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Dagorn, L., Holland, K. N., & Filmalter, J. (2010). Are drifting FADs essential for testing the ecological trap hypothesis ? Fisheries Research, 106, 60–63.
Résumé: Because tropical tunas are known to aggregate around floating objects, it has been suggested that the large number of drifting fish aggregating devices (FADS) built and deployed by purse seiners could act as an 'ecological trap'. This hypothesis states that these networks of drifting FADS could take fish to areas where they would not normally go or retain them in places that they would otherwise leave. Because the ecological trap hypothesis was first advanced for drifting FADs, some have argued that only studies using drifting FADs can test this hypothesis. However, because working with drifting FADs is difficult, accepting this precept would preclude the scientific community from providing urgently needed information to organizations charged with management of fisheries that exploit drifting FADs. We argue that because both anchored and drifting FADs alter the natural environment, the more easily accessible anchored FADs can be used to test the ecological trap hypothesis. Also, based on a comparative scientific approach, we argue that understanding the behaviour of tunas around anchored FADs can improve our general understanding of tunas around all types of floating objects and help design new, well focused studies for drifting FADs. As anchored FADs are easier to access and offer a greater potential for research, we encourage scientists to design and conduct studies (in particular on the behaviour of fish at FADS) around the moored structures.
Mots-Clés: ecological; Fad; hypothesis; trap; Tunas
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Gounand, I., Daufresne, T., Gravel, D., Bouvier, C., Bouvier, T., Combe, M., et al. (2016). Size evolution in microorganisms masks trade-offs predicted by the growth rate hypothesis. Proc. R. Soc. B-Biol. Sci., 283(1845), 20162272.
Résumé: Adaptation to local resource availability depends on responses in growth rate and nutrient acquisition. The growth rate hypothesis (GRH) suggests that growing fast should impair competitive abilities for phosphorus and nitrogen due to high demand for biosynthesis. However, in microorganisms, size influences both growth and uptake rates, which may mask trade-offs and instead generate a positive relationship between these traits (size hypothesis, SH). Here, we evolved a gradient of maximum growth rate (mu(max)) from a single bacterium ancestor to test the relationship among mu(max), competitive ability for nutrients and cell size, while controlling for evolutionary history. We found a strong positive correlation between mu(max) and competitive ability for phosphorus, associated with a trade-off between mu(max) and cell size: strains selected for high mu(max) were smaller and better competitors for phosphorus. Our results strongly support the SH, while the trade-offs expected under GRH were not apparent. Beyond plasticity, unicellular populations can respond rapidly to selection pressure through joint evolution of their size and maximum growth rate. Our study stresses that physiological links between these traits tightly shape the evolution of competitive strategies.
Mots-Clés: Bacteria; bacterial community; biological stoichiometry; cell-size; escherichia-coli; experimental evolution; fresh-water; growth rate hypothesis; inorganic polyphosphate; intrinsic growth; mechanistic approach; Pseudomonas fluorescens; resource competition; r/K strategies; Stoichiometry; variable environment
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Louw, G. G., Freon, P., Huse, G., Lipinski, M. R., & Coetzee, J. C. (2014). Pelagic fish species assemblages in the southern Benguela. African Journal of Marine Science, 36(1), 69–84.
Résumé: Patterns in the co-occurrence of small pelagic fish species within single shoals were investigated using data from 6 814 throws of commercial purse-seiners in South Africa. Assuming that the throw composition reflected the true composition of the assemblage, it was shown that: (1) mixed pelagic assemblages were as prevalent as pure shoals; (2) assemblages of anchovy Engraulis encrasicolus and sardine Sardinops sagax exhibited a seasonal distribution pattern; (3) there was a highly skewed species ratio in terms of abundance by mass; and (4) patterns in the size distributions of two-species shoals were complex and dependent on the L. and the relative abundance of the species concerned. We hypothesise that the observed patterns reflect the 'net gain of the subordinate', whereby fish occurring in small numbers are less conspicuous and/or less energetically attractive for potential predators if they are smaller than the dominant component of the school. If the subordinate fish grow larger than the dominant fish, this advantage persists. Potential sources of bias are alluded to but are not considered to have had a major impact on the conclusions reached, although they may form the basis for further work.
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Valladares, F., Matesanz, S., Guilhaumon, F., Araujo, M. B., Balaguer, L., Benito-Garzon, M., et al. (2014). The effects of phenotypic plasticity and local adaptation on forecasts of species range shifts under climate change. Ecology Letters, 17(11), 1351–1364.
Résumé: Species are the unit of analysis in many global change and conservation biology studies; however, species are not uniform entities but are composed of different, sometimes locally adapted, populations differing in plasticity. We examined how intraspecific variation in thermal niches and phenotypic plasticity will affect species distributions in a warming climate. We first developed a conceptual model linking plasticity and niche breadth, providing five alternative intraspecific scenarios that are consistent with existing literature. Secondly, we used ecological niche-modeling techniques to quantify the impact of each intraspecific scenario on the distribution of a virtual species across a geographically realistic setting. Finally, we performed an analogous modeling exercise using real data on the climatic niches of different tree provenances. We show that when population differentiation is accounted for and dispersal is restricted, forecasts of species range shifts under climate change are even more pessimistic than those using the conventional assumption of homogeneously high plasticity across a species' range. Suitable population-level data are not available for most species so identifying general patterns of population differentiation could fill this gap. However, the literature review revealed contrasting patterns among species, urging greater levels of integration among empirical, modeling and theoretical research on intraspecific phenotypic variation.
Mots-Clés: change impacts; climate change; climate variability hypothesis; ecological niche models; edge populations; environments; evolution; genetic differentiation; global change; intraspecific variation; local adaptation; niche; phenotypic plasticity; population differentiation; quercus-coccifera; reaction norms; thermal tolerance; tree populations
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Wasof, S., Lenoir, J., Hattab, T., Jamoneau, A., Gallet-Moron, E., Ampoorter, E., et al. (2018). Dominance of individual plant species is more important than diversity in explaining plant biomass in the forest understorey. J. Veg. Sci., 29(3), 521–531.
Résumé: QuestionsHow does plant community diversity influence variation in plant biomass? There are two competing hypotheses: the biomass ratio' hypothesis, where biomass is influenced by the abundance and traits of the most dominant species, and the diversity' hypothesis, where the diversity of organisms influences biomass through mechanisms such as niche complementarity. However, no studies have tested which one of these two hypotheses better explains the variation in plant biomass in the forest understorey. LocationTemperate deciduous forests in northern France. MethodsFor the forest understorey, we assessed species diversity and biomass as well as soil and light conditions in 133 forest plots of 100m(2) each. Using mixed-effect models and after controlling for potential confounding factors, we tested the biomass ratio' hypothesis by relating the relative abundance of the most dominant species across our study sites and the CWM of plant traits (leaf area and plant height) to biomass. The diversity' hypothesis was tested by relating biomass to various measures of taxonomic, functional and phylogenetic diversity. ResultsBiomass of the forest understorey was mainly related to the relative abundance and the trait values of the most dominant species, supporting the biomass ratio' hypothesis. In contrast to the diversity' hypothesis, functional diversity indices had a negative impact on biomass. We found no contribution of taxonomic or phylogenetic diversity indices. ConclusionThe abundance and traits of the most dominant species matter more than taxonomic, functional or phylogenetic diversity of the forest understorey in explaining its biomass. Thus, there is a need for experiments that aim to fully understand keystone species' responses to on-going changing biotic and abiotic conditions and to predict their effects on ecosystem functioning and processes.
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