Scientists studying monarch butterflies have traditionally focused on two sources for their decline -- winter habitat loss in Mexico and fewer milkweed plants in the Midwest. New research, however, shows that a critical piece of the butterfly's annual cycle was missing -- the fall migration. By focusing on this southerly trek, as well as changing the scale at which winter populations are examined, scientists reveal a wider, more-accurate spectrum of threats that have contributed to the monarch population's downward trend. The research team built a multi-scale model and included a summer population index to account for year-to-year variations. They then added environmental factors occurring during the fall migration, such as temperature and landscape greenness. A brown, dry fall means fewer nectar resources -- where monarchs obtain their energy -- to fuel them along their journey.
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