Facultative mutualism increases survival of an endangered ant-tended butterfly

Abstract

Much of our understanding of mutualism is derived from pairwise obligate interactions between specialized participants. These observations may not adequately inform conservation efforts for facultative mutualist species, especially for areas in which invasive species are present and affect the mutualism. We evaluate the effects of ant attendance on larval survival for the endangered Fender’s blue butterfly (Icaricia icarioides fenderi Macy) in situ and examine the effects of biotic and abiotic factors—specifically ant community composition and microhabitat structure—on this mutualistic interaction in Oregon, USA. Fender’s blue larval survival was as much as three times higher in plots with a high proportion of ant attendance compared to plots in which larvae were rarely tended, and ant recruitment was reduced by dense, invasive European grasses. Thatch from these dense invasives, specifically tall oatgrass (Arrhenatherum elatius Beauv.) and tall fescue (Schedonorus arundinaceus Schreb.), are a threat to larval survival. In addition, variation in microhabitat structure determined which ant species occurred in our plots. Larvae were associated with ten ant species but primarily tended by only two: Prenolepis imparis Say and Aphaenogaster occidentalis Emery, and we observed no difference in butterfly larval survival between the two predominant ant species. Conservation efforts for this species would benefit from actions that reduce invasive grasses and facilitate ant tending interactions. Our study demonstrates how invasive grasses and forbs in degraded prairie systems can negatively impact the conservation of an at-risk species by disrupting an important mutualistic interaction.