Zalba and Cozzani (2004) found intense grazing by feral horses resulted in increased predation on bird eggs in grasslands, leading to reduced avian richness and diversity. However, avian richness and diversity were higher in areas subject to moderate levels of grazing than areas in which horses had been excluded (Zalba and Cozzani 2004).
Species richness and diversity are slightly greater in areas of moderate grazing than in enclosures – intermediate disturbance hypothesis (Connell, 1978) (Willig et. al. 2018).
Grazing animals can have beneficial impacts on native plants and animals (Schultz, 2011) and protect endangered plants (Gilfedder & Kirkpatrick 1994).
Williams in the publication Biodiversity and Environmental Change talks of evidence that vegetation change is occurring after 13 years of exclosure from horse grazing. The most significant effect of exclosure at both sites was:
- “Increase in the height of the vegetation, with a reduction in small-scale species richness also emerging”, and
- Increases in the coverage of the dominant grasses and sedges have been noted within exclosures, partially at the expense of low-growing perennial herbs.
Lowers fuel levels thus fire severity (Silvers 1993 and Davies et al.2015)
Other publications citing positive effects of grazing on species diversity (Fahnestock and Detling (1999), Austrheim and Eriksson (2001), Fahnestock and Detling (2002), Ostermann-Kelm et al. (2009) and Stroh, et al. (2012)
Connell, Joseph H., 1978, Diversity in Tropical Rain Forests and coral Reefs, Science, New Series, Vol. 199, No. 4335 (Mar. 24, 1978), pp. 1302-1310 (9 pages), American Association for the Advancement of Science
Davies, KW, Boyd, CS, Bates, JD and Hulet, A 2015, ‘Dormant season grazing may decrease wildfire probability by increasing fuel moisture and reducing fuel amount and continuity’, International Journal of Wildland Fire 24: 849–856.
Fahnestock, J.T., Detling, J.K. The influence of herbivory on plant cover and species composition in the Pryor Mountain Wild Horse Range, USA. Plant Ecology 144, 145–157 (1999). https://doi.org/10.1023/A:1009899509067
Fahnestock, Jace T., and Detling, James, 2002, Bison Prairie Dog Plant Interactions in North American Mixed-grass Prairie, June 2002, Oecologia 132(1):86-95
Gilfedder, L., and Kirkpatrick, J.B., 1994, Climate, Grazing and Disturbance, and the Population Dynamics of Leucochrysum albicans at Ross, Tasmania, Australian Journal of Botany 42(4) 417 – 430
Ostermann-Kelm, Stacey D. et al., 2009, Impacts of feral horses on a desert environment, BMC Ecology 2009, 9:22, This article is available from: http://www.biomedcentral.com/1472-6785/9/22
Schultz, Nick L., et al., 2011, Effects of grazing exclusion on plant species richnessand phytomass accumulation vary across a regionalproductivity gradient, Journal of Vegetation Science22(2011) 130–142
Silvers, L 1993, ‘The effects of grazing on fuel loads and vegetation in the Barmah Forest’, Honours thesis, School of Environmental and Information Sciences, Charles Sturt University, Albury, NSW.
Stroh, Peter A., et al., 2012, The potential for endozoochorous dispersal of temperate fen plant species by free-roaming horses, Applied Vegetation Science Vol. 15, No. 3 (August 2012), pp. 359-368
Williams, Richard, et al., 2014, Alpine ecosystems, in Biodiversity and Environmental Change (CSIRO Publishing, 2014)
Willig, M.R. , et al., 2018, Latitudinal Gradients of Biodiversity: Pattern, Process, Scale, and Synthesis, Annual Review of Ecology, Evolution, and Systematics Vol. 34:273-309 (Volume Publication date November 2003) First published online as a Review in Advance on July 11, 2003
Zalba, Sergio M., and Cozzani, Natalia C., 2004, The impact of feral horses on grassland bird communities in Argentina, Animal Conservation (2004)7, 35–44, The Zoological Society of London.