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Structural complexity strongly influences the outcome of predator-prey interactions in benthic marine communities affecting both prey concealment and predator hunting efficacy. How habitat structure interacts with species-specific differences in predatory style and antipredatory strategies may therefore be critical in determining higher trophic functions. We examined the role of structural complexity in mediating predator-prey interactions across several macrophyte habitats along a gradient of structural complexity in three different bioregions: western Mediterranean Sea (WMS), eastern Indian Ocean (EIO) and northern Gulf of Mexico (NGM). Using sea urchins as model prey, we measured survival rates of small (juveniles) and medium (young adults) size classes in different habitat zones: within the macrophyte habitat, along the edge and in bare sandy spaces. At each site we also measured structural variables and predator abundance. Generalised linear models identified biomass and predatory fish abundance as the main determinants of predation intensity but the efficiency of predation was also influenced by urchin size class. Interestingly though, the direction of structure-mediated effects on predation risk was markedly different between habitats and bioregions. In WMS and NGM, where predation by roving fish was relatively high, structure served as a critical prey refuge, particularly for juvenile urchins. In contrast, in EIO, where roving fish predation was low, predation was generally higher inside structurally complex environments where sea stars were responsible for much of the predation. Larger prey were generally less affected by predation in all habitats, probably due to the absence of large predators. Overall, our results indicate that, while the structural complexity of habitats is critical in mediating predator-prey interactions, the direction of this mediation is strongly influenced by differences in predator composition. Whether the regional pool of predators is dominated by visual roving species or chemotactic benthic predators may determine if structure dampens or enhances the influence of top-down control in marine macrophyte communities. [ABSTRACT FROM AUTHOR]

Densities of submerged vegetation and those of associated animals tend to co-vary. This relationship is often attributed to the positive correlation between the density of vegetation and its protective value against predators. However, two counteracting basic elements underlying this paradigm limit its generality. That is, increasing vegetation density should result in decreased predator-prey encounters, whilst at the same time predator-prey encounters should increase as animal densities increase. These two mechanisms should thus counteract each other when the densities of vegetation and associated animals, including both prey and predators, co-vary. Experimental designs that expose fixed densities of prey and predators to varying densities of vegetation assess only the former mechanism and may thus not properly evaluate the protective value of vegetation in such conditions. By contrast, designs that mimic the naturally co-varying organism densities test both mechanisms and thus their counteractive impacts on predator-prey encounters. We compared the outcomes of the two alternative designs and carried out additional experiments to explain the putative discrepancy. Increasing vegetation density (mimics of Potamogeton pectinatus) enhanced prey ( Daphnia magna) survival only when fixed densities of prey and predators ( Perca fluviatilis or Rutilus rutilus) were used. When the animal densities were allowed to co-vary with vegetation density, vegetation had no impact on prey survival. Instead, prey survival was determined by the aggregate density of prey and predators, shaped by the species-specific traits of the latter. Thus, the impact of the increased animal densities overrode the impact of the increased vegetation density on predator-prey encounters. It may be insufficient to attribute the co-variation of vegetation, prey and predator densities simply to the association between vegetation density and its protective value. Increased food resources and reduced competition within vegetation may promote prey and thereby also predator abundance to a greater extent than previously thought. [ABSTRACT FROM AUTHOR]