Differences in predator composition alter the direction of structure‐mediated predation risk in macrophyte communities

Abstract

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

The Seagrass Effect Turned Upside Down Changes the Prospective of Sea Urchin Survival and Landscape Implications

“…The scenario is even more complicated by the evidence that the effects of habitat structure are strongly dependent on regional predator pools: indeed, they would be largely a function of predator identity, which determines whether habitat complexity either reduces or enhances top-down control within the ecosystem [16,17]. …”

Evaluating potential artifacts of tethering techniques to estimate predation on sea urchins

“…This does not, however, imply that these predators were unable to detect and respond to chemical cues but that differences between the emitted signals by a pierced and unpierced sea urchin were undistinguishable for the predator under natural field conditions. In fact, it is known that benthic predators, such as gastropods and starfish, are cryptic chemotactic organisms, relying heavily on chemical cues for their predation success in highly structured habitats such as seagrass meadows (Farina et al, 2014). However, the lack of preference for pierced urchins indicates that second order artifacts are minimal for fish and benthic predators.…”

“…When sea urchin outbreaks take place, these communities can shift to a less productive and diverse state-termed "urchin barrens" (Pinnegar et al, 2000). In this context, estimating the ability of predators to control urchin numbers is critical to understand ecosystem functioning (Clemente et al, 2007;Farina et al, 2014;Heck and Valentine, 1995;Heck and Wilson, 1987;McClanahan, 1999;McClanahan and Muthiga, 1989;Pederson and Johnson, 2006;Shears and Babcock, 2002). Tethering experiments can provide insight on the degree to which differences in predation rates between different localities contribute to barren formation through cascading effects (Clemente et al, 2008).…”

Herbivore control in connected seascapes: habitat determines when population regulation occurs in the life history of a key herbivore

“…In contrast, benthic predators (i.e. starfish and whelks) in seagrass meadows likely contribute to overall predation in seagrasses (Boada et al 2015a, Farina et al 2014). …”

“…Seagrass meadows with large canopies protect sea urchins from predation much more than low macroalgal canopies or turf algae (Farina et al 2009, Farina et al 2014, although in this habitat the presence of refugia reduces predation impact since sea urchins tend to stay in small crevices (Boada et al 2016). While it is not particularly surprising that predation is so important in regulating urchin populations, it is of particular relevance for macroalgal communities, given their susceptibility to shift to overgrazed, unproductive barrens (Pinnegar et al 2000).…”