Comparison of two milkweed (Asclepias) sampling techniques on eastern Nebraska grasslands

Authors: MANZANAREZ, MERCY - US Department Of Transportation; PANELLA, MELISSA - Nebraska Game & Parks Commission; Wonkka, Carissa; STEINAUER, GERALD - Nebraska Game & Parks Commission; STONER, KRISTAL - National Audubon Society

Submitted to: Prairie Naturalist
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/18/2022
Publication Date: 10/13/2022
Citation: Manzanarez, M.L., Panella, M.J., Wonkka, C.L., Steinauer, G.A., Stoner, K.J. 2022. Comparison of two milkweed (Asclepias) sampling techniques on eastern Nebraska grasslands. Prairie Naturalist. 1:54-64.

Interpretive Summary: In recent years, the interest in estimating the number of milkweeds on the landscape has grown because the plants and their close relatives serve as the hosts for the larvae of monarch butterflies. Monarchs are a species of great conservation concern because of declining numbers accross their range. Widespread loss of milkweed is one of the primary factors implicated in the butterfly’s decline. We conducted a study to compare two techniques for assessing milkweed abundance and richness. A plot-based method was compared to a transect method to determine the accuracy/efficiency trader-offs associated with each for sampling in remnant and rehabilitated grasslands in eastern Nebraska. We surveyed a total of 48 grassland areas using two distinct sampling methods, belt-transects and randomly selected quadrats along transects, on properties under four different land management types: 1) high-diversity local ecotype plantings, 2) privately-owned agricultural lands with grassland enhancement, 3) public wildlife management area grassland rehabilitations, and 4) unmanaged pastures. Both the plot and transect surveys indicated differences in milkweeds among property types. However, more nuanced differentiation was indicated by the transect method that was not captured by plot sampling. High-diversity planting sites and wildlife management areas had more milkweeds than did unmanaged pastures and privately-owned sites. The plot method did not detect these differences as well. Both methods required equal amounts of time to accomplish. The findings for our sites suggest that the belt transect method is more effective than the plot method for predicting milkweed per hectare as well as milkweed species diversity The ability to choose methods that will accurately identify and quantify milkweed on a variety of land management types is paramount for implementing a conservation plan for the monarch butterfly not only in Nebraska but throughout the Midwestern United States.

Technical Abstract: In recent years, the interest in estimating the number of milkweeds (Asclepias spp.) on the landscape has grown, because the plants and their close relatives serve as the hosts for the larvae of monarch butterflies (Danaus plexippus). Monarchs are a species of greatest conservation need. US Fish and Wildlife recently determined that federal listing for monarchs is warranted but that it is precluded currently because of species of higher listing priority. Widespread loss of milkweed (Asclepias spp.) throughout the monarch’s range is one of the primary factors implicated in the butterfly’s decline. We conducted a study to compare two techniques for assessing milkweed abundance and richness. A plot-based method was compared to a transect method to determine the accuracy/efficiency trader-offs associated with each for sampling in remnant and rehabilitated grasslands in eastern Nebraska. We surveyed a total of 48 grassland areas using two distinct sampling methods, belt-transects and randomly selected quadrats along transects, on properties under four different land management types: 1) high-diversity local ecotype plantings, 2) privately-owned agricultural lands with grassland enhancement, 3) public wildlife management area grassland rehabilitations, and 4) unmanaged pastures. Both the plot and transect surveys indicated differences in Asclepias abundance and richness among property types. However, more nuanced differentiation was indicated by the transect method that was not captured by plot sampling. High-diversity planting sites and wildlife management areas had more Asclepias species per hectare than did unmanaged pastures and the privately-owned sites. The variance was much higher for counts obtained via plot-based sampling relative to counts obtained via belt-transects; therefore, the transect method resulted in statistical differences among some management types that was not significant due to higher variance using data collected in plots. Both methods required equal amounts of time to accomplish. The findings for our sites suggest that the belt transect method is more effective than the plot method for predicting milkweed per hectare as well as milkweed species richness. The ability to choose methods that will accurately identify and quantify milkweed on a variety of land management types is paramount for implementing a conservation plan for the monarch butterfly not only in Nebraska but throughout the Midwestern United States.