With millions of dollars being poured into urban and suburban stream restoration projects across the Chesapeake Bay watershed, a recent study suggests location matters when trying to assess how effective those efforts have been.
After surveying 13 Baltimore highly degraded suburban streams that had undergone makeovers, a pair of researchers found that aquatic insect populations were larger and more diverse in isolated headwaters than in larger downstream reaches.
“If biodiversity is important, (gains are) more apparent in smaller streams. And smaller streams, I would think, are going to be much cheaper to restore,” said Christopher Swan, the study’s lead author and professor of geography and environmental sciences at the University of Maryland Baltimore County.
Swan and co-author Bryan Brown, associate professor of biological sciences at Virginia Tech, published their findings in the September issue of Ecological Applications, the journal of the Ecological Society of America. Swan is affiliated with UMBC's Center for Urban Environmental Research and Education.
The streams they checked in Baltimore County had undergone typical restoration work, including stabilization of eroding banks, manipulation of their stream beds and extensive tree planting. They sampled them quarterly for a year, along with 13 nearby streams for comparison.
Their conclusion: restored headwaters had 15 to 37 percent more populations of aquatic insects than neighboring unrestored streams. But in larger downstream reaches, they saw almost no difference in species richness between restored and unrestored streams.
In an interview, Swan cautioned that the lack of noticeable improvement in aquatic insects in the larger streams does not necessarily mean the projects were a failure. In those larger downstream reaches, insect populations are heavily influenced by adjacent areas from which insects and fish can move back and forth. In isolated headwaters, by contrast, local water quality conditions appear to have a significant effect on the number and diversity of aquatic insects.
“If you’re on a more well-connected part of a river network,” he said, “the movement of animals in streams could overwhelm any effect that restoration might produce.”
And while restored headwaters saw an increase in aquatic life, it wasn’t enough to transform their ecological health, the researchers found. The insect populations seen in all the restored streams were generally low, meaning they were all “heavily impaired” regardless of location. Even an extensive restoration of an urban stream, it seems, can’t make up for all the pavement and runoff in its watershed.
Streams often get restored for other reasons, of course, Swan noted. In the Bay watershed, many projects are primarily aimed at reducing the amounts of sediment and nutrient pollution getting washed down into the Chesapeake. But even those projects typically identify improved biodiversity as an additional goal.
“What we have done is offered another component, or another thing to consider when restoring streams,” Swan said. “If biodiversity is an endpoint, position within a river network does seem to matter.”
To demonstrate the study’s findings, Swan took a reporter to see a restored stretch of Stony Run, a tributary of the Jones Falls that wends its way through North Baltimore. We encountered hikers using a streamside trail as we worked our way down to the water.
“It looks great,” he said, “But you see exactly what they did: They armored the banks to keep it from eroding, and provided step pools.” Though the water was clear, with aquatic insects skating across its surface, he noted, there was “very little riffle habitat” in the stream bed for fish and bottom-dwelling insects. “But it looks good … they built a trail for people, how could you not like it?”
Even so, with streams like this, he added, “If you take a look at the number of species we saw, on average, it wasn’t a lot. And when we saw improvement, percentage wise it was significant, but a high percent of a low number is still a low number.”