Can restored wetlands make up for the loss of natural systems?
For the past twenty years, federal law has mandated that natural wetlands destroyed by land development be replaced in greater quantity by manmade wetlands. Do these restored wetlands make up for the loss of natural systems? Does wetland mitigation work?
Wetland ecosystems have become the poster child for the science of restoration ecology, in large part due to the federal policy known as “no-net-loss” of wetlands. Established by the first President Bush, the policy mediates the conflict between land development and the protection of wetlands. When wetlands are destroyed through development, agriculture, or other landscape alterations, the law stipulates that for every acre destroyed, at least 1.5 acres must be replaced elsewhere on the landscape. This mitigation policy assumes that restored wetlands are structurally and functionally equivalent to the natural wetlands they replace.
However, recent studies show that created and reclaimed sites often do not achieve structural or functional equivalency with natural wetlands, nor do they appear to achieve natural function over time. Several Kenyon students and I have been researching this question, in an attempt to identify the mechanistic differences in natural and restored wetlands and define what factors limit their success.
First, let's look at what wetlands are. The characteristics of wetlands vary widely, sometimes making them difficult to recognize even when you are standing in one. Wetlands are a diverse group of ecosystems, defined as shallowly to intermittently flooded lands where water affects both the landscape's structure (e.g., species diversity) and associated ecosystem processes (productivity, carbon storage). Wetlands are known by a variety of names, including bogs, fens, tidal marshes, swamps, billabongs, playas, and moors.
Because wetlands were once seen as obstacles to productive land use, for many years government policy encouraged their destruction with the view that the "best wetland was a drained wetland." Estimates are that over the past two hundred years, more than half of the original wetland area in the continental United States has been lost due to human activities, and in Ohio an estimated 90 percent of wetlands have been destroyed or substantially degraded.
As wetlands were eliminated, their losses directly or indirectly brought about changes in the ecosystem services they provide, such as water-quality improvement, flood control, carbon sequestration (they hold approximately 20 percent of the total land-based carbon pool), and the ability to support a diversity of aquatic plant and animal species. Over the past few decades, the reputation of wetlands has undergone nothing short of a public-relations miracle, as their value was recognized.
The no-net-loss mandate for replacement has generated a profusion of wetland restoration projects and spurred research to determine if those projects are working. Our studies here at Kenyon, like several others, have concluded that the policy is failing, as complex and variable natural systems are replaced by ones that are simpler and less diverse. This kind of wetland trading often fails because the science of recreating a fully functioning ecosystem has lagged behind the legal mandate to repair environmental damage. We are asked to recreate ecosystems whose dynamics and intricacies we do not fully grasp.
Why do mitigation projects sometimes fail? In our research, my students and I have found that the most common explanations are linked to the different ecosystem compartments that define wetlands:
Water. The pattern of water levels over time is a particularly important predictor of wetland restoration success. Too much or too little water drastically alters ecosystem processes and species composition. Getting the hydrology wrong, as many projects have, leads to project failure.
Soil. Soil quality is a measure of ecological health and provides a way to gauge the development or progress of wetland restoration projects. Many restoration sites' soils are lacking in organic matter and nutrients, which limits the development of ecosystems.
Plants. How plant communities "assemble" within a wetland is determined by the initial conditions of the site and associated environmental factors, such as flooding, temperature, and nutrient availability. Debates rage about the best techniques for getting the species right, much of it conforming to the philosophy of "build it and they will come." However, results show that if you build it, only a few will come—but not the complete lineup of species that a fully functional site should have.
The extent of human alteration to ecological systems makes it imperative that we learn how to undo some of that damage. Progress must be quick; in the wetlands world, we are faced by the rapid loss of the very ecosystems we study.