Extra terrestrials: drought creates niche space for rare invertebrates in a large-scale and long-term field experiment

Abstract

Freshwater habitats are drying more frequently and for longer under the combined pressures of climate change and overabstraction. Unsurprisingly, many aquatic species decline or become locally extinct as their benthic habitat is lost during stream droughts, but less is known about the potential 'winners': those terrestrial species that may exploit emerging niches in drying riverbeds. In particular, we do not know how these transient ecotones will respond as droughts become more extreme in the future. To find out we used a large-scale, long-term mesocosm experiment spanning a wide gradient of drought intensity, from permanent flows to full streambed dewatering, and analysed terrestrial invertebrate community assembly after 1 year. Droughts that caused stream fragmentation gave rise to the most diverse terrestrial invertebrate assemblages, including 10 species with UK conservation designations, and high species turnover between experimental channels. Droughts that caused streambed dewatering produced lower terrestrial invertebrate richness, suggesting that the persistence of instream pools may benefit these taxa as well as aquatic biota. Particularly intense droughts may therefore yield relatively few 'winners' among either aquatic or terrestrial species, indicating that the threat to riverine biodiversity from future drought intensification could be more pervasive than widely acknowledged.

Keywords: biodiversity; community assembly; drying; invertebrates; mesocosm experiment; stream drought.

Figure 1.

Non-metric multidimensional scaling output (stress = 0.1) illustrating differences in assemblage composition between treatments, with species scores displayed by taxonomic order. Shaded polygons are the minimum convex hulls that encompass all the channels in each treatment and photos depict an example channel from each treatment (clockwise from bottom left: connected, fragmented and dry). Labelled species are those designated nationally scarce in the UK.

Figure 2.

Pairwise Sørensen dissimilarities between the pooled communities of connected, fragmented and dry treatments, partitioned into turnover (β_sim) and nestedness-resultant (β_nes) components (top) and multiple site dissimilarities among channels within each treatment (bottom). Error bars in both panels display 95% CI.