University of Cape Town
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All savanna islands of fertility are not equal: colonial birds influence soil nutrient stoichiometries with consequences for tree seedling growth

posted on 2023-05-31, 07:27 authored by Timothy Aikins KhanTimothy Aikins Khan

Islands of fertility associated with tree/shrub patches in arid grasslands create spatial heterogeneity of soil nutrients. Faunal activities under these trees/shrubs may contribute to diverse characteristics of these fertile patches of soil due to different faecal inputs. This study used the islands of fertility created by camelthorn Vachellia erioloba and shepherd Boscia albitrunca trees (tree islands of fertility- TIFs) and also trees that host sociable weavers Philetairus socius (bird islands of fertility-BIFs) in the Kalahari Desert to understand the diversity in islands of fertility. We hypothesized that grasslands, the tree islands of fertility (TIFs), and the bird islands of fertility (BIFs) differ in both the concentrations and stoichiometries of soil nutrients due to the sources of nutrients and that this subsequently determines the growth and foliar nutrient concentrations and stoichiometries of plants that grow on these soils. We predicted seedlings grown on soils from BIFs would have greater growth, higher biomass yield, and higher foliar nutrients than TIFs and Grasslands. We sampled and grew camelthorn seedlings in soils from BIFs, TIFs, and matrix grasslands. Despite the higher soil nutrients in TIFs than in grasslands, there were no significant differences in seedling growth. However, we observed significantly higher seedling growth in BIF soils compared to TIF soils. Seedlings grown in soils from BIFs and grasslands allocated more growth to shoots and roots, respectively,while TIFs were intermediate. The foliar nutrient stoichiometries of seedlings grown in BIF soils were similar to the stoichiometric ratios in BIF soils and sociable weaver faecal matter. This shows that the faecal input of the sociable weaver accounts for the growth differences in these islands of fertility. The ecological engineering activities of the sociable weaver address nutrient limitations essential for camelthorn seedling growth, which TIFsoils could not address despite the high faecal input of mammals.


National Research Foundation-Department of Science and Technology Center of Excellence of the FitzPatrick Institute of African Ornithology

University of Cape Town

Tswalu Foundation

Kalahari Endangered Ecosystem Project (KEEP) sponsors



University of Cape Town, Biological Sciences, FitzPatrick Institute of African Ornithology

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