Predicting ecological connectivity in urbanizing landscapes

Nearly half the world's population lives in urban centers, and these areas are increasingly important components of regional and global land cover. However, their ecological attributes are often overlooked, despite the presence of species, ecosystem services, and risks associated with the spread of pests or threatening processes such as fire. Movement and dispersal of organisms contribute to species persistence in urban landscapes; however, landscape patterns that promote ecological connectivity may also facilitate the spread of undesirable organisms or processes. I investigate how urban form can be used to predict ecological connectivity and assist in prioritizing urban landscapes for conservation activities and risk management. I examine the value of qualitative and quantitative descriptions of urban morphology as predictors of ecological connectivity by comparing sixty-six cities in the USA. Results show that qualitative categories are not adequate for describing ecological connectivity; multivariate descriptions are much better predictors, with urban area, number of urban patches, urban patch extent, level of aggregation, and perimeter area fractal dimension composing the significant synthetic variables. The dominance of area as a differentiating variable led to the development of a new urban connectivity index using a combination of urban area and state population size. This metric, based on readily available aspatial data, explains 78% of variation in ecological connectivity. These results provide a simple but novel tool for beginning to understand the role of urban morphology in promoting desirable environmental outcomes and managing environmental risks in urbanizing landscapes. DOI:10.1068/b31134 ôPresent address: Global Change Research Program, National Center for Environmental Assessment, ORD^US EPA (MC 8601N), 1200 Pennsylvania Avenue NW, Washington DC 20460, USA, e-mail: [email protected] The encroachment of urban areas on natural habitats has many detrimental impacts, such as habitat loss, fragmentation (Alberts et al, 1993; Bolger et al, 1997; Brown et al, 2000; Soulë et al, 1988; 1992; Swenson and Franklin, 2000; van Dyck and Matthysen, 1999), edge habitat creation (Moran, 1984), invasive species influx (Crooks and Soulë, 1999; Drayton and Primack, 1996; Guilden et al, 1990; Kowarik, 1990; Rapoport, 1993; Wetterer, 1997; Zipperer et al, 1997), and increased pollution (Charbonneau and Kondolf, 1993). However, many species persist in urbanizing landscapes, sometimes utilizing new habitats and even expanding their ranges (Gompper, 2002; Sol et al, 1997). This means that the impacts of urbanization are not uniformly negative. In this paper I focus on the consequences of urbanization for ecological or environmental processes related to movement, and therefore connectivity (table 1). The increasingly common wildland ^ urban interface (Platt, 2001) contributes to the positive and negative effects of urbanization on ecological and environmental processes linked to habitat connectivity. This suggests a need to couple the understanding of urbanization with an assessment of the potential for organism movement and process spread. One approach is to examine the spatial structure of urbanization through morphology and its impact on ecological connectivity, which is directly linked to species persistence through movement and dispersal and to the spread of other spatial processes such as fire. Movement is critical to organism persistence (Hanski, 1994) because it includes foraging, mate location, territory defense, dispersal, and migration and is a product of the intrinsic qualities of organisms and their interaction with landscape features (Bierwagen, 2003). The product of this interaction can be analyzed by means of connectivity metrics. The most widely used metrics examine the spatial distribution of habitat and provide estimates of how Table 1. Ecological and environmental processes affected by urbanization through changes in connectivity. Type of impact Mechanism Reference Negative effects of urbanization Spread of invasive species Increased connectivity between urban and natural landscapes increases movement potential Aragon and Morales (2003), Boet et al (1999), Lim et al (2003), Woo and Zedler (2002) Spread of pest species Increased connectivity between human-dominated and natural landscapes increases movement potential Nowak and McBride (1992) Spread of disease through population Fragmentation and decreased connectivity foster outbreaks in local populations Hess (1994), Lafferty and Gerber (2002), McCallum and Dobson (2002), O'Neill et al (1992) Fire spread and ignition Increased access and conducive spatial patterns in landscapes allow fire spread Platt (2001), Russell and