10.1 INTRODUCTION

Interest in studying wetlands has escalated greatly within the last decade, largely in recognition of the ecological and economic damage already suffered because of wetland destruction. In the United States alone, drainage and conversion to agriculture have destroyed more than half the wetlands in the lower 48 states (Tiner, 1984). Sensitivity to these losses in recent years has resulted from improved understanding of the subtle ways in which wetlands affect not only other natural ecosystems but also human societies.

   Many wetlands are connected hydrologically to terrestrial and/or aquatic ecosystems; migrating animals, especially birds, connect non-contiguous ecosystems. Recognition of the importance of wetlands in the United States increased dramatically once declines in waterfowl populations migrating to the southern states were associated with increased conversion of freshwater marshes to agriculture in the prairie pothole region in north-central United States and south-central Canada. In other parts of the world declines in fisheries have been associated with wetland destruction, such as removal of mangroves or submersion of floodplains in reservoirs. Therefore, evaluation of the importance of an individual wetland must often encompass a larger region, such as a distant wetland or a downstream water body. This degree of connectedness increases the difficulty of achieving an understanding of the global impacts of wetlands, such as the effect of increased methane release on the ozone layer (summarized by Dickinson and Cicerone, 1986) or the interactions between evapotranspiration (ET) and climate patterns (e.g. Echternacht, 1982).

   Although wetland losses have been largely checked in many developed countries, their ecological and economic impacts are still not fully appreciated elsewhere in the world. A major reason for this lag in perception is the necessity of studying wetlands as whole ecosystems before an understanding can be attained of their role within a landscape and a human economy. Such an approach requires simultaneous analysis of flows of carbon, nutrients, and water as they interact with climate and other forcing functions. The ecosystem approach is particularly important in wetlands because of the close coupling among carbon, nutrient, and hydrologic cycles (e.g. Chapin et al., 1980; Howarth and Teal, 1980). Small changes in hydroperiod, nutrient supply, or fire frequency, for example, can have dramatic impacts on other phenomena in a wetland and on the values of the wetland itself. These may be difficult and expensive to quantify, as are the more indirect effects on downstream ecosystems or more remote areas that may be just as important.

   An ecosystem analysis may be conducted for any of at least three reasons. First, it may be intended simply to determine how a particular ecosystem functions. At what rate does it fix carbon and accumulate organic matter? What nutrients limit its productivity? What kinds of animals does it produce and support? This basic approach of documentation was a characteristic of the International Biological Program. Today, an ecosystem analysis is more likely to be guided by hypotheses regarding response to perturbations, including explicit management practices such as wastewater disposal, drainage, or timber harvesting, and inadvertent pollution with acid rain or agricultural runoff. Finally, an ecosystem analysis might address hypotheses about the nature of a specific pathway or set of pathways in order to shed light on how ecosystems in general function. This requires a very different approach, but it can be important in interpreting the patterns that are observed in the first two kinds of studies. This chapter describes three kinds of approaches to ecosystem analysis in wetlands in order to provide a basis for determining future research strategies.