Fairy circles, patches of barren land surrounded by a ring of grasses in arid ecosystems, are found in the Namib and Australian deserts and are primarily thought to form and persist via one of two competing hypotheses: competition among grasses for limited soil resources or the creation of favorable environments by termite communities. Longer-term experimental manipulations are necessary to test these competing hypotheses and compare against alternatives. Specifically, large herbivores dominate the Namibian landscape, yet their impact on fairy circles’ life cycles has not previously been considered. In this study, we examined the role of wildlife herbivory, soil resource constraints, and soil fauna on plant establishment and vegetative regrowth in fairy circles. We enclosed fairy circles within a fence perimeter and treated the circle gaps with fertilizer (N, P, and K), water, water and fertilizer, or insecticide. After two years, we measured within-gap grass recruits and perimeter grass re-sprouting. Using images taken at the time of measurement, we analyzed the spatial arrangement of recruits using point pattern analysis to test whether plant recruitment occurs non-randomly throughout fairy circles during gap collapse and if treatments alter the spatial arrangement of recruits within circles.
Results/Conclusions
All treatments impacted circle maintenance by altering within-circle plant recruitment, perimeter grass regrowth, or both. Fairy circles within the fenced enclosure contained 12 times more recruits than circles exposed to wildlife herbivory, suggesting that large herbivores play a critical role in the long-term maintenance of landscape gap patterns. Nutrient addition led to an increase in plant recruitment, while water and fertilizer addition resulted in an increase in perimeter regrowth. Reduction of soil fauna with insecticide did not alter recruit density, but yielded a two fold increase in perimeter grass regrowth. Additionally, point pattern analysis showed that within-gap recruits are arranged in a non-random pattern, with recruit density higher near the edge of fairy circles. These results suggest that small-scale environmental heterogeneity or a facilitative effect from established perimeter plants shape the spatial arrangement of circle collapse. Overall, we show that multiple biotic and abiotic factors, including herbivory and within-gap soil resources, influence the density and spatial arrangement of plant recruitment in fairy circles. Given that many of these factors can vary through space and time, fairy circle maintenance and collapse are likely mediated by a complex interplay of processes that vary in their impact on plant growth within fairy circles.