From ecosystems to ecosystem services: Stream restoration as ecological engineering

Ecosystem restoration was originally founded upon recovering ecosystems using wildlands as a reference state. More recently there has been interest in shifting to the restoration of ecosystem services – the benefits that natural systems can provide to humans. This shift is resulting in new restoration goals as well as new methodological approaches. The pace at which restoration goals and methods are changing is particularly fast for running-water ecosystems, which calls for a rigorous assessment of the environmental and economic costs and benefits associated with such changes. In this paper, we explore the environmental costs and benefits of an emerging form of urban stream restoration, in which ecosystems are vastly transformed in order to enhance specific ecosystem functions and support desirable services. These projects are usually implemented in highly incised low-order perennial, intermittent, or ephemeral stream reaches. In either case, the stream channel is transformed into a stormwater management structure designed to reduce peak flows and enhance hydraulic retention of stream flow with the goals of reducing bank erosion and promoting retention of nutrients and suspended sediments. Results to date indicate that this novel ecological design approach does modify the hydrologic responses of streams during some storm events, but there is no consistent pattern of nitrogen retention or removal that would lead to net annual benefits. While additional data are needed, results suggest there is the potential for sediment retention, at least during some flows. Ongoing work which includes monitoring both pre- and post-project implementation will help resolve this uncertainty. If sediment retention does occur, it is likely to decrease over time making the lifespan of these highly engineered projects is finite. Furthermore, environmental impacts associated with these projects can include loss or damage of riparian forests and export of sediment pulses during construction which may offset project benefits depending on their lifespan. Therefore, the use of approaches where entire existing ecosystems are modified to enhance a few specific biophysical processes should be limited to the most degraded systems where less invasive techniques, such as upland reforestation, reduced lawn fertilization, or better stormwater management at the source of runoff generation have first been exhausted.