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

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          Abstract

          Threats to migratory animals can occur at multiple periods of the annual cycle that are separated by thousands of kilometres 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. Our 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. We 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. We used perturbation analysis to determine the sensitivity of population abundance to changes in vital rate among life stages, seasons and geographical regions. Next, we compared the singular effects of each threat to the full model where all factors operate concurrently. Finally, we generated predictions 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. Our 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. Our model predictions also suggest that mitigating the negative effects of genetically modified crops results in higher population size and lower extinction risk. 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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          Links between worlds: unraveling migratory connectivity

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            Regional forest fragmentation and the nesting success of migratory birds.

            Forest fragmentation, the disruption in the continuity of forest habitat, is hypothesized to be a major cause of population decline for some species of forest birds because fragmentation reduces nesting (reproductive) success. Nest predation and parasitism by cowbirds increased with forest fragmentation in nine midwestern (United States) landscapes that varied from 6 to 95 percent forest cover within a 10-kilometer radius of the study areas. Observed reproductive rates were low enough for some species in the most fragmented landscapes to suggest that their populations are sinks that depend for perpetuation on immigration from reproductive source populations in landscapes with more extensive forest cover. Conservation strategies should consider preservation and restoration of large, unfragmented "core" areas in each region.
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              Variation in survivorship of a migratory songbird throughout its annual cycle

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                Author and article information

                Journal
                Journal of Animal Ecology
                J Anim Ecol
                Wiley
                00218790
                January 2015
                January 2015
                June 25 2014
                : 84
                : 1
                : 155-165
                Affiliations
                [1 ]Department of Integrative Biology; University of Guelph; Guelph ON N1G2W1 Canada
                [2 ]Climate Adaptation Flagship; CSIRO; Ecosystem Sciences; GPO 2583 Brisbane QLD 4001 Australia
                [3 ]ARC Centre of Excellence for Environmental Decisions; the NERP Environmental Decisions Hub; Centre for Biodiversity & Conservation Science; University of Queensland; Brisbane Queensland 4072 Australia
                Article
                10.1111/1365-2656.12253
                24903085
                442c365a-9146-4fcb-9d4c-2a50e9cfcb0e
                © 2014

                http://doi.wiley.com/10.1002/tdm_license_1.1

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