The role of functional traits in species distributions revealed through a hierarchical model

Quantifying how functional traits relate to environmental gradients provides insight into mechanisms governing species distributions. Here, we bring together the fi elds of species distribution modelling and functional trait ecology with hierarchical modelling by explicitly incorporating traits into a multi-species distribution model. We combined traits from the leaf-height-seed strategy scheme (specifi c leaf area (SLA), plant height and seed mass) with a distribution model for 20 eucalypt taxa in Victoria, Australia. Th e key insight of this approach is how traits modulate species responses to environmental gradients. Th e strongest link was between SLA and percent rock cover (species with low SLA had positive responses to rockiness, whereas high SLA species responded negatively to rockiness). We found evidence for complex yet potentially important interactions. For instance, the probability of species occurrence increased with rainfall and solar radiation on average yet the response varied depending on species height and SLA. Tall species were predicted to increase with rainfall and solar radiation across the range of SLA values (tall species with low SLA were especially sensitive to rainfall). Short species responded positively to rainfall and solar radiation only if they had low SLA. Th is framework readily accounts for interactions between combinations of traits and environmental variables unlike multi-step approaches. Further application of this concept will contribute to a generalized mechanistic understanding of how traits infl uence species distributions along environmental gradients, with implications for understanding the response of species to global change.