Biogeography: Latitudinal and Elevational Trends

Abstract

Vascular epiphytes show a stronger latitudinal gradient in species richness and abundance than most other plant groups and are therefore frequently used as a defining feature of tropical rainforests. Although well established, the autecological basis for this pattern is still unresolved. Although frost and drought are commonly stated as major factors, there is hardly any experimental evidence to substantiate such claims. There are also shifts in the taxonomic composition of epiphyte communities with latitude. Communities are generally dominated by orchids in the tropics and by ferns in the temperate zones. A similar pattern can be found when moving up in elevation in the tropics: epiphytic ferns have been found at up to 4500 m a.s.l. The elevational distribution of epiphytes is typically characterized by a mid-elevation bulge. Possible mechanistic explanations for this observation are discussed. A final section deals with diversity patterns within the tropics, from a regional to a continental scale.