Soil Biota in Banded Landscapes

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.

Variation in savanna vegetation on termite mounds in north-eastern Namibia

In savanna, termite mounds support more diverse vegetation than off-mound areas, but little is known of the patterns in plant assemblages on mounds. To explain vegetation differentiation between (1) component structures of termite mounds (conical centre vs. pediment), (2) active and inactive mounds (termites present vs. termites absent), and (3) sites on and off mounds (on mounds vs. surrounding savanna), species composition, richness and abundances of woody plants were recorded on 70 mounds and in 13 savanna plots (each 20 × 20 m) in north-eastern Namibia, focusing on soil hardness, mound status (active or not) and mound micro-topography as explanatory factors. Woody plants were absent from 33% of active mounds (54% of active cones) but were absent from only 5% of inactive mounds. Species richness and abundance per mound (mean ± SD) were lower on active mounds with (2.0 ± 1.8 and 4.6 ± 6.6, respectively) and without pediments (0.6 ± 0.6 and 0.9 ± 1.1, respectively) than on inactive mounds (4.4 ± 2.7, 19.4 ± 18.8, respectively). Despite the lower woody plant cover, some characteristic species, such as Salvadora persica, occurred preferentially on active mounds; this species occurred on 42% of active mounds. Mean soil hardness (± SD) was higher on conical parts of active mounds (4300 ± 2620 kPa) than on adjacent pediments (583 ± 328 kPa) and inactive mounds (725 ± 619 kPa). This study suggested that mound status, mound micro-topography, and soil hardness promote variability in the vegetation on mounds.

Emergent properties from organisms to ecosystems: towards a realistic approach

More realistic approaches are needed to understand the complexity of ecological systems. Emergent properties of real systems can be used as a basis for a new, neither reductionist nor holistic, approach. Three systems, termed here BUBBLEs, WAVEs and CRYSTALs, have been identified as exhibiting emergent properties. They are non-hierarchical assemblages of individual components, with amplification and connectedness being two main principles that govern their build-up, maintenance and mutual relationships. Examples from various fields of biological and ecological science are referred to, ranging from individual organisms to landscapes.