Asclepias dynamics on US rangelands: implications for conservation of monarch butterflies and other insects

The genesis of this study is in response to the United States (US) Fish and Wildlife Service (USFWS) listing of the monarch butterfly (Danaus plexippus plexippus) on 17 December 2020 in the US Federal Register as a candidate species under the Endangered Species Act of 1973. Annual censuses have identified that the eastern and western North American monarch migratory populations have been generally declining over the last 20 yr due to a myriad of environmental factors. Monarch reproduction at the larval stage is dependent on the presence of milkweed (Asclepias) plant species. The United States Department of Agriculture-Natural Resources Conservation Service (USDA-NRCS) National Resource Inventory range-land data set (similar to 23,400 on-site samples; 2032 sites with milkweed presence) was used to evaluate milkweed species densities, geolocations, and environmental gradients. Twenty-two milkweed species were identified on rangelands across 17 sampled US western states, with seven species comprising 65.5% of milkweed frequency of occurrence. The most dominant milkweed species on non-federal rangelands were Asclepias viridis, A. syriaca, A. verticillata, and A. speciosa (constancy >10% where milkweed was present). Asclepias speciosa was the dominant species from the standpoint of total plant density for the data set, whereas A. viridis was the most frequently occurring species. Total milkweed density estimates based on low, midpoint, and high estimates were 1.3, 4.1, and 6.9 B plants on 13.2 M ha. Seven US states (Kansas, Texas, Nebraska, South Dakota, North Dakota, Oklahoma, and Montana) contained 88.8% of the total estimated ha with milkweed presence. In the Central Great Plains, Northwestern Great Plains, Nebraska Sand Hills, and Flint Hills, Southwestern Tablelands, High Plains, Northwestern Glaciated Plains, and Cross Timbers, Omemik level III ecoregions contained 76.7% of the estimated milkweed plants. Milkweed species density was highest at latitude N35-40 with decreasing populations toward south (N25-30) and north (N45-50) latitudes. Milkweed species densities were greatest at longitude W-95-100 and decreased toward the western US with lowest population numbers at W-120-125. Analysis of environmental variables showed milkweed species dominance on mollisols, non-saline sites, neutral pH, well-drained soils, loam and sandy loam soil textures, and sites with soil organic matter at 1.5-3%. Disturbance gradients and habitat dynamics relating to ecological condition and rangeland health differed among the dominant milkweed species identified in this study.