Dynamics of Intermittent Stream Habitat Regulate Persistence of a Threatened Fish at Multiple Scales

Traditionally, threatened species management has emphasized conservation of individual populations and has assumed that abundance within suitable habitats is primarily governed by local environmental factors. However, recent research has revealed that landscape-level processes such as disturbance, dispersal, and habitat patch mosaic structure may also strongly influence local populations. We studied the population and habitat dynamics of a threatened fish, the Arkansas darter (Etheostoma cragini) at four spatial scales (pool, reach, segment, watershed) in two intermittent Colorado plains streams. At each scale, information on hydrology, habitat, and fish populations was combined to evaluate factors influencing darter persistence. At the pool scale, Arkansas darters persisted in most permanent pools during summer drought, tolerated extremes of hyperthermia and hypoxia, and were extirpated only when pools dried. Deeper pools had a higher probability of persisting during summer drought. At the reach scale, survival measured during an intensive mark-recapture study was high in a stenothermal spring pool, but low in associated downstream pools that froze during winter and had harsh, variable thermal regimes during summer. However, juvenile darters hatched earlier, grew faster, and were relatively more abundant in the warmer downstream pools compared to juveniles in the spring pool, where older fish dominated. Movement rates between the two habitats were relatively low, but adult darters were more mobile, dispersing out of the spring pool in early spring and returning upstream before the onset of summer drought. At the segment scale, flow variation among seasons and years controlled habitat connectivity, thereby affecting dispersal and reproduction and shaping the distribution and age composition of darter populations. At the watershed scale, nonnative northern pike (Esox lucius) preyed on darters, reducing their distribution and abundance in 13 km of Big Sandy Creek and isolating upstream darter populations from those downstream. Large floods scoured new spring-pool refugia and filled other pools, altering the mosaic of physical habitats. A comprehensive picture of factors controlling Arkansas darter populations and their habitat emerged only when physical and biotic processes were integrated across all four scales. To be effective, recovery efforts for species inhabiting variable environments must consider multiple scales, from landscape-level processes that c