Understanding the ecological processes that govern species’ range margins is a fundamental question in ecology with practical implications in conservation biology. The centre-periphery hypothesis (CPH) predicts that organisms have higher abundance at the centre of their geographic range. However, most tests of the CPH often used raster data, assuming that climatic conditions are consistent across one square km. This assumption is not always justified, particularly for species that live in mountainous regions where climatic conditions often vary widely across a small special scale. When using occurrence data, most previous studies do not evenly sample across the species’ distribution. We sampled 54 plots of an endemic perennial herb, Thunbergia atacorensis (Acanthanceae), throughout its range in West-Africa to collect biotic and abiotic variables including, Thunbergia density, leaf mass per area, and basal diameter. The basal diameter was used to estimate population demographic structure (skewness). We used these data to build a structural equation model testing the direct and indirect effects of distance from geographic and climatic niche centres, and altitude on density of Thunbergia atacorensis as mediated by abiotic and biotic factors, population skewness, and individual size. Contrary to the prediction of the CPH, plant density did not vary with distance from geographic or climatic niche centre, indicating that even the climatic centre does not necessarily have optimal ecological conditions. However, soil nitrogen and soil potassium mediated the relationship between altitude and plant density. Further, plant size increased with soil nitrogen and soil potassium. We found no direct or mediating effect of interspecific competition on plant density. Our study highlights the relative role that abiotic factors play in shaping species range limits, and the critical role of altitudinal gradient.

We randomly established five 5 m x 5 m permanent plots to collect data on herbivory rate, interspecific competition among surrounding species, species diversity and abiotic soil data in 12 populations of Thunbergia atacorensis.