Grazing-induced trampling not only reduces vegetation cover (Pathway 3) but also disrupts the connectivity of surface soil macropores (Marquart and others 2019), reducing infiltration (Pathway 9 via 1), or compacting the soil layers, exposing the surface to raindrop impact, and leading to reduced surface stability and greater water erosion (Gifford and Hawking 1978 Pathway 7 via 1). Indirectly, this reduces the capacity of soils to conduct water and can increase the likelihood that surface moisture is evaporated from the uppermost soil layers (for example, Thurow 1991 Pathway 6 via 2). For example, herbivory can positively and negatively alter plant community composition, favouring short-statured annual species with fibrous root systems over taller perennials with deep-rooted systems (Clay and others 2005), and enhanced the richness of both native and exotic grasses and forbs (Stahlheber and D’Antonio 2013 Sasha and others 2017 Figure 1 Pathway 2). Livestock grazing directly and indirectly impacts hydrological processes by direct removal of plants through herbivory and effects on soils by trampling (Figure 1). One of the most important impacts of overgrazing is its effect on soil hydrological processes the movement of water through the soil profile (infiltration) or across the surface (runoff), its capture and storage in the soil (soil moisture), and the entrainment and transport of eroded sediment in runoff (Thurow 1991). Predicted drier climates and increasing demand for water to support an ever-growing population and demand for agricultural products will place increasing pressure on dwindling water supplies and ecosystem stability in water-limited regions such as drylands, where grazing of livestock is the predominant land-use (Steinfeld and others 2006 Huang and others 2016). Yet despite its importance, poorly managed livestock activity is often blamed for contributing to widespread land and water degradation (for example, desertification, water pollution), habitat destruction (for example, land degradation in grassland, rangeland) and contributes heavily to global CO 2 emissions (Fleishner 1994 Eldridge and others 2016 Sanderman and others 2017). Livestock grazing will continue to exhibit an upward trend globally (Asner and others 2004). It provides billions of people with multiple resources such as milk, meat, hide, fuel and fertiliser (dung), security, transportation, and the potential to accumulate capital (Campos and others 2016). Grazing of livestock on native pastures accounts for 70% of all agricultural land and 30% of Earth’s land surface (Steinfeld and others 2006). The livestock sector is a substantial global industry, accounting for 40% of GDP, and employing 1.3 billion people, of which 1 billion are from some of the world’s poorest societies (Steinfeld and others 2006). Livestock grazing is unlikely to result in positive environmental outcomes for infiltration and sediment production. Our study demonstrates the nuanced effects of livestock activity, with herbivore type and intensity, and environmental context modifying the hydrological outcomes. The impact of livestock activity also varied with climatic region, soil texture, and the interaction between aridity and plant cover. These impacts varied markedly among herbivore types and intensity, with greatest reductions in infiltration at high and low grazing contrasts, and more pronounced negative effects of grazing when sheep and cattle grazed together than either alone. Overall, we found that livestock activity increased sediment production (+ 52 ± 19.8% mean ± 95% CI), reduced infiltration (− 25 ± 5.2%), but had no significant effect on either runoff (+ 27 ± 29.6%) or soil moisture (− 5 ± 5.9%). We used a global meta-analysis to examine the effects of livestock activity on hydrological inflows (infiltration, soil moisture), and outflows (runoff, sediment) using a dataset compiled from 3044 contrasts of grazed and ungrazed data from 129 publications between 19. This makes it difficult to provide ecologically based advice on how best to manage grazing to minimise environmental damage and maximise hydrological functions. Despite numerous studies and reviews of the effects of livestock activity on hydrological functions, a global synthesis of their effects on different biotic (vegetation, herbivore type) and abiotic (climate, soil, spatial scales) factors remains elusive. The livestock sector supports billions of people worldwide, yet when poorly managed, can have substantial negative effects on soils and ecosystem functions.
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