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Hyvàrinen, O., 2022. Megaherbivores and Earth system functioning: Landscape-scale effects of white rhino loss on vegetation, fire and soil carbon dynamics. Thesis presented to the Swedish University of Agricultural Sciences, Umea, pp. 1-160

Location: Africa - Southern Africa - South Africa
Subject: Ecology
Species: White Rhino

Original text on this topic:
Megaherbivores (> 1000 kg) have been suggested to strongly influence ecosystem functioning with consequences potentially scaling up to the global climate. However, due to poaching, we may lose some of the most vulnerable species at functional densities in the wild in the coming decades. In this thesis, I investigated the consequences of white rhinoceros (Ceratotherium simum) loss on savanna functioning in Hluhluwe-iMfolozi Park, South Africa. I predicted that the loss of rhino leads to ecosystem-level changes in vegetation, particularly in terms of grass structure and grass-woody ratios, and fire patterns, and that these changes further affect climate drivers, more specifically, soil carbon dynamics. To start with a broader perspective, I conducted a systematic review which revealed major taxonomic, thematic and geographical biases and knowledge gaps in the empirical peer-reviewed literature on ecosystem and Earth system effects of extant megaherbivores. In a more in-depth study, I utilized a “natural experiment” and remote sensing imagery, which showed that rhino loss was associated with increased fire occurrence and, indirectly, a higher rate of woody encroachment. Moreover, grazing lawn, which has been previously linked to rhino presence, was associated with lower fire occurrence and, indirectly, lower rate of woody encroachment. Finally, through a field study I show that soil organic carbon (SOC) stocks or the size of functional SOC pools with varying persistence did not differ among grazing intensity contrasts even at the highest grazing intensities. Instead, I show that SOC stocks were higher in woody encroached grassland plots compared to not encroached reference plots for soils with low clay content but lower for soils with high clay content. Moreover, the least persistent SOC pool was smaller in the woody encroached plots compared to the reference plots in open grazing lawn habitat characterized by high grazing intensity. Instead, in hillslope grassland habitat characterized by low grazing intensity and high rainfall, both the least persistent and most persistent SOC pools were larger in the woody encroached plots than in the reference plots. These results support the hypothesis that white rhino loss leads to an increase in fire occurrence and thus indirectly an increase in woody encroachment. However, there was no evidence to support the hypothesis that white rhino grazing directly reduces SOC stocks, although the data suggest that rhino can influence soil carbon dynamics indirectly through limiting woody encroachment. I concur that white rhino are influential ecosystem-engineers and propose that their loss has meaningful landscape-scale consequences on savanna functioning where the dynamics of vegetation, fire and soil carbon are strongly coupled.

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