Flow variation at multiple scales filters fish life histories and constrains community diversity in desert streams

Abstract

Environmental regimes, which encompass decadal-scale or longer variation in climate and disturbance, shape communities by selecting for adaptive life histories, behaviors, and morphologies. In turn, at ecological timescales, extreme events may cause short-term changes in composition and structure via mortality and recolonization of the species pool. Here, we illustrate how short-term variation in desert stream fish communities following floods and droughts depends on the context of the long-term flow regime through ecological filtering of life history strategies. Using quarterly measures of fish populations in streams spanning a 10-fold gradient in flow variation in Arizona, USA, we quantified temporal change in community composition and life history strategies. In streams with highly variable flow regimes, fish communities were less diverse, fluctuation in species richness was the principle mechanism of temporal change in diversity, and communities were dominated by opportunistic life history strategies. Conversely, relatively stable flow regimes resulted in more diverse communities with greater species replacement and dominance of periodic and equilibrium strategies. Importantly, the effects of anomalous highand low-flow events depended on flow regime. Under more stable flow regimes, fish diversity was lower following large floods than after seasons without floods, whereas diversity was independent of high-flow events in streams with flashier flow regimes. Likewise, community life history composition was more dependent on antecedent anomalous events in stable compared to more temporally variable regimes. These findings indicate that extreme events may be a second-level filter on community composition, with effects contingent on the long-term properties of the disturbance regime (e.g., overall degree of variation) in which extremes take place. Ongoing changes to global environmental regimes will likely drive new patterns of community response to extreme events.

Food web efficiency in desert streams

“…The identity of predators can influence ecological efficiencies through mismatched body stoichiometry, foraging preferences, and changes in food chain length (Rock et al 2016). Flow regime variability can additionally structure the composition of fish communities due to differences in life history strategies that are selected for under contrasting regimes, such as fecundity, energy allocation to reproduction, and body size (Mims and Olden 2013; Baruch et al 2022). This study provides estimates of uncertainty in food web efficiency that encompass stochasticity through time and across multiple streams, including inherent variability in community composition.…”

“…We calculated semi‐partial R 2 (R β 2 ) and associated confidence limits (CL) for each fixed effect using the standardized generalized variance approach to account for uncertainty in the estimated relationships (Edwards et al 2008). Annual estimates of GPP, fish production, FCE, and environmental attributes were derived from Baruch et al (2021) and Harms et al (2022) and are available in File S1.…”

“…As such, this metric of hydrologic disturbance is related to food web dynamics across a broad geographic extent (Sabo et al 2010, b ). See Baruch et al (2022) for detailed site and flow regime descriptions and Table S2 for additional metrics of streamflow magnitude and variation at each site.…”

“…Data used in these analyses are freely available online through the Environmental Data Initiative (EDI) https://doi.org/10.6073/pasta/02e0c0ef771cc31b75bca053561d8616 (Baruch et al 2021) and https://doi.org/10.6073/pasta/eb28ac2a44ea8c17084e229d431d17bd (Harms et al 2022).…”

Geomorphological habitat type drives variation in temporal species turnover but not temporal nestedness in Amazonian fish assemblages

“…These events can significantly alter turnover and nestedness rates beyond typical seasonal ranges (Hatosy et al 2013, Langer et al 2016, Pilotto et al 2020). In aquatic ecosystems, such events may include extreme dry periods (Baruch et al 2022), exceptional spates or flood levels (Resh et al 1988, Saito et al 2021), sudden extreme deoxygenation events (Henderson et al 1998), and fluctuations in nutrient availability or other physico‐chemical water properties (Cook et al 2018, Kraus et al 2019).…”