Namibian fairy circles: Hostile territory for soil nematodes

Abstract

Fairy circles are rings of grass with centers of bare soil that are found in some arid grasslands. Above- and belowground chemical and biological attributes of fairy circles have been explored in an ongoing debate about the ultimate causes of this pattern. We studied the soil nematode communities associated with Stipagrostis fairy circles along a 900-km range in the Namib Desert of Namibia in southern Africa. Nematode abundance and diversity were highest in soils along the vegetation rings that define fairy circles and in soils in the vegetated matrix surrounding the bare circles, demonstrating the positive impact of plant-derived resources on nematode communities. In contrast, soils from the bare centers of fairy circles had lower organic matter content and were nearly defaunated, averaging only 9.9 {+/-} 1.7 nematodes 100 g-1 soil. Bacterial-feeding Acrobeloides nematodes were the only taxa over-represented in center soils in comparison to ring soils. Our results indicate that nematode communities are influenced by the unique soil environments that the fairy circle vegetation pattern generates and suggest that the soils at the centers of fairy circles are uniquely hostile habitat for soil organisms as well as plants. Co-occurrence network analysis of nematode communities elucidated relationships among the taxa. For example, the abundances of dorylaims and Nothacrobeles were positively correlated across all soil positions, suggesting they have overlapping ecological niches. In ring soils, the abundances of fungal-feeding Aphelenchoides, Ditylenchus, and Hexatylus were positively correlated, likely due to enhanced fungal communities in these plant-influenced soils. Panagrobelus demonstrated niche specialization by being negatively correlated to two other bacterial-feeding taxa (Elaphonema in matrix soils and Acrobeloides in ring soils). The co-occurrence patterns revealed by these relatively low diversity communities provide insights into the potential roles of nematode interactions as well as environmental factors in community assembly.