Nutrient pollution and reduced grazing each can stimulate algal blooms as shown by numerous experiments. But because experiments rarely incorporate natural variation in environmental factors and biodiversity, conditions determining the relative strength of bottom-up and top-down forcing remain unresolved. We factorially added nutrients and reduced grazing at 15 sites across the range of the marine foundation species eelgrass (Zostera marina) to quantify how top-down and bottom-up control interact with natural gradients in biodiversity and environmental forcing. Experiments confirmed modest top-down control of algae, whereas fertilisation had no general effect. Unexpectedly, grazer and algal biomass were better predicted by cross-site variation in grazer and eelgrass diversity than by global environmental gradients. Moreover, these large-scale patterns corresponded strikingly with prior small-scale experiments. Our results link global and local evidence that biodiversity and top-down control strongly influence functioning of threatened seagrass ecosystems, and suggest that biodiversity is comparably important to global change stressors.

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...

Trophic cascades occur in many ecosystems, but the factors regulating them are still elusive. We suggest that an overlooked factor is that trophic interactions (TIs) are often scale-dependent and possibly interact across spatial scales. To explore the role of spatial scale for trophic cascades, and particularly the occurrence of cross-scale interactions (CSIs), we collected and analysed food-web data from 139 stations across 32 bays in the Baltic Sea. We found evidence of a four-level trophic cascade linking TIs across two spatial scales: at bay scale, piscivores (perch and pike) controlled mesopredators (three-spined stickleback), which in turn negatively affected epifaunal grazers. At station scale (within bays), grazers on average suppressed epiphytic algae, and indirectly benefitted habitat-forming vegetation. Moreover, the direction and strength of the grazer-algae relationship at station scale depended on the piscivore biomass at bay scale, indicating a cross-scale interaction effect, potentially caused by a shift in grazer assemblage composition. In summary, the trophic cascade from piscivores to algae appears to involve TIs that occur at, but also interact across, different spatial scales. Considering scale-dependence in general, and CSIs in particular, could therefore enhance our understanding of trophic cascades.

Regime shifts in ecosystem structure and processes are typically studied from a temporal perspective. Yet, theory predicts that in large ecosystems with environmental gradients, shifts should start locally and gradually spread through space. Here we empirically document a spatially propagating shift in the trophic structure of a large aquatic ecosystem, from dominance of large predatory fish (perch, pike) to the small prey fish, the three-spined stickleback. Fish surveys in 486 shallow bays along the 1200 km western Baltic Sea coast during 1979-2017 show that the shift started in wave-exposed archipelago areas near the open sea, but gradually spread towards the wave-sheltered mainland coast. Ecosystem surveys in 32 bays in 2014 show that stickleback predation on juvenile predators (predator-prey reversal) generates a feedback mechanism that appears to reinforce the shift. In summary, managers must account for spatial heterogeneity and dispersal to better predict, detect and confront regime shifts within large ecosystems.