Ecological filters and variability in stream macroinvertebrate communities: do taxonomic and functional structure follow the same path?

We examined the community-environment relationships of lotic macroinvertebrates in near-pristine headwater streams, and the correlation between patterns in taxonomic and functional structure at two regional extents. The across-ecoregion scale comprised five ecoregions spanning all of Finland, while the within-ecoregion scale comprised of north boreal and middle boreal ecoregions. We expected that taxonomic structure should exhibit stronger relationships than functional structure to spatial gradients, while the reverse should be true for local environmental factors. We found some support for this notion, because spatial variables were marginally more important for taxonomic than functional structure. Furthermore, within the two ecoregions, local environmental variables were slightly more important for functional than taxonomic structure. Geographical location (i.e. spatial variables) was more influential at the across-ecoregions extent than within the two ecoregions. Largely the same local environmental variables accounted for variation in both taxonomic and functional structure, including water pH, nutrients, colour, and stream size. Similar responses to local environmental features likely contributed to the significant correlation between patterns in taxonomic and functional structure. It has been suggested that functional traits may be rather insensitive to natural variation, yet our functional categorization showed distinct variation along local environmental, catchment, and spatial gradients across near-pristine streams. Thus, natural ecological filters should be accounted for prior to the examination of the effects of anthropogenic filters on stream macroinvertebrate community structure.