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Stress Gradient Hypothesis (stress + gradient_hypothesis)
Selected AbstractsFacilitation research in marine systems: state of the art, emerging patterns and insights for future developmentsJOURNAL OF ECOLOGY, Issue 6 2009Fabio Bulleri Summary 1. Positive species interactions are increasingly recognized as important drivers of community structure and ecosystem functioning. Although the literature on facilitative interactions in terrestrial environments has been reviewed and emerging patterns have been synthesized, comparable attempts are lacking for the marine realm. 2. By means of a quantitative survey of the literature, I provide a critical summary of current knowledge on positive species interactions in marine environments. In particular, I (i) assess how marine facilitation research compares to that carried out in terrestrial environments in terms of focus and philosophical approach; (ii) illustrate the mechanisms by which facilitation takes place in different habitats; (iii) assess whether benefactor and beneficiary species are more likely to belong to the same or to a different trophic level; and (iv) provide examples of how including facilitation into ecological theory might advance our understanding of the mechanisms that regulate ecosystem functioning. 3. Except for some studies in intertidal habitats, few studies in marine environments have been framed within mainstream facilitation theory (e.g. the Stress Gradient Hypothesis) and research does not seem to be organized in a self-contained theme. Amelioration of physical conditions appears to be the most common mechanism of facilitation in intertidal habitats, whilst associational defence predominates in the subtidal. 4. In contrast to the terrestrial literature, dominated by plant,plant interactions, marine benefactors and beneficiary species often belong to different trophic levels. This might imply little overlapping of resource niches or a differential response to environmental conditions or consumer pressure, with implications for the persistence of facilitative effects at the extreme ends of stress gradients. 5. Recent research shows that facilitation can enhance temporal variability and invasibility of marine communities and emphasizes the central role of positive species interactions in regulating the functioning of natural ecosystems. 6.Synthesis. Studies encompassing a wide variety of life histories and environmental conditions are central to achieving a unified facilitation theory. Research in marine environments can provide new insights into the mechanisms underlying variations in the strength and direction of species interactions, but this will require greater awareness and consideration of facilitation. [source] Facilitation in the conceptual melting potJOURNAL OF ECOLOGY, Issue 6 2009Rob W. Brooker Summary 1. Here we present an introduction to this issue's Special Feature arising from the British Ecological Society Symposium: Facilitation in Plant Communities (20,22 April 2009). 2. Papers in the Special Feature demonstrate the benefits that arise from cross-system application of general concepts, for example, the well-known stress gradient hypothesis. Such comparisons challenge our definition of facilitation, as well as our pre-conceptions on the nature of intermediary organisms. 3. We suggest that under some circumstances a clear definition of the two-way nature of interactions is essential, e.g. when considering the evolutionary implications of facilitation. In other cases, however, we can perhaps be more relaxed, e.g. when facilitation is a component of conservation ecology. 4.Synthesis. Overall we believe that establishing facilitation as an independent concept has driven substantial progress towards a clearer understanding of how ecological systems work. Through the links established by work such as that presented in this Special Feature, we believe this field will continue to make rapid progress and aid ecological understanding in general. [source] Inclusion of biotic stress (consumer pressure) alters predictions from the stress gradient hypothesisJOURNAL OF ECOLOGY, Issue 6 2009Christian Smit Summary 1. ,The stress gradient hypothesis (SGH) predicts a shift from net negative interactions in benign environments towards net positive in harsh environments in ecological communities. While several studies found support for the SGH, others found evidence against it, leading to a debate on how nature and strength of species interactions change along stress gradients, and to calls for new empirical and theoretical work. 2. ,In the latest attempt in this journal, it is successfully argued how the SGH should be expanded by considering different life strategies of species (stress tolerance versus competitive ability) and characteristics of abiotic stress (resource versus non-resource based) over wider stress gradients (opposed to low,high contrasts), but the crucial role of biotic stress by consumers is largely ignored in this refinement. 3. ,We point out that consumers strongly alter the outcome of species interactions in benign and harsh environments, and show how inclusion of consumer-incurred biotic stress alters the predicted outcome of interactions along resource- and non-resource-based stress gradients for stress-tolerant and competitive benefactors and beneficiaries. 4. ,Synthesis. New studies should include stress gradients consisting of both abiotic and biotic components to disentangle their impacts, and to improve our understanding of how species interactions change along environmental gradients. [source] What does the stress-gradient hypothesis predict?OIKOS, Issue 10 2010Resolving the discrepancies In recent years the importance of facilitative interactions in ecological communities is increasingly recognized. This phenomenon has been observed repeatedly, particularly in vegetation communities, in a wide range of environmental conditions. The current hypothesis predicts that the role of facilitation becomes increasingly important in conjunction with increasing stress. Several empirical studies, however, failed to detect such patterns, particularly at the extreme ends of the stress gradients. Herein, we present a conceptual model that may resolve discrepancies between expected and observed and provides a more precise framework of the existing hypotheses. By relaxing two common assumptions commonly used by the stress-gradient hypothesis (SGH) we are able to demonstrate that under some circumstances the importance of facilitation may be less at the extreme ends of these gradients. Namely, we first re-emphasize the notion that physiological response is not linear with respect to environmental changes along stress gradients. Second, it is argued that the net outcome of facilitative and competitive interactions is reflected in the fitness of individuals as a product of these two processes, in contrast to the commonly applied assumption of additivity. Accordingly, a synthesis of the concepts of population biology (measures of fitness) and plant physiology (nonlinear responses) with the stress gradient hypothesis while retaining the original simplicity of the SGH model contributes to a better specification of the predictions of the stress-gradient hypothesis and the resolution of observed contradictions. [source] | ||||||||||