1. Many aspects of global change affect the variability of species population densities, in terms of both the magnitude and pattern of density fluctuations. However, we have limited empirical understanding of the consequences of altered temporal variability of populations, independent of changes in their mean densities, for the structure and stability of natural communities and the responses of ecosystems to additional stressors. 2. We used a field experiment to test the effects of altered temporal variability of a single consumer species on community structure and stability. Specifically, we manipulated the temporal variability of populations of a key grazer species on temperate rocky shores (Littorina littorea), independent of their mean densities, over 12 months and measured the responses of algal communities in terms of multiple measures of structure and stability. Further, we tested whether consumer variability determined the effects of an additional perturbation, elevated sedimentation, on algal communities. 3. The effects of sedimentation on the structure and stability of algal communities were regulated by the temporal variability of consumer populations. In particular, elevated sedimentation led to a decrease in algal evenness, but only when consumer densities were held constant, and resulted in a decrease in the rate of local algal extinctions, but only when consumer temporal variability was increased. 4. Independent of sedimentation, increased temporal variability of consumer populations led to a shift in algal assemblage structure and affected the stability of algal communities in terms of both compositional turnover and resistance to environmental perturbations. Further, these effects varied according to the temporal pattern of consumer density fluctuations. 5. Synthesis. Our results demonstrate that changes in the temporal variability of a single species can modify multiple aspects of both the structure and stability of natural communities and alter their responses to perturbations. However, the effects of consumer variability cannot be predicted without knowledge of the temporal pattern of density fluctuations. These findings have profound implications for our understanding of the effects of multiple disturbances on ecosystems.
Abstract. To understand the consequences of biodiversity loss, it is necessary to test how biodiversity-ecosystem functioning relationships may vary with predicted environmental change. In particular, our understanding will be advanced by studies addressing the interactive effects of multiple stressors on the role of biodiversity across trophic levels. Predicted increases in wave disturbance and ocean warming, together with climate-driven range shifts of key consumer species, are likely to have profound impacts on the dynamics of coastal marine communities. We tested whether wave action and temperature modified the effects of gastropod grazer diversity (Patella vulgata, Littorina littorea, and Gibbula umbilicalis) on algal assemblages in experimental rock pools. The presence or absence of L. littorea appeared to drive changes in microalgal and macroalgal biomass and macroalgal assemblage structure. Macroalgal biomass also decreased with increasing grazer species richness, but only when wave action was enhanced. Further, independently of grazer diversity, wave action and temperature had interactive effects on macroalgal assemblage structure. Warming also led to a reversal of grazer-macroalgal interaction strengths from negative to positive, but only when there was no wave action. Our results show that hydrodynamic disturbance can exacerbate the effects of changing consumer diversity, and may also disrupt the influence of other environmental stressors on key consumer-resource interactions. These findings suggest that the combined effects of anticipated abiotic and biotic change on the functioning of coastal marine ecosystems, although difficult to predict, may be substantial.
Abstract:Our knowledge of the effects of consumer species loss on ecosystem functioning is limited by a paucity of manipulative field studies, particularly those that incorporate inter-trophic effects. Further, given the ongoing transformation of natural habitats by anthropogenic activities, studies should assess the relative importance of biodiversity for ecosystem processes across different environmental contexts by including multiple habitat types. We tested the context-dependency of the effects of consumer species loss by conducting a 15-month field experiment in two habitats (mussel beds and rock pools) on a temperate rocky shore, focussing on the responses of algal assemblages following the single and combined removals of key gastropod grazers (Patella vulgata, P. ulyssiponensis, Littorina littorea and Gibbula umbilicalis). In both habitats, the removal of limpets resulted in a larger increase in macroalgal richness than that of either L. littorea or G. umbilicalis. Further, by the end of the study, macroalgal cover and richness were greater following the removal of multiple grazer species compared to single species removals. Despite substantial differences in physical properties and the structure of benthic assemblages between mussel beds and rock pools, the effects of grazer loss on macroalgal cover, richness, evenness and assemblage structure were remarkably consistent across both habitats. There was, however, a transient habitat-dependent effect of grazer removal on macroalgal assemblage structure that emerged after three months, which was replaced by non-interactive effects of grazer removal and habitat after 15 months. This study shows that the effects of the loss of key consumers may transcend large abiotic and biotic differences between habitats in rocky intertidal systems. While it is clear that consumer diversity is a primary driver of ecosystem functioning, determining its relative importance across multiple contexts is necessary to understand the consequences of consumer species loss against a background of environmental change. Further, given the ongoing transformation of natural habitats by anthropogenic activities, 20 studies should assess the relative importance of biodiversity for ecosystem processes across 21 different environmental contexts by including multiple habitat types. We tested the context-22 dependency of the effects of consumer species loss by conducting a 15-month field 23 experiment in two habitats (mussel beds and rock pools) on a temperate rocky shore, 24 Oikosfocussing on the responses of algal assemblages following the single and combined removals 25 of key gastropod grazers (Patella vulgata, P. ulyssiponensis, Littorina littorea and Gibbula 26 umbilicalis). In both habitats, the removal of limpets resulted in a larger increase in 27 macroalgal richness than that of either L. littorea or G. umbilicalis. Further, by the end of the 28 study, macroalgal cover and richness were greater following the removal of multiple grazer 29 species compared to single species removals. D...
Recent studies have improved our understanding of nearshore marine ecosystems surrounding Ascension Island (central Atlantic Ocean), but little is known about Ascension's benthic environment beyond its shallow coastal waters. Here, we report the first detailed physical and biological examination of the seabed surrounding Ascension Island at 100 -1000 m depth. Multibeam swath data were used to map fine scale bathymetry and derive seabed slope and rugosity indices for the entire area. Water temperature and salinity profiles were obtained from five Conductivity, Temperature, Depth (CTD) deployments, revealing a spatially consistent thermocline at 80 m depth. A camera lander (Shelf Underwater Camera System; SUCS) provided nearly 400 images from 21 sites (100 m transects) at depths of 110 -1020 m, showing high variability in the structure of benthic habitats and biological communities. These surveys revealed a total of 95 faunal morphotypes (mean richness .14 per site), complemented by 213 voucher specimens constituting 60 morphotypes collected from seven targeted Agassiz trawl (AGT) deployments. While total faunal density (maximum .300 m 22 at 480 m depth) increased with rugosity, characteristic shifts in multivariate assemblage structure were driven by depth and substratum type. Shallow assemblages ( 100 m) were dominated by black coral (Antipatharia sp.) on rocky substrata, cup corals (Caryophyllia sp.) and sea urchins (Cidaris sp.) were abundant on fine sediment at intermediate depths (250 -500 m), and shrimps (Nematocarcinus spp.) were common at greater depths (.500 m). Other ubiquitous taxa included serpulid and sabellid polychaetes and brittle stars (Ophiocantha sp.). Cold-water corals (Lophelia cf. pertusa), indicative of Vulnerable Marine Ecosystems (VMEs) and representing substantial benthic carbon accumulation, occurred in particularly dense aggregations at ,350 m but were encountered as deep as 1020 m. In addition to enhancing marine biodiversity records at this locality, this study provides critical baseline data to support the future management of Ascension's marine environment.
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