Definition, identification, and impact: Getting to grips with overgrazing: A literature review

Monday 17th February 2025

We often hear commentators throwing around the term 'overgrazing' - yet what does this term actually mean, and how might overgrazing - which results in many negative biodiversity and broader environmental impacts - best be tackled?

ABSTRACT

Key findings include:
• Overgrazing is ill-defined so one of the key challenges is to provide an accurate definition
• Many methodologies and techniques are utilised to both identify overgrazing, and measure the impacts of overgrazing – however, these often lack breadth – partly because of the complexity of the many dynamic and variable aspects that relate to overgrazing, which restricts the scope of research
• There may be an opportunity to develop agritech that would empower land managers by communicating overgrazing-related data to graziers and land managers in real time.

INTRODUCTION

One challenge is the issue of definition, the complexity of which is highlighted by Fielding and Haworth (1999) who assert that overgrazing does not mean the same thing to ecologists and farmers, noting how ecologists may see overgrazing as something that moves vegetation away from its natural state, whereas agriculturalists often assess overgrazing by focusing on productivity and profitability.

Despite the broad and significant environmental impacts of overgrazing, which is one of the key ecological problems confronting graziers and land managers internationally, many studies tend to focus on a limited number of overgrazing indicators and impacts, in very specific geographical areas; this reality is noted by Eldridge and Delgado-Baquerizo (2017) who overcome the issue by synthetizing relatively focused research to draw conclusions relating to ecosystem impacts on a larger geographical scale.

In this literature review I examine some of the key literature to understand what overgrazing is, where the research sits, and where there may be gaps that need filling.

DEFINING OVERGRAZING – complexity unleashed…

To set the scene… grasslands and savanna biomes cover around 40% of the terrestrial surface, of earth as referenced by Banerjee et al. (2023), and are dominated by grasses (Poaceae) together with grass-like and herbaceous plants.

More than 25% of the earth’s surface – over 33 million square kilometres – is managed grazing according to Asner et al. (2004), whose paper defines managed grassland as “as any geographically extensive operation designed for the production of animals for consumption, including for meat, milk, and any major animal products.”

As with the term overgrazing, there are challenges associated with the definition of grassland, due to inexact and varying definitions, as overviewed by Dixon et al. (2014) – also, it should be noted that grazing, and hence overgrazing, can take place in biomes that are not classified as grassland and savanna.

Mysterud (2006) emphasises how the definition of overgrazing is determined by the management and/or conservation objectives – on that basis, grassland farmers, forestry managers, wildlife managers and nature conservationists should apply varying definitions that focus on and align with their end goals.

Carrying capacity is often used to define the point above which overgrazing occurs and MacNab (1985) provides a traditional agricultural-sector definition to the carrying capacity of land: “For range management, the density of cattle providing maximum sustained production of beef is the carrying capacity of the land” before making a similar point to Mysterud (2006) above by highlighting how carrying capacity will be determined by the end goal, so – for example – the optimal carrying capacity for maximising agricultural productivity on land will likely differ from the optimal carrying capacity for biodiversity maximisation.

Wilson and Macleod (1991) argue that defining overgrazing is problematic because of its broad range of impacts on aspects such as botanical composition, forage cover, erosion, livestock production and wildlife habitat, which underlines the need for overgrazing definitions to be specifically aligned with management goals, while encompassing the breadth of such objectives.

Wilson and Macleod (1991) apply a relatively narrow definition of overgrazing in their paper: “Overgrazing ls defined as occurring where there is a concomitant vegetation change and loss of animal productivity arising from the grazing of land by herbivores” – indeed, much of the literature on overgrazing is the practice of grazing livestock in a way that exceeds the carrying/productive capacity of the land – for example, see the definitions used in the papers by Niu et al. (2019) and Fragnière et al. (2022). Yet these definitions may in themselves be too narrow because, for example, the management objective may not relate to livestock/agricultural productivity, and livestock many not even be the key grazing animals.

In conclusion, the concept of overgrazing could be better understood if a suitably broad and all-encompassing definition could be universally agreed upon and applied – alongside sub-definitions that could be applied to accurately define the term in a way that aligns with specific management objectives. Much of the literature and research that relates to overgrazing fails to explore in detail what overgrazing is, in terms of general and in-context definitions, so there is certainly an opportunity to explore definitions further, and support the application of accurate and relevant definitions that better communicate overgrazing.

IDENTIFYING OVERGRAZING – a varied picture…

Sales-Baptista et al. (2016) identifies many of the challenges associated with identifying and preventing overgrazing in the Montado in Spain, such as the complexity arising from spatial and temporal variations; this paper proceeds to make the point that overgrazing is to do with more than just animal numbers, and that grazing pressure is a better early indicator of overgrazing risk than stocking rate, because it captures more of the complexities that need to be grappled with.

This paper overviews many of the complex interactions that need to be assessed when identifying and preventing overgrazing, ranging from tree cover and feed supplements to grazing management and wildlife grazing competition:

Sales-Baptista et al. (2016) also argues that knowledge is needed at a paddock scale rather than whole farm or regional scale, and that technologies such as a WSNs (wireless sensor networks) and GPS could be used to monitor paddock pressures by monitoring long term pasture and animal stocking rates, and variations.

Meanwhile, Hilker et al. (2014) studied an area of 2 million square kilometres, using daily MODIS data from the Aqa spacecraft (satellite data) to identify changes, including overgrazing issues, in the Mongolian Steppe. And a paper by Hao et al. (2018) outlined the use of satellite lead area (LAI) data to map grazing patterns and temporal dynamics to better understand the impacts of grazing, and potential overgrazing, on the upper reach of the Heihe River in China.

And Harmse et al. (2022) outlines how imagery from the Sentinel-2 satellite, constellation combined with GPS location data, can be used to monitor semi-arid rangelands – and identify and quantify overgrazing; in contrast, Liu and Lu (2021) apply a very different approach to assess grassland degradation in the Tibetan Plateau, applying methodologies and calculations to pre-existing mapping work.

Meanwhile Wang et al. (2020) conclude that metabolites in plant roots change in response to overgrazing – a finding that could potentially be utilised to identify overgrazing; furthermore, Huhe et al. (2017) confirm that soil bacteria/fungi are responsive to, and sensitive to, grazing activity.

Primary and secondary data sources were identified and synthesized via a specific methodology, overviewed by Gebeyehu et al. (2021) in their attempt to identify overgrazing hotspots in Ethiopia’s Lower Omo Valley – ranging from questionnaires involving graziers to satellite data.

Meanwhile, in an unusual approach Kakonge (2012) applies chaos theory to both understand and address environmental degradation – including overgrazing, in Lesotho, Africa, arguing that chaos theory helps us both understand and respond to highly complex, dynamic and variable environmental challenges, such as overgrazing.

Omuto et al. (2014) have developed a framework for assessing land degradation in Somalia, utilising a mix of remote sensing and expert opinion to create the framework for both national and more localised assessments that encompass land degradation from overgrazing.

While Sartorello et al. (2020) has carried out a systemic review and meta-analysis to provide an overview and insights into the impact of pastoral activities on biodiversity – encompassing aspects relating to overgrazing; concluding that Arthropoda (particularly insects) can be monitored to detect overgrazing.

The most extreme cases of overgrazing can result in soil erosion and the loss of plant nutrients, leading to a long-term decline in productivity, as outlined by van de Koppel et al. (1997) who describe how what they term ‘catastrophic vegetation shifts’ occur.

In conclusion, there are a broad range of different approaches and methodologies used to identify overgrazing – from the traditional to the hi-tech – however, they often lack breadth in terms of the aspects and impacts covered, because of the complexities involved in examining overgrazing; also, I question how actionable much of this work is by graziers and other land managers. It would be good to see more of a research focus on actionable early-warning indicators of overgrazing that can be responded to in good time by graziers and land managers.

Finally, it is clear from the literature that I have examined that that economics is a key driver- a paper by Fang and Wu (2022), focused on the overgrazing of the grasslands of inner Mongolia, discovering that herders attributed land degradation to a variety of causes, including economic pressures.

Boles et al. (2021) also highlights the role that economic pressure plays as a driver of overgrazing in Kenya, and argues that adaptive mobility is key to protect both the rangeland ecosystems and the livelihood of the pastoralists themselves. While Bonn (2010) identifies rural subsidies and payments as key drivers of overgrazing in Britain’s uplands.

So, beyond the identification of overgrazing in the practical terms… if we are to prevent overgrazing, it is clearly essential to understand the bigger picture, including key contributory factors and drivers that occur beyond the specific biomes we are studying and working with.

THE BROAD IMPACTS OF OVERGRAZING – a quick overview…

Breidenbach et al. (2022) studies the Tibetan Plateau’s Kobresia pastures, noting that overgrazing and climate change make the topsoils vulnerable to irreversible degradation; the paper identifies the critical threshold of grazing intensity, on a microbiological basis, above which pasture degradation and the associated negative environmental impacts, become irreversible.

Kemp et al. (2013) refers to problems on 90% of China’s grasslands, such as decreased ground cover and increased erosion – in part driven by overgrazing.
And Dlamini et al. (2016) asserts that grasslands store around 10% of the world’s SOC (soil organic carbon), while the degradation of grassland soils reduces SOC, illustrating how the SOC pool is strongly determined by grassland management, with strong implications for climate change; meanwhile Dlamini et al. (2014) examines the way in which overgrazing depletes soil nitrogen.

And a paper by Donovan and Monaghan (2021) highlight that it is globally recognised that overgrazing is a key contributor to accelerated soil degradation while Fragnière et al. (2022) points out that livestock farming is a key driver of worldwide biodiversity loss.

While Sartorello et al. (2020) examined the impact of pastoral activities on biodiversity in Europe and uncovered a generalised negative impact from overgrazing across all the habitats and geographical areas examined, other than mountain shrubland.

Whereas Wang and Tang (2019) found that “the key drivers for the response of diversity to grazing intensity were varied among taxa, which indicated that comprehensive factors, including multi-taxa diversity and multi-functionality, should be considered when applying grazing management in grasslands.” – underlining the complexity in terms of the response of biodiversity to grazing intensities.

Finally, it is important to point out that grazing and hence overgrazing is not limited to pastureland – there is plenty of research that examines the impact of grazing on non-grass biomes, such as the paper by Li and Jiang (2021) that examines the impact livestock can have on forests.

In conclusion, overgrazing is a huge environmental challenge as overviewed by the papers above, yet the interactions between grazing and the environment are highlight complex and variable.

CONCLUSIONS

Having reviewed some of the key aspects in this literature review, which covers the definition of overgrazing alongside the identification of and impacts of overgrazing, I would advocate additional research to guide graziers and land managers on how to define, identify and mitigate overgrazing.

Regarding the identification of overgrazing, a broad range of techniques are advocated and utilised by researchers, both in-the-field and in terms of desk research – varying from satellite data through to field work, yet – despite the variety of approaches – I could find very few options that could be utilised in the field in real time, by graziers and land managers.

So I advocate a three pronged approach to fill the literature and research gaps surrounding overgrazing, empowering graziers and land managers seeking to get to grips with, protect against and reverse overgrazing:
• The development of a broad and all-encompassing definition of overgrazing, supported by sub-definitions specific to land management plans
• A synthesis of early warning indicators into a single database to support graziers and land managers who need help identifying the early warning signs of overgrazing
• Research into the development of a practical on-farm/on-land agritech solution that identifies overgrazing in its complexity, and in context – providing real-time feedback and guidance to land-managers and graziers – perhaps even utilising AI; this research project could draw on the world of precision livestock farming tech, such as that detailed by Aquilani et al. (2022).

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APPENDIX

Asner, G. P., Elmore, A. J., Olander, L. P., Martin, R. E., and Harris, A. T. (2004). Grazing systems, ecosystem responses, and global change. Annu. Rev. Environ. Resour. 29, 261–299. doi: https://www.annualreviews.org/doi/abs/10.1146/annurev.energy.29.062403.102142

Banerjee, Das, D., Zhang, H., & John, R. (2023). Grassland-woodland transitions over decadal timescales in the Terai-Duar savanna and grasslands of the Indian subcontinent. Forest Ecology and Management, 530, 120764–. https://doi.org/10.1016/j.foreco.2022.120764

Boles, Shoemaker, A., Mustaphi, C. J. C., Petek, N., Ekblom, A., & Lane, P. J. (2019). Historical Ecologies of Pastoralist Overgrazing in Kenya: Long-Term Perspectives on Cause and Effect. Human Ecology : an Interdisciplinary Journal, 47(3), 419–434. https://doi.org/10.1007/s10745-019-0072-9
Bonn. (2010). Drivers of environmental change in uplands. Routledge.
https://doi.org/10.1007/s10980-010-9471-4

Breidenbach, A., Schleuss, P. M., Liu, S., Schneider, D., Dippold, M. A., de la Haye, T., Miehe, G., Heitkamp, F., Seeber, E., Mason-Jones, K., Xu, X., Huanming, Y., Xu, J., Dorji, T., Gube, M., Norf, H., Meier, J., Guggenberger, G., Kuzyakov, Y., & Spielvogel, S. (2022). Microbial functional changes mark irreversible course of Tibetan grassland degradation. Nature communications, 13(1), 2681. https://doi.org/10.1038/s41467-022-30047-7

Díaz-Pereira, Romero-Díaz, A., & de Vente, J. (2020). Sustainable grazing land management to protect ecosystem services. Mitigation and Adaptation Strategies for Global Change, 25(8), 1461–1479. https://doi.org/10.1007/s11027-020-09931-4

Dlamini, Chivenge, P., & Chaplot, V. (2016). Overgrazing decreases soil organic carbon stocks the most under dry climates and low soil pH: A meta-analysis shows. Agriculture, Ecosystems & Environment, 221, 258–269. https://doi.org/10.1016/j.agee.2016.01.026

Dlamini, Chivenge, P., Manson, A., & Chaplot, V. (2014). Land degradation impact on soil organic carbon and nitrogen stocks of sub-tropical humid grasslands in South Africa. Geoderma, 235-236, 372–381. https://doi.org/10.1016/j.geoderma.2014.07.016

Donovan, & Monaghan, R. (2021). Impacts of grazing on ground cover, soil physical properties and soil loss via surface erosion: A novel geospatial modelling approach. Journal of Environmental Management, 287, 112206–. https://doi.org/10.1016/j.jenvman.2021.112206

Dixon, Faber‐Langendoen, D., Josse, C., Morrison, J., Loucks, C. J., & Ebach, M. (2014). Distribution mapping of world grassland types. Journal of Biogeography, 41(11), 2003–2019. https://doi.org/10.1111/jbi.12381

Eldridge, & Delgado‐Baquerizo, M. (2017). Continental‐scale Impacts of Livestock Grazing on Ecosystem Supporting and Regulating Services. Land Degradation & Development, 28(4), 1473–1481. https://doi.org/10.1002/ldr.2668

Fielding, & Haworth, P. F. (1999). Upland habitats. Routledge. https://doi.org/10.4324/9780203061152

Fragnière, Gremaud, J., Pesenti, E., Bétrisey, S., Petitpierre, B., Guisan, A., & Kozlowski, G. (2022). Mapping habitats sensitive to overgrazing in the Swiss Northern Alps using habitat suitability modeling. Biological Conservation, 274, 109742–. https://doi.org/10.1016/j.biocon.2022.109742

Gebeyehu, Ben G. J. S. Sonneveld, & Denyse Snelder. (2021). Identifying Hotspots of Overgrazing in Pastoral Areas: Livestock Mobility and Fodder Supply–Demand Balances in Nyangatom, Lower Omo Valley, Ethiopia. Sustainability (Basel, Switzerland), 13(3260), 3260–. https://doi.org/10.3390/su13063260

Hao, Pan, C., Fang, D., Zhang, X., Zhou, D., Liu, P., Liu, Y., & Sun, G. (2018). Quantifying the effects of overgrazing on mountainous watershed vegetation dynamics under a changing climate. The Science of the Total Environment, 639, 1408–1420. https://doi.org/10.1016/j.scitotenv.2018.05.224

Harmse, Hannes Gerber, & Adriaan van Niekerk. (2022). Evaluating Several Vegetation Indices Derived from Sentinel-2 Imagery for Quantifying Localized Overgrazing in a Semi-Arid Region of South Africa. Remote Sensing (Basel, Switzerland), 14(1720), 1720–. https://doi.org/10.3390/rs14071720

Hilker, Natsagdorj, E., Waring, R. H., Lyapustin, A., & Wang, Y. (2014). Satellite observed widespread decline in Mongolian grasslands largely due to overgrazing. Global Change Biology, 20(2), 418–428. https://doi.org/10.1111/gcb.12365

Huhe, Huhe, Xianjiang Chen, Fujiang Hou, Yanpei Wu, & Yunxiang Cheng. (2017). Bacterial and Fungal Community Structures in Loess Plateau Grasslands with Different Grazing Intensities. Frontiers in Microbiology, 8. https://doi.org/10.3389/fmicb.2017.00606

Kemp, Guodong, H., Xiangyang, H., Michalk, D. L., Fujiang, H., Jianping, W., & Yingjun, Z. (2013). Innovative grassland management systems for environmental and livelihood benefits. Proceedings of the National Academy of Sciences – PNAS, 110(21), 8369–8374. https://doi.org/10.1073/pnas.1208063110

Kakonge. (2002). Application of chaos theory to solving the problems of social and environmental decline in Lesotho. Journal of Environmental Management, 65(1), 63–78. https://doi.org/10.1006/jema.2001.0519

van de Koppel, Rietkerk, M., & Weissing, F. J. (1997). Catastrophic vegetation shifts and soil degradation in terrestrial grazing systems. Trends in Ecology & Evolution, 12(9), 352–356. https://doi.org/10.1016/S0169-5347(97)01133-6

Li, & Jiang, B. (2021). Responses of forest structure, functions, and biodiversity to livestock disturbances: A global meta‐analysis. Global Change Biology, 27(19), 4745–4757. https://doi.org/10.1111/gcb.15781

Liu, & Lu, C. (2021). Quantifying Grass Coverage Trends to Identify the Hot Plots of Grassland Degradation in the Tibetan Plateau during 2000-2019. International Journal of Environmental Research and Public Health, 18(2), 416–. https://doi.org/10.3390/ijerph18020416

MacNab. (1985). Carrying Capacity and Related Slippery Shibboleths. Wildlife Society Bulletin, 13(4), 403–410. https://www.jstor.org/stable/3782664

Mysterud (2006). The concept of overgrazing and its role in management of large herbivores. https://doi.org/10.2981/0909-6396(2006)12[129:TCOOAI]2.0.CO;2

Niu, Zhu, H., Yang, S., Ma, S., Zhou, J., Chu, B., Hua, R., & Hua, L. (2019). Overgrazing leads to soil cracking that later triggers the severe degradation of alpine meadows on the Tibetan Plateau. Land Degradation & Development, 30(10), 1243–1257. https://doi.org/10.1002/ldr.3312

Omuto, Balint, Z., & Alim, M. S. (2014). framework for national assessment of land degradation in the drylands: a case study of somalia. Land Degradation & Development, 25(2), 105–119. https://doi.org/10.1002/ldr.1151

Sales-Baptista, d’Abreu, M. C., & Ferraz-de-Oliveira, M. I. (2016). Overgrazing in the Montado? The need for monitoring grazing pressure at paddock scale. Agroforestry Systems, 90(1), 57–68. https://doi.org/10.1007/s10457-014-9785-3

Sartorello, Pastorino, A., Bogliani, G., Ghidotti, S., Viterbi, R., & Cerrato, C. (2020). The impact of pastoral activities on animal biodiversity in Europe: A systematic review and meta-analysis. Journal for Nature Conservation, 56, 125863–. https://doi.org/10.1016/j.jnc.2020.125863

Wang, Delgado‐Baquerizo, M., Zhao, X., Zhang, M., Song, Y., Cai, J., Chang, Q., Li, Z., Chen, Y., Liu, J., Zhu, H., Wang, D., Han, G., Liang, C., Wang, C., Xin, X., & Veen, C. (2020). Livestock overgrazing disrupts the positive associations between soil biodiversity and nitrogen availability. Functional Ecology, 34(8), 1713–1720. https://doi.org/10.1111/1365-2435.13575

Wang, & Tang, Y. (2019). A global meta-analyses of the response of multi-taxa diversity to grazing intensity in grasslands. Environmental Research Letters, 14(11). https://doi.org/10.1088/1748-9326/ab4932

Wilson, & Macleod, N. (1991). Overgrazing: present or absent? Journal of Range Management, 44(5), 475–482. https://doi.org/10.2307/4002748

Xuening Fang, & Jianguo Wu. (2022). Causes of overgrazing in Inner Mongolian grasslands: Searching for deep leverage points of intervention. Ecology and Society, 27(1), 8–. https://doi.org/10.5751/ES-12878-270108

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