Shaping the niche of Taxus baccata, a modelling exercise using biologically meaningful information

Widely used Correlative Species Distribution Models (C-SDMs) usually make some simplifying assumptions, often failing to consider important ecological and evolutionary attributes potentially hindering the characterization of the species niche. Here, we use the tree species Taxus baccata to explore the effects of including biologically meaningful information on processes and features beyond purely abiotic factors that are expected to determine its niche and range size. To elucidate how these often neglected factors affect C-SDM results, we modelled the current niche in the species’ southernmost European range using Maxent. More specifically, we included available basic information regarding biotic interactions, local adaptation and non-equilibrium demographic dynamics. The potential effect of biological interactions was introduced using habitat suitability of co-occurring tree species as predictive variables. Local adaptation was included modelling two distinct regional adaptive groups. We also used individual growth estimated under field conditions as a surrogate for demographic behaviour to control for the quality of model predictions and empirically assess the effect of biotic interactions. Including information on co-occurring tree species improved model performance and decreased the projected range size in most cases. These effects were not a result of biological interactions per se, but instead a consequence of co-occurring species accounting for fine-scale environmental variability not described by any of the climatic variables used. Considering local adaptation allowed detecting the role of different climatic variables in shaping the niche of each adaptive group that could potentially also act as selective pressures in the near future. Finally, and more importantly, we found that including populations that are probably currently found under non-equilibrium suboptimal conditions might largely overestimate the species niche.