Demographic response of a range-limited tropical plant to climatic and geographic gradients shapes its population dynamics from center to periphery

Jacob K. Moutouama, Ecology and Evolutionary Biology, University of Tennessee, Knoxville, TN and Orou G. Gaoue, Department of Ecology and Evolutionary Biology, University of Tennessee, Knoxville, TN

Background/Question/Methods
The center-periphery hypothesis predicts a decline in population density towards the periphery of species distribution range, reflecting a spatially explicit variation in environmental conditions. However most demographic test of this hypothesis failed to disentangle the role of geography from that of climatic or ecological niche. Because species density varies over time and space, using density instead of population dynamics to explain range limitation can lead to erroneous conclusions.
We hypothesized that because species are expected to experience optimal abiotic conditions at their climatic niche center, (1) central populations will have better demographic growth, survival and fruit production than peripheral populations. Consequently, (2) central populations are expected to have higher growth rates than peripheral populations which, if they are declining over time, will limit species range expansion beyond these boundaries. We also expect that with decreasing ecological conditions toward peripheral populations, (3) the population dynamics will be more sensitive to perturbation of the rate of survival of remnant individuals plants and this contribution of survival to population persistence will increase from central to peripheral populations. To test these hypotheses, we studied the demography of Thunbergia atacorensis (Acanthaceae), a range-limited herb in West-Africa. We used these demographic data to parametrize an integral projection model to test how population dynamics, demographic vital rates, and the sensitivity of population growth rates to perturbation of these vital rates vary with increasing distance from the species’ climatic versus geographic centers.
Results/Conclusions
Survival, growth, and fruit production decrease with distance from the climatic center suggesting a reduction in demographic performance mediated by changes in abiotic conditions. In contrast, demographic vital rates increased with distance from the geographic center. Population growth rate was higher in central population and decreased toward peripheral populations. Surprisingly, the relative influence of plant shrinkage on population growth rates overweighted that of survival-growth particularly in peripheral populations. Our study highlights the need to use population dynamics instead of density as metric in studies testing the center-periphery hypothesis. Our study also suggest that using geographic distribution in lieu of climatic or ecological niche to explain species center-periphery dynamics can be misleading.