Diversity mediates the responses of invertebrate density to duration and frequency of rivers' annual drying regime

Abstract

Predicting the impacts of global change on highly dynamic ecosystems requires a better understanding of how communities respond to disturbance duration, frequency and timing. Intermittent rivers and ephemeral streams are dynamic ecosystems that are recognized as the most common fluvial ecosystem globally. The complexity of the drying process can give rise to different annual and antecedent hydrological conditions, but their effect on aquatic communities remains unclear. Here, using aquatic invertebrates from 33 streams across a flow‐intermittence gradient, we assessed how annual (drying duration and frequency) and recent drying characteristics (duration of the last dry period and flowing duration since the last rewetting) affect the density and diversity metrics of communities and trophic groups while controlling for other key abiotic factors (dissolved oxygen and altitude). We characterized invertebrate communities using taxonomy and functional traits to capture biological features that increase vulnerability to drying. In addition, using structural equation modelling (SEM), we evaluated pathways by which drying characteristics directly impact invertebrate density and whether diversity indirectly mediates such relationships. We show that drying frequency drove reductions in diversity at the community level and within trophic groups, whereas both the drying duration and frequency had a negative influence on density metrics. Reductions in taxonomic richness were linked to increased annual drying duration, whereas functional diversity declined in response to annual drying frequency. Filterer, predator and shredder trophic groups exhibited the strongest negative responses to drying. Recent drying characteristics had a minor effect on density and diversity metrics. Our SEM results demonstrated that diversity mediates the negative impacts of annual drying duration and frequency on invertebrate density through reductions in their taxonomic richness and functional diversity. Our results underscore the importance of considering multiple drying characteristics together with the interdependence of density and diversity to better anticipate drying responses in freshwater ecosystems.

Ecological relevance of non‐perennial rivers for the conservation of terrestrial and aquatic communities

“…Although this metric has been widely used to effectively capture spatial differences in riparian vegetation and aquatic macroinvertebrates (Bruno et al., 2014 ; Sánchez‐Montoya et al., 2007 ), other metrics, such as the maximum duration of the dry period or the frequency of drying events, can also be relevant (Arias‐Real et al., 2021 ; Crabot et al., 2020 ; Pineda‐Morante et al., 2022 ; Sánchez‐Montoya et al., 2018 ). The use of in situ hydrological measurements (e.g., data loggers) can better capture recent (e.g., duration of the last dry period) and annual (e.g., annual drying duration and frequency) variability (Arias‐Real et al., 2021 ; Pineda‐Morante et al., 2022 ), but long‐term metrics that represent average conditions in the last decades, such as the mean number of dry days per year and proportion of years with no flow, are particularly relevant and indicated when sampling sites have been surveyed across several years (Bruno, Gutiérrez‐Cánovas, Velasco, et al., 2016a ; Bruno, Gutiérrez‐Cánovas, Sánchez‐Fernández, et al., 2016b ; Belmar et al., 2019 ; Stubbington et al., 2022 ). Finally, that biodiversity surveys were conducted at the end of the wet phase might also have masked flow intermittence effects on biological groups.…”

Integrating spatiotemporal hydrological connectivity into conservation planning to protect temporary rivers

“…Presence/absence of water was registered from July 2018 to December 2019 using temperature data loggers deployed at each sampling site (Pineda‐Morante et al, 2022). This information was used to calculate different hydrological metrics: the total number of dry days or duration of drying events (hereafter TotDur), the frequency of drying events or number of drying events (hereafter TotNum) and the average length of each drying event (hereafter TotLeng) (Arias‐Real et al, 2021; Crabot et al, 2020; Pineda‐Morante et al, 2022). In addition, the flow permanence was calculated as the percentage of the days that reaches were wet along two different time windows: 50 days before each sampling campaign (FP50), and 100 days before each sampling campaign (FP100).…”

Navigating through space and time: A methodological approach to quantify spatiotemporal connectivity using stream flow data as a case study

“…In this work, we used the entire 513‐day period to calculate all the spatiotemporal indices. Using the same flow state database obtained with the data loggers, we also calculated several local hydrological metrics commonly used to characterise surface flow state in temporary streams (Arias‐Real et al, 2021; Crabot et al, 2020; Pineda‐Morante et al, 2022), such as the total number of dry days (TotDur), the number of changes from wet to dry days (TotNum) and the average number of dry days for each drying event (TotLeng).…”

“…So far, flow data obtained using high‐frequency temperature data loggers have been used to characterise the local hydrological conditions of temporary streams (e.g. drying duration, number of consecutive dry days, days with disconnected pools, days after flow return) and its effects on aquatic communities (Arias‐Real et al, 2021; B‐Béres et al, 2019; Beesley & Prince, 2010; Crabot et al, 2020). However, this information has not yet been used to develop spatiotemporal connectivity measures.…”