Drought intensification alters the composition, body size, and trophic structure of invertebrate assemblages in a stream mesocosm experiment

Abstract

Predicted trends towards more intense droughts are of particular significance for running water ecosystems, as the loss of critical stream habitat can provoke sudden changes in biodiversity and shifts in community structure. However, analysing ecological responses to the progressive loss of stream habitat requires a continuous disturbance gradient that can only be generated through large‐scale manipulations of streamflow. In the first experiment of its kind, we used large artificial stream channels (mesocosms) as analogues of spring‐fed headwaters and simulated a gradient of drought intensity that encompassed flowing streams, disconnected pools, and dry streambeds. We used breakpoint analysis to analyse macroinvertebrate community responses to intensifying drought, and identify the taxa and compositional metrics sensitive to small changes in drought stress. We detected breakpoints for >60% of taxa, signalling sudden population crashes or irruptions as drought intensified. Abrupt changes were most pronounced where riffle dewatering isolated pools. In the remnant wetted habitat, we observed a shift to larger body sizes across the community, primarily driven by irruptions of predatory midge larvae and coincident population collapses among prey species (worms and smaller midges). Our results suggest that intense predation in confined, fragmented stream habitat can lead to unexpected changes in body sizes, challenging the conventional wisdom that droughts favour the small. Pool fragmentation might thus be the most critical stage of habitat loss during future droughts, as the point at which impacted rivers and streams begin to exhibit major shifts in fundamental food web properties.

Effects of experimental multi-season drought on abundance, richness, and beta diversity patterns in perennially flowing stream insect communities

“…Drift immigration likely increased after the 2015 flood event also, with the increased discharge bringing in new individuals. In areas without drift immigration, such as headwater systems, drought scenarios could result in high community turnover similar to what has been found in drought studies with no immigration in recirculating mesocosms (Aspin et al, 2019;Leigh et al, 2019).…”

How freshwater biomonitoring tools vary sub‐seasonally reflects temporary river flow regimes

“…However, DEHLI displayed stronger temporal variations compared with Family LIFE and Species LIFE in both case studies and broadly tracked hydrological conditions, most notably within temporary reaches. This provides further evidence highlighting that riverine ecosystems display a stepped response to flow cessation events as key habitats become disconnected within the channel, rather than instream communities responding to a linear continuum of reduced flow velocities (Boulton, ; Bogan et al, ; Chadd et al, ; Aspin et al, a; b).…”

Tipping points and multiple drivers in changing aquatic ecosystems: A review of experimental studies

“…He and colleagues used ANOVA to identify this regime shift. Another positive example, this time on the identification of breakpoints, was provided by Aspin et al (2019) who wished to identify a breakpoint in macroinvertebrate community composition along 21 levels of drought in streams using segmented regression analysis. These two experimental approaches had distinctive objectives (identifying a regime shift vs. a breakpoint) and used appropriate designs and statistical analyses to reach their objectives.…”