79 WETLANDS, Vol. 26, No. 1, March 2006, pp. 79–96  2006, The Society of Wetland Scientists THE EFFECTS OF ADJACENT LAND USE ON WETLAND SPECIES RICHNESS AND COMMUNITY COMPOSITION Jeff E. Houlahan, 1 Paul A. Keddy, 2 Kristina Makkay, 1 and C. Scott Findlay 3,4 1 Department of Biology University of New Brunswick at Saint John P.O. Box 5050 Saint John, New Brunswick, Canada K1N 6N5 E-mail: jeffhoul@unbsj.ca 2 Ottawa-Carleton Institute of Biology University of Ottawa 30 Marie Curie Ottawa, Ontario, Canada K1N 6N5 3 Edward G. Schlieder Endowed Chair for Environmental Studies Department of Biological Sciences Southeastern Louisiana University Hammond, Louisiana, USA 70402 4 Institute of Environment University of Ottawa 555 King Edward Street Ottawa, Ontario, Canada K1N 6N5 Abstract: Wetlands provide important ecosystem functions and values, such as wildlife habitat, water fil- tration and flood protection. Wetland plant communities play a fundamental role in maintaining these func- tions but are thought to be increasingly threatened by human modifications of the landscape, such as defor- estation and road construction. Here, we examine the quantitative relationships between two dependent variables (plant species richness, community composition) and a set of independent variables describing land use (e.g., forest cover, road density, and building density). As independent variables, we further include wetland characteristics that may be related to landuse practices (e.g., area and nutrient status). Wetland size is the most important predictor of both total plant species richness and the species richness within most functional groups. In addition, landscape properties, such as forest cover, presence of streams and nutrient status of water and sediment are significant predictors of plant species richness. Adjacent land use 250–300 m from the wetland affects plant diversity. Differences in the land-use-diversity relationship among different plant functional groups suggest that adjacent land use affects wetland plant communities in two important ways. First, it alters the abundance and distribution of propagules in adjoining habitats. Second, it alters the number of dispersal routes. Our results suggest that current management practices are inadequate and that regulation of adjacent land use is a critical component of wetland conservation. Key Words: plant diversity, land use, wetlands, forest cover, species-area, dispersal, streams, exotics, scale, functional groups, eutrophication, fertilizer INTRODUCTION Wetlands provide many important functions and values in the landscape, including reduction in flood- ing, fish production, and carbon storage (Mitsch and Gosselink 1986, de Groot 1992, Keddy 2000). Most wetland conservation policies therefore aim to prevent direct loss of wetland habitat. Restrictions, if any, on adjacent land generally apply only to a narrow buffer zone. In Ontario, Canada, for example, the buffer is 120 m (OMMA and OMNR 1992). Yet, there is mounting evidence that wetland functions can be im- paired not only by modification of the wetland itself, but also by adjacent land use (Moss 1984, Burbridge 1994, Detenbeck et al. 1996). Vegetated buffers are a potentially effective tech- nique for reducing damage caused by incompatible ad-