Each year, North American monarch butterflies undergo a spectacular two-way migration from breeding locations in Canada and the US to overwintering sites in Mexico. Throughout their annual cycle, monarchs utilize habitats in three different countries and require strikingly different resources and habitats at each life stage. Like other migratory animals, this shifting spatial distribution poses challenges for identifying key determinants of monarch population dynamics and assessing their conservation status. Monarchs are an incredibly popular insect that has been exceptionally well studied; multiple long-term monitoring programs exist within North America that span timescales of 3 to over 30 years. This virtually unprecedented wealth of data on a single animal species represents a rare scientific resource for understanding how natural and anthropogenic factors affect the population dynamics and movement patterns of migratory species. At the same time, a great need exists to integrate existing data sets for analysis and interpretation of both within-season and longerterm population trends. Our proposed working group includes a team of experts in monarch ecology, migration biology, statistical model building, climate modeling and geography to answer the most fundamental questions of monarch biology throughout their range in North America. We will explore data from throughout the monarchs’ annual life cycle to identify major ecological mechanisms that shape large-scale patterns of abundance and movement, and to predict the consequences of human activities, including shifting agricultural practices, deforestation and climate change, for long-term dynamics. We will also work directly with NCEAS informatics staff to develop a web-based portal that allows public access to and use of monarch butterfly observational data, much of which has been collected by volunteer observers. Although our efforts focus primarily on a single species, our questions, approaches and findings will have great relevance to understanding the dynamics of other pollinator species and neotropical migrants across North America. collapse