and phases of dry soil, when the water table can be well below the root region. Soils range from high levels of sand in higher areas to higher amounts of clay and silt in riverine areas. Elevation of the Pantanal ranges from above sea level. Annual rainfall over the flood basin is between , with most rainfall occurring between November and March. Annual average precipitation ranged from 920 to 1,540 mm in the years 1968-2000. In the Paraguay River portion of the Pantanal, water levels rise between two meters to five meters seasonally; water fluctuations in other parts of the Pantanal are less than this. Flood waters tend to flow slowly ( per second) due to the low gradients and high resistance offered by the dense vegetation. When rising river waters first contact previously dry soil, the waters become oxygen-depleted, rendering the water environs anoxic. Many natural fish kills can occur if there are no oxygenated water refuges available. The reason for this remains speculative: it may be due to the growth of toxin-producing bacteria in the deoxygenated water rather than as a direct result of lack of oxygen. Flora The vegetation of the Pantanal, often referred to as the "Pantanal complex", is a mixture of plant communities typical of a variety of surrounding biome regions: these include moist tropical Amazonian rainforest plants, semiarid woodland plants typical of northeast Brazil, Brazilian cerrado savanna plants and plants of the Chaco savannas of Bolivia and Paraguay. Forests usually occur at higher altitudes of the region, while grasslands cover the seasonally inundated areas. The key limiting factors for growth are inundation and, even more importantly, water-stress during the dry season. According to Embrapa, approximately 2,000 different plants have been identified in the Pantanal biome and classified according to their potential, with some presenting significant medicinal promise. Fauna The Pantanal ecosystem is home to some 463 species of birds, 269 species of fishes, more than 236 species of mammals, 141 species of reptiles and amphibians, and over 9,000 subspecies of invertebrates. The apple snail is a keystone species in Pantanal's ecosystem. When the wetlands are flooded once a year, the grass and other plants will eventually die and start to decay. During this process, decomposing microbes deplete the shallow water of all oxygen, suffocating larger decomposers. Unlike other decomposing animals, the apple snails have both gills and lungs, making it possible for them to