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

Authors: SPAETH JR., K - Natural Resources Conservation Service (NRCS, USDA); BARBOUR, P.J. - Natural Resources Conservation Service (NRCS, USDA); MORANZ, R. - Natural Resources Conservation Service (NRCS, USDA); DINSMORE, S.J. - Iowa State University; Williams, Christopher - Jason

Submitted to: Ecosphere
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/11/2021
Publication Date: 1/20/2022
Citation: Spaeth Jr., K., Barbour, P., Moranz, R., Dinsmore, S., Williams, C.J. 2022. Asclepias dynamics on US rangelands: Implications for conservation of monarch butterflies and other insects. Ecosphere. 13(1). Article e03816. https://doi.org/10.1002/ecs2.3816.
DOI: https://doi.org/10.1002/ecs2.3816

Interpretive Summary: Milkweed plants serve a critical role in various life stages of many ecologically important insects and are therefore a vital conservation concern throughout North America, particularly for habitat of the monarch butterfly. Populations of monarch butterflies in the US have experienced significant declines over the last 20 years and this decline is partially attributed to declines in milkweed plants. This study used an extensive vegetation dataset (the USDA-NRCS National Rangeland Inventory) from non-federal rangelands in the US to examine the extent and distribution of milkweed plants and evaluate rangeland health associations and environmental gradients associated with milkweed species as a critical food source for monarch butterflies and other insects. The results document the previously unknown extent of milkweed availability throughout non-federal rangelands in the western US and the rangeland health and environmental factors that affect milkweed population dynamics. This broad assessment of milkweed provides an invaluable reference for future assessments of pollinator habitat, aids in recovery efforts for the monarch butterfly, and serves as a model for plant-environment interaction assessments over large spatial scales.

Technical Abstract: The genesis of this study is in response to the US Fish and Wildlife Service (USFWS) listing of the monarch butterfly (Danaus plexippus plexippus) on December 17, 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 years due to a myriad of environmental factors. Monarch reproduction at the larval stage is dependent on the presence of milkweed (Asclepias) plant species. The USDA-NRCS National Resource Inventory rangeland dataset (~ 23,400 on-site samples; 2,068 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 60.9% of the hectares with milkweed presence. Estimated density for all milkweed species shows 4.03 B plants on 11.5 M ha. Kansas, Texas, Nebraska, and Oklahoma were the four leading states hosting milkweed plants, with 55% of milkweed plant density occurring in the Central Great Plains, Northwestern Great Plains, Nebraska Sand Hills, and Flint Hills Omernik level III ecoregions. The most dominant milkweed species on non-federal rangelands were Asclepias viridis, A. syriaca, A. verticillata, and A. speciosa each with a constancy of >10% of random field NRI samples. 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.