Failures of Replacement Wetlands
"The debate related to 'No Net Loss' is most often a numbers game and what is usually lost in the debate is the fact that wetlands continue to be lost or degraded and wetlands designed to replace them often have lower biodiversity and do not function similarly to the natural wetlands."
- Dennis Whigham, 1999
Replacement wetlands have been studied extensively by the biology community (Sawhill and Ferguson 1998, Moore et al. 2001, Stolt et al. 2000, Whigham 1999, Redmond 2000, Gopal 1999, Ehrenfeld and Toth 1997, Galatowitsch et al. 1999). Researchers have found several major differences in these replacement wetlands from natural wetlands, including landscape setting, hydrology, level of biodiversity, soil composition, and nutrient availability. These environmental variables cooperate to regulate and support wetland processes. Understanding the failures in replacement wetlands is important because if even one variable is non-functional, the rest are affected. Many of the problems with replacement wetlands have been attributed to the design parameters (Whigham 1999).
The landscape setting of wetlands is very important to take into consideration when proposing a restoration or mitigation project. Wetlands are not isolated ecosystems; they are parts of larger landscapes, such as rivers, streams, and coasts (Whigham 1999). Natural wetlands depend on their surrounding environment for water, nutrient inflows, seeds, migrating species, and protection. There are exchanges between wetlands and the surrounding landscape that allow them to function (Whigham 1999). However, many environmental architects did not take this into consideration, designing flat wetlands with defined boundaries. It has been found that created wetlands are generally more open than natural wetlands. In a study performed by Cole and Brooks (2000) focusing on the differences in natural and mitigation wetlands, it was noted that the created wetlands had many more open patches than the reference wetlands. Furthermore, these replacement wetlands are generally placed in an environment they could never naturally occur in, such as grass plains (Whigham 1999, Stolt et al. 2000). Not only does this affect the environment in which they constructed the wetland, it does not even give the wetland a chance to function properly.
A typical change in a wetland hydrology during the hot months of summer.
It has been stated that "the long-term success of any wetlands restoration or creation project is, to a very large extent, dependent upon restoring, establishing, or developing and managing the appropriate hydrology" (Cole and Brooks 2000). Wetlands have very characteristic hydrology that contributes to wetland function. Wetland hydrology is determined in part by several geological characteristics, including surface relief, land surface slope, thickness and permeability of the soils, and the properties of the underlying geological materials (Bedford 1996). The hydrology of a natural wetland, however, is difficult to replicate because it is dependent on so many geological characteristics as well as the source of water and the surrounding landscape (Bedford 1996, Cole and Brooks 2000). Natural wetlands tend to have frequent water level changes depending on precipitation and temperature, and generally they dry down in the late summer and fall. However, this characteristic is not easily reproduced, so created wetlands are generally flooded with very little variation all year (Gopal 1999).
In a comparison between replacement wetland and natural wetland hydrology, it was found that the mean water level of replacement wetlands was significantly higher than natural wetlands. Water in created wetlands was at or above the water table an average of 32 days before drawing down, compared to only 9 at natural wetlands (Cole and Brooks 2000). Furthermore, water in natural wetlands was above the water table only 30 percent of the time, whereas water in created wetlands was above the water table twice as often, 65 percent of the time (Cole and Brooks 2000). This study is evidence that hydrology in created wetlands has yet to reach parity to natural wetlands.
There are major implications when a wetland does not have characteristic hydrology. Not only is function and sustainability lost, biodiversity is negatively impacted as well. The plants and animals that depend on wetlands are adapted to the natural fluctuations in water levels, and some even require it for seed dispersal. Without this hydrologic diversity, there is less biological diversity in a wetland ecosystem (Gopal 1999).
Wetland biodiversity is extremely important to the health of this ecosystem. It thought that species diversity increases the biological stability of an ecosystem (Tilman 1999). While this is a largely disputed statement in the scientific community, accepted biological theory, such as competitive compensation and niche differentiation effects, fully supports the diversity-stability idea.
There is evidence that the loss of wetlands is having an impact on biodiversity: it is estimated that 46 percent of the nation's endangered species are closely tied to wetlands (Whigham 1999). In replacement wetlands, the species diversity has been reported to be lower than that in natural wetlands (Whigham 1999), and generally the species that dominate in created wetlands are different than in natural wetlands (Stolt et al. 2000). In some studies, it has been claimed that species diversity in created wetlands is lower than that of natural wetlands (Whigham 1999). This affects the productivity of an ecosystem because the organisms that inhabit the system keep it functioning.
Wetland soils are "the physical foundation of every wetland ecosystem"; without proper soil, plant and microbial communities that contribute to the ecosystem processes will be lost (Stolt et al. 2000). Created wetlands have been found to have a soil composition that is much coarser and lower in organic material than natural wetlands (Whigham 1999). In a comparison study between created and reference wetlands, it was found that soil from created wetlands is composed of almost twice as much sand as the reference sites (67% and 37% composition respectively). Silt and clay were found in higher percentages in the soil in reference wetlands (42% and 21% respectively) than in created wetlands (19% and 14% respectively) (Stolt et al. 2000). Because sand does not hold water or exchange very well, and the amount of silt and clay is relatively low in created wetlands, there is concern that wetland function is compromised by mitigation (Stolt et al. 2000).
The soil pH in natural wetland is also lower than the pH found in created wetlands. In the same study mentioned above using reference wetlands to assess the soil of created wetlands, it was found that the mean pH of created wetlands was 5.9, and the mean pH in reference wetlands was 5.3 (Stolt et al. 2000). While the numerical difference between the pH's is not very large, even small changes in pH can potentially affect sensitive plants and animals.
Creating soil with the proper characteristics is near impossible. Restoring a wetland using the original, degraded soil is thought to be more successful than creating a wetland with soil that lacks the definitive wetland soil characteristics (Whigham 1999, Stolt et al. 2000).
Decomposition is an extremely important process in wetlands because it allows the release of nutrients that plants and animals thrive on from dead organic matter (Mitsch and Gosselink 2000). Replacement wetlands have been found to lack certain decomposition characteristics that are found in natural wetlands. In a study comparing decomposition rates in twenty-year-old and two-year-old depressional wetlands, litter bags with 2 grams each of two types of wetland plants, Scirpus cyperinus and Typha latifolia, were placed in the wetlands. The litter bags were picked up 5 times over a period of 507 days and the remaining mass was weighed. It was found that decomposition was faster in the older wetlands (twenty years) than the two year old wetlands. However, the decomposition rates in both wetlands were much lower than that found in natural wetlands (Atkinson and Cairns 2001). In created wetlands, 82 percent and 86 percent (20 year old and 2 year old respectively) of the litter remained after one year, but in natural wetlands only 50 percent of the litter remained (Atkinson and Cairns 2001). This study suggests that there are functional differences in the decomposition rates and nutrient availability in created wetlands.