Positive feedbacks between savanna tree size and the nutritional characteristics of “Islands of fertility” are amplified by sociable weaver colonies

In arid environments, areas of high soil nutrient concentrations are common under woody plants due to ecosystem functions, including faunal deposition. Sociable weavers (Philetairus socius) are birds that build nests on camelthorn (Vachellia erioloba) and shepherd (Boscia albitrunca) trees and their faeces enrich the soil. Nevertheless, the soil under the nests is devoid of vegetation. We evaluated the hypotheses that the presence of trees and weaver nests alters soil properties, which vary with tree species and size as well as nest size, but that the area devoid of vegetation under nest trees is due to physiological drought, lack of viable seeds or nutrient toxicity. We sampled and analysed soil properties under trees with and without a sociable weaver nest and the grassland between them. Soils under trees were characterized by higher N (2.3-fold) and P (1.3-fold) compared to grassland areas. Furthermore, soils under nest-trees had higher N (3.5-fold), and P (4.1-fold) concentrations than soils under trees without nests. Some soil properties also varied with increasing tree and colony size. Soil water infiltration rate decreased under nest-trees with increased colony size, limiting seed germination, despite good seed banks and nutrient-rich soils. This combination of feedbacks establishes these trees as powerful ecosystem engineers.