States and rates: Complementary approaches to developing flow‐ecology relationships

Abstract

In recognition of the influence of flow on riverine habitats and organisms, stream ecologists have devoted considerable effort to the development of quantitative predictive relationships describing ecological responses to flow variability, i.e. flow‐ecology relationships. Methods used to generate flow‐ecology relationships can be thought of as a continuum bookended by pure states approaches on one end and by rates approaches on the other. In pure states approaches, the ecological response is a snapshot of a condition or property (i.e. a state) derived from a single measurement in time. In contrast, ecological responses in rates approaches reflect temporal change (i.e. a rate) and are thus reliant on repeated measurements made over time. Here, we elaborate on the fundamental characteristics of different approaches (pure states, rates and an intermediate approach we call repeated states) for generating flow‐ecology relationships, examine how commonly the different approaches are used in the flow‐ecology literature, conduct an independent analysis to illustrate the different outcomes achieved by applying repeated‐states and rates approaches using a dataset for stream fish diversity in relation to flow magnitude, and identify some of the different ways ecologists are applying rates approaches in flow ecology. Our literature review revealed that repeated‐states approaches (53% of reviewed studies) were used far more commonly than either pure states (19%) or rates (12%) approaches to generate flow‐ecology relationships. The remaining hybrid studies (17%) used both state and rate responses, and thus also relied on repeated measurements over time. Despite frequent collection of data suitable for rates approaches, flow‐ecology relationships have generally been developed using states approaches that relate changes in ecological states to different long‐term average flow conditions, rather than to specific flow sequences over much shorter time intervals. Such flow‐ecology relationships cannot generate temporally specific predictions of ecological responses to changing flow conditions (i.e. the expected change in state following a specific flow sequence), nor can they describe demographic processes underlying observed changes. While there are different scenarios in which a pure or repeated‐states approach would be useful, more frequent use of rates approaches would increase our ability to test flow‐ecology hypotheses and our mechanistic understanding of flow‐ecology relationships.

Beyond the natural flow regime? Broadening the hydro‐ecological foundation to meet environmental flows challenges in a non‐stationary world

“…Wheeler et al. () report that measurement of ecological states, including their repeated measurement over time, comprise 72% of reviewed approaches for characterising flow–ecology relationships. Against this trend, some have argued that ecological processes, rather than habitat limitation per se, are a more appropriate focus for e‐flows (e.g.…”

“…For example, recruitment failure by insects (Kennedy et al., ) or riparian trees (Lytle & Merritt, ; Rood et al., ) does not manifest in species abundances for many months, and shifts in community structure under flow regulation (Mims & Olden, ) is a relatively slow process that can be measured across many sites simultaneously using monitoring data. The repeated measurement of snapshot state variables over time constitutes a kind of temporally varying ecological properties and is a predominant form of flow–ecology relationship reported in the literature (Wheeler et al., ).…”

Recent advances in environmental flows science and water management—Innovation in the Anthropocene

“…However, their literature review shows that many studies involve a third approach, “repeated states,” characterised by “temporally repeated measurements of ecological state responses.” Repeated‐state responses expressed as rate responses can generate predictions of biotic response to specific flow sequences over time, although they do not reveal the actual mechanisms underlying the dynamics of ecological change. While the authors recognise the strengths and limitations of all these approaches, they argue that “rates approaches have been underused to date and could facilitate substantial progress” in mechanistic understanding of flow–ecological relationships (Wheeler et al., ).…”

Seasonal streamflow extremes are key drivers of Brook Trout young‐of‐the‐year abundance

“…For freshwater populations, predictions that extreme hydrologic events, such as floods and droughts, will increase (Ingram et al 2013, National Academies of Sciences 2016) are of particular concern. We have a limited understanding of how aquatic populations will respond to hydrologic changes (Walters 2016, Wheeler et al 2017. We have a limited understanding of how aquatic populations will respond to hydrologic changes (Walters 2016, Wheeler et al 2017.…”