ABSTRACT
The Venezuelan Llanos, a well-defined ecoregion in northern South America, occupy an area of approximately 240,000 km2 lying between 7° and 10°N. These wide lowland plains consist mainly of Quaternary alluvial sediments covered by a mosaic of savannas, gallery forests and dry to semideciduous forests. The climate is markedly macrothermic (>24°C) and tropophilous, with a strong alternation between one rainy and one dry season, and with an average rainfall between 800 and 2200 mm/year. A recent thorough inventory of vascular plants collected in this region during the past two centuries has yielded a surprisingly high number of 3219 species in 1117 genera and 190 families, including 127 species of ferns, 860 monocotyledons and 2232 dicotyledons. The ten most important families account for almost 42% of the total flora, of which the grasses and sedges alone comprise c.450 species; the Leguminosae is the most diverse family (c.350 species) of the dicotyledons, followed by the Rubiaceae, Asteraceae, Euphorbiaceae and Melastomataceae. Despite the large area and the great number of taxa, the level of endemism of the Llanos is low 96(approximately only 1% of the total flora), probably because of the very young (Quaternary) alluvial landscape and the consequently short evolutionary time available for speciation, together with the lack of major geographical barriers across the entire region. The highest species diversity is concentrated in the semideciduous forest types of the western Llanos and in the extensive gallery forests, but other typical Llanos families, such as grasses and sedges, have their greatest diversity in the open savanna and shrubland communities. β-diversity in the extensive grassland ecosystems has never been studied for the entire region, but it is suspected to be extraordinarily high, as a result of the relatively complex pattern of soil and climatic parameters in each of the six or seven major landscape types recognized in the region.
