Diversity and Biogeography of Neotropical Vascular Epiphytes

In his classic work Schimper (1888), emphasizing the taxonomic diversity of epiphytes, listed 33 families and 232 genera of epiphytes. Until very recently, subsequent authors have generally accepted Schimper's figures (Richards, 1957; Johansson, 1974). However, epiphytism (here used in a broad sense to include hemiepiphytes, see Kress, 1986) was recently reported to exist in 65 different vascular plant families (56 families exluding ferns), 38 of these with epiphytes in the Neotropics (Madison, 1977). Our own data (Appendix 1) and additional records compiled by Kress (1986) now record 83 vascular plant families with epiphytic species.3 At least 876 genera include at least one epiphytic species and there are perhaps 29,000 epiphytic species, ca. 10% of all vascular plants (Table 1). Thus at first sight epiphytism seems a very widespread and successful life-style, which very many unrelated taxa have evolved. However, a closer examination suggests that even though there are both many species and higher taxa of epiphytes, few of the higher taxa account for most of the species. Burger (1985), for example, emphasized that relatively few lineages have been able to enter the epiphytic niche, presumably because of the complex suite of adaptations needed. Thus even though it is true that the evolution of an epiphytic habit has been a relatively common feature of vascular plant evolution, it is equally true that very few of the taxa that have evolved an epiphytic habit have radiated successfully to produce other epiphytic species (Table 2). In most of the epiphyte-containing families, epiphytism is a rather insignificant anomaly. Indeed, eliminating a mere 85 such oddball species from the roster of the world's epiphytes removes 31 families from the epiphytic ranks. Only 32 seed plant families have as many as five or more epiphytic species, 26 of these with epiphytes in the Neotropics. It is on the 42 families (Table 3) that contain epiphytes in the Neotropics that this paper will focus. Even though this analysis of epiphyte diversity and distribution is largely focused on the Neotropics, a few comparisons with the Paleotropics are instructive. There are actually slightly more families with epiphytes in the Paleotropics (43) than in the Neotropics (42), with all of the paleotropical epiphytic families having epiphytic representatives in Australasia but only 15 in Africa and Madagascar. If only the 32 seed plant families with five or more epiphytic species are considered, there are also roughly equal representations of epiphyte-containing families in the Neotropics (26) and Australasia (25) but only about half as many in Africa (14). At the species level the story is very different. There are many more epiphytes in the Neotropics, at least half again as many as in Australasia and six times as many as in Africa. Although similar numbers of genera and families evolved epiphytism in the different regions, subsequent speciation as epiphytes was dramatically greater in the Neotropics. A major objective of this paper, then, will be to try to explain why there is so much epiphyte diversity in the Neotropics.