Alpha and beta diversity of connected benthic–subsurface invertebrate communities respond to drying in dynamic river ecosystems

Abstract

Drying disturbances are the primary determinant of aquatic community biodiversity in dynamic river ecosystems. Research exploring how communities respond to disturbance has focused on benthic invertebrates in surface sediments, inadequately representing a connected community that extends into the subsurface. We compared subsurface and benthic invertebrate responses to drying, to identify common and context-dependent spatial patterns. We characterized community composition, alpha diversity and beta diversity across a gradient of drying duration. Subsurface communities responded to drying, but these responses were typically less pronounced than those of benthic communities. Despite compositional changes and in contrast to reductions in benthic alpha diversity, the alpha diversity of subsurface communities remained stable except at long drying durations. Some primarily benthic taxa were among those whose subsurface frequency and abundance responded positively to drying. Collectively, changing composition, stable richness and taxon-specific increases in occurrence provide evidence that subsurface sediments can support persistence of invertebrate communities during drying disturbances. Beta-diversity patterns varied and no consistent patterns distinguished the total diversity, turnover or nestedness of subsurface compared to benthic communities. In response to increasing drying duration, beta diversity increased or remained stable for benthic communities, but remained stable or decreased for subsurface communities, likely reflecting contrasts in the influence of mass effects, priority effects and environmental filtering. Dissimilarity between subsurface and benthic communities remained stable or increased with drying duration, suggesting that subsurface communities maintain distinct biodiversity value while also supporting temporary influxes of benthic taxa during drying events. As temporary rivers increase in extent due to global change, we highlight that recognizing the connected communities that extend into the subsurface sediments can enable holistic understanding of ecological responses to drying, the key determinant of biodiversity in these dynamic ecosystems.

Local and regional drivers influence how aquatic community diversity, resistance and resilience vary in response to drying

“…taxonomic richness) increased with local flow permanence, measured as TotalFlow, and decreased with the prevalence of both ponded (PoolMonth) and dry (DryMonth) conditions in the year preceding sample collection. The negative relationship between flow intermittence and the α diversity of aquatic communities is well‐established (Datry et al 2014b, Leigh and Datry 2017, Stubbington et al 2019). Our long‐term monthly observations of instream conditions advance understanding of this relationship, by distinguishing between responses to partial (PoolMonth) and complete drying (DryMonth).…”

“…However, taxonomic richness did not change between years as flowing durations increased. This stability suggests that after a dry phase ends, local changes in diversity are either random (Shackelford et al 2017) or result from factors other than disturbance regime and connectivity, such as local refuge availability (Chester and Robson 2011, Stubbington et al 2019).…”

“…In addition, low richness at disturbed sites enhances differences among communities (Chase and Myers 2011). Alternatively, spatial β diversity can decline if extended dry phases homogenize communities through convergent niche selection (Chase 2007, Stubbington et al 2019). Alpha diversity and community temporal stability may also decrease with increasing disturbance extent, if drying reduces catchment‐wide colonist sources, thus reducing stabilizing mass effects (Zelnik et al 2018).…”

Stochastic processes and ecological connectivity drive stream invertebrate community responses to short‐term drought

“…An organism's capacity to persist in drought‐affected systems is regulated by a combination of morphological, behavioural and life‐history traits, including capacity to enter diapause or accelerate development, dietary flexibility, body size, mobility, oxygen acquisition mode and thermal tolerance (Aspin et al., 2019). For example, small invertebrates are better able to manoeuvre within the substrate and may therefore persist during drying events by seeking subsurface refuges (Stubbington et al., 2019). Furthermore, organisms with specialist feeding habits might be more affected by drought than generalists, as drying limits access to some resources (Aspin et al., 2019; Ledger et al., 2011).…”

“…The presence of refuges allows organisms to persist locally and recolonize after disturbance (Lake, 2000). During drought, stream organisms may find refuge within the subsurface sediments (Stubbington et al., 2019) or among patches of leaf litter (Chester & Robson, 2011) where moisture is retained for longer. Community responses to disturbance depend on the size, quality and arrangement of habitat patches (Altermatt & Holyoak, 2012), suggesting that the spatial arrangement of resources and refuges may affect the resistance of a community to, and recovery from, drought.…”