The political boundaries of Chile provide relatively natural biogeographic boundaries because of the topography of desert, mountain, and ocean boundaries. To the north is the hyperarid Atacama Desert, which reaches its extreme of conditions in the Tacna-Arica region along the Peruvian border. Here, a virtual absence of rainfall separates the Peruvian floristic elements of the desert from the Chilean elements. To the east, the mediterranean-climate region of central Chile is strongly delineated by the high Andean Cordillera.Many peaks in this range reach well above 6000 m and effectively shield Chile from weather fronts moving westward across Argentina.
Although major uplift of the Andes began in the mid-Tertiary, at least 14 million years ago, the range continues to be tectonically active today. The elevation of mountain passes along the Andes in northern and central Chile are too high to allow easy migration of either plants or animals, and thus have helped to isolate the flora and fauna of Chile. Only in southern Chile where the Andes are lower has there been easy migration across this range. However, the severe climatic conditions of cold that characterize Patagonia in southern Chile act to strongly reduce the biological diversity in this area.
Comparisons of species diversity between Chilean organisms and those of other mediterranean-climate regions deserve some caution in terms of the area included. As with California, the political boundaries of Chile include desert and wet forest ecosystems that are not comparable with core mediterranean-climate habitats. Most figures for Chilean diversity in the literature are based on political boundaries. An additional issue of political boundaries comes in assessing levels of endemism for the Chilean flora or fauna. Levels of endemism for all groups are much lower if strict adherence to the political boundaries of Chile rather than the more natural boundaries of the Chilean/Patagonian biogeographic province.
Much of central Chile, which covers an area of about 140 x 103 km2, shares a physiographic structure parallel to that of California. Moving inland from the Pacific Ocean, there is a coastal range of mountains, a broad central valley, and a high mountain range to the east. The geologically recent Cordillera de la Costa is relatively tall. West of Santiago at about 331S latitude, the major peaks are Cerro Campana (1910 m), Campanita (1510 m), and El Roble (2220 m). These peaks are high enough to intercept moisture from humid southwestern winds, producing woodland areas with significant fog interception and thus improved water relations.
The dominant vegetation in central Chile is matorral, an evergreen shrubland similar in general form to chaparral. Along the coast and to the north this community grades into a coastal matorral with a greater dominance by drought deciduous shrubs. At higher elevations and on sites with greater water availability, matorral grades into a sclerophyll woodland community, and to the south into hygrophilopous woodlands with many species characteristic of the Valdivian forest region. Much of the central valley of Chile today is dominated by a savanna community termed espinal, with Acacia caven as the sole dominant. This community is almost certainly the result of human intervention on landscape processes over the last four centuries. Sclerophyll woodland and matorral would once have covered much of this area.
Unlike California and the Mediterranean Basin, the high mountains of the Andean Cordillera in central Chile do not have a forest zone. The young age of these mountains and lack of soil weathering has produced unstable geological conditions on the west-facing slope of the Andes. Matorral communities on the lower foothills of the Andes give way to a low and scrubby montane matorral community at about 2000 m elevation.
The biological diversity of Chile has an ancient origin that dates back to Gondwanaland. Southern Chile in particular exhibits many broad biogeographical linkages with New Zealand and Australia. Central Chile shows other biogeographical connections with southeastern Brazil, a linkage dating back to mid-Tertiary times before the uplift of the Cordillera de los Andes. Since the Andean uplift, however, the biota of Chile has evolved largely in isolation from other biogeographic regions.
Biosystematic and biogeographic knowledge of the flora and fauna of Chile has increased greatly in recent decades, and thus the biodiversity of most groups of vascular plants and vertebrates is relatively well known. These syntheses have been applied more generally to the country as a whole, however, rather than to discrete regional areas. Thus, there is a strong need for more regional studies that evaluate patterns of alpha, beta, and gamma diversity in relation to environmental gradients.