Metrics of disturbance in a redwood forest ecosystem: responses of stream amphibians to repeated sediment infusions

Abstract

Disturbances are part of the natural dynamics of Earth's ecosystems, with these events more common now in the Anthropocene. Yet metrics for calibrating these impacts and measuring an ecosystem's capacity to recover are lacking. Highway construction in 1989 to bypass Prairie Creek Redwoods State Park in California resulted in storm-driven infusion of exposed sediments into five streams; five nearby streams that were not intersected by the bypass construction were not affected by this event and served as controls for a natural experiment. A second large storm event in 1995 contributed sediment loads into all ten streams resulting in a disturbance gradient that allowed us to examine the effects of repeated sediment disturbances. We evaluated the impacts of these stresses on three resident stream amphibian species in 1990 and again in 1996. In 1990, the impacted streams had sixfold higher pool sediment loads and significantly lower larval tailed frog (Ascaphus truei) densities and lower densities of coastal giant salamanders (Dicamptodon tenebrosus) and southern torrent salamanders (Rhyacotriton variegatus) compared with the un-impacted streams. During the six years between these storm events, pool bowl sediment loads increased 14-fold in the previously un-impacted streams and threefold in the previously impacted streams. Larval tailed frogs and torrent salamanders further declined in both sets of streams in 1996 although non-significantly. In contrast, giant salamander densities increased in both stream sets. Of the three species, giant salamanders appeared the most resistant to the depositional events, while the other two species appeared to decline relative to the intensity of the sediment disturbances but still persist. We believe these results demonstrate the usefulness of these three amphibians as metrics for measuring the effects of this common disturbance type on the ecological resilience of stream networks in this and other temperate northwest forest ecosystems.