Phenotypic plasticity: A missing element in the theory of vegetation pattern formation

Regular spatial patterns of vegetation are a common sight in drylands. Their formation is a population-level response to water stress that increases water availability for the few via partial plant mortality. At the individual level, plants can also adapt to water stress by changing their phenotype. Phenotypic plasticity of individual plants and spatial patterning of plant populations have extensively been studied independently, but the likely interplay between the two robust mechanisms has remained unexplored. In this paper, we incorporate phenotypic plasticity into a multi-level theory of vegetation pattern formation and use a fascinating ecological phenomenon, the Namibian "fairy circles," to demonstrate the need for such a theory. We show that phenotypic changes in the root structure of plants, coupled with pattern-forming feedback within soil layers, can resolve two puzzles that the current theory fails to explain: observations of multi-scale patterns and the absence of theoretically predicted large-scale stripe and spot patterns along the rainfall gradient. Importantly, we find that multi-level responses to stress unveil a wide variety of more effective stress-relaxation pathways, compared to single-level responses, implying a previously underestimated resilience of dryland ecosystems.

Evolution at the arid extreme: the influence of climate on sand termite colonies and fairy circles of the Namib Desert

In the hyperarid Namib Desert, the sand termite Psammotermes allocerus Silvestri, 1908 (Isoptera: Rhinotermitidae) establishes colonies that create conspicuous, barren patches known as 'fairy circles' on permeable, sandy soils. The central bare areas of fairy circles serve the key function of storing moisture received from sparse rainfall. The sandy soil texture allows rapid infiltration and percolation of precipitation, while localized herbivory by the termites creates the bare patch, thereby reducing the rapid loss of soil moisture by the uptake and transpiration of water by plants. The resulting storage of rain water even during prolonged periods of drought enables perennial life in hyperarid desert environments and forms a globally unique example of ecosystem engineering by social insects. During the past decade, most publications primarily debated the origin of fairy circles. Here, we contribute to the special issue with a focus on the functional and evolutionary dimension of the structure of the Psammotermes colony with two differing nest types and two spatially separated key resources, as a successful adaptation to extreme desert environment. The paper is primarily a review and a synthesis of previous work, with the inclusion of new, relevant findings. This article is part of the theme issue 'The evolutionary ecology of nests: a cross-taxon approach'.

Spatial Self-Organization of Ecosystems: Integrating Multiple Mechanisms of Regular-Pattern Formation

Large-scale regular vegetation patterns are common in nature, but their causes are disputed. Whereas recent theory focuses on scale-dependent feedbacks as a potentially universal mechanism, earlier studies suggest that many regular spatial patterns result from territorial interference competition between colonies of social-insect ecosystem engineers, leading to hexagonally overdispersed nest sites and associated vegetation. Evidence for this latter mechanism is scattered throughout decades of disparate literature and lacks a unified conceptual framework, fueling skepticism about its generality in debates over the origins of patterned landscapes. We review these mechanisms and debates, finding evidence that spotted and gapped vegetation patterns generated by ants, termites, and other subterranean animals are globally widespread, locally important for ecosystem functioning, and consistent with models of intraspecific territoriality. Because these and other mechanisms of regular-pattern formation are not mutually exclusive and can coexist and interact at different scales, the prevailing theoretical outlook on spatial self-organization in ecology must expand to incorporate the dynamic interplay of multiple processes.

Gradual regime shifts in fairy circles

Large responses of ecosystems to small changes in the conditions--regime shifts--are of great interest and importance. In spatially extended ecosystems, these shifts may be local or global. Using empirical data and mathematical modeling, we investigated the dynamics of the Namibian fairy circle ecosystem as a case study of regime shifts in a pattern-forming ecosystem. Our results provide new support, based on the dynamics of the ecosystem, for the view of fairy circles as a self-organization phenomenon driven by water-vegetation interactions. The study further suggests that fairy circle birth and death processes correspond to spatially confined transitions between alternative stable states. Cascades of such transitions, possible in various pattern-forming systems, result in gradual rather than abrupt regime shifts.

Strong interaction between plants induces circular barren patches: fairy circles

Fairy circles consist of isolated or randomly distributed circular areas devoid of any vegetation. They are observed in vast territories in southern Angola, Namibia and South Africa. We report on the formation of fairy circles, and we interpret them as localized structures with a varying plateau size as a function of the aridity. Their stabilization mechanism is attributed to a combined influence of the bistability between the bare state and the uniformly vegetation state, and Lorentzian-like non-local coupling that models the competition between plants. We show how a circular shape is formed, and how the aridity level influences the size of fairy circles. Finally, we show that the proposed mechanism is model-independent.