A new study led by Chris Swan has found that stream restoration efforts such as bank stabilization, tree planting, and streambed manipulations have a greater impact on stream life in small headwaters than in larger streams. “Larger streams are more well-connected to populations of species that are tolerant of disturbance, but smaller streams are more isolated, so the actual manipulation for the restoration has a stronger effect on who’s there,” he explains.
The findings suggest that “restoration practices would be most effective in areas where community structure and biodiversity are driven primarily by local factors, i.e., isolated sections of networks,” write Swan, professor of geography and environmental systems, and co-author Bryan Brown of Virginia Tech, in Ecological Applications. “Our results amplify the call for a larger scale perspective on river restoration.”
The study compares adjacent restored and unrestored sections of five headwaters and eight large streams in Maryland. Over the course of a year, Swan, Brown, and lab technician Charles Wahl spent hundreds of hours at the 13 sites, kicking up the streambed to drive tiny invertebrates, just two to 10 millimeters long, into nets. Back in Swan’s lab, the researchers used microscopes to painstakingly identify each organism. Caddisfly and may fly larvae were most common, with a variety of beetle larvae and snails rounding out the catch.
Data analysis revealed that restored sections of headwaters supported 15 to 37 percent more species than unrestored sections next door. However, in large streams, the restoration had no discernible effect on biodiversity. Also, the restored headwaters were more likely to sustain stable communities of organisms, whereas large streams were more likely to gain and lose species in both restored and unrestored sections. Swan suggests the differences may exist “simply because we haven’t appreciated completely the way these organisms move around.”
Swan is also quick to point out that restorations are undertaken for a variety of reasons, and invertebrate diversity is not the only measure of success or failure. In general, “Just because invertebrates didn’t respond, doesn’t mean restoration didn’t do its job.”
Still, Swan notes, these results are important because “this is one of the few studies that finds restoration is context-dependent.” He hopes the study will encourage people to think more thoroughly about how factors like species dispersal and river network connectivity impact biodiversity, ideally trickling up “to the state level, or even the EPA level” to inform future investments in restoration efforts.
It’s notable that the small headwaters most improved by restoration are also the most vulnerable. “When we develop a landscape, the first thing we do is mow over or bury headwater streams, because it’s easy to do—or we put them in a pipe,” Swan says. Plus, “Headwater streams are 60 percent or more of all stream miles in a river network, so they’re disproportionately at risk of being lost.” The good news? “They’re the cheapest to restore, because they’re so small.”
Swan, Brown, and additional colleagues at UC-Riverside and the National Science Foundation’s National Ecological Observatory Network have received a new grant from the NSF, which also funded the current study, to expand on this work by looking at species dispersal in river networks at the watershed level. If a species’ population is declining in a well-connected stream, it may be replenished (or “rescued”) by migration of individual organisms from a nearby stream. In an isolated headwater, that’s much less likely, and the species is more likely to disappear in that location. In their upcoming research, Swan and colleagues will ask, “How does that scale up to the entire watershed and help us understand how biodiversity persists through time?”
Swan hopes the team’s work on species dispersal patterns will further inform practical decisions about investments in supporting healthy streams. The current study was a step in the right direction, he says, demonstrating that “we can do this at a realistic scale,” and showing scientists as well as non-profits, citizens, and lawmakers that this work is “something that matters.”
Learn more from the complete article, “Metacommunity Theory Meets Restoration: Isolation May Mediate How Ecological Communities Respond to Stream Restoration“ in Ecological Applications.
Chris Swan is also known for his research on urban ecology. Learn more from “Urban nature: What kinds of plants and wildlife flourish in cities?” in The Conversation.
Image: Chris Swan speaks at the Green Lab at Light City in Baltimore, April 2017. Photo by Marlayna Demond ’11 for UMBC.