Monarch butterfly (Danaus plexippus) populations east of the Rocky Mountains migrate annually to central Mexico. The overwintering population has declined more than 80% over two decades. One prevailing theory for this is the milkweed-limitation hypothesis. Increased efficiency in agricultural practices and widespread conversion of grasslands has resulted in a precipitous decline in milkweed species (Asclepias spp) that monarch caterpillars depend on as their obligate hosts. National scale research suggests a 5-fold increase in milkweed stems is needed to address extinction risks of the eastern monarch population. To accomplish this increase despite continued land conversion, the US Fish and Wildlife Service has recommended extensive conservation efforts across all sectors including urbanized areas. Currently, little is known about the existing density and habitat potential of milkweed in urban landscapes. We developed a field method for randomly sampling milkweed abundance across the urban gradient using “metro-transects” and field sampling in naturalized areas. We conducted our study with support from USF&WS and local partners across four major metropolitan areas along the monarch migratory route: Chicago, Minneapolis-Saint Paul, Kansas City, and Austin.
Milkweed density was estimated across 16 consolidated land use categories. To better understand potential milkweed density across land use sectors, our field team also did targeted sampling of “exemplary” and “enhanced” greenspace conversion areas across land-use sectors. Two geospatial tools were developed in collaboration with USGS staff, 1) a “baseline” tool to estimate existing milkweed abundance values across municipalities and metropolitan areas, and 2) a “scenario planning” tool for estimating projected milkweed abundancies and densities under various user-supplied scenarios that vary landowner participation by land use sectors. The amount of land that could be monarch habitat varied widely across metropolitan areas and across land use categories within metropolitan areas. Only a very small percentage of land available for urban monarch habitat currently contains milkweed. The potential for increasing milkweed stems in urban landscapes is enormous and depends on engaging different audiences in each land use category. Existing and potential milkweed estimates are summarized and visualized across US Census Bureau census blocks. Our results, the geospatial tools, a manual, and a guidebook are made available to the public with a primary intended audience of municipal and regional planning agencies, major landowners, and federal, state, and not-for-profit conservation organizations interested in or working on pollinator issues.