Long‐ and short‐term responses ofAsclepiasspecies differ in respect to fire, grazing, and nutrient addition

Abstract

Premise of the Study The tallgrass prairie ecosystem has experienced a dramatic reduction over the past 150 yr. This reduction has impacted the abundance of native grassland species, including milkweeds (Asclepias). Methods We used two long‐term (27 yr) data sets to examine how fire, grazing, and nutrient addition shape milkweed abundance in tallgrass prairie. We compared these results to those of a greenhouse experiment that varied nutrient levels in the absence of competition, herbivory, and mutualistic relationships. Key Results Asclepias species exhibited broad patterns in response to burning regimes that did not include grazing, but experienced more species‐specific patterns in other combinations. Asclepias syriaca was the only species to increase in abundance in plots that included burning and nutrient addition. In the greenhouse we found that nitrogen significantly increased biomass, while no effect of phosphorus was detected. Conclusions These results indicate that A. syriaca will do best in settings with high nutrient loads, low competition, and no grazers. These characteristics define a small portion of the tallgrass prairie but exemplify modern agricultural settings, which have replaced prairies. However, other milkweeds examined did not share this pattern, which indicates that milkweed species will respond differently when exposed to agricultural settings, with some less able to cope with land conversion to pasture or row‐crop agriculture.

Adult Monarch (Danaus plexippus) Abundance Is Higher in Burned Sites Than in Grazed Sites

“…Indirect impacts due to ways in which fire affects monarch host plants or nectar plants are more likely. The effects of fire on common milkweed (Asclepias syriaca) are unclear; Towne and Kemp (2008) reported that A. syriaca declined in frequency after summer fire, but a recent study in Kansas found that it increased after fire and decreased with grazing, while seven other species of milkweeds increased with grazing (Ricono et al, 2018). In the southern Great Plains, Asclepias viridis density and re-growth were significantly higher immediately after summer burns, leading to an increase in observed monarch eggs and larvae in burned areas (Baum and Sharber, 2012).…”

Response of pollinator taxa to fire is consistent with historic fire regimes in the Sierra Nevada and mediated through floral richness†

“…For example, fire may improve habitat for monarch butterflies (Nymphalidae: Danaus plexippus) by increasing the density of firefollowing Asclepias host species (Baum & Sharber, 2012;Ricono et al, 2018).…”

“…Furthermore, butterflies, even within families, are highly variable in host plant specificity (Dyer et al., 2007), and this will likely affect butterfly response to fire at the species level (Gaigher et al., 2019; Huntzinger, 2003). For example, fire may improve habitat for monarch butterflies (Nymphalidae: Danaus plexippus ) by increasing the density of fire‐following Asclepias host species (Baum & Sharber, 2012; Ricono et al., 2018).…”