In this study, we investigated the emergence of spatial self-organized patterns on intertidal flats, resulting from the interaction between biological and geomorphological processes. Autocorrelation analysis of aerial photographs revealed that diatoms occur in regularly spaced patterns consisting of elevated hummocks alternating with water-filled hollows. Hummocks were characterized by high diatom content and a high sediment erosion threshold, while both were low in hollows. These results highlight the interaction between diatom growth and sedimentary processes as a potential mechanism for spatial patterning. Several alternative mechanisms could be excluded as important mechanisms in the formation of spatial patterns. We developed a spatially explicit mathematical model that revealed that scale-dependent interactions between sedimentation, diatom growth, and water redistribution explain the observed patterns. The model predicts that areas exhibiting spatially selforganized patterns have increased sediment accretion and diatom biomass compared with areas lacking spatial patterns, a prediction confirmed by empirical evidence. Our study on intertidal mudflats provides a simple but clear-cut example of how the interaction between biological and sedimentary processes, through the process of self-organization, induces spatial patterns at a landscape level.

Cross-habitat interactions among bivalve species control com m unity structure on intertidal flats S e r e n a D o n a d i , 1,8 T j i s s e v a n d e r H e i d e ,2 E l s M. v a n d e r Z e e , 3'4 J o h a n S . E k l ö f , 1'5'6 J o h a n v a n d e K o p p e l ,2'7 E l l e n J . W e e r m a n ,2 T h e u n i s P i e r s m a , 3'4 H a n O l f f ,2 a n d B r i t a s K l e m e n s E r i k s s o n 1 D epartm ent o f M arine B enthic E cology and Evolution, Centre f o r Ecological and E volutionary Studies (C .E E S ), U niversity o f Groningen, P .O . B o x 1IOS, 9700 C C Groningen, The Netherlands ~Com m unity and Conservation E cology Group, Centre fo r Ecological and E volutionary Studies (C E E S ), University o f Groningen, P.O . B o x 11103, 9700 C C Groningen, The Netherlands 3 A n im a l E cology Group, Centre f o r Ecological and E volutionary Studies (C E E S ), University o f Groningen, P.O . B o x 11103, 9700 C C Groningen, The Netherlands 4D epartm ent o f M arine Ecology, R o ya l Netherlands Institute f o r Sect Research ( N I O Z ) , P .O . B o x 59, 1790 A B D en Burg, Texel, The Netherlands 3D epartm ent o f B iology and E nvironm ental Science, University o f Gothenburg, B o x 461, SE -405 30 Gothenburg, Sweden 6D epartm ent o f S ystem s Ecology, Stockholm U niversity , SE -106 9Stockholm , Sweden Spa tia l E cology D epartm ent, R o y a l Netherlands Institute fo r Sea Research (NIOZ), P .O . B o x 140, 4400 A C , Yerseke, The NetherlandsAbstract. Increasing evidence shows th a t spatial interactions between sedentary organisms can structure communities and prom ote landscape complexity in m any ecosystems. Here we tested the hypothesis that reef-forming mussels (M ytilus edulis L.), a dom inant intertidal ecosystem engineer in the W adden Sea, prom ote abundances of the burrowing bivalve Cerastoderma edule L. (cockle) in neighboring habitats at relatively long distances coastw ard from mussel beds. Field surveys within and around three mussel beds showed a peak in cockle densities at 50-100 m tow ard the coast from the mussel bed, while cockle abundances elsewhere in the study area were very low. Field transplantation of cockles showed higher survival of young cockles (2-3 years old) and increased spat fall coastw ard of the mussel bed com pared to within the bed and to areas w ithout mussels, whereas growth decreased within and coastw ard of the mussel bed. O ur measurements suggest that the observed spatial patterns in cockle numbers resulted from (1) inhibition effects by the mussels close to the beds due to preemptive algal depletion and deteriorated sediment conditions and (2) facilitation effects by the mussels farther away from the beds due to reduction of wave energy. O ur results imply that these spatial, scale-dependent interactions between reef-forming ecosystem engineers and surrounding communities of sedentary benthic organisms can be an im portant determ inant of the large-scale community structure in intertidal ecosystems. Understanding this interplay between neighboring communi...

Recently, Lévy walks have been put forward as a new paradigm for animal search and many cases have been made for its presence in nature. However, it remains debated whether Lévy walks are an inherent behavioural strategy or emerge from the animal reacting to its habitat. Here, we demonstrate signatures of Lévy behaviour in the search movement of mud snails (Hydrobia ulvae) based on a novel, direct assessment of movement properties in an experimental set-up using different food distributions. Our experimental data uncovered clusters of small movement steps alternating with long moves independent of food encounter and landscape complexity. Moreover, size distributions of these clusters followed truncated power laws. These two findings are characteristic signatures of mechanisms underlying inherent Lévy-like movement. Thus, our study provides clear experimental evidence that such multi-scale movement is an inherent behaviour rather than resulting from the animal interacting with its environment.

Theoretical models predict that spatial self-organization can have important, unexpected implications by affecting the functioning of ecosystems in terms of resilience and productivity. Whether and how these emergent effects depend on specific formulations of the underlying mechanisms are questions that are often ignored. Here, we compare two alternative models of regular spatial pattern formation in mussel beds that have different mechanistic descriptions of the facilitative interactions between mussels. The first mechanism involves a reduced mussel loss rate at high density owing to mutual protection between the mussels, which is the basis of prior studies on the pattern formation in mussels. The second mechanism assumes, based on novel experimental evidence, that mussels feed more efficiently on top of mussel-generated hummocks. Model simulations point out that the second mechanism produces very similar types of spatial patterns in mussel beds. Yet the mechanisms predict a strikingly contrasting effect of these spatial patterns on ecosystem functioning, in terms of productivity and resilience. In the first model, where high mussel densities reduce mussel loss rates, patterns are predicted to strongly increase productivity and decrease the recovery time of the bed following a disturbance. When pattern formation is generated by increased feeding efficiency on hummocks, only minor emergent effects of pattern formation on ecosystem functioning are predicted. Our results provide a warning against predictions of the implications and emergent properties of spatial self-organization, when the mechanisms that underlie self-organization are incompletely understood and not based on the experimental study.

Self‐organized spatial patterns have been proposed as possible indicators for regime shifts in ecosystems. Until now, this hypothesis has only been tested in drylands. Here, we focus on intertidal mudflats where regular spatial patterns develop in early spring from the interaction between diatom growth and sedimentation but disappear when benthic herbivore abundance increases in early summer, accompanied by a dramatic shift to a bare mudflat. We followed the patch‐size distributions of diatom biofilms during this degradation process. As time progressed, we found a temporal change in the spatial configuration occurring simultaneously with the loss of the diatom–sediment feedback. This indicates a gradual failure in time of the self‐organization process that underlies regular patterning in this ecosystem. The path to degradation co‐occurred with the loss of the larger patches in the ecosystem, which resulted in a decrease of the truncation in the patch‐size distribution. Hence, our study in mudflat ecosystems confirms the general hypothesis that spatial patterns can provide important clues about the level of degradation. Nevertheless, our study highlights the need for thorough study about the type of spatial patterns and the nature of the underlying feedbacks before a reliable assessment of ecosystem status can be made, as changes in patch‐size distribution differed markedly with those observed in other ecosystems.

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Abstract. Regular, self-organized spatial patterns in primary producers have been described in a wide range of ecosystems and are predicted to affect community production and resilience. Although consumers are abundant in most systems, the effect of trophic interactions on pattern formation in primary producers remains unstudied. We studied the effects of top-down control by herbivores on a self-organized landscape of regularly spaced, diatom-covered hummocks alternating with water-filled hollows on an intertidal mudflat in The Netherlands. Spatial patterns developed during spring but were followed by a rapid collapse in summer, leading to a flat landscape with low diatom densities and little variation in sediment bed level. This dramatic decline co-occurred with a gradual increase of benthic herbivores. A manipulative field experiment, where benthic herbivores were removed from the sediment, revealed that both diatom growth and hummock formation were inhibited by the activity of benthic herbivores. Our study provides clear evidence of top-down control of spatial selforganized patterns by benthic herbivores within a biological-geomorphic landscape.

Self-facilitation through ecosystem engineering (i.e., organism modification of the abiotic environment) and consumer-resource interactions are both major determinants of spatial patchiness in ecosystems. However, interactive effects of these two mechanisms on spatial complexity have not been extensively studied. We investigated the mechanisms underlying a spatial mosaic of low-tide exposed hummocks and waterlogged hollows on an intertidal mudflat in the Wadden Sea dominated by the seagrass Zostera noltii. A combination of field measurements, an experiment and a spatially explicit model indicated that the mosaic resulted from localized sediment accretion by seagrass followed by selective waterfowl grazing. Hollows were bare in winter, but were rapidly colonized by seagrass during the growth season. Colonized hollows were heavily grazed by brent geese and widgeon in autumn, converting these patches to a bare state again and disrupting sediment accretion by seagrass. In contrast, hummocks were covered by seagrass throughout the year and were rarely grazed, most likely because the waterfowl were not able to employ their preferred but water requiring feeding strategy (‘dabbling’) here. Our study exemplifies that interactions between ecosystem engineering by a foundation species (seagrass) and consumption (waterfowl grazing) can increase spatial complexity at the landscape level.