Unravelling the annual cycle in a migratory animal: breeding-season habitat loss drives population declines of monarch butterflies

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1. Threats to migratory animals can occur at multiple periods of the annual cycle that are separated by thousands of kilometers and span international borders. 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 the 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. 2. The authors’ objectives were to assess population viability; determine which life stage, season, and geographical region are contributing the most to population dynamics; and test the three hypotheses that explain the observed population decline. 3. 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 annual cycle. Perturbation analysis was used to determine the sensitivity of population abundance to changes in vital rate among life stages, seasons, and geographical regions. Next, the singular effects of each threat were compared to the full model where all factors operate concurrently. Finally, predictions were generated to assess the risk of host plant loss as a result of genetically modified crops on current and future monarch butterfly population size and extinction probability. 4. The year-round population model predicted population declines of 14% and a quasi-extinction probability (<1000 individuals) >5% within a century. Monarch abundance was more than four times more sensitive to perturbations of vital rates on the breeding grounds than on the wintering grounds. Simulations that considered only forest loss or climate change in Mexico predicted higher population sizes compared to milkweed declines on the breeding grounds. The model predictions also suggest that mitigating the negative effects of genetically modified crops results in higher population size and lower extinction risk. 5. Recent population declines stem from reduction in milkweed host plants in the United States that arise from 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 of host plant loss on the breeding grounds is the top conservation priority to slow or halt future population declines of monarch butterflies in North America.

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ELECTRONIC FILE - Zoology

Citation: Flockhart, D. T. Tyler, Jean-Baptiste Pichancourt, D. Ryan Norris, and Tara G. Martin. 2015. Unravelling the annual cycle in a migratory animal: breeding-season habitat loss drives population declines of monarch butterflies. Journal of Animal Ecology 84(1): 155-165.

CSE-style Citation: Flockhart DTT, Pichancourt JB, Norris DR, Martin, TG. 2015. Unravelling the annual cycle in a migratory animal: breeding-season habitat loss drives population declines of monarch butterflies. Journal of Animal Ecology. 84(1):155-165.