific reviews consistently conclude that the use of glyphosate-based herbicides in accordance with product labels do not pose a significant risk of direct toxicity to wildlife (PMRA 2015, Thompson 2011; Durkin 2003, Tatum et al. 2004, Guynn 2004, Sullivan and Sullivan 2003, Solomon & Thompson 2003, Giesy et al. 2000, Couture et al 1995). These strongly congruent conclusions are further supported by the fact that after more than 30 years of registered use in forest vegetation management, there has never been a scientifically documented case of direct mortality of moose, deer or other animals attributed to glyphosate-based herbicide exposure.

Indirect effects, through herbicide induced change to the plant community on treated sites do occur and have been scientifically studied and documented, including with respect to altered habitat utilization by moose. The most important change induced by glyphosate-based herbicide treatments is reduction in relative amounts of woody deciduous brush (e.g. maple, aspen, birch, pin cherry and alder species) that moose favour as browse. Reduced amounts of this food source will typically result in a transient pattern of lower moose abundance on treated sites. For the period in which their preferred woody browse material remains suppressed, typically for several years after treatment (Sullivan and Sullivan 2003; Lautenschlager et al. 1999; Raymond et al. 1996; Escholz et al. 1996), moose will tend lower use of these sites in favour of others that contain higher amounts of browse at their preferred browsing height. In that context, it should be recognized that only ~ 1/3rd of the forest area harvested in each year is treated with glyphosate-based herbicides, while the majority (2/3rds) is left to regenerate without chemical treatment. Most of these untreated cutover areas will contain copious amounts of woody browse material. Additionally, moose utilize a wide-variety of plant foods for sustenance and particularly aquatic vegetation through much of the summer. Aquatic systems are not intentionally sprayed with glyphosate-based herbicides and larger permanent wetlands, ponds and lakes are protected by no spray-buffers, ensuring no significant changes to the aquatic plant food base that are utilized heavily by moose during summer. Finally, it should be noted that plant communities on recently harvested forest cutovers are highly dynamic, growing and changing rapidly through the first few years of successional development. Most animals which naturally use these early successional habitats are well adapted to plant community dynamics and to seeking out feeding sites that meet their nutritional requirements.

Connor, J.F., and McMillan, L.M. 1990. Winter utilization by moose of glyphosate-treated cutovers. Alces, 26: 91–103.

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Crawford, H.S., R.A. Lautenschlager, M.R.Stokes and T.L.Stone. 1993. Effects of forest disturbance and soil depth on digestible energy for moose and white-tailed deer. Res. Pap. NE-682. USDA For. Serv., Northeastern For. Exp. Sta, Radnor, PA. (as cited in Lautenschlager and Sullivan 2002).

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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.