Every year monarch butterflies in southern Canada and the north-central United States travel over 4,000 kilometers (2,485 miles) southward. Triggered by cool weather and the slow death of their host plant, milkweed, monarchs make their long journeys to the oyamel fir forest in Mexico. However, this stunning migration is at risk. The population of monarchs in California alone has declined by 86% between 2017 and 2018. 

Monarch butterflies face a wide range of threats including bad weather, climate change, and exposure to chemicals and contaminants, as well as the dual dangers of predation and pathogens. Deforestation in Mexico where the butterflies overwinter and the loss of breeding habitat and milkweed on the northern breeding range also pose significant risk at different stages in the monarch life cycle. Despite mounds of research in each of these areas, it turns out that we still don’t fully understand the contribution of each to monarch declines.

To answer this question, my colleagues and I examined 115 peer-reviewed papers, classifying them by the type of risk and whether  the potential threat currently had a positive or negative effect on  monarch butterflies. Using papers with multi-year datasets or results from predictive models, we also assessed whether each threat is thought to pose a continued risk to the population (as opposed to a one-time problem). We found that poor environmental conditions and loss of habitat in Mexico and on the northern breeding grounds are the most severe threats to monarch butterflies.  With this in mind, researchers can design studies and conservation interventions that that directly address these threats to reduce the decline of this charismatic species.