An index to track the ecological effects of drought development and recovery on riverine invertebrate communities

Abstract

In rivers, the ecological effects of drought typically result in gradual adjustments of invertebrate community structure and functioning, punctuated by sudden changes as key habitats, such as wetted channel margins, become dewatered and dry. This paper outlines the development and application of a new index (Drought Effect of Habitat Loss on Invertebrates-DEHLI) to quantify the effects of drought on instream macroinvertebrate communities by assigning weights to taxa on the basis of their likely association with key stages of channel drying. Two case studies are presented, in which the DEHLI index illustrates the ecological development of drought conditions and subsequent recovery. These examples demonstrate persistent drought effects months or several years after river flows recovered. Results derived using DEHLI are compared with an established macroinvertebrate flow velocity-reactive index (Lotic-invertebrate Index for Flow Evaluation-LIFE score) and demonstrates its greater sensitivity to drought conditions. Data from a number of rivers in south east England were used to calibrate a statistical model, which was then used to examine the response of DEHLI and LIFE to a hypothetical multi-year drought. This demonstrated a difference in response between sampling seasons, with the spring model indicating a lagged response due to delayed recolonisation and the autumn model differentiating habitat loss and flow velocity-driven responses. The application of DEHLI and the principles which underlie it allow the effects of drought on instream habitats and invertebrates associated with short or long term weather patterns to be monitored, whilst also allowing the identification of specific locations where intervention via river restoration, or revision of existing abstraction licensing, may be required to increase resilience to the effect of anthropogenic activities exacerbated by climate change.

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

“…Shifts in community composition as drought intensified appeared to reflect differences in habitat preference and abiotic resistance, as dominance progressively changed from flow‐dependent EPT species to a variety of habitat generalists (chironomids, flatworms) to drought‐resistant specialists and/or air‐breathers, such as arachnids, soldierflies, and biting midges (Boulton & Lake, ; Lake, ). However, the abundance trends of several core taxa were not well explained by physicochemical stress, as we observed contrasting responses of different chironomids with similar tolerances of flow cessation and oxygen depletion (Chadd et al., ; Moog, ). Indeed, channel fragmentation coincided with irruptions of certain taxa typically associated with well‐oxygenated conditions (e.g.…”

“…Despite this context dependence of ecological responses to drying, prevailing theory, developed by Boulton () and others (e.g. Bogan, Boersma, & Lytle, ; Boulton & Lake, ; Chadd et al., ), depicts drought as a stepped ramp disturbance. Here, gradual biotic response to steadily escalating environmental stress (the ramp) is punctuated by sudden changes in community structure as critical habitats are lost (the steps).…”

Extreme drought pushes stream invertebrate communities over functional thresholds

“…More generally, the high sensitivity of many traits to drought intensification highlights their value as functional biomarkers for resistance and resilience at both species and community level, potentially supplementing existing taxonomy-based biomonitoring metrics (e.g., DELHI index; Chadd et al, 2017). For instance, the tendency for most taxa with high physiological resistance to drying to be aerial dispersers underscores the importance of maintaining a network of refugia to act as sources of recolonists.…”

Complexity in hydroecological modelling: A comparison of stepwise selection and information theory

“…In this case, it could be argued that as an evidence‐based methodology, information theory offers clearer, more robust statistical inference. Indeed, this is recognized by two of the authors of the 1999 LIFE paper, who have recently looked to information theory when developing and applying a new index, the Drought Effect of Habitat Loss on Invertebrates (DELHI; Chadd et al ()).…”

“…Hydroecological models are predominantly developed through statistical methods such as regression analysis, including multiple linear regression (e.g., Clarke & Dunbar, , and Monk et al, ), and multilevel models (recent examples include Bradley et al, , and Chadd et al, ). Algorithms are commonly employed to do the “heavy lifting” in the determination of model structure; in hydroecology specifically, stepwise multiple regression is widely used.…”