Résumé: Predictive models accounting for the effect of bioturbation on sediment resuspension must be based on ecological theory as well as on empirical parametrization. The scaling trend of individual metabolic and activity rates with body mass may be a key to the mechanistic understanding of the observed patterns. With this study we tested if general size scaling rules in bio-mediated sediment resuspension may apply to a broad range of physical contexts for the endobenthic bivalve Cerastoderma edule. The effect on sediment resuspension of populations of C. edule differing by individual size was measured across physical gradients of current velocity and sediment composition in terms of fraction of fine particles. C. edule were able to enhance the resuspension of sediment containing silt, while they had scarce effect on the resuspension of coarse sediment. The effect of bioturbation was maximal at intermediate current velocity, when the hydrodynamic forcing is not strong enough to overcome the abiotic sediment resistance but it is able to suspend the bioturbated sediment. Although differences in sediment silt content and intensities of hydrodynamic stress have a relevant influence in determining the bioturbators individual contribution to sediment resuspension, the observed mass scaling trend is consistent across all treatments and close to theoretical expectation for size scaling of individual metabolic rates. This observation supports the hypothesis that the contribution of individual bioturbators to sediment resuspension is directly related to their energy use. Therefore, the proposed approach allows the formulation of expectations of biotic contribution to sediment resuspension based on the general size scaling laws of individual energy use.

Résumé: Salinity tolerance is an important trait that governs the ecology of disease-vector mosquitoes by determining their choice of larval habitat, and consequently their ecological and geographical distribution. Here, we used laboratory strains to determine the osmotic responses of larvae of obligate freshwater disease-vector mosquitoes (Aedes aegypti, Aedes albopictus, Anopheles coluzzii, An. gambiae, Culex pipiens, and Cx. quinquefasciatus) and assessed their relationship with salinity tolerance. First, we analyzed the acute dose-mortality response of fourth-instar larvae to salinity; then, we measured their hemolymph osmolality after 24-h exposure to varying salinities. We found that Ae. albopictus was the most tolerant species, followed by An. coluzzii, Ae. aegypti, Cx. quinquefasciatus, and An. gambiae, in decreasing order. Cx. pipiens was the least tolerant species. All mosquitoes were hyper-iso-osmoregulators, but with species-specific differences. Specifically, hemolymph osmolality in deionized water varied among species, and Cx. pipiens and the two Aedes species showed the lowest and highest osmolality. Although all species were osmoconformers at higher salinity values, hemolymph osmolality approached environmental osmolality more rapidly in species of the Culex genus, compared with Aedes species where it increased slowly. Moreover, hemolymph osmolality in deionized water was significantly correlated with tolerance to salinity across species. This could allow predicting the salinity tolerance of untested species on the basis of their osmoregulatory ability. However, this correlation disappeared when considering the hemolymph osmolality of larvae exposed to salinities higher than deionized water.

Résumé: Agricultural land use is a primary driver of environmental impacts on streams. However, the causal processes that shape these impacts operate through multiple pathways and at several spatial scales. This complexity undermines the development of more effective management approaches, and illustrates the need for more in-depth studies to assess the mechanisms that determine changes in stream biodiversity. Here we present results of the most comprehensive multi-scale assessment of the biological condition of streams in the Amazon to date, examining functional responses of fish assemblages to land use. We sampled fish assemblages from two large human-modified regions, and characterized stream conditions by physical habitat attributes and key landscape-change variables, including density of road crossings (i.e. riverscape fragmentation), deforestation, and agricultural intensification. Fish species were functionally characterized using ecomorphological traits describing feeding, locomotion, and habitat preferences, and these traits were used to derive indices that quantitatively describe the functional structure of the assemblages. Using structural equation modeling, we disentangled multiple drivers operating at different spatial scales, identifying causal pathways that significantly affect stream condition and the structure of the fish assemblages. Deforestation at catchment and riparian network scales altered the channel morphology and the stream bottom structure, changing the functional identity of assemblages. Local deforestation reduced the functional evenness of assemblages (i.e. increased dominance of specific trait combinations) mediated by expansion of aquatic vegetation cover. Riverscape fragmentation reduced functional richness, evenness and divergence, suggesting a trend toward functional homogenization and a reduced range of ecological niches within assemblages following the loss of regional connectivity. These results underscore the often-unrecognized importance of different land use changes, each of which can have marked effects on stream biodiversity. We draw on the relationships observed herein to suggest priorities for the improved management of stream systems in the multiple-use landscapes that predominate in human-modified tropical forests.

Résumé: The costs and benefits of group living often depend on the spatial position of individuals within groups and the ability of individuals to occupy preferred positions. For example, models of predation events for moving prey groups predict higher mortality risk for individuals at the periphery and front of groups. We investigated these predictions in sardine (Sardinella aurita) schools under attack from group hunting sailfish (Istiophorus platypterus) in the open ocean. Sailfish approached sardine schools about equally often from the front and rear, but prior to attack there was a chasing period in which sardines attempted to swim away from the predator. Consequently, all sailfish attacks were directed at the rear and peripheral positions of the school, resulting in higher predation risk for individuals at these positions. During attacks, sailfish slash at sardines with their bill causing prey injury including scale removal and tissue damage. Sardines injured in previous attacks were more often found in the rear half of the school than in the front half. Moreover, injured fish had lower tail-beat frequencies and lagged behind uninjured fish. Injuries inflicted by sailfish bills may, therefore, hinder prey swimming speed and drive spatial sorting in prey schools through passive self-assortment. We found only partial support for the theoretical predictions from current predator-prey models, highlighting the importance of incorporating more realistic predator-prey dynamics into these models. This article is part of the themed issue 'Physiological determinants of social behaviour in animals'.

Résumé: Aim We investigate patterns of phylogenetic diversity in relation to species diversity for European birds, mammals and amphibians to evaluate their congruence and highlight areas of particular evolutionary history. We estimate the extent to which the European network of protected areas (PAs) network retains interesting evolutionary history areas for the three groups separately and simultaneously. Location Europe Methods Phylogenetic (QE(PD)) and species diversity (SD) were estimated using the Rao's quadratic entropy at 10 ' resolution. We determined the regional relationship between QE(PD) and SD for each taxa with a spatial regression model and used the tails of the residuals (QE(RES)) distribution to identify areas of higher and lower QE(PD) than predicted. Spatial congruence of biodiversity between groups was assessed with Pearson correlation coefficient. A simple classification scheme allowed building a convergence map where a convergent pixel equalled to a QE(RES) value of the same sign for the three groups. This convergence map was overlaid to the current PAs network to estimate the level of protection in convergent pixels and compared it to a null expectation built on 1000 randomization of PAs over the landscape. Results QE(RES) patterns across vertebrates show a strong spatial mismatch highlighting different evolutionary histories. Convergent areas represent only 2.7% of the Western Palearctic, with only 8.4% of these areas being covered by the current PAs network while a random distribution would retain 10.4% of them. QE(RES) are unequally represented within PAs: areas with higher QE(PD) than predicted are better covered than expected, while low QE(PD) areas are undersampled. Main conclusions Patterns of diversity strongly diverge between groups of vertebrates in Europe. Although Europe has the world's most extensive PAs network, evolutionary history of terrestrial vertebrates is unequally protected. The challenge is now to reconcile effective conservation planning with a contemporary view of biodiversity integrating multiple facets.

Résumé: Aestivation or daily torpor is an adaptive tactic to survive hot and dry periods of low food availability, and has been documented for species of lungfishes, teleost fishes, amphibians, reptiles, birds, and mammals. Among these species, aestivation is characterized by inactivity and fasting, and for lungfishes and amphibians the formation of a cocoon around the body to retard water loss. While metabolic and physiological changes to aestivation have been well examined, few studies have explored the morphological responses of organs and tissues to aestivation. Predictably, inactive tissues such as skeletal muscles and those of the gastrointestinal tract would regress during aestivation, and thus aid in the reduction of metabolic rate. African lungfishes experience changes in the structure of their skin, gills, lungs, and heart during aestivation. For anurans, the group most thoroughly examined for morphological responses, aestivation generates significant decreases in gut mass and modification of the intestinal epithelium. Intestinal mucosal thickness, enterocyte size, and microvillus length of anurans are characteristically reduced during aestivation. We can surmise from laboratory studies on fasting reptiles, birds, and mammals that they likewise experience atrophy of their digestive tissues during torpor or aestivation. Aestivation-induced loss of tissue structure may be matched with a loss of cellular function generating an integrative decrease in tissue performance and metabolism. Ample opportunity exists to remedy the paucity of studies on the morphological plasticity of organs and tissues to aestivation and examine how such responses dictate tissue function during and immediately following aestivation.