Range size and climatic niche correlate with the vulnerability of epiphytes to human land use in the tropics
Correspondence: Nils Köster, Botanischer Garten und Botanisches Museum Berlin-Dahlem, Freie Universität Berlin, Königin-Luise-Straße 6–8, 14195 Berlin, Germany.
Range-restricted species account for a large proportion of global biodiversity, and many such species are highly threatened by deforestation and intensifying land use in the tropics. The effects of land use on the diversity of range-restricted species have rarely been studied and remain unknown for vascular epiphytes – diverse and important elements of tropical forests. This study analyses the vulnerability of range-restricted epiphyte species to human land use, compared to that of widespread species.
Western Ecuador (Chocó ecoregion): lowland rain forest (Bilsa, 0°21′ N 79°44′ W, 450–650 m a.s.l.) and Andean cloud forest (Otonga, 0°25′ S 79°01′ W, 1650–2250 m a.s.l.).
The epiphytic vegetation of 220 study plots was surveyed. Distribution data based on herbarium specimens were compiled for all identified species in order to estimate their geographical and elevational ranges as well as the climatic conditions within the species' ranges. These range characteristics were compared for species found in contiguous primary forests, primary forest fragments, secondary forests, and for isolated remnant trees in pastures.
Of the 587 identified epiphyte species, 252 were endemic to the Chocó ecoregion (42.9%). Chocó endemics were not more strongly affected by human land use than non-endemics. However, small geographical ranges and narrow climatic niches were associated with higher vulnerability to habitat changes caused by land use. Epiphyte assemblages in young secondary forests had the lowest proportions of range-restricted species and were dominated by species with broad elevational ranges. Species in secondary forests occupied the broadest ranges of mean annual temperature and precipitation; species on isolated remnant trees at the lowland site inhabited on average warmer and drier geographical ranges than species in the primary forest.
Epiphyte species with restricted geographical distributions and narrow climatic tolerances are particularly vulnerable to human-induced habitat changes, but endemism to national or biogeographical entities alone is a poor indicator of vulnerability. Instead, narrower ecological niches correlate with lower plasticity towards changes in microclimatic habitat conditions. Besides a higher extinction risk due to their small range sizes, many range-restricted species might be threatened by their lower tolerance to the impacts of human land use.