Ecological drivers of range limitation in species

Understanding the ecological and evolutionary drivers of species range limitation is a central debate in evolutionary ecology with direct implications for conservation ecology. This is important because such understanding provides insights into the mechanistic drivers of species adaptation or extinction in peripheral populations where species are often exposed to severe and stochastic biotic and abiotic stressors. The center-periphery hypothesis (CPH) posits that species performance decreases toward range periphery, a potential mechanism to explain range limitation. However, after several decades of testing the center-periphery hypothesis, we still lack general support for this hypothesis. In my dissertation, I combined meta-analysis, field observation, and modeling to gain general understanding of the drivers of range limitation. First, I ask whether there is general support for the center-periphery hypothesis. To address this question, I conducted a meta-analysis on studies that tested the CPH. I found that only genetic diversity decreased from center to periphery. Population dynamics and population abundance did not decrease from center to periphery. This lack of support of the CPH was explained by high heterogeneity in effect size due to species kingdom, pollination type, and methods used to test the CPH. This works also revealed that most tests of the CPH are only conducted in temperate regions. This motivated me to use a tropical herb, Thunbergia atacorensis, as a study system to test the role of soil heterogeneity, and altitude on the variation of herbivory, population abundance and population dynamics between central and peripheral populations. I found that there was no change in plant density, herbivory rate and population dynamics from niche centroid to periphery. Instead, soil nutrients were the primary drivers of population abundance, herbivory rate and population growth rate in Thunbergia populations. While intuitive, the CPH is not common across taxa and the mixed support for the CPH indicates that range limitation is not a direct result of alteration of conditions toward the periphery but rather the result of the combined influences of biotic and abiotic factors on population performance across the range.