Biological trait composition of European stream invertebrate communities: assessing the effects of various trait filter types

Understanding the action of filters on the biological trait composition of communities is constrained by the multitude of filter types (e.g. abiotic vs biotic, actual vs historical) that may cause changes of a multitude of traits (e.g. small vs large body size, short vs long life cycle) at a multitude of spatial scales (e.g. continent vs landscape vs local site). Using published data on the as natural as possible abundances and 11 biological traits (described through 63 categories) of 254 European stream invertebrate genera, we assessed how already available knowledge can serve to identify the importance of the action of different types of trait filters at two spatial scales. Therefore, we analysed observed and simulated abundance-weighted trait compositions at the local scale of 384 running water sites (RWS) and at the landscape scale of 14 large biogeographical regions (LER). Actual abiotic filters acted significantly and independently of the taxonomic richness on the invertebrate traits at the RWS- and LER-scale, whereas biotic filters had no significant effect. Evidence for the action of historical trait filters across Europe was only weak at both scales. Size, reproductive cycle, respiration and locomotion technique, feeding habits and vulnerability to disturbance responded to altitude and stream width of the RWS according to existing views about the effects of riparian, physiological, interstitial or disturbance controls of these traits. These controls acted independently on trait categories that did not co-occur within the genera, because correlations of size categories with other trait categories were higher in the abundance-weighted trait array (across communities) than in the original trait array (across genera). Overall, many of the 63 trait categories were scarcely affected by the trait filters considered in this study. Therefore, we briefly discuss potential effects of continental filters and of stream system-specific, local physical filters, as the latter should produce similar trait patterns on a global scale. Our study suggests that analyses of the currently available knowledge can simplify the complicated hypothetical framework on trait filter actions, which sharpens the focus on future research needs.