Invasive species influence macroinvertebrate biomonitoring tools and functional diversity in British rivers.
Biological invasions could have major implications for the management and conservation of freshwater systems if they lead to a misclassification of waterbodies. However, there is limited understanding of the sensitivity of existing biomonitoring tools to invasive species in rivers; and even less known regarding how they influence community taxonomic and functional measures. This research explores the response of freshwater macroinvertebrate communities to biological invasion using taxonomic and functional indices. Utilising a long-term dataset (spanning 2000-2019, 5,988 samples) from rivers in England, the performance of four biomonitoring tools (WHPT, WHPT-ASPT, LIFE and PSI) and two community functional indices (functional richness and redundancy) was examined before and after the colonisation of the invasive species, Dikerogammarus haemobaphes (Eichwald, 1841; Crustacea: Gammaridae). This species represents a recent (first record 2012) and highly successful invader, allowing its range expansion within waterbodies to be examined in detail. Spatial (national and basin level) and seasonal (spring and autumn) effects were investigated using a before-after control-impact (BACI) experimental framework and linear mixed effects models. Results indicated that invasion by D. haemobaphes resulted in significant reductions to the WHPT index and functional diversity metrics (richness and redundancy) while more subtle patterns were observed for other metrics. Analysis of seasonal and individual river basins (River Trent and R. Thames) identified largely consistent responses. The establishment of D. haemobaphes also resulted in some modifications to the functional composition of aquatic communities primarily associated with voltinism and resistance features. Synthesis and applications. Our findings indicate that Dikerogammarus haemobaphes should be considered a significant pressure to riverine communities. These results have implications for biomonitoring, which informs managerial actions as effects may not be detected using a single taxonomic index. Community functional measures are useful in characterising the effects of invasive species and may form a valuable part of the 'toolbox' used for studying biological invasions in rivers. The research illustrates the need to consider the wider threats posed by invasive species on the long-term integrity of freshwaters and the efficacy of freshwater biomonitoring tools.