December 2005 1893 LAND-USE CHANGE IN RURAL AMERICA 1893 Ecological Applications, 15(6), 2005, pp. 1893���1905 q 2005 by the Ecological Society of America EFFECTS OF EXURBAN DEVELOPMENT ON BIODIVERSITY: PATTERNS, MECHANISMS, AND RESEARCH NEEDS ANDREW J. HANSEN,1,4 RICHARD L. KNIGHT,2 JOHN M. MARZLUFF,3 SCOTT POWELL,1,5 KATHRYN BROWN,1,6 PATRICIA H. GUDE,1,7 AND KINGSFORD JONES1 1Ecology Department, Montana State University, Bozeman, Montana 59717 USA 2Department of Forest, Rangeland, and Watershed Stewardship, Colorado State University, Fort Collins, Colorado 80523 USA 3College of Forest Resources, University of Washington, Seattle, Washington 98195 USA Abstract. Low-density rural home development is the fastest-growing form of land use in the United States since 1950. This ������exurban������ development ( 6���25 homes/km2) includes urban fringe development (UFD) on the periphery of cities and rural residential development (RRD) in rural areas attractive in natural amenities. This paper synthesizes current knowl- edge on the effects of UFD and RRD. We present two case studies and examine the patterns of biodiversity response and the ecological mechanisms that may underlie these responses. We found that many native species have reduced survival and reproduction near homes, and native species richness often drops with increased exurban densities. Exotic species, some human-adapted native species, and species from early successional stages often in- crease with exurban development. These relationships are sometimes nonlinear, with sharp thresholds in biodiversity response. These effects may be manifest for several decades following exurban development, so that biodiversity is likely still responding to the wave of exurban expansion that has occurred since 1950. The location of exurban development is often nonrandom relative to biodiversity because both are influenced by biophysical factors. Consequently, the effects on biodiversity may be disproportionately large relative to the area of exurban development. RRD is more likely than UFD to occur near public lands hence it may have a larger influence on nature reserves and wilderness species. The ecological mechanisms that may underlie these responses involve alteration of habitat, ecological processes, biotic interactions, and increased human disturbance. Research on the patterns and mechanisms of biodiversity remains underdeveloped, and comparative and experimental studies are needed. Knowledge resulting from such studies will increase our ability to understand, manage, and mitigate negative impacts on biodiversity. Key words: biodiversity biotic interactions ecological mechanisms fire habitat fragmentation landscape management land cover land use rural residential development urban fringe development weeds. INTRODUCTION Rural America is undergoing a dramatic transition. For the first time in more than a century, more people are moving to rural areas than from rural lands (Johnson 1998). Fleeing the cities, many retirees, entrepreneurs, and others are seeking the small-town lifestyles and natural amenities of rural landscapes (Rudzitis 1999). Manuscript received 21 July 2003 revised 10 September 2004 accepted 8 November 2004 final version received 10 De- cember 2004. Corresponding Editor: M. G. Turner. For reprints of this Invited Feature, see footnote 1, p. 1849. 4 E-mail: hansen@montana.edu 5 Present address: USDA Forest Service, Pacific Northwest Research Station, 3200 SW Jefferson Way, Corvallis, Oregon 97331. 6 Present address: 14445 Buffalo St., Anchorage, Alaska 99516. 7 Present address: P.O. Box 283, King Salmon, Alaska 99613. This rural in-migration is driving large changes in land use. The typical trajectory of land use change across the United States prior to 1950 was from wild land and resource extraction uses to agriculture and to suburban and urban uses. An entirely new land use has become prevalent in many parts of the United States since 1950. Many people are choosing to live ������out of town������ on small ������ranchettes������ and in rural subdivisions. Termed exurban development, low-density housing ( 6���25 homes/km2) within a landscape dominated by native vegetation is now the fastest growing form of land use in the United States (Brown et al. 2005). Land long used for forestry or ranching is now being converted to home sites. The effects of exurban development on native species and ecological communities have only recently been the topic of ecological studies. Since 1950, there has been a five-fold increase in the area within the conterminous United States that is oc- cupied at exurban densities (Brown et al. 2005). The

1894 INVITED FEATURE Ecological Applications Vol. 15, No. 6 PLATE 1. Rural residential development in the Greater Yellowstone Ecosystem near Red Lodge, Montana, USA. The rural homes are placed near low-elevation riparian forests that are especially important for biodiversity. Photo by A. Hansen. exurban land use type currently covers nearly 25% of the area of the lower 48 states. The most rapid gains were in the eastern deciduous forest, the southwest, the western seaboard, the Rocky Mountains, and the upper Midwest. This exurban development is manifest in two forms. Urban fringe development is the expansion of exurban densities on the periphery of cities. This urban fringe development (UFD) is largely driven by urban dwellers seeking more rural lifestyles while still having access to urban jobs and services (Ulmann 1954, Healy and Short 1987, Raish et al. 1997). Exurban development in counties adjacent to metropolitan counties increased six fold since 1950 (Brown et al. 2005). Over time, these exurban developments often transition to sub- urban and urban land uses. A second form of exurban development is occurring distant from cities. It is focused on rural areas attractive in scenery, climate, outdoor recreation and other ������nat- ural amenities������ (Rasker and Hansen 2000). Rural coun- ties not adjacent to metropolitan counties increased fivefold in exurban area since 1950 (Brown et al. 2005). This rural residential development (RRD) is common in the rural counties of the Rocky Mountain West, the Pacific Northwest, the upper Midwest, and the south- eastern United States (Gersh 1996). Rather than being randomly distributed, this development is often asso- ciated with the borders of national parks and other pub- lic lands rivers, lakes, or coastal areas areas of mod- erate climate and good outdoor recreational opportu- nities and towns and small cities that offer national airports, high-speed internet access, and cultural ame- nities (Cromartie and Wardwell 1999, McGranahan 1999, Nelson 1999 see Plate 1). The effects of both forms of exurban development on wildlife and biodiversity are poorly known. Relative to other types of land use, exurban development is substantially understudied. Miller and Hobbs (2002) found that only 6% of the papers on human landscapes published in Conservation Biology dealt with exurban and urban places. The majority of these consider the general gradient from rural to urban in and around cit- ies. While these studies typically do not cleanly sep- arate biodiversity in exurban places relative to subur- ban and urban places, they do provide a context for assessing general trends in biodiversity under land use intensification. RRD has been examined in only a few recent studies, with most of them being in the Rocky Mountain West. Understanding the effects of exurban development on biodiversity is important to public policy. With a quarter of the nation���s land area in this land use type, policies on exurban development may have a substan- tial effect on biodiversity nationwide. The general view among conservationists and the public is that exurban development alters ecological processes and biodiver- sity to a greater extent than forestry and agriculture (Marzluff and Ewing 2001). Hence, many initiatives have emerged to protect ������open space������ from exurban development through conservation easements and other approaches. There is also the view that the effects of exurban development are proportional to home density. Thus, zoning for lower density housing is often used to protect ecological resources.