Diversity and composition of plants species along elevational gradient: research trends

Studies on species richness patterns along elevation gradients are fascinating and gaining attention. We compiled data from 118 studies of elevational gradients in a broad range of organisms of plant species throughout the world between 2001 and 2021 to estimate the patterns of species richness and their determinants. The present study showed that more than half (57%) of studies had found unimodal hump-shaped richness patterns (maximum diversity at the middle and lower edges), followed by a monotonic decline (26%), in different taxa of the plant. Nearly one-fifth of studies (17.5%) followed either monotonic incline, reverse hump-shaped, or non-distinct patterns. A more uniform model as a unimodal hump shape was present in extensive studies in mountainous regions. Despite the latitudinal and elevational variation, both hemispheres showed similar patterns of species richness for all plant taxa. Some taxa showed bimodality, and some others showed multi-model patterns of elevation-species richness patterns with peak diversity due to topographical and climatic factors. Upper and lower elevational ranges may provide temperature and rainfall extremes, reducing species diversity. The mid-domain effect may explain this species richness pattern. However, the current study confirmed the presence of a hump-shaped species richness pattern as a result of the size of the midpoint elevation or elevation range considered for an individual species. The higher the midpoint value, the longer the elevation range that favors a unimodal pattern. Due to inclusion of limited number of species, universal pattern of species richness along elevational gradient and their determinants could not be anticipated. We need more succinct and regional researches focusing specific taxa and delimiting factors.