ABSTRACT
Human population growth and the associated economic activities have a major influence on the health and evolution of the natural environment. Humans have influenced natural environments in two ways: by reducing the extents of natural areas, and consequently their biodiversity, due to overexploitation of resources; and by creating protected areas where biodiversity is both regulated and conserved with a view to preserving the services provided to humans, or with the intent to repair previous damage. Throughout human history, the tendency to reduce the extents of natural habitats has dominated over the impulse to restoration and, consequently, humans are now facing a rapidly increasing rate of loss of biodiversity. This trend is aggravated by climate change, particularly in semi-arid areas such as the southwestern United States. Dryland ecosystems1 are particularly susceptible to climate variation and, in such regions, the (lack of) availability of water (surface water, groundwater and air moisture) acts as the main constraint on biological activity.
