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

Abstract

Freshwater systems are projected to experience increased hydrologic extremes under climate change. To determine how small streams may be impacted by shifts in flow regimes, we experimentally simulated flow loss over the span of three summers in nine 50 m naturally fed stream channels. The aquatic insect community of these streams was sampled before, during, and after experimental drought treatments as well as following one unforeseen flood event. Abundance, richness, and beta diversity were measured as indicators of biotic effects of altered flow regimes. Abundance declined in proportion to flow loss. In contrast, we observed a threshold response in richness where richness did not decrease except in channels where losses of surface flow occurred and disconnected pools remained. The flood reset this pattern, but communities continued their prior trajectories shortly thereafter. Beta diversity partitions suggested no strong compositional shifts, and that the effect of drought was largely experienced uniformly across taxa until flow cessation. Pools served as a refuge, maintaining stable abundance gradients and higher richness longer than riffles. Upon flow resumption, abundance and richness returned to pre-treatment levels within one year. Our results suggest that many taxa present were resistant to drought conditions until loss in surface flow occurred.

Species Composition of Aquatic (Nepomorpha) and Semiaquatic (Gerromorpha) Heteroptera (Insecta: Hemiptera) in Kaeng Krachan National Park, Phetchaburi Province, Thailand

“…Pools became the only available refuges for aquatic insects during these unfavorable periods [71][72][73]. These aquatic true Heteroptera have been reported to colonize new suitable habitat by short flights, and some are known to estivate during unfavorable conditions [74,75]. In this study, numerous species of semiaquatic Heteroptera were found at high densities in pools during the drought period.…”

Climate change is poised to alter mountain stream ecosystem processes via organismal phenological shifts

“…Third, we did not measure immigration or drift of individuals. While immigration from nearby Convict Creek during the experiment could have reduced treatment differences, drift into the channels was negligible in a past study ( 56 ). However, emigrant drift may have differed among our treatments, as described by studies that found short-term increases in drift under reduced flow conditions followed by drift declines ( 20 ).…”

“…Convict Creek also provided natural substrate consisting of cobbles, sand, and silt. This experimental array has been used in past research questions investigating fish growth and stream invertebrate community composition ( 56 , 61 ). The artificial channels have the advantage of mimicking natural ecosystems better than recirculating field mesocosms or laboratory flumes, while allowing for replication that is difficult to obtain in natural streams.…”

Aspects of flow variability and spatial context predict temporal beta diversity in river metacommunities

“…Overall, our results indicate that temporal beta diversity of freshwater benthic macroinvertebrates may be strongly shaped by both long-term hydrological context and spatial context, and that these factors may serve as better predictors of long term community variability than variability in point estimates of environmental measurements. Recent research on flow variability has emphasized extreme flow events such as drought and floods as well as intermittency in driving community patterns (Datry et al 2014, Schneck et al 2017, Saffarinia et al 2022, Rolls et al 2022). In our study, the principal component axis with strong loadings for the magnitude and rates at which the hydrograph rises or falls showed the strongest predictive ability of temporal beta diversity.…”