Ecosystems of South America on the old erosion surfaces of the shields The materials originated from these rocks are at an advanced stage of in situ weathering and are millions of years old A shallower layer of waste material covers the landscapes of residual relief The valley bottoms and lowlands are covered with drift material that can reach considerable depths Along the Andean belt (Figure 1) there are Precambrian rocks masked by different younger deposits Since the Paleozoic, the ocean has invaded pre-Andean areas several times, and extensive marine deposits mixed with metamorphic and intrusive rocks occur all along the range from Venezuela to Chile One important consequence of marine transgressions (i.e., ingressions and regressions) is the inland deposition of rich marine sediments During marine regressions, erosion increased the loss of previously deposited layers In the Early Devonian (408–362 Â106 years ago), the largest of all marine transgressions covered the Amazon and Parnaiba basins (Figure 1), which remained submerged until the middle Devonian In the upper Paleozoic (290–245 Â106 years ago) only the Parnaiba basin received mixed marine and continental sediments In the Early Mesozoic (245B Â106 years ago), eolian and fluviatile sediments were deposited in the Parnaiba and Amazon basins The Triassic sediments (208B Â106 years ago) came from continental areas and were deposited in an arid environment During the Jurassic (208–145 Â B106 years ago), the products of major volcanic activity in central and western Argentina were deposited in the Parana´, Sao Francisco, Parnaiba, and Amazon basins (Figure 1) During the Late Cretaceous, marine depositions also occurred over large areas of eastern South America (Harrington, 1973) Most of the current physical features of South America – topography, coastline, and river systems – developed toward the end of the Tertiary period during the Pliocene (2–8 Â B106 years ago) when the major uplift of the Andes took place During the Tertiary, several tectonic movements accompanied by acid intrusions and volcanic activity lifted the western part of the continent forming what is now Andean Cordillera (Figure 1) During the uplifting, continuous sedimentation occurred in the inter-Andean valleys and along the eastern piedmont, forming deposits that were later involved in Andean orogenic movements that resulted in complex folded geological sequences During the Miocene–Pliocene, most of the northern Andean ranges were strongly lifted while the southern Pampas were broken into faulted blocks These movements were accompanied by intense volcanic activity in Colombia, Ecuador, Peru, Bolivia, Argentina, and Chile, which also cause the uplifting of the Brazilian and Guianan highlands in the eastern part of the continent These orogenic and volcanic activities continue today, particularly in Colombia, Ecuador, and Chile, and the areas affected have very fertile soils At present, the Andean mountains have fluctuating altitudes from 3000 to 5000 m with higher summits between Ecuador and Central Chile and the highest peak of the Western Hemisphere – Cerro Aconcagua – reaching an altitude of 7005 m in Argentina Owing to the position of the Andes, most of the hydrological networks of the continent drain westward to the Atlantic Ocean via three main river systems: the Amazon River, the Parana´– Paraguay Rivers, and the Orinoco River A wide belt of low sedimentary plains extends along the eastern piedmont of the Andes, from the Llanos of Colombia 107 and Venezuela to the Argentinean Pampas, and eastward along the valley of the Amazon River (Figure 1) The majority of these plains reach altitudes from 200 to 500 m These plains are composed of Tertiary and Quaternary sediments deposited from the South American and Brazilian shields and more recently from the Andes (Harrington, 1973) The intercratonic lowlands of the Amazon basin are made of heavy sedimentary clays originated from erosion of uplifted deposits washed from the Andes (Figure 1) Those deep clay sediments were deposited on the flat bottom of an ancient inland sea that covered most ofthe basin during the Late Tertiary–Early Pleistocene when the ocean level was higher Climate Climate is an important factor that determines to some degree the type of soils and vegetation of any given region and, therefore, influences the structure and functioning of ecosystems Figure illustrates the major South American climate types according to the classification of Koăppen The shape of South America essentially determines its large-scale climatic patterns, which in turn influence the geographical distribution of ecosystems South America has the shape of an acute triangle with most of its area situated in tropical latitudes and its southern portion extending well into high latitudes Although the southern portion of South America reaches more than 201 farther South than the tip of Africa, the continent is so narrow at the southern end that the landmass of this region rather constitutes a continental peninsula Because most of South America occupies tropical regions and the continental area decreases at higher latitudes, the property of climatic continentality is absent Accordingly, the more important factors that determine the diversity of climatic regimes observed across South America are the major global atmospheric circulation patterns, the proximity to oceans and their currents, the Andean relief, and other coastal and inland topographic variations (Colinvaux, 1989) In general, tropical temperatures dominate the northern part of the continent, declining smoothly toward the south Major variations in temperature are due to the Andean Mountains and ocean currents The presence of the Andes causes climatic diversity over the continent due to altitudinal variations that change wind circulation patterns and to adiabatic responses of air masses along the altitudinal gradients of the mountain chains South American environments can range from hot deserts and warm rainy climates in the lowlands to cold deserts, temperate, and iced polar climates in the Andean highlands (Figure 2) Two ocean currents are particularly important to the regional temperatures: the cold Humboldt Current that brings lower temperatures from the south up north along the West Coast and the warm Brazilian current that brings warmer temperatures down south along the eastern part of the continent (Edit, 1968) Owing to the lack of continentality, the distinct climates of South America are often characterized by differences in precipitation Rainfall across most of the northern and central parts of the continent depends on the intertropical convergence (ITC) The ITC occupies its most northerly position from June to September, when it is located between 71 and 91 N Its advance and retreat migration does not take place