Spatial dynamics of species distributions in an anthropogenic landscape in the context of climate change

Abstract

Introduction: Climate change and land-use (intensity) change are factors that force biological organisms to change their geographical distributions. These two global drivers, both alone and in combination, have a large impact on the geographical distribution of species. Explaining current distributions and predicting species’ future distributions are principal activities of biogeography, but very few such studies have been undertaken in the Himalayan region. This is despite climate change being more rapid in the Himalaya than the global average. Additionally, significant out-migration (to urban areas and/or abroad) and a change from pastoralism to tourism as the main livelihood in the mountains of Nepal have caused a drastic reduction in grazing pressure from domestic animals. Objectives: This thesis aims (i) to evaluate the spatial dynamics of a species – Abies spectabilis – at its upper species limit in response to recent warming and reduced landuse intensity in the central Himalayan mountains; (ii) to investigate species potential responses to climate change under future projected warming (a case study of three Rhododendron sister taxa) and evaluate their climatically potential current and future distribution as well as distributional overlaps; and (iii) to investigate climatic niche similarity between parapatrically disjunct subspecies of Macaca assamensis (Assamese macaque) and whether their climatically potential distribution areas with current and future climate overlap or not. Methodology: To evaluate the spatial dynamics of species in current climate, an empirical study was carried out in the alpine treeline ecotone of the central Himalayan region (Paper I). The potential response of species to future projected climate was analysed based on predicted distributions from species distribution model (SDMs). The SDMs were fitted on binomial presence vs pseudo-absence and presence vs background data against associated bioclimatic variables using the Random Forest algorithm. The potential distributions were predicted for the current and future projected climate (Paper II). In a third case study (Paper III), the bioclimatic niche similarity between Macaca assamensis ssp. pelops and M. assamensis ssp. assamensis was tested with a multivariate analysis of variance. The potential distributions of both subspecies based on bioclimatic variables under current and future climate were predicted using Random Forest and MaxEnt algorithms. Their overlap was analysed with Ecological Niche Modelling Tools (ENMTools) software. Main results: The upper species limit of Abies spectabilis is moving towards higher elevation, i.e. towards a cooler climate, in a backdrop of recent climate change and reduced land-use intensity (Paper I). The advancement of the leading edge can be interpreted as an attempt to remain within their climatic niche. The potential response under future projected warming for Rhododendron sister taxa is species-specific (Paper II). Rhododendron lowndesii, a species from a dry region of Nepal may not move to higher elevations, while predictions suggest that R. cowanianum may shift its leading edge along with its optimum and R. lepidotum may shift its whole range along with its optimum to track their climatic niches. The distribution models predicted higher overlaps of climatically suitable areas between Rhododendron sister taxa in the future compared to current modelled distributional overlaps. The closely related parapatrically disjunct Macaca assamensis ssp. pelops and M. assamensis ssp. assamensis have rather different climatic niches, but with some overlap (Paper III). In geographical space, however, the subspecies are separated by a zoogeographic barrier – the river Brahmaputra. The modelled current potential geographical distributions are smaller than the predicted climatically suitable areas in the future, but it is uncertain whether the species will disperse and whether biotic interactions will allow them to occupy these areas in the future. In the projected future climate, they will lose some of their current potential geographic space and some new geographic space may become available for them. Conclusions: Species responses to climate change are species-specific. Species maintain their spatial dynamics to remain within their climatic niches. Closely related sister taxa do not necessarily have similar climatic niches.