Emergent intraspecific multiple predator effects shape estuarine trophic dynamics across a gradient of habitat complexity

Estuarine ecosystems are characterized by heterogeneity in species assemblages and habitat complexity, so prey in these systems are often threatened by multiple predators across a mosaic of habitats. When several predator species or conspecifics co-occur, behavioral interactions between them can introduce non-independence to their predator-prey dynamics, which can alter the fate of shared prey. Habitat complexity may regulate these interactions and further impact prey survivorship. We conducted an experiment that manipulated the densities and identities of two ecologically similar estuarine-dependent predators, Red Drum (Sciaenops ocellatus) and Spotted Seatrout (Cynoscion nebulosus), within three habitat complexities. The resulting survivorships of two shared prey types were utilized to determine whether these predators are substitutable, to examine their inter- and intra-specific trophic interactions, and to investigate the impact of habitat structural complexity on their trophic dynamics. The predators differed in their prey preferences and predation pressures, indicating they are generally not substitutable. Interactions between conspecific predators were particularly important in shaping trophic dynamics, and often resulted in combined predation impacts that either enhanced or reduced the survivorship of shared prey as compared to expected values based on foraging rates of individuals. Minimal evidence of interspecific trophic interactions was detected. The observed emergent multiple predator effects differed among the habitats, and were generally intense in intermediate habitat complexity but were either intense or weak in high habitat complexity, depending on the predator combination. These results collectively indicate that prey regulation in this estuarine trophic system is highly dependent on the predator-prey assemblage and habitat context, and enhances our understanding of how multiple predators interactively shape their shared ecosystem

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

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

On the origin and evolution of the material in 67P/Churyumov-Gerasimenko

International audiencePrimitive objects like comets hold important information on the material that formed our solar system. Several comets have been visited by spacecraft and many more have been observed through Earth- and space-based telescopes. Still our understanding remains limited. Molecular abundances in comets have been shown to be similar to interstellar ices and thus indicate that common processes and conditions were involved in their formation. The samples returned by the Stardust mission to comet Wild 2 showed that the bulk refractory material was processed by high temperatures in the vicinity of the early sun. The recent Rosetta mission acquired a wealth of new data on the composition of comet 67P/Churyumov-Gerasimenko (hereafter 67P/C-G) and complemented earlier observations of other comets. The isotopic, elemental, and molecular abundances of the volatile, semi-volatile, and refractory phases brought many new insights into the origin and processing of the incorporated material. The emerging picture after Rosetta is that at least part of the volatile material was formed before the solar system and that cometary nuclei agglomerated over a wide range of heliocentric distances, different from where they are found today. Deviations from bulk solar system abundances indicate that the material was not fully homogenized at the location of comet formation, despite the radial mixing implied by the Stardust results. Post-formation evolution of the material might play an important role, which further complicates the picture. This paper discusses these major findings of the Rosetta mission with respect to the origin of the material and puts them in the context of what we know from other comets and solar system objects

Predation on seeds of the seagrass Posidonia australis in Western Australia

Despite much evidence that predation governs seed abundance, and ultimately seedling and adult plant distribution and abundance in terrestrial ecosystems, there is a dearth of information from seagrass dominated ecosystems. We report here on the first study to examine predation rates from seeds of Posidonia australis measured during field tethering experiments at 5 locations in Western Australia. Seeds that were recently dehisced from ripe fruits and at a similar stage of development were tethered in seagrass and adjacent unvegetated sand for 24 h and then assessed for damage. Seed predation was noted at all sites and ranged from partially to completely eaten seeds. Higher daily proportional damage was observed in seagrass (34 to 53%) than on unvegetated sand (3 to 20%), but was significantly greater at only 3 of the 5 sites. There was no significant difference in proportional mortality for seeds among seagrass meadows, whereas in sand, there was a significant site effect. While we were unable to identify specific seed predators, the type of damage we observed on the seeds suggest small fish or invertebrates are the primary causative agents. Our results add to the growing body of evidence that seagrass seed predation does occur, that it has the potential to affect recruitment, and has implications for understanding the dynamics of P. australis meadows. Finally, our data present an interesting contrast to the paradigm for seagrass faunal studies, which almost invariably have shown higher proportional mortality in bare sand than in seagrass

Critical evaluation of the nursery role hypothesis for seagrass meadows

The vast majority of published papers concerning seagrass meadows contain statements to the effect that seagrass beds serve as important nurseries for many species. We reviewed more than 200 papers that were relevant to the nursery role hypothesis. We used both vote counting and meta-analytic techniques to evaluate whether the body of previous studies that report seagrass meadows to be nursery grounds actually contain data that support this proposition. We restricted our analyses to papers that compared seagrass beds to other habitats, and examined data on a variety of well-studied species concerning their density, growth, survival and migration to adult habitat. Within this group of papers, we considered potential factors that could influence the nursery function (e.g. location, or laboratory vs field studies). We also evaluated case histories of well-documented large-scale seagrass losses on the nursery function. Major results were consistent with the expectations that abundance, growth and survival were greater in seagrass than in unstructured habitats. Abundance data also suggested that seagrass beds in the Northern Hemisphere might be more important as nursery areas than those in the Southern Hemisphere. Surprisingly, few significant differences existed in abundance, growth or survival when seagrass meadows were compared to other structured habitats, such as oyster or cobble reefs, or macroalgal beds. Nor were there decreases in harvests of commercially important species that could clearly be attributed to significant seagrass declines in 3 well-studied areas. However, there were decreased abundances of juveniles of commercially important species in these areas, suggesting a strong link between seagrass abundance and those of juvenile finfish and shellfish. One important implication of these results is that structure per se, rather than the type of structure, appears to be an important determinant of nursery value. Clearly, more rigorous studies that test all aspects of the nursery role hypothesis are clearly needed for seagrass meadows as well as other structured habitats. The results of such studies will allow better decisions to be made concerning the conservation and restoration of marine habitats