An Evaluation of Studies on the Potential Threats Contributing to the Decline of Eastern Migratory North American Monarch Butterflies (Danaus plexippus)

Abstract

The migratory monarch butterflies (Danaus plexippus) of eastern North America have undergone large-scale declines, which may be attributable to a variety of underlying causes. The uncertainty about the primary cause of declines and whether individual threats are likely to increase in the future presents challenges for developing effective conservation management and policy initiatives that aim to improve population viability. This paper identifies five potential threats and classifies these threats according to the types of studies (observational, experimental, simulation/models) and their current impact and anticipated risk. Broadly, the threats can be classified into five categories: (1) change in suitable abiotic environmental conditions; (2) deforestation in the overwintering range;(3) exposure to contaminants including the bacteria Bacillus thuringiensis, herbicides, and insecticides; (4) loss of breeding habitat; and (5) predation, parasitism, and species-specific pathogens. The vast distribution of the monarch butterfly makes it likely that population declines are attributed to a suite of interacting factors that vary spatially and temporally in their contribution. Nonetheless, the published papers we reviewed suggest the decline in suitable environmental conditions in addition to overwintering (i.e., deforestation) and breeding habitat loss are the most likely threats to continue to affect the population viability of monarch butterflies.

Further evidence for a global decline of the entomofauna

“…In North America, population declines of the monarch butterfly ( Danaus plexippus ) have been linked to increasing use of neonicotinoid insecticides (James 2019; Wilcox et al . 2019; Halsch et al . 2020).…”

Grappling with uncertainty in ecological projections: a case study using the migratory monarch butterfly

“…This means that continual monitoring and reassessment of the monarch will remain a critical need both to understand its dynamics but also to provide the US Fish and Wildlife Service the information they require for their yearly deliberations. These studies reveal that many of the biggest drivers of global change on biodiversity, including climate, land use, and agricultural practices such as chemical spraying impact the monarch (Oberhauser et al 2017, Malcolm 2018, Wilcox et al 2019 but that those forces change over time (Saunders et al 2018.…”

Distribution, phenology, and land cover preference risk assessments of four South-Central US milkweeds (Asclepias spp.) important for monarch butterflies

“…The signi cant long-term decline in the eastern monarch overwintering population (Semmens et al 2016) has been primarily attributed to the loss of milkweed habitat in the Midwest US for second and third generation larvae (Oberhauser et al 2017;Pleasants 2017;Thogmartin et al 2017;Stenoien et al 2018;Taylor et al 2020), and adult mortality within the Central Mexico overwintering grounds (Brower et al 2004(Brower et al , 2012(Brower et al , 2017Ries et al 2015). A combination of factors, including milkweed breeding habitat loss, climate change, and deforestation of overwintering habitat, may represent the greatest risks to eastern migratory monarch butter ies (Malcolm 2018;Wilcox 2019). Demographic models of Oberhauser et al (2017) indicate that increasing milkweed and nectar availability for monarchs in both the northern and southern parts of the breeding range may be the most e cient strategy for promoting monarch population growth.…”