Exploring anthropogenic and natural processes shaping fern species richness along elevational gradients

Abstract

Aim (1) To explore the impact of land use, climate and environmental heterogeneity on fern species richness along a complete elevational gradient, and (2) to evaluate the relative importance of the three groups of variables within different elevational intervals. Location A temperate mountain region (55,507 km2) of Italy on the southern border of the European Alps divided into a regular grid of 1476 cells (grain 35.7 km2). Methods We applied multiple regression (spatial and non-spatial) to determine the relative influence of the three groups of variables on species richness, including variation partitioning at two scales. We considered the whole gradient (all 1476 cells) to explain the overall elevational pattern of species richness, and we grouped the cells into elevational intervals of 500 m in order to evaluate the explanatory power of the predictors within different zones along the gradient. Results Species richness showed a hump-shaped pattern with elevation, forming a plateau between 800 and 1500 m. The lowest species richness was found in warm and relatively dry disturbed lowlands. Moving upwards, the greatest species richness was found in forest-dominated mid-elevations with high environmental heterogeneity. At high elevations dominated by open natural habitats, where temperature and precipitation were relatively low, species richness declined but less sharply than in the lowlands. Although it was impossible to separate the effects of the three groups of predictors along the whole gradient, the analysis of separate elevational intervals shed light on their relative importance. The decline of species richness within lowlands was mainly related to a combined effect of deforestation and low environmental heterogeneity. In the middle part of the gradient, habitat heterogeneity and topographic roughness were positively associated with species richness. The richness decline within high-elevation areas was related mostly to climatic constraints. Main conclusions Human impact due to land-use modifications strongly affects the elevational pattern of species richness. It is therefore increasingly important to adopt a multiple-hypothesis approach, taking anthropogenic effects explicitly into account when describing ecological processes along elevational gradients.