The benefit of public transportation: physical activity to reduce obesity and ecological footprint.

The 20th century witnessed exponential growth of the human population. A concurrent change was urbanization, with close to half of the world's population residing in urban centers at the beginning of the 21st century (McGranahan and Satterthwaite, 2003). Humans have domesticated nature with a net benefit to themselves such as enhancing food supplies, reducing exposure to predators and other dangers, and promoting commerce, often at a cost to other species (Kareiva et al., 2007). A concept, ecological footprint (EF, the amount of land required to produce the resources needed by a person annually), was first introduced in 1996 and immediately applied for cities (Wackernagel and Rees, 1996; Folke et al., 1997). The EF is comprised of resources used to meet a person's food, water, and most significantly, energy demands for housing and transportation that account for up to 90% of the EF. The concept provides a reasonable tool to demonstrate natural resource dependence of human activities to politicians and the public, although much work is needed to employ it as a cohesive analytical tool for management. Fortunately, advances have been made to assess the EF of water supplies (Jenerette and Larsen, 2006; Jenerette et al., 2006a; Jenerette, et al., 2006b), of transportation (Muniz and Galindo, 2005), and of household energy demand (Lenzen et al., 2004) and of its application in urban settings (Luck et al., 2001; Kaye et al., 2006; Du et al., 2006) and other land uses, barring the difficulty of estimating the EF of aquaculture production (Roth et al., 2000). The attractiveness of the EF is that it represents a single, quantifiable measure of human impact on the environment. Using 2003 economic activity data, the EF of low, middle, and high income countries have been estimated at 0.8, 1.9 and 6.4 ha per person, respectively (Websource, 2007a). The EF of all countries averages 2.2 ha per person. In comparison, the biocapacity, e.g., the resource that is available for sustained human consumption, is 1.8 ha per person (Websource, 2007a). Rapid