Habitat Restoration in a Changing World: Determining the Indirect Effects of Warming on Monarch Butterflies (Danaus plexippus) as Mediated by Changes in Nectar Quality

Abstract

Loss of natural habitats is one of the most critical threats to biodiversity. Habitat restoration is a key strategy to re-establish degraded ecosystems and support the persistence and colonization of pollinators and insects. Pollinator-focused restoration includes introducing floral resources to landscapes to increase native insect pollinator abundance. However, climate change is a major, intensifying threat globally, and insects are among the most affected groups experiencing population declines across the biosphere. Habitat restoration projects and decisions must consider climate change to ensure the effective functioning and resilience of restored habitats, but anticipating the effects of climate change on insects is difficult. Both direct and indirect effects of warming temperatures are expected to impact insect populations, but the indirect effects remain poorly studied. It is unclear how nectar plants will respond to warming temperatures and how these responses may impact insects. Since floral seed mixes are a key component of habitat restoration, exploring how different floral species will respond to warming temperatures, and how those responses may impact pollinators, is essential. The endangered Eastern migratory population of monarch butterflies (Danaus plexippus) is at risk due to climate change. Monarchs rely on nectar from late-season flowering plants in their breeding range in Canada to fuel their migration south to Mexico. Thus, warming-induced declines in the nectar quality of these plants may have critical conservation repercussions. Here, I used a field warming experiment at the monarch's northern range limit in Ontario, Canada, to examine i) the vegetative, floral, and nectar responses of 3 highly visited flowering species to warming, ii) the variation of those responses across the 3 species, and iii) the subsequent impacts of these responses on the body composition of adult monarchs. I found that the warming treatment lowered nectar quality and availability of late-season flowering plants. These warming-induced plant and nectar responses led to a decrease in the fat mass of monarchs who fed on the nectar of warmed plants. These body composition measurements are important metrics for monarch migration and overwintering survival. All three late-season flowering plant species experienced declines in nectar quality; it is unclear if a pattern of adaptive capacity exists in these plants. Therefore, habitat restoration projects should make planting decisions that are context-specific for the needs of the pollinators and plant communities in a specific area.