Drought effects on invertebrate metapopulation dynamics and quasi‐extinction risk in an intermittent river network

Abstract

Ecological communities can remain stable in the face of disturbance if their constituent species have different resistance and resilience strategies. In turn, local stability scales up regionally if heterogeneous landscapes maintain spatial asynchrony across discrete populations—but not if large‐scale stressors synchronize environmental conditions and biological responses. Here, we hypothesized that droughts could drastically decrease the stability of invertebrate metapopulations both by filtering out poorly adapted species locally, and by synchronizing their dynamics across a river network. We tested this hypothesis via multivariate autoregressive state‐space (MARSS) models on spatially replicated, long‐term data describing aquatic invertebrate communities and hydrological conditions in a set of temperate, lowland streams subject to seasonal and supraseasonal drying events. This quantitative approach allowed us to assess the influence of local (flow magnitude) and network‐scale (hydrological connectivity) drivers on invertebrate long‐term trajectories, and to simulate near‐future responses to a range of drought scenarios. We found that fluctuations in species abundances were heterogeneous across communities and driven by a combination of hydrological and stochastic drivers. Among metapopulations, increasing extent of dry reaches reduced the abundance of functional groups with low resistance or resilience capacities (i.e. low ability to persist in situ or recolonize from elsewhere, respectively). Our simulations revealed that metapopulation quasi‐extinction risk for taxa vulnerable to drought increased exponentially as flowing habitats contracted within the river network, whereas the risk for taxa with resistance and resilience traits remained stable. Our results suggest that drought can be a synchronizing agent in riverscapes, potentially leading to regional quasi‐extinction of species with lower resistance and resilience abilities. Better recognition of drought‐driven synchronization may increase realism in species extinction forecasts as hydroclimatic extremes continue to intensify worldwide.

Extreme low‐flow effects on riverine fauna: A perspective on methodological assessments

“…Hydrological characterisation of present and future ELF events: Flow regimes can vary significantly in chalk headwater environments where a gradient of flow permanence from temporary systems dry for >9 months of the year to perennial environments can occur within small spatial scales (e.g., <7 km - White et al, 2018). Recurring monitoring (White et al, 2018;Sarremejane et al, 2021b) and modelling (Eastman et al, 2021) of flow conditions or states can provide spatially continuous hydrological data over longer time-periods, which could be more widely used to better characterise ELF thresholds within chalk river networks. Such information could be more routinely combined with information including groundwater models, human population growth estimates and hydroclimatic change projections (e.g.…”

“…Metacommunity dynamics: Some field-based assessments have explored faunal assemblage metacommunity dynamics during ELF conditions within chalk rivers (e.g. Sarremejane et al, 2021b). However, there remains a limited understanding of how demographic (e.g.…”

“…Such information could also help target sampling sites within field-based assessments, including deep pools where salmonid species (brown trout [S. trutta] and Atlantic salmon [S. salar]) congregate during ELF events to better understand population collapse risks from stranding and habitat and future ELF conditions could also help directly inform flow conditions within controlled experiments, which could explore the vulnerability of different communities seeded from sites yielding different degrees of flow permanence to respective ELF conditions (e.g. perennial communities subjected to slow-flow conditions; temporary communities exposed to prolonged drying).5.4.2 | Metacommunity dynamicsSome field-based assessments have explored faunal assemblage metacommunity dynamics during ELF conditions within chalk rivers (e.g Sarremejane, Stubbington, et al, 2021)…”

Drought in intermittent river and ephemeral stream networks

“…More recently, research has explored the influence of the longitudinal configuration and extent of drying on the aquatic biodiversity of river networks (Crabot et al, 2020; Sarremejane et al, 2020; Sarremejane, Stubbington, et al, 2021). The dynamics of aquatic invertebrate communities in river networks where drying occurs in headwaters, for example, is very different from those in rivers in which drying occurs in downstream sections (Crabot et al, 2020).…”

Intermittency as an indicator of drought in streamflow and groundwater

“…It is seen that intermittency and drought have been studied separately although they are related to each other. Although fundamental hydrology knowledge suggests indeed that increasing droughts will increase intermittency in non-perennial rivers, there still exists a gap to better understand the relationship between the intermittency and various types of drought in non-perennial river basins which has been studied more in terms of ecohydrology (Kleine et al, 2021;Sarremejane, Messager, & Datry, 2021;Sarremejane, Stubbington, et al, 2021;Sauquet, Beaufort, et al, 2021). The significance of this study is the following hypothesis based on the so-called intermittency cycle (Figure 1), which is a novel concept aiming to better understand this relationship: Due to ever-increasing higher water demand of population and water-related sectors, water abstraction from rivers and groundwater withdrawal from aquifers increase day by day, groundwater level drops down, and it recharges rivers less.…”