Environmental filters – including those resulting from biotic interactions – play a crucial role during the assembly of ecological communities. The importance of scale has thereby been acknowledged but filters at different scales have rarely been quantified in relation to each other, although these hierarchically nested filters eventually determine which communities assemble from a regional species pool. Saproxylic beetles offer an ideal system to study such hierarchically nested environmental filters. Three steps of filtering during the community assembly of these deadwood-dependent beetles are proposed. First, starting from a regional species pool, species must disperse to forest sites. Second, within a site, individuals need to find a patch with preferred microclimatic conditions. Third, the conditions of a single deadwood object (i.e. tree species identity, decomposition stage) at this patch will determine, which species colonise and establish. To study these hierarchical filters, we used unique long-term data sets of saproxylic beetle diversity from trap catches at 29 sites and from emergence traps on 694 experimentally installed deadwood logs at the same sites in three regions in Germany. To relate different environmental filters to beetle assemblages, we used a set of 13 functional traits that are hypothesised to relate to different filters at different scales. We show that all three hierarchical filtering steps resulted in reductions of functional diversity and simultaneous shifts in the functional composition of beetle assemblages, reflecting the roles of different traits in response to different filters. Trait composition changed most strongly at the last filtering step, i.e. depended on tree species identity and decomposition stage. We showed that if community assembly is analysed as a hierarchical multi-step process based on data from different spatial scales, environmental filters can be quantified at these scales. As such, a better understanding of the role that different filters play at different spatial scales can be reached.
