Large herbivores maintain a two‐phase herbaceous vegetation mosaic in a semi‐arid savanna

Abstract

Many arid and semi‐arid rangelands exhibit distinct spatial patterning of vegetated and bare soil‐dominated patches. The latter potentially represent a grazing‐induced, degraded ecosystem state, but could also arise via mechanisms related to feedbacks between vegetation cover and soil moisture availability that are unrelated to grazing. The degree to which grazing contributes to the formation or maintenance of degraded patches has been widely discussed and modeled, but empirical studies of the role of grazing in their formation, persistence, and reversibility are limited.We report on a long‐term (17 years) grazing removal experiment in a semi‐arid savanna where vegetated patches composed of perennial grasses were interspersed within large (>10 m2) patches of bare soil.Short‐term (3 years) grazing removal did not allow bare patches to become revegetated, whereas following long‐term (17 years) grazing removal, bare soil patches were revegetated by a combination of stoloniferous grasses and tufted bunchgrasses. In the presence of grazers, stoloniferous grasses partially recolonized bare patches, but this did not lead to full recovery or to the establishment of tufted bunchgrasses.These results show that grazers alter both the balance between bare and vegetated patches, as well as the types of grasses dominating both patch types in this semiarid savanna.Synthesis: Large herbivores fundamentally shaped the composition and spatial pattern of the herbaceous layer by maintaining a two‐phase herbaceous mosaic. However, bare patches within this mosaic can recover given herbivore removal over sufficiently long time scales, and hence do not represent a permanently degraded ecosystem state.

Grasses continue to trump trees at soil carbon sequestration following herbivore exclusion in a semiarid African savanna

“…The topography at the study sites consists of gently rolling hills interspersed with granitic inselbergs (Augustine and McNaughton ). Mean annual rainfall for the period 1972–2009 was 514 mm (Sankaran et al ), whereas from 2003 to 2016, annual precipitation averaged 633 mm (Augustine et al ). The vegetation is characterized by semiarid savannas with an Acacia ‐dominated tree and shrub community and a discontinuous layer of perennial grasses (Augustine ).…”

Positive impacts of livestock and wild ungulate routes on functioning of dryland ecosystems

“…For example, in a semiarid Kenyan savanna, cattle grazing was reported to form degraded bare spaces throughout the landscape. Yet, long‐term grazing exclusion has led to revegetation of the bare spaces by herbaceous plants, demonstrating the potential reversal of degradation processes and recovery of these rangelands (Augustine et al., 2019 ). Further, overgrazing may entirely remove vegetation, consequently simplifying the ground surface and preventing ecosystem self‐organization.…”

“…Despite the expected increase in the ecosystem's NPP, this chain of effects decreases ecosystem complexity, reduces plant species diversity, and lowers the economic value of rangelands (Schlesinger et al., 1990 ; Stavi, Shem‐Tov, et al., 2015 ). At the same time, if animal rate is too low, or where animal grazing is excluded, herbaceous (Augustine et al., 2019 ) or woody vegetation cover may increase, decreasing or eliminating the bare interpatch spaces (Archer et al., 2017 ; Stavi, Lavee, et al., 2009 ; Turner et al., 2003 ). A similar effect was reported for the northern Negev region, where goats grazing on the fresh foliage of the dominant shrub species, Sarcopoterium spinosum (L.) Spach, regulate its cover (Stavi et al., 2008 ).…”

Variable effects of long‐term livestock grazing across the western United States suggest diverse approaches are needed to meet global change challenges

“…The effects of grazing by wild and domesticated herbivores on rangeland ecosystem structure and function can vary widely, driven by the complex combinations of climate, soil type, natural disturbance regimes, plant communities, and historic and contemporary grazing practices (Milchunas & Lauenroth, 1993; Bakker et al, 2006; Koerner et al, 2018; Forbes et al, 2019; Maestre et al, 2022). The rate of change in plant community composition and/or bare soil exposure in response to differing grazing intensities can vary substantially across ecosystems and continents (Harrison & Shackleton, 1999; O'Reagain et al, 2014; Veblen et al, 2016; Augustine et al, 2019; Wilmer et al, 2021), and at fine scales across contrasting soil types (e.g. Goheen et al, 2018).…”