SUMMARY1. This paper focuses on the premise that the habitat provides the templet upon which evolution forges characteristic species traits. Alternative hypotheses are that there are historic and phylogenetic constraints on the match between organism and environment. 2. In our analysis of river systems, as one dimension of the templet we choose temporal heterogeneity and assume some relationship between this and the frequency of disturbance. The second dimension is spatial heterogeneity in the physicochemical environment and we pay particular attention to the way such variation can ameliorate the influence of disturbances through the provision of refugia. 3. We derive predictions about the traits of species, including size, generation time, reproductive tactics, body form, mobility and potential for regeneration, that are likely to occur in particular regions of the two-dimensional templet. We also present predictions about community-level characteristics such as number of species per resource category and total species richness. The general predictions are intended to apply to a wide range of plants, micro-organisms and animals, provided the measurements have been made at a scale appropriate to them. 4. Hypotheses can be tested by comparing traits predicted for particular habitats, whose spatial and temporal heterogeneity have been quantified, with those actually observed. If the fit turns out to be good, we will be able to conclude that the habitat templet approach offers a sound framework within which to pose questions in river ecology. Habitat templets in ecology
Summary1. Body-size may be an important feature of the structure of food webs. Detailed food web data are however scarce, particularly those including ontogenetic dietary shifts within species. We examined the predator guild in a well characterized food web, that of Broadstone Stream (UK), to assess the importance of body-size within and among species in relation to intraguild predation and niche overlap. 2. In agreement with recent food web theory, mutual predation and cannibalism were frequent and occurred in many pairwise permutations. This intraguild predation was strongly asymmetric, being determined by relative body-size within and among species, and seasonal 'ontogenetic reversals' in trophic status arose when generations overlapped. 3. Predator size determined dietary overlap, with ontogenetic shifts often outweighing taxonomic differences. Small predators had the narrowest diets, regardless of species, and were limited to feeding on a restricted subset of the total prey size-spectrum. Niche overlap decreased as pairwise differences in body-size increased among and within species. Overlap in diet also tracked seasonal changes in resource availability, being highest in summer, when prey were abundant and small, and declining progressively over time, as prey became scarcer and/or larger. The small predators also became more detritivorous as prey abundance declined and the larger prey species attained size-refugia. 4. The body-size constraints driving feeding relationships within the predator guild, in terms of both resource partitioning and intraguild predation, lend support to recent niche models of food web structure (Warren 1996;Williams & Martinez 2000). The highly interconnected food web of Broadstone Stream appeared to be structured by relatively simple rules, with seasonal and ontogenetic shifts in the size-spectrum accounting for most of the changes in predator diet and trophic position. Encounter rate in time (prey and predator mobility) and space (microhabitat use) and foraging mode also influenced prey vulnerability and niche overlap, but were secondary to the effect of body-size.
SUMMARY1. The fundamental importance of freshwater resources, the rapid extinction rate among freshwater species and the pronounced sensitivity of freshwater ecosystems to climate change together signal a pre-eminent need for renewed scientific focus and greater resources. Against this background, the Freshwater Biological Association in 2008 launched a new series of 'summit' Conferences in Aquatic Biology intended to develop and showcase the application of ecological science to major issues in freshwater management. 2. This collection of studies arose from the first summit entitled 'Multiple Stressors in Freshwater Ecosystems'. Although freshwater science and management are replete with mutiple-stressor problems, few studies have been designed explicitly to untangle their effects. 3. The individual case studies that follow reveal the wide array of freshwaters affected by multiple stressors, the spatial and temporal scales involved, the species and ecosystem processes affected, the complex interactions between ecology and socioeconomics that engender such effects, the approaches advocated to address the problems and the challenges of restoring affected systems. The studies also illustrate the extent to which new challenges are emerging (e.g. through climate change), but also they develop a vision of how freshwaters might be managed sustainably to offset multiple stressors in future. 4. More generically, these case studies illustrate (i) how freshwaters might be at particular risk of multiple-stressor effects because of conflicts in water use, and because the hydrological cycle vectors stressor effects so effectively and so extensively; (ii) that dramatic, nonlinear, 'ecological surprises' sometimes emerge as multiple-stressor effects develop and (iii) that good ecology and good ecologists add considerable value to other freshwater disciplines in understanding multiple stressors and managing their effects.
The use of species traits in basic and applied ecology is expanding rapidly because trait-based approaches hold the promise to increase our mechanistic understanding of biological responses. Such understanding could transform descriptive field studies in community ecology into predictive studies. Currently, however, trait-based approaches often fail to reflect species-environment relationships adequately. The difficulties have been perceived mainly as methodological, but we suggest that the problem is more profound and touches on the fundamentals of ecology and evolution. Selection pressures do not act independently on single traits, but rather, on species whose success in a particular environment is controlled by many interacting traits. Therefore, the adaptive value of a particular trait may differ across species, depending on the other traits possessed by the species and the constraints of its body plan. Because of this context-dependence, trait-based approaches should take into account the way combinations of traits interact and are constrained within a species. We present a new framework in which trade-offs and other interactions between biological traits are taken as a starting point from which to develop a better mechanistic understanding of species occurrences. The framework consists of 4 levels: traits, trait interactions, trait combinations, and life-history strategies, in a hierarchy in which each level provides the building blocks for the next. Researchers can contribute knowledge and insights at each level, and their contributions can be verified or falsified using logic, theory, and empirical data. Such an integrated and transparent framework can help fulfill the promise of traits to transform community ecology into a predictive science
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