Distribution Modeling ofAsclepiasspp. Predicts Potential Conservation Benefits of Marginal Ecosystems as Nature Reserves in Northwest Ohio

Abstract

Due to grassland and prairie conversion for agriculture, milkweed species (Asclepias spp.) are now confined to small natural areas, agricultural fields, and marginal ecosystems (e.g., ditches and field margins). Monarch butterflies (Danaus plexippus) rely exclusively on milkweed and are in decline in part due to declining milkweed populations. We conducted a species distribution modeling exercise using MaxEnt to model milkweed species and common milkweed (Asclepias syriaca) presence in natural areas and ditches throughout northwest Ohio in order to identify which local or regional landscape attributes most strongly influence milkweed presence. We found that while natural areas and ditches have similar presence probability ranges (milkweed: natural areas 3–98% and ditches 1–98%; common milkweed: natural areas and ditches 0–98%), natural areas have a higher average probability of presence than ditches (milkweed: natural areas = 30 ± 26% and ditches = 7 ± 7%; common milkweed: natural areas = 22 ± 26% and ditches = 5 ± 6%). However, due to the large area covered by ditches and field margins in our study system, ditches and field margins with a probability of milkweed occurrence greater than the average probability of occurrence in natural areas encompass a larger total area than natural areas. We also observed that the presence of milkweed is positively correlated with the presence of shrub and C4 grasses. We conclude that marginal ecosystems in ditches and field margins are an important component of regional milkweed and, thusly, monarch conservation by providing potential habitat across a diffuse network.