Many people think it’s OK to develop coastal marshes as long as we compensate by restoring them elsewhere. But can we really create marshes? It’s too soon to tell, according to new research in the December 2000 issue of Conservation Ecology.
“Unfortunately, the apparently successful attempts have often been monitored only for a short time period…, if at all,” say Neil Dawe of the Canadian Wildlife Service in Qualicum Beach, British Columbia, and four co-authors.
Coastal marshes cleanse freshwater of silt and nutrients before it flows into the ocean and provide nurseries for fish and feeding grounds for migratory birds. People have damaged many marshes in North America by converting them to agriculture, filling them for industry and housing, and fragmenting them with roads.
To restore marshes, people have tried to both rehabilitate degraded marshes and create new ones. Today, many restoration projects are undertaken to mitigate the loss of natural marshes to development, raising the question of whether we know enough about marshes to create them.
While a good number of these “restored” marshes have been deemed successful, most were monitored for only a year or two. To see what can happen to man-made marshes over a longer time period, Dawe and his colleagues monitored one of the largest marsh creation projects in British Columbia over a 13-year period (1982-1994).
In 1981, four islands were created in Vancouver Island’s Campbell River estuary to mitigate the loss of a natural marsh, which was developed into a log-sorting facility. Altogether, there were nearly six acres of man-made marsh, about two acres of which were planted with 23,000 marsh vegetation plugs that were salvaged when the nearby log-sorting facility was built.
To assess the success of these man-made marshes, Dawe and his colleagues compared the vegetation (coverage, diversity, and aboveground biomass) on the four created islands to that on the natural marshes on Nunn’s Island, which is nearby.
The good news is that by the 13th year, vegetation coverage and diversity were quite similar in both the man-made and natural low marshes. The bad news is that while the vegetation was stable in the natural marshes, it was still changing in the man-made marshes. After growing steadily for at least six years, the vegetation in large areas (up to 40 by 65 feet) died back on three of the man-made islands.
The researchers attribute these die-backs to island settling, which presumably created low areas that held water longer than the plants could tolerate. Because the researchers were monitoring the man-made marshes, they were able to make the simple fix of digging channels to drain the low areas.
Dawe and his colleagues draw two main conclusions from their study. First, we need to monitor restored marshes at least until their vegetation is as stable as that of nearby natural marshes. “Ecosystems are ‘moving targets,’ having uncertain and unpredictable futures,” say the authors. “Had our monitoring survey ended [after the sixth year], we might have concluded that the project was a success.” Second, we need to recognize that we may not know enough about marshes to create them. Otherwise, “the result may be the trading of natural coastal wetlands or mudflats for human-made marshes that ultimately fail to become productive systems,” say the authors.
For more Information
Dawe, N.K., G.E. Bradfield, W.S. Boyd, D.E.C. Trethewey, and A.N. Zolbrod. 2000. Marsh creation in a northern Pacific estuary: Is thirteen years of monitoring enough? Conservation Ecology 4(2):12