Abstract
The migratory monarch butterflies (Danaus plexippus) of eastern North America have undergone large-scale declines, which may be attributable to a variety of underlying causes. The uncertainty about the primary cause of declines and whether individual threats are likely to increase in the future presents challenges for developing effective conservation management and policy initiatives that aim to improve population viability. This paper identifies five potential threats and classifies these threats according to the types of studies (observational, experimental, simulation/models) and their current impact and anticipated risk. Broadly, the threats can be classified into five categories: (1) change in suitable abiotic environmental conditions; (2) deforestation in the overwintering range; (3) exposure to contaminants including the bacteria Bacillus thuringiensis, herbicides, and insecticides; (4) loss of breeding habitat; and (5) predation, parasitism, and species-specific pathogens. The vast distribution of the monarch butterfly makes it likely that population declines are attributed to a suite of interacting factors that vary spatially and temporally in their contribution. Nonetheless, the published papers we reviewed suggest the decline in suitable environmental conditions in addition to overwintering (i.e., deforestation) and breeding habitat loss are the most likely threats to continue to affect the population viability of monarch butterflies.
Highlights
Insect populations are experiencing rapid declines globally (Dirzo et al, 2014; Stork et al, 2015) that may have implications for ecosystem function and contributions to economic services (AllenWardell et al, 1998; Potts et al, 2010)
Though it is probable that other threats exist beyond those identified in this review, we focus on five broad potential threats commonly reported for the eastern migratory North American population of the monarch butterfly: (1) change in suitable abiotic environmental conditions, (2) deforestation in the overwintering range, (3) exposure to contaminants including the bacteria Bt, herbicides, and insecticides, (4) loss of breeding habitat, and (5) predation, parasitism, and pathogens
Field studies constituted one of the principal methods documenting the effects of sudden changes in environmental conditions and adverse weather patterns in the decline of monarch butterflies (50% of total studies on adverse weather events, Brower et al, 2015, 2017), but only a single study considered the effect of extreme weather patterns before fall migration (25% of total studies on adverse weather events, Hunt and Tongen, 2017)
Summary
Insect populations are experiencing rapid declines globally (Dirzo et al, 2014; Stork et al, 2015) that may have implications for ecosystem function and contributions to economic services (AllenWardell et al, 1998; Potts et al, 2010). Changes in the suitability of environmental conditions driven by extreme weather and climate change (Batalden et al, 2007; Barve et al, 2012; Brower et al, 2017), habitat loss (Didham et al, 1996; Fattorini, 2011; Thogmartin et al, 2017b), exposure to contaminants (Stanley-Horn et al, 2001; Thogmartin et al, 2017b), and changes in species interactions (e.g., invasive species, Burghardt and Tallamy, 2015) can have profound effects on biodiversity. The substantial loss of biodiversity within the insect taxon emphasizes their sensitivity
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