Connecting herbivore effects to population dynamics of common milkweed (Asclepias syriaca) using an Integral Projection Model
Common milkweed is the primary host for monarch butterflies and milkweed decline has been implicated as the largest contributing factor in the drastic declines of monarch butterflies. Numerous studies have examined the influence of milkweed defensive traits on the ecology and evolution of milkweed-insect interactions and documented individual milkweed response to herbivory. However, no studies have examined the effects of herbivores on milkweed population dynamics. We constructed an integral project model (IPM) of common milkweed. IPMs are a modification of classic matrix models that project a continuously size structured population through discrete time. With IPMs, vital rates (survival, growth and fecundity) are modeled as continuous statistical functions of plant size (height and/or stem diameter). Our initial model included only size as a predictor, but can easily incorporate variation in herbivory and other environmental covariates as additional predictors in the statistical equations. Field data were collected over three years at four sites in Virginia including plant size, reproduction, survival, density, and herbivore damage.
Plant size was a significant predictor of reproductive output, with larger plants having a much higher probability of flowering and producing four times the number of viable pods (75 cm tall plants produced an average of 1 pod per stem; 150 cm tall plants produced an average of 4 pods per stem; P = 0.0004). Plant size did not have a significant effect on annual survival P = 0.59). Across the different sites, milkweed density ranged from 1 to 4.5 stems per square meter. Preliminary analysis of density effects revealed that nearest neighbor distance had no effect on any aspect of milkweed demography (survival, growth or reproduction). However, when measured as plants per square meter, density correlated with an increase in flowering and pod production. Herbivory per plant also declined with density of plants per square meter. Density and herbivory both are promising covariates to add to our IPM.