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

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 one 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.