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 high- and 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.