Cooperative Fish and Wildlife Research Units Program: Nebraska
Education, Research and Technical Assistance for Managing Our Natural Resources

North American monarch butterflies are a charismatic pollinator, in part, because of their unique long-distance migration and formation of large clusters of individuals at winter roosting sites. Through monitoring at roost sites, marked declines in monarch numbers were detected. The eastern population segment of monarchs that winters in Mexico declined by 90% with the western population segment estimated to have declined by 95% since the late 1990s. Further, eastern and western segments of monarchs are genetically differentiated, indicating the two regions are demographically isolated. The marked declines experienced by monarchs across North America prompted concerns about the persistence of the unique behavioral segments of the species and capacity of the species to respond to current and emerging threats.

The effective population size (and thereby retention of genomic diversity) of species underlies a populations’ capacity to adapt and respond to changes in the environment in both current and future conditions. Dramatic reductions in census sizes not only reduce the capacity of populations to retain genomic diversity but also reductions allow for the expression of deleterious mutations that are present in the population at low frequency, which can further erode the viability of populations in decline. Evaluation of the impact that the decline has on the effective population size of monarchs, therefore, is needed to inform resiliency and population persistence forecasts under projected future conditions.