An ~80% decline in the eastern population of the monarch butterfly (Danaus plexippus) has prompted conservation efforts to increase summer reproductive success in the Midwest United States. Implementation of conservation practices will create a patchwork of milkweed (mainly Asclepias spp.) habitat within agricultural landscapes dominated by corn and soybean production. Since the monarch butterfly is a vagile species, reproductive success is, in part, a function of both the amount and spatial arrangement of habitat patches in a fragmented landscape. To inform conservation planning we developed a spatially-explicit, agent-based model for summer breeding, non-migratory female monarch butterfly movement and egg-laying on an Iowa, USA landscape. Our model employs a unique movement algorithm when monarch agents encounter habitat edges that incorporates monarch perceptual range to their host plant and spatial memory of previously visited habitat. These behavioral factors are rarely incorporated into animal movement algorithms; however, they can influence estimates of resource utilization. Model exploration assessed the distribution and density of eggs laid on a spatially-explicit 148,665 ha landscape comprised of 17 land cover classes with varying milkweed densities. Uncertainty analysis was undertaken by sampling 25 combinations of perceptual range, spatial memory, flight step length and flight directionality parameters from a total of 256 (44) possible combinations. Movement paths simulated with our new movement algorithm show preferential use of high density milkweed areas that would not be simulated using a correlated random walk. Increasing perceptual range caused a decrease in the area used by monarch agents and caused a skewed egg distribution where most eggs were laid in relatively few habitat patches. Increasing spatial memory caused an increase in the area used but decreased the median number of eggs laid in roadside habitat. Current national and regional monarch conservation goals assume a uniform distribution of milkweed in different land cover classes. Translating these goals into spatially-explicit, heterogeneous habitat patches is essential for predicting realized fecundity in the landscape. Our model provides the foundation to link national and regional monarch conservation goals to fine scale spatial configurations of habitat patches in defined landscapes.
Available at: http://works.bepress.com/steven_bradbury/69/