The role of wildfires in the recovery strategy for the endangered southern California steelhead

Abstract

Southern California steelhead (Oncorhynchus mykiss) occupy wildfire-prone watersheds from the Santa Maria River in Santa Barbara County to the Tijuana River at the U.S.-Mexico border. This tectonically active landscape is characterized by a Mediterranean climate, highly erosive soils, and a fire-dependent chaparral/ coastal sage scrub-dominated plant community. These features create an unstable landscape to which the southernmost steelhead populations have adapted over the past 20 m.y. Wildfires help to create and maintain essential features of the species’ freshwater habitats, including boulder-forced and step pools, which provide oversummering rearing habitat, and spawning gravels, which are essential for reproduction. Disturbance events can also periodically render steelhead spawning and rearing habitat locally inaccessible or unsuitable for the freshwater reproductive phase of their life-history. The episodic nature of wildfires, floods, and droughts characteristic of southern California is reflected in river and stream evolution as a cyclical rather than a linear process. These disturbance events have become more frequent, intense, and extensive as a result of anthropogenic climate change and the increased extent of the urban-wildland human interface with chaparral/coastal sage scrub and forested lands, including the four U.S. national forests in southern California. The long-term viability of southern California steelhead populations requires that they be able to persist under the foreseeable natural disturbance regime characteristic of southern California. The recovery strategy pursued by the National Marine Fisheries Service (NMFS) for the listed endangered southern California steelhead has recognized the essential role of wildfire in the species’ life-history and its role as one of the major natural disturbances that pose a risk to the listed species. Using a wildfire-frequency analysis, NMFS has adopted a recovery strategy consisting of population redundancy and spatial separation to maximize the persistence of the species in the face of wildfire and associated geomorphic processes and facilitate the species’ ability to evolve adaptations in response to changing environmental conditions.