Abstract: Modelling the Effects of Density Dependence and Leaf Chemistry on Population Dynamics of Common Milkweed

The monarch butterfly has seen dramatic population declines in the past two decades (81% since 1999). A major contributing factor of this is the simultaneous decline in populations of common milkweed, on which the monarch is entirely dependent for much of its life cycle. As part of a plan to combat this decline, major replanting programs have begun in order to rejuvenate both species. However, while we know a great deal about the biology of monarchs, we know much less about the ecology of milkweed. This project aims to fill that gap by examining the role that leaf chemistry and plant density play in common milkweed population dynamics. To do this, I will collect demographic, spatial, and chemical data on milkweed in the field, and use that to expand a mathematical population model for milkweed.

The current model applies Integral Projection Modelling techniques to predict the number of individuals in the future, based on statistical relationships between environmental variables (i.e. herbivory) and population vital rates (i.e. survival, growth, reproduction).

Using this model we have learned that herbivory by specialist species, (like monarchs) reduces milkweed population growth rate. Leaf chemistry and population density are expected to be important factors affecting population dynamics, through changing the plants’ nutrition content and toxic chemical defenses, or by reducing milkweed fitness, respectively. Therefore the inclusion of these two factors will make the current model much more robust.

Determining the relationship that exists between these two factors and the vital rates of common milkweed and incorporating this relationship into our existing Integral Projection Model will make it possible to more accurately project future population decline/recovery under different management scenarios, thus aiding in conservation efforts