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.