It is widely recognized that species diversity frequently increases towards the equator. This observation has led to the prediction that the strength of species interactions (e.g., herbivory) and the intensity of coevolution are greatest at lower latitudes. The study of plant-herbivore interactions have long served as a model for the study of latitudinal gradients in species interactions and coevolution, where plants found closer to the equator are expected to have evolved increased defences.
Ph.D. student Daniel Anstett is studying whether these predictions can explain biogeographic patterns in herbivory and plant defences. Studying the plant Oenothera biennis (common evening primrose), he found that the relationship between herbivory and latitude exhibits every possible pattern (positive, negative and no relationship at all), depending on the herbivore species inflicting the damage (Anstett et al. 2014). He also found that gradients in temperature explain the most variation in biogeographic patterns of herbivory.
Current research on this topic is dissecting the roles of genetic variation among plant populations and environmental variation in explaining the observed biogeographic patterns in plant defenses and herbivory. Specifically, with collaborators at the University of Turku (J.-P. Salminen and J. Ahern), we are quantifying the concentration of over 100 individual putative defence chemical compounds (ellagitannins, flavonoids, and caffeic acid derivatives) and relating genetic variation in these compounds to latitude, the environment, and herbivory. We are simultaneously testing whether latitudinal patterns in defense exist at macroevolutionary scales, by examining how compounds vary across >100 species in the evening primrose family (Onagraceae).