Invasion of temperate riparian forests by Acacia dealbata affects macroinvertebrate community structure in streams

Abstract

The riparian forest determines many of the stream characteristics such as physicochemical variables and the availability and quality of food resources for aquatic communities. In central Portugal, native mixed deciduous forests are being heavily invaded by Acacia dealbata, which is an evergreen and nitrogen-fixing tree species, but effects on stream benthic macroinvertebrate communities are unknown. We assessed invasion effects on macroinvertebrate communities by comparing streams flowing through native forests (native streams) and streams flowing through forests heavily invaded by A. dealbata (invaded streams), in central Portugal. We did not find major differences in macroinvertebrate abundance, total taxa richness or diversity between native and invaded streams, due to high variation among native streams. However, taxonomic structure of macroinvertebrate communities differed between stream types, with invaded streams showing lower percentage of sensitive taxa. Macroinvertebrate communities structure based on functional feeding groups also differed between stream types, with a tendency for higher abundance of shredders and scrapers/grazers and lower abundance of filter feeders in native than invaded streams, probably due to altered composition of leaf litter inputs and canopy cover within invaded streams. Our findings suggest that invasion by A. dealbata affect macroinvertebrate community mostly in terms of taxonomic and functional structure. Since A. dealbata is rapidly spreading in all southern Europe, this study highlights the importance of enhancing the comprehension of its impact on aquatic communities to identify ecological risks and formulate effective mitigation strategies.