Ecosystem restoration has received an increasing amount of interest and funding as natural habitats have become progressively degraded. A primary aim of most restoration efforts is to recreate the physical habitat needed to support a focal species. This aim, which has been called the “Field of Dreams” hypothesis (or “if you build it, they will come”), has proven controversial because it represents a somewhat narrow view of what limits the recovery of populations. For example, stream restoration efforts in the western U.S. often focus on the enhancement of channel structure, gravel beds, or other geomorphic features that are designed to improve spawning habitat for adult salmon, a group that has experienced severe population declines and local extinctions. But salmon restoration efforts have been minimally successful in many areas of the west coast. One reason may be that the Field of Dreams hypothesis does not take in to account how physical alteration of spawning habitat might alter food webs in ways that could indirectly influence other life-stages of the focal species, such as juvenile salmon. Here we use a case study to examine how restoration of adult salmon spawning habitat indirectly alters the macroinvertebrate portion of a food web that supports juvenile salmon. We conducted two studies at a restoration site in the Merced River, California to ask 1) has physical habitat restoration altered invertebrate assemblages and, if so, 2) what physical characteristics of a restored stream explain these alterations?
Results/Conclusions
We show that restoration of adult spawning habitat decreases invertebrate abundance and dramatically alters species composition and dominate life-history types. Using an in situ manipulation, we demonstrate that shifts in the invertebrate community are driven by increased substrate mobility and decreased substrate heterogeneity that are typical of restored rivers. Our work suggests that river restoration efforts that lead to geomorphic changes can alter food webs in ways that may indirectly limit population sizes of focal species.
