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Spatial Heterogeneity of Resources: a mediator of large herbivore population stability?

Final Report Summary - SPHERE (Spatial Heterogeneity of Resources: a mediator of large herbivore population stability?)

Based at the Centre for Ecological and Evolutionary Synthesis, Department of Biology of the University of Oslo, SPHERE explored the role of spatial heterogeneity of resources as a mediator of large herbivore population stability in African savannas. Recent studies show that the increased variability in rainfall that is predicted under local climate change scenarios and the resulting temporal variability in resource availability might lead to severe declines in African herbivore populations. However, new theory predicts that spatial heterogeneity of resources can act as a buffer against such temporal heterogeneity. Up to recently this theory awaited empirical testing. African savanna grasslands are an ideal system to test this theory, with characteristic grazing lawns as a clearly visible aspect of spatial resource heterogeneity. Grazing lawns represent clearly confined patches of specific grazing-tolerant grass species with high nutrient quality. As such grazing lawns are nutrient hotspots surrounded by grassland of relatively low food quality. Several mammalian grazer species concentrate foraging in these lawns but it is unclear how lawn abundance and spatial distribution affects grazer population dynamics on a larger scale. We used this system to investigate the hypothesized effect of resource heterogeneity on population stability in two protected savanna reserves in South Africa, Hluhluwe-iMfolozi Park (HiP) and Kruger National Park (KNP). We choose these two reserves because they lie at different ends of the heterogeneity continuum. Both parks host a complete and similar set of indigenous grazer species and have comparable variation in rainfall and general abiotics. However, there is a strong contrast between both parks in terms of size, herbivore density and resource heterogeneity (availability and distribution of lawns). HiP is a small reserve, 900 km2 , with high overall grazer densities (> 8000 kg/km2 ) and KNP a large reserve, 20,000 km2 , with low grazer densities (~ 2000 kg/km2 ). HiP is characterized by a very heterogeneous grazing system, where high habitat diversity is coupled with small-scale (~ 10-100 m) grassland patch diversity (grazing lawns). KNP, on the other hand, is much more homogeneous.
The first overall objective was to quantify resource heterogeneity in both reserves in terms of grazing lawn distribution and understand what drives these distribution patterns. We mapped grazing lawn distribution across Hluhluwe-iMfolozi Park on 24 line transects of ~8 kilometre long. In this way we repeated a mapping exercise in 2004. This allowed us to look at factors that drive the distribution and persistence of grazing lawns over time. Grazing lawns were more abundant in the parts of the reserve with relatively low rainfall and in areas with high density of features of white rhino presence (e.g. dung). This confirms previous work that grazing lawns are more likely to develop under relatively low rainfall conditions and that white rhino play a crucial role in creating lawns. Interestingly, our results also indicated that the persistence of grazing lawns was higher close to former boma sites. These boma sites are former cattle kraal sites that were abandoned when the last people were moved from the reserve in the 1960s. Hence, former human land use might be part of the explanation behind the high grassland heterogeneity in HiP. We also mapped grazing lawn distribution in the southern part of KNP. The first preliminary field surveys indicated that the type of grazing lawns, as known from HiP, are hard to find in KNP if present at all. Clear grassland nutrient hotspots, so-called sodic sites, are prevalent in KNP, but it is unclear to what extent they are functionally similar to the grazing lawns in HiP. Current studies suggest that they are more constrained in the landscape by abiotic factors than the lawns in HiP. As mentioned, the lawns in HiP seem to be for a large part created by white rhino. In contrast to HiP, white rhino went extinct in KNP towards the end of the 19th century and were only fairly recently re-introduced into KNP (1960s). This might explain why rhino-mediated grassland heterogeneity (especially grazing lawns) are not so obvious in KNP. Hence, before being able to compare populationlevel response of herbivores to variation in similar nutrient hotspots between both parks, it was necessary to assess whether rhino-created grazing lawns similar to the ones in HiP are present in Kruger. Therefore, we walked transects in KNP in areas with high versus low rhino densities, and long versus recent colonization history and mapped structural heterogeneity in terms of variation in grass height, and presence of grazing lawns. Preliminary results suggest that there are still almost no rhino created grazing lawns in KNP but structural heterogeneity seems to be higher in grasslands with high rhino density and long colonization density. Final results should confirm this and will also give more insight in the recolonization patterns of KNP by white rhino and the factors that drive these patterns.