Conserving migratory animals has always been a challenge because they face a range of threats at different parts of their migration, often separated by vast geographical distances (consider the threat to migratory birds from the loss of tidal flats in Asia). Addressing threats to population viability of migratory animals therefore requires integrating information of how individuals move, survive and reproduce throughout their annual cycle.
Populations of the iconic monarch butterfly (Danaus plexippus) of eastern North America have declined over the last 21 years. Three hypotheses have been posed to explain this decline: habitat loss on the overwintering grounds in Mexico, habitat loss on the breeding grounds in the United States and Canada, and extreme weather events. Scientists in Canada and Australia, including EDG researchers, have recently reported on their analysis of the migration of this butterfly. They assessed its population viability, and determined which life stage, season and geographical region are contributing the most to the decline of the species.
They developed a spatially structured, stochastic and density-dependent periodic projection matrix model that integrates patterns of migratory connectivity and demographic vital rates across the butterfly’s annual cycle. What they found was that monarch abundance was more than four times more sensitive to perturbations of vital rates on the breeding grounds (in the US) than on the wintering grounds (in Mexico).
Recent population declines of the butterfly stem from a reduction in milkweed host plants in the United States (which is connected to an increasing adoption of genetically modified crops and land-use change), not from climate change or degradation of forest habitats in Mexico. Therefore, reducing the negative effects from the loss of host plants on the breeding grounds is the top conservation priority to slow or halt future population declines of monarch butterflies in North America.
Flockhart DTT, JB Pichancourt, DR Norris & TG Martin (2014). Unravelling the annual cycle in a migratory animal: breeding-season habitat loss drives population declines of monarch butterflies. Journal of Animal Ecology. doi: 10.1111/1365-2656.12253