Preferred nectar sources for the monarch butterfly (Danaus plexippus plexippus) along the Great Plains migration pathway

Population declines of the monarch butterfly (Danaus plexippus plexippus) in North America have largely been attributed to the distribution and condition of species-specific preferred nectar sources. In 2020, the US Fish and Wildlife Service (USFWS) listed the monarch butterfly in the US Federal Register as a candidate species under the Endangered Species Act of 1973. The USFWS ranked the availability, quality, and spatial distribution of nectar plants during autumn migration as the fourth most contributing factor to US monarch population declines. During the autumn migration through the Great Plains, monarchs seek nectar plants to accumulate lipid reserves for further migration to and overwintering in Mexico. We applied vegetation and rangeland health data from the US Department of Agriculture, Natural Resources Conservation Service, National Resources Inventory (NRI) to quantify species density and richness of monarch-preferred nectar plants, associated rangeland conditions, and diversity of nectar sources along this autumn migration pathway. We focused specifically on longitudinal gradients W-095-100 and W-100-105 and discrete 5 degrees latitudinal-longitudinal cells within those gradients. The respective NRI dataset spans 8211 rangeland sites sampled between 2009 and 2018. Approximately 84.4% of sites in W-095-100 and 72.5% of sites in W-100-105 contained monarch-preferred nectar plants. Preferred nectar plants made up 7.4% of 2438 identified plant species in W-095-100 and 6.1% of 2371 identified plant species in W-100-105. For W-095-100, preferred nectar plant densities were highest for the 5 degrees cell covering portions of US states Oklahoma and Kansas and lowest for the 5 degrees cell at the US-Mexico border. In W-100-105, preferred nectar plant densities decreased linearly from north to south. Preferred nectar plant densities were greater for 5 degrees cells in W-100-105 (50.5 billion plants) as compared with W-095-100 (44.4 billion plants). Consistent with trends in preferred nectar source density, rangeland conditions assessed by similarity indices and rangeland health protocols were generally lowest for 5 degrees cells spanning the US-Mexico border. The results provide the most comprehensive assessment to date for preferred nectar sources of the monarch butterfly along the Great Plains autumn migration to Mexico and document generally decreasing nectar sources and habitat conditions at southern latitudes in this ecologically important pathway.