Vascular epiphyte global distributions

Aim: Vascular epiphytes are ubiquitous components of wet tropical forests
where they contribute substantially to local and regional plant diversity.
While some basic epiphyte distribution patterns are relatively well
studied, little effort has been made to understand the drivers responsible
for constraining their global distribution. This study quantifies the
substantial contribution of epiphytes to global gradients and centres of
vascular plant diversity and explores whether epiphytes vary from
terrestrial plants in relation to contemporary and historical
environmental variables. Location: Global. Time period: Present. Major
taxa studied: Tracheophyta. Methods: Using a comprehensive epiphyte
species list (EpiList 1.0), and distribution information for 27,850
epiphyte species derived from numerous literature sources, we describe the
global biogeography of epiphytes. We used generalized linear mixed effects
models to assess the relationship between epiphytic and terrestrial plant
diversity, and contemporary and historical environmental
predictors.  Results: We find that epiphytes substantially
contribute to global centres of vascular plant diversity, accounting for
up to 39% of the vascular flora in Neotropical regions. Epiphytes decrease
in species numbers with increasing latitude at a rate three times faster
than terrestrial plants, a trend that is driven mainly by the distribution
of tropical forests and precipitation. Further, large regional differences
emerge that are explained by several large endemic angiosperm families
(e.g., Neotropical Bromeliaceae) that are absent in other tropical
regions. Main conclusions: Our results show that epiphytes are
disproportionately diverse in most global centres of plant diversity and
play an important role in driving the global latitudinal diversity
gradient for plants. The distribution of precipitation and tropical
forests emerge as major drivers of the latitudinal diversity gradient in
epiphyte species richness. Finally, our findings demonstrate how epiphyte
floras in different biogeographical realms are composed of different
families and higher taxa revealing an important signature of historical
biogeography.