Species Ranges, Richness and Replacement of Trees in the Evergreen Forests of the Western Ghats

Abstract

It has been more than two centuries since the latitudinal pattern of increase in taxonomic richness from poles to equator was first documented. After two centuries of research, and with more than two dozen hypotheses proposed, an understanding of the mechanisms underlying this pattern and their relative importance remains incomplete. Factors such as evolutionary history, area and latitude associated variables such as temperature, solar energy, climatic stability and seasonality are known to influence species richness by affecting geographic range size and location over ecological and evolutionary time. Understanding the forces that affect geographic range size is, therefore, integral to our understanding of latitudinal patterns in species richness. Using woody plants as a study system, my dissertation deciphers the latitudinal pattern, if any, in species richness within the evergreen forests of the Western Ghats. These wet evergreen forests form an evolutionarily distinct biogeographic zone, which has remained isolated from its counterparts. This has resulted in a high percentage of endemism among the evergreen woody plants and, therefore, the global geographic ranges of most of these plants are restricted within the boundaries of the Western Ghats. The first main objective of this dissertation is to understand the determinants of geographic range size in the evergreen woody plants of the Western Ghats. Further, the Western Ghats are characterized by a sharp climatic gradient in temperature and rainfall seasonality that is not correlated with mean annual temperature or annual rainfall. This allows a direct test of the hypotheses and predictions that are based on climatic seasonality, without the confounding effect of other climatic correlates of latitude. Therefore, the second main objective of this dissertation is to understand the mechanisms underlying latitudinal patterns in species richness of evergreen woody plants in the Western Ghats. Regional species richness is an outcome of two factors- local species richness of each location within the region and turnover in species composition among the locations, which in turn are a result of patterns in range size, range location and range overlap. To address these two objectives, I first test the effect of climatic niche of a species in determining geographic range size and then examine the effect of latitude associated climatic seasonality on range location and range overlap. Next, I link the observed pattern in range geometry to latitudinal patterns in species turnover and finally to latitudinal patterns in species richness. While the first part of my dissertation study deals with factors that generate spatial variation in species richness, the second part deals with the factors underlying spatial variation in species composition. Environmental heterogeneity and dispersal are considered the most important determinants of species turnover i.e. change in species composition. However, their relative importance in structuring in diverse plant communities within tropical regions across different scales is poorly understood. Hence, the third objective of this dissertation is to understand the processes that influence change in species composition of woody plants within the Western Ghats. Geographic range size and population size are important attributes of species rarity, which are directly linked to their extinction risk. Hence, data on distribution and population status of species can help us focus our efforts on those species that require conservation attention. This is achieved through carrying out species threat assessments based on attributes such as range and population size and then assigning then to a threat category. A majority of species endemic to the Western Ghats have not yet been assessed, largely due to lack of data on their population and distribution status. Therefore, the fourth and the final part of my dissertation explores the application of information on species range size and abundance in prioritizing species for conservation. To address these objectives, I sampled the wet evergreen forests of the Western Ghats along a series of locations distributed across its entire latitudinal gradient. Based on 156 plots, covering a latitudinal gradient of more than 1200 km and comprising of more than 20,000 occurrence locations belonging to more than 450 species of woody plants, I derived quantitative estimates of latitudinal gradients in range size, local and regional richness as well as species turnover. I used a combination of statistical and simulation approaches to discern the mechanisms underlying large-scale pattern in species ranges, richness and turnover. My dissertation is structured as follows.