Species Richness and Abundance of Benthic Invertebrates Are Multiplied by Beaver (Castor sp.) Activities in Small Floodplains

Abstract

Beaver activities modify floodplains of small streams fundamentally. Prevailing lotic sections are supplemented with several additional habitats (e.g., ponds, dams and side channels). While there are several investigations into the invertebrate fauna of single beaver-induced habitats, the ways beavers modify aquatic invertebrate assemblages of entire floodplain sections have not been explored. We investigated three beaver territories and three upstream floodplain sections (“non-beaver territories”). We quantified the areas covered by different aquatic habitat types and collected 188 habitat-specific invertebrate samples that yielded more than 82,000 individuals. We used GLMMs with logit links and PCoA to quantify the differences between territory types in terms of aquatic habitat area, abundance, species richness, general community composition, and flow preferences. The area of aquatic habitats increased six-fold in beaver territories. The abundance of benthic invertebrates increased by a factor of 4.5. From species extrapolation analysis, we inferred that species richness in beaver territories increased by factor of 2.7, compared to non-beaver territories. Of the 254 taxa recorded, 141 occurred exclusively in beaver territories and only four taxa occurred exclusively in non-beaver territories. The most taxa-rich habitat type was side channels in beaver territories. Community composition was most similar between habitat types with comparable flow patterns (e.g., beaver ponds and pools in non-beaver territories). In both territory types, rheophile taxa accounted for the largest percentage, with a median of 57% in beaver territories and 61 % in non-beaver territories. The number of rheobiont taxa (21 in total) was higher in beaver territories (20 taxa) than non-beaver territories (16 taxa), while the overall proportion of rheobiont taxa was higher in non-beaver territories. Beaver activities greatly enhanced species richness and abundance of aquatic invertebrates. While a predominantly lotic community shifts to an assemblage with more heterogeneous flow preferences, almost no taxa (not even those dependent on high currents) were eliminated from beaver territories. Beavers thus enhance aquatic invertebrate biodiversity while maintaining the original community. Beaver activities are cost-effective means by which the biodiversity of small floodplains can be enhanced and should be an integral part of restoration plans in regions where beavers were formerly present.