Non-random species loss under human disturbance increases the nestedness component of macroinvertebrate beta diversity in tropical streams

In the perspective for partitioning beta diversity proposed by Baselga (2010), species loss was assumed to increase the nestedness component. As species loss is generally non-random under human disturbance, we hypothesised that non-random species loss tends to increase both taxonomic and functional nestedness components, and that such increases can be explained by environmental gradients. We surveyed the macro-invertebrates in two pairs of tropical degraded stream groups (with strong human disturbance) and reference stream groups (with much less human disturbance). We measured and compared both taxonomic and functional beta diversities and their nestedness components between sampling sites in and among the reference and degraded streams. Macroinvertebrate communities indeed underwent significant non-random loss from the reference to degraded streams, which is linked to the traits of taxa responding to disturbances (e.g., body size, tolerance, and feeding habit). Under non-random loss, both taxonomic and functional beta diversities signifi-cantly increased between the reference and degraded streams, mainly due to an increase in their nestedness components. In particular, the nestedness component dominated the functional beta diversity. As expected, taxonomic and functional beta diversities and their nestedness components were explained by environmental distance, specially by the variation of environmental variables such total nitrogen and water velocity. Our study provides evidence at a regional scale for non-random species loss increasing nestedness components of not only taxonomic but also functional beta diversities, and strongly suggests paying attention to reference streams for the protection of regional biodiversity in the tropics.