Posted by: oikosasa | July 9, 2013

Editor’s Choice July

The Editor’s choice papers in the July issue of Oikos are “A critical analysis of the ubiquity of linear local–regional richness relationships” by Goncalves-Souza et al. and “Bottom–up and top–down forces structuring consumer communities in an experimental grassland” by Rzanny et al.  Both are available free online! Here’s the EiC’s motivation for the choice:

DriesBesides promoting synthesis, Oikos has a tradition in publishing studies that challenge widely accepted ecological paradigms. The ubiquity of linear relationship between local and regional species richness is such an idea that found its way to many textbooks. The potential impact of regional and local processes on community structure has been traditionally tested by regressing local against regional species richness. This approach was justified by the idea that communities controlled by regional processes are unsaturated, while those affected by local processes are not. However, while such a linear relationship has been theoretically criticized, a critical reevaluation has so far not been done. Thiago Gonçalves-Souza and colleagues reanalysed published studies with a new unbiased method and found no prevalence of linear relationships and more than 40% of misclassifications. Its apparent ubiquity appeared to be due to the use of biased statistical methods (linear regressions to detect linearity). The study demonstrated such local-regional diversity relationships to follow other ‘rules’ than linear ones. The metacommunity perspective provides a framework to study the importance of processes acting and interacting at different spatial scales. A full understanding of these mechanisms will ultimately generate synthesis on the form and strength of the local-regional diversity scaling rules.

While this framework likely advances our understanding of the processes leading to species diversity, we still lack proper insights on the relative strength of different local mechanisms (food web interactions for instance) that structure species communities. Species at intermediate trophic levels (consumers) are expected to be affected by the interplay between bottom-up and top-down effects, but synthesis on the relative importance of these effects is lacking. By analysing data from a long-term grassland diversity experiment, Michael Rzanny and colleagues demonstrate bottom–up forces to account for the major part of the explainable variation within the composition of all functional groups of consumers (plant chewers, suckers , saprophages) but also predators and parasitoids. Legumes appeared to be an especially important driver of consumer community structure. Predator-mediated top–down forces also influenced the majority of consumer functional groups, but were much weaker. In order to partition the different sources of variation, redundancy analysis was applied. Equally interesting, and again emphasising the interactive effects between local and regional processes, was the importance of different spatial components for explaining, especially, predator community structure.

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