Soil nitrogen mediates the effect of distance from climatic center on herbivory rates in a tropical herb

The center-periphery hypothesis predicts that species are most abundant at the center of their range. Differential herbivory rates between center and periphery populations can explain this variation in species abundance. However, if the geographic center of a species distribution coincides with its ecological optimum, the resource availability hypothesis predicts higher herbivory rates and tolerances at the center compared to the periphery. Biogeographic studies on herbivory have treated these two mechanisms separately, limiting our mechanistic understanding of the role of herbivory in shaping species range limits. We analyzed the role of resource availability on herbivory variation from center to periphery using data collected across the distribution of Thunberbia atacorensis, a range limited species of West Africa. We used two types of distances: geographic distance (the distance from each plot to the geographic center of all populations) and climatic distance (the distance from each plot to the preferendum of the species). We found no increase in herbivory toward the periphery of the climatic and geographic ranges. However, herbivory rates increased with soil nitrogen. Nitrogen in soil decrease from center to periphery of climatic range. Phylogenetic diversity and surrounding plants' competition did not affect herbivory rates. Our study provides insights into how nutrient limitation can shape species center-periphery distribution by altering spatial variation in herbivory rates.

Methods
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.