Monarch butterflies are partial migrants ( 40), as not all migrate ( 48, 171 ). Secondary threats to monarch population viability include ( d) climate change effects on the distribution and abundance of milkweed host plants and the dynamics of breeding, overwintering, and migration ( e) the influence of invasive plants and natural enemies ( f) habitat fragmentation and coalescence that promote homogeneous, species-depleted landscapes and ( g) deliberate culture and release of monarchs and invasive milkweeds. Given that monarch larvae are specialist herbivores of milkweeds, particularly of the genus Asclepias, and that most monarchs migrate each year to locate these host plants across North American ecosystems ( Figure 2) now dominated by agriculture, monarch populations face three threats to their viability: ( a) loss of milkweed resources for larvae due to genetically modified (GM) crops, pesticides, and fertilizers ( b) reduced nectar resources from flowering plants to fuel adult migration and ( c) degraded overwintering forest habitats due to commercially motivated deforestation and other economic activities. These data show a steady and consistent decrease in overwintering numbers for monarch populations both east and west of the Rocky Mountains ( Figure 1) ( 28, 133, 160). Regular monitoring of overwintering colony areas in Mexico began in 1993, followed in 1997 by regular counts of monarchs at overwintering sites in California ( Figure 1). A similar but more diffuse pattern of migration occurs west of the Rocky Mountains, with overwintering in approximately 400 wooded locations along the coast of California ( 83, 118, 162). Such a remarkable annual coalescence of almost an entire population prompted action to protect overwintering sites in Mexico and to monitor the size of the aggregations each winter because they may serve as an indicator of environmental threats across large spatial areas ( 20, 26). Each year, between 100 million and 1 billion monarchs migrate from southern Canada and the United States east of the Rocky Mountains to aggregate tightly in central Mexico. plexippus, which has been revealed by a wide range of scientists working on ecology, evolution, behavior, genetics, chemistry, physiology, cell biology, and disease dynamics ( 1, 18). Over the next 40 years, this discovery morphed into thriving citizen science initiatives ( 113) driven by interest in the biology of D. In 1975, he discovered that monarch butterflies east of the Rocky Mountains fly to the Trans-Mexican Volcanic Belt of central Mexico, where they spend the winter tightly aggregated in oyamel fir trees ( Abies religiosa) at elevations above 3,000 m ( 165, 166). Secondary threats to population viability include ( d) climate change effects on milkweed host plants and the dynamics of breeding, overwintering, and migration ( e) the influence of invasive plants and natural enemies ( f) habitat fragmentation and coalescence that promote homogeneous, species-depleted landscapes and ( g) deliberate culture and release of monarchs and invasive milkweeds.įred Urquhart ( 164– 166) began marking monarch butterflies ( Danaus plexippus) in 1937 to find out what happens to them after they leave Ontario in southern Canada each September. Both populations face three primary threats to their viability: ( a) loss of milkweed resources for larvae due to genetically modified crops, pesticides, and fertilizers ( b) loss of nectar resources from flowering plants and ( c) degraded overwintering forest habitats due to commercially motivated deforestation and other economic activities. Eastern migrants overwinter in high-elevation forests in Mexico, and western monarchs overwinter in trees on the coast of California. Monarch butterflies ( Danaus plexippus) are familiar herbivores of milkweeds of the genus Asclepias, and most monarchs migrate each year to locate these host plants across North American ecosystems now dominated by agriculture.
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