Assessing elevational shifts in the species-abundance distribution
(SAD): a case study of vascular plants in the Alborz Mts., Iran
Abstract
Species Abundance Distributions (SADs) describe the commonness and
rarity of species in communities, but few studies explored the
ecological factors influencing SAD patterns. Elevational gradients may
provide useful opportunities to understand SAD variations in response to
environmental characteristics. We investigated how plant SADs changed
along a 2,500 m elevational gradient in the Alborz Mountains (Iran). We
sampled plots at 100 m intervals from 2,000 to 4,500 m elevation to
model SADs using several probability distributions. Most communities
were best fitted by the exponential and gamma distributions as well as
the Weibull distribution. Weibull parameters showed a unimodal pattern
and identified different degrees of dominance and rarity along
elevational gradient. Communities at lower elevations were characterized
by the presence of many species with low dominance. At middle
elevations, communities had many rare and many species with intermediate
abundances. At high elevations, communities were characterized by
reduced taxonomic diversity and low dominance. Less stressful climatic
conditions that characterize lower elevations allow more species to
coexist, but competition may constrain species abundances. Most species
are unable to cope with harsher conditions of higher elevations, and are
filtered out, leading to a reduced taxonomic diversity and dominance in
these communities.