Landscape- and local-level variables affect monarchs in Midwest grasslands

Abstract

Context It is estimated that over one billion milkweed stems need to be restored to sustain the eastern North American migratory population of monarch butter ies; where and in what context the stems should be placed on the landscape is key to addressing habitat de cits. Objectives We assessed how the amount of appropriate habitat surrounding a particular patch of monarch habitat affects monarch presence and reproduction. To ensure that habitat restoration efforts are targeted towards areas that maximize monarch population growth, it is important to understand the effects of landscape heterogeneity on monarch occurrence in habitat patches (i.e. grasslands with milkweeds) across the landscape. Methods Over two summers (2018-2019), we surveyed monarch adults, larvae, and eggs at sixty grassland sites in Wisconsin that varied in patch size and landscape context (proportion grassland, forest edge density, and road density). We also estimated milkweed density and oral richness to characterize local patch quality. Results Adult monarch abundance was highest at patches with the lowest proportion of surrounding grassland and lowest road density, and was heavily in uenced by patch quality variables. Egg and larva density in a patch increased with milkweed density and oral richness within a patch. Patch size was unrelated to monarch abundance. Conclusions These results suggest that optimal sites for monarch habitat restoration are within landscapes with less surrounding habitat and that high milkweed density and oral richness should be conservation goals.