From ecosystems to ecosystem services: Stream restoration as ecological engineering
Introduction
The concept of ecosystems as life-support systems and as providers of goods and services that have quantifiable value has now become widely adopted by the scientific and management communities (Cowx and Aya, 2011). The concept has been extremely useful in educating the public about our reliance on natural systems, but it also has implications for the science and practice of restoration. Historically, the focus of restoration ecology was on how best to recover “wildlands,” and the choice of reference systems or a nearby least disturbed ecosystem of similar type for guiding restoration was typically a prior condition (Swetnam et al., 1999, White and Walker, 1997). Of course, the use of such references for restoration has been challenged by two persistent questions: What past? When has a system been free of human disturbance?
These questions are particularly germane given the dramatic changes in land use that have occurred worldwide and the potential impacts of climate change (Davies, 2010). But, if a wildlands concept was not to guide restoration efforts, ecologists had to come up with an alternative. A variety of options have been proposed, including restoration targeting the historical range of variability (Morgan et al., 1994) or some guiding image of that (Palmer et al., 2005), restoration to maximize biodiversity or recover a valued species (Feld et al., 2011), and restoration to recover lost ecosystem processes (Beechie et al., 2010). For river systems in particular, Dufour and Piegay (2009) suggest the use of a restoration framework that incorporates both the historical context of a site (and its potential functions as observed in reference sites) as well as the societal needs for that site when developing restoration objectives. This is an appealing perspective but may be particularly difficult to achieve since current societal needs may conflict with the services an ecosystem provided historically (Sanon et al., 2012).
At the same time that restoration ecologists were broadening perspectives on goals and guidelines for restoration, the formalization and rise in broad use of the ecosystem services concept was occurring (MEA, 2005). Initially, the term “ecosystem services” meant essentially the benefits of nature to households, communities, and economies, and most attention was placed on the valuation of these ecosystem services. More recently, however, understanding when and where specific services are produced has become of great interest in the environmental management community (Daily et al., 2009). Whereas the ecosystem services concept largely arose independent of the concept of ecological restoration, we suggest they are increasingly intersecting. An ecosystem services framework does provide a new way to think about restoration goals and interventions. However, the very act of categorizing services implies an independence of the different components that support an ecosystem (e.g., soils, wetlands, forests) and the processes that sustain it (e.g., carbon cycling, primary production) (Muridan and Rival, 2012). This assumption combined with separate valuation of components and processes (Mehan, 2009) and emerging markets for restoration of specific services has placed additional pressure on ecologists to identify which biophysical processes and ecosystem components must be restored to recover specific ecosystem types and functions (Palmer and Filoso, 2009). If we understand these relationships well and a specific service is desired then restoration can target the subset of processes and components that will lead to the production of that service; however, targeting only a subset could limit the provision of other ecosystem services (Gilvear et al., 2013). For example, work by Sanon et al. (2012) indicated that restoration specifically targeting hydraulic connectivity of an Austrian floodplain would provide habitat for native biodiversity but reduce the provision of drinking water for local citizens. There are also a number of studies that have shown loss of terrestrial ecosystem services related to biodiversity or the provision of water when reforestation restoration is undertaken to enhance carbon sequestration (Hall et al., 2012, Jackson et al., 2005).
The concept of restoration of ecosystem services differs from single- or multi-species management in that the former necessarily is focused on the human use or desire for the service, whereas the latter is often but not necessarily motivated by utilitarian objectives. In both cases, however, concerns have been raised over the potential loss or degradation of ecosystem attributes that are not the focus of management or restoration efforts. Despite these concerns, the trend to focus on ecosystem services as part of ecological restoration and management is increasing (Trabucchi et al., 2012). Oyster restoration has been recommended as a strategy to help reverse eutrophication in coastal waters, and the costs and benefits of forest and wetland restoration are increasingly being evaluated in an ecosystem services framework (Birch et al., 2010, Cerco and Noel, 2007, Jenkins et al., 2010). Adoption of this framework seems to be happening at a particularly rapid pace with respect to running-water ecosystems, in part because of the potential linkage of stream restoration to environmental mitigation markets, but also because of the strong human dependency on the services that rivers provide (Doyle and Yates, 2010, Palmer, 2009). To illustrate how ecological restoration can shift from efforts to recover whole ecosystems and the full suite of their services to efforts undertaken to recover specific attributes or processes, we focus below on Coastal Plain streams. However, this phenomenon is not unique to running-water systems. Similar shifts can be found in very different types of ecosystems and parts of the world (e.g., forest restoration shifting to managed timberland for carbon offsets (Ecotrust, 2013); biodiversity conservation and restoration shifting to habitat creation for selected bird species (Morris et al., 2006)).
Section snippets
Running-water ecosystems and restoration
Streams and their floodplains provide ecosystem services essential to human well-being (Palmer and Richardson, 2009), and have become increasingly managed to optimize these services (Tockner et al., 2011). As a result, the rate of biodiversity loss in running waters exceeds that of terrestrial and marine systems and the water quality status of the world's rivers is declining; this is particularly evident in urban areas. Urban expansion is a major global issue (Seto et al., 2011). In some
The Coastal Plain urban stream example
Many healthy stream ecosystems can store or remove sediment and nutrients before they reach coastal areas. However, the ecosystem processes responsible for storage and removal are closely tied to infiltration and water retention capacity of entire watersheds and may become impaired in urban tributaries. Recovery of these processes has been the motivation for many restoration projects that have led to widely variable outcomes. Increases in stream bank denitrification (Kaushal et al., 2008) and
Restoration as design: implications
We began this paper discussing factors that are contributing to shifting frameworks that guide ecosystem restoration. The move from an ecosystem restoration perspective in which efforts are made to restore historical wildland or least-impacted communities may be giving way to efforts to restore specific ecosystem services. This shift may be associated with a focus on restoring, recovering, or engineering ecosystems to maximize a subset of biophysical processes or ecosystem attributes that
Conclusions
There may be no way to avoid the dramatic urbanization trend occurring worldwide and therefore impacts to natural systems are inevitable. However, there are ways to limit those impacts and even reverse some using principles from restoration science and ecological engineering. Both of these disciplines emphasize identification of the underlying cause of impacts and then determining how lost or impaired biophysical processes can be sustainably recovered or replaced to reverse impacts. Moving to a
Acknowledgements
This work was supported by grants from EPA's Network for Sustainability (#R383220601) and Global Climate Change (#GS-10F-0502N) Programs, by NOAA (#NA100AR4310220), Maryland-MDE and Anne Arundel County; and by a grant from the NSF (DBI-1052875).
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