A Comparison of Created and Natural Wetlands and the Effect of Landscape Characteristics on Vegetation, Amphibian and Bird Variables in Freshwater Marshes of Upstate New York
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AuthorPorter, David W.
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AbstractWetlands provide a number of ecological and social functions, including flood abatement, improvement of water quality, recharge ground water and support for a great diversity of flora and fauna. Despite their many functions and values, wetlands have not always been appreciated. Since the 1780s, it is estimated that 53% of wetland acreage in the United States has been lost due to draining, filling and the subsequent development of land for roads or farms. In 1972, the Clean Water Act (CWA) was established to try to reverse declining wetland acreage in the U.S. According to Section 404 of the CW A, any loss of wetland acreage due to development or other means must be compensated in the form of mitigation wetlands. Mitigation wetlands are created wetlands built to replace both wetland acreage and function. The success of mitigation wetlands is varied, and there are questions as to our ability to replace lost wetland functions. In 2000 and 2001, I surveyed plants, amphibians and birds at nine created and nine natural wetlands and calculated a series of twelve response variables. I used the Wilcoxon paired sample test to look for differences among created and natural wetlands. I found no significant differences between created and natural wetlands for any of the response variables. Wetlands are important components of the landscape and their functions are influenced by their position within the landscape and the watershed. Many wetland animals require and depend on surrounding terrestrial land for foraging, migration, breeding, cover and hibernation. If we are to create wetlands that are ecologically similar to natural wetlands, then we must view and understand them from a larger, landscape perspective. Since I found no significant differences among created and natural wetlands, I combined the wetlands into a larger set of 18. GIS software was used to map each wetland and its surrounding land, and calculate a set of landscape-level predictor variables. I used simple linear regression and best subset multiple regression analyses to look for predictor variables that might account for the variation found in amphibian, bird and plant response variables. I found positive significant relationships between open water classification and plant, amphibian and bird species richness in both years. In addition, open water class was positively associated with bird and amphibian species diversity, and also the number of birds per census in both 2000 and 2001. The only negative significant association with open water class was with the proportion of T. latifolia. Watershed area was a significant positive predictor of amphibian in 2001, and of bird species richness, plant species richness, the number of birds per census and plant species diversity in both years. Wetland area was significant and positively related, in both years, to the number of birds per census and plant species richness. Invasive plant species richness was significant and positively associated with the length of the road in the watershed in both years, while invasive plant species cover was significant and positively associated with the percentage of urban, commercial and industrial land within 1 km of wetlands. Plant species richness and diversity were significant and positively associated with bird species richness and bird species diversity in both years, and with the number of birds per census in 2001. Open water class was part of four of the best subset models in 2000 and five models in 2001. Watershed area was part of seven best subset models in 2000 and five in 2001. The functions of wetlands are influenced by their position within a landscape. Therefore, the selection of an appropriate site for a mitigated wetland project should consider the surrounding landscape properties. The results of my study suggest that mitigated wetlands should be located 1) in large watersheds, 2) far from roads and urban areas, and near other wetlands. Also, mitigated wetlands should be constructed and maintained in a "hemimarsh" state so that the cover to water ratio is between 1:1 and 1:2, and the establishment and spreading of aggressive plant species such as T. latifolia should be controlled to help maintain a high level of structural diversity.