Last-stand population ecology in fragmented rivers and conservation in de-facto refugia

Authors: Christopher W. Hoagstrom, Thomas P. Archdeacon, Stephen R. Davenport, David L. Propst, James E. Brooks

Year: 2014 (xlvi)

Abstract

Habitat destruction and subdivision associated with dams increasingly restrict sensitive riverine taxa to isolated reaches with residual habitat (i.e., de-facto refugia). Population dynamics in these reaches must be well understood to determine critical habitat features. We used 20 years of population-monitoring data to assess population trends and habitat associations for three imperiled, pelagic-broadcast spawning minnows (Notropis girardi, Notropis jemezanus, and Notropis simus pecosensis). The study reach had a regulated flow regime and three distinct sub-reaches with, from upstream to downstream, “discontinuous” (i.e., degraded) habitat, unchannelized (i.e., high quality) habitat, and channelized (i.e., degraded) habitat. We assessed population and flow-regime influences on recruitment and summarized distributional patterns of cohorts. Each population was persistent and dominated by a single year class. In two species, high-flow events regulated recruitment, which is typical for riverine, pelagic-broadcast spawning minnows. Populations of all three species centered in unchannelized habitat. Intra-reach habitat connectivity was important to facilitate movement to unchannelized habitat from climatic refugia and, in the case of N. s. pecosensis, to allow upstream return of juveniles displaced into channelized habitat as propagules or small age-0 individuals. Over-winter recruitment and climatic refugia appeared to help species withstand streamflow intermittence. Recognition that fragmentation has destroyed all but isolated refugia should motivate proactive place-based conservation where species make a “last stand”. Monitoring provides the basis for comprehensive assessment, but only threat alleviation ensures conservation.

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