Monarch butterflies (Danaus plexippus, Linnaeus, 1758) are comprised of two migratory populations separated by the Rocky Mountains and are renowned for their long-distance movements among the United States, Canada, and Mexico. Both populations have declined over several decades across North America prompting all three countries to evaluate conservation efforts. Monitoring monarch distribution and abundance is a necessary aspect of ongoing management in Canada where they are a species at risk. We used presence-only data from two citizen science data sets to estimate the annual breeding distribution of monarch butterflies in Canada between 2000 and 2015. Monarch breeding distribution in Canada varied widely among years owing to natural variation, and when considering the upper 95% of the probability of occurrence, the annual mean breeding distribution in Canada was 484 943 km2 (min: 173 449 km2; max: 1 425 835 km2). The area of occurrence was approximately an order of magnitude larger in eastern Canada than in western Canada. Habitat restoration for monarch butterflies in Canada should prioritize productive habitats in southern Ontario where monarchs occur annually and, therefore, likely contribute most to the long-term viability of monarchs in eastern North America. Overall, our assessment sets the geographic context to develop successful management strategies for monarchs in Canada.
Monarch butterflies (Danaus plexippus, Linnaeus, 1758) in North America are known for their long-distance movements among the United States, Canada, and Mexico. Monarchs are comprised of one genetic population with two migratory flyways separated by the Rocky Mountains (Brower 1995; Lyons et al. 2012). The eastern North American population overwinters in the highlands of central Mexico and migrates to breeding areas in the midwestern and eastern United States and Canada over successive breeding generations before a final generation migrates back to Mexico (Brower 1995; Flockhart et al. 2013). The western population overwinters in coastal areas of California and some individuals migrate north over multiple breeding generations to all western states and occasionally reaching very southern portions of Canada in British Columbia before a final generation migrates back to coastal California (Brower 1995; Dingle et al. 2005; James et al. 2018). Both migratory populations have declined over the last several decades because of multiple threats (Brower et al. 2012; Schultz et al. 2017), and monitoring their distribution and abundance is a necessary aspect of their ongoing management (Commission for Environmental Cooperation 2008).
Their small population size and large fluctuations in population growth rate (Brower et al. 2012; Flockhart et al. 2015; Semmens et al. 2016; Schultz et al. 2017) were primary reasons why monarchs were designated a species of ‘special concern’ in Canada in 1997, a status that was reconfirmed in 2010 (COSEWIC 2010). Monarch population trajectories currently meet the threshold to be considered endangered in Canada and the Committee on the Status of Endangered Wildlife in Canada has recommended this listing to the federal government for a classification decision due in 2019 (COSEWIC 2016). Both North American populations are at risk from multiple factors such as disease, extreme weather, and overwinter habitat loss (Brower et al. 2012). There is increasing evidence to support that population viability is three times more sensitive to breeding habitat loss compared with habitat loss on the wintering grounds (Flockhart et al. 2015) and, therefore, the monarch population decline is driven, at least in part, by decreasing abundance of their obligate host plants, milkweeds (Asclepias spp.) across the United States and Canada (Pleasants and Oberhauser 2013; Flockhart et al. 2015; Stenoien et al. 2016; Oberhauser et al. 2017; Thogmartin et al. 2017a). Prioritizing locations based on the greatest probability of occurrence for breeding habitat restoration in Canada, therefore, relies on spatially explicit mapping the distribution of monarchs across space and time that extends beyond simple generalized maps of distribution.
A variety of long-term citizen science monitoring programs across North America provides data to map the distribution and phenology of many organisms. Monarchs are the focus of multiple citizen science programs that document locations, migration patterns, reproduction, and disease rates (e.g., Journey North, Monarch Alert, Monarch Watch; for a more complete list see, Ries and Oberhauser 2015). Citizen science programs that document monarch occurrence data may record first monarch observations of the season (e.g., Journey North; Davis and Howard 2005) or a compilation of observations throughout the spring and summer for monarchs as well as other North American butterflies (e.g., eButterfly; Prudic et al. 2017). All programs record the date and location where monarchs are observed, giving rise to so-called presence-only data, and have been used to document migration routes (Howard and Davis 2009), migration speed (Davis and Howard 2005), and breeding distributions (Batalden et al. 2007; Flockhart et al. 2013). Presence-only citizen science data from different programs, if not biased to only include specific locations or portions of the annual cycle, can be combined to increase sample sizes to document annual variation in distribution patterns inherent in insects. Understanding annual variation in distribution would provide valuable information to aid in management and conservation planning (Warren et al. 2001).
The annual breeding distribution of monarchs in Canada depends on the colonization of individuals that migrate from the United States (Brower 1995) and the conditions (habitat, physiological, and geographic) under which such migrations occur (Brown et al. 1996). The eastern North American monarch butterfly population often reaches into southern parts of Canada as far west as Alberta (Brower 1995; Flockhart et al. 2013; Flockhart et al. 2019). In some extreme cases, individuals observed early in the breeding season have likely migrated directly from their overwintering grounds in Mexico (Miller et al. 2012). In contrast, monarchs that reach the extreme southern portions of British Columbia are likely from the western North American population, which overwinters in California (Environment and Climate Change Canada 2016; Yang et al. 2016; James et al. 2018). Introgression among the eastern and western populations (Lyons et al. 2012) implies that climatic, physiologic, and geographic factors may consistently influence migration and hence colonization of the breeding distribution in Canada. Monarch breeding distribution may correlate with open habitats typical of grassland ecoregions (Oberhauser et al. 2001), geographic limits of migration and recruitment (McKinnon et al. 2010), or climatic thresholds for flight in insects. Additionally, these factors may not be mutually exclusive if, for instance, host plant availability may be limited by climate (Lemoine 2015). Because Canada legislates species at risk, including monarchs, independently of the US or Mexico, effective conservation planning requires understanding the extent of the breeding range in Canada, the natural fluctuation from year to year (Brown et al. 1996; Prysby and Oberhauser 2004), and what might be the primary drivers of annual variability.
In this study, we use the citizen science programs eButterfly (Prudic et al. 2017) and Journey North (Davis and Howard 2005) to estimate the annual breeding distribution of monarch butterflies in Canada between 2000 and 2015. In doing so, we test predictions from several hypotheses to explain variation in monarch occurrence (Table 1). Our results provide the geographic context for decision-makers to identifying priority areas to engage in restoration and other conservation-related activities.
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