A theoretical foundation for multi-scale regular vegetation patterns

Abstract

Self-organized regular vegetation patterns are widespread and thought to mediate ecosystem functions such as productivity and robustness, but the mechanisms underlying their origin and maintenance remain disputed. Particularly controversial are landscapes of overdispersed (evenly spaced) elements, such as North American Mima mounds, Brazilian murundus, South African heuweltjies, and, famously, Namibian fairy circles. Two competing hypotheses are currently debated. On the one hand, models of scale-dependent feedbacks, whereby plants facilitate neighbours while competing with distant individuals, can reproduce various regular patterns identified in satellite imagery. Owing to deep theoretical roots and apparent generality, scale-dependent feedbacks are widely viewed as a unifying and near-universal principle of regular-pattern formation despite scant empirical evidence. On the other hand, many overdispersed vegetation patterns worldwide have been attributed to subterranean ecosystem engineers such as termites, ants, and rodents. Although potentially consistent with territorial competition, this interpretation has been challenged theoretically and empirically and (unlike scale-dependent feedbacks) lacks a unifying dynamical theory, fuelling scepticism about its plausibility and generality. Here we provide a general theoretical foundation for self-organization of social-insect colonies, validated using data from four continents, which demonstrates that intraspecific competition between territorial animals can generate the large-scale hexagonal regularity of these patterns. However, this mechanism is not mutually exclusive with scale-dependent feedbacks. Using Namib Desert fairy circles as a case study, we present field data showing that these landscapes exhibit multi-scale patterning-previously undocumented in this system-that cannot be explained by either mechanism in isolation. These multi-scale patterns and other emergent properties, such as enhanced resistance to and recovery from drought, instead arise from dynamic interactions in our theoretical framework, which couples both mechanisms. The potentially global extent of animal-induced regularity in vegetation-which can modulate other patterning processes in functionally important ways-emphasizes the need to integrate multiple mechanisms of ecological self-organization.

The interplay between facilitation and habitat type drives spatial vegetation patterns in global drylands

“…Similarly, large patches are considered dynamically instable due to competitive interactions between plants which tend to divide large patches (Bordeu et al 2016). Moreover, this suggests that regular and irregular patterns are often mixed in the field (something already suggested by Tarnita et al 2017). 3).…”

Behavioral self-organization underlies the resilience of a coastal ecosystem

“…Biological complexity at different spatial scales, driven by a multitude of behavioral, population-level, or ecosystem-level processes, is a defining characteristic of natural ecosystems (22,(34)(35)(36). A recent theoretical study highlighted that the interplay between two different self-organization processes created spatial patterns at two different scales in mussel beds, which proved a crucial factor in defining mussel bed persistence (22).…”

Contrasting Global Patterns of Spatially Periodic Fairy Circles and Regular Insect Nests in Drylands

“…The origin of these FCs in water‐limited drylands has puzzled scientists for decades (Sahagian, ; van Rooyen et al, ). The hypotheses include (a) allelopathic influences of a plant that no longer occurs in FCs (Meyer et al, ; Theron, ); (b) seepage of carbon monoxide, hydrocarbons, or other volatiles from below ground that kills grass in regular patterns (Albrecht et al, ; Jankowitz et al, ; Naude et al, ); (c) an unspecified microbial effect (Eicker et al, ; Ramond et al, ; van der Walt et al, ); (d) faunal removal of grasses by termites (Becker & Getzin, ; Juergens, ; Moll, ; Tarnita et al, ; Vlieghe et al, ) or detrimental influence on grasses by ants (Picker et al, ); and (e) self‐organized emergent vegetation patterning arising from competitive and facilitative interactions between grasses (Cramer et al, ; Cramer & Barger, ; Getzin et al, , , , ; Ravi et al, ; Tschinkel, ; van Rooyen et al, ).…”