The study site is located inside Las Chinchillas National Reserve, at 31° 30' S, 71° 06' W, about 300 km N of Santiago, Chile, and 60 km E of the Pacific coast.
The Reserve spans 400-1700 m elevation, has a rugged topography, and is dominated physiographically by an alternation of ridges and ravines, with few flat areas in between.
The climate is classified as semiarid, with sporadic precipitation concentrated during austral winter months (June through August). Mean annual rainfall 1986-1997 was 170 mm, but with marked increases in 1987 (513 mm), 1992 (307 mm), and 1997 (367 mm) associated with the respective El Niņo events (1986-87, 1991-92, 1997-98).
The vegetation is thornscrub composed mainly of spiny dicots, bromeliads, and cacti, depending on sun exposure.
Slopes 
that face to the
north (equatorial) receive higher solar radiation, and have sandy soils with scattered
gravel patches and rock outcrops. Because of the xeric character of these slopes, grasses
are scarce (mainly Nassela spp. and Stipa spp.), but cacti (Trichocereus spp., Tephrocactus
ovatus), bromeliads (Puya berteroniana), and schlerophyllous shrubs (Bahia
ambrosioides, Cordia decandra, Bridgesia incisaefolia, Flourensia
thurifera) are abundant. In contrast, slopes that face to the south (polar) receive
less radiation, are thus more mesic, grasses are more abundant, there are no cacti or
bromeliads, and a different assemblage of shrubs prevail (Adesmia spp., Porlieria
chilensis, Colliguaya odorifera, Proustia spp.). Despite these floristic
differences between slopes, they are more similar between them than with the dissecting
ravines (see below).
