Ecology and Vulnerability

Monarch

Insects, including lepidoptera, play an integral role in ecosystems as pollinators and food sources, and studying their interactions and indirect effects of climate change on them is critical (Harvey et al., 2022). Lepidoptera are extremely sensitive to changes in their environment and are being threatened by herbicides, insecticides, habitat loss, pollution, and direct and indirect effects of climate change (Wagner 2021). As ectotherms, climate change can be a major physiological stressor that can cause changes to misalignment with Lepidoptera host plant and emergence, shifts in range, behavior, development and more (Hellmann et al., 2016, Patterson et al., 2019). Species with longer developmental overwintering stages, narrower diets, or restricted ranges may be more at risk of decline or extinction due to changing temperatures and precipitation compared to generalist species (Patterson et al., 2019). Temperature increases have led to many species emerging earlier than host plants, with spring species advancing faster than summer species. Phenological mismatch can reduce survival due to emerging before their host plant or after the host plant senesces (Patterson et al., 2019). Early emergence of adult butterflies could mean limited nectar resource availability, and exposure to late snowstorms and freezing temperatures (Patterson et al., 2019). Asynchrony in some lepidoptera larvae has shown increased rates of parasitism and predation (including cannibalism) due to mismatched timing of host plant emergence and the butterfly’s life cycle (Despland 2018). Research on the effects of climate change on Lepidoptera species is still largely understudied and little is known about the response of many of the RCGN species listed above. More research is needed at both the fine scale and large scale, temporally and spatially.

Shifts in Range, Elevation, or Depth

Monarch butterflies have a wide distribution range, with different regions showing different responses to climate change when modeled at a broad and fine-scale level (Svancara et al., 2019). Monarch butterflies may be limited by climatic conditions and partially by the range of their obligate milkweed (Asclepias spp.) host plant (Lemoine, 2015). Lemoine (2015) projected the Monarch butterfly’s distribution alongside its host plant genus Asclepias and found that Asclepias distribution accounted for ~35% of the variation in Monarch sightings. Climate change could shift the geographic range of some Asclepias species, moving the Monarch’s host plant northward further into Canada. This shift in optimal habitat could cause monarchs to expand their range north during summer breeding seasons (Lemoine, 2015). Fine-scale modeling on western populations showed a shift inland and upward elevation of California's overwinter populations as temperatures increased (Svancara et al., 2019).

Shifts in Phenology

Research from the eastern portion of the Monarch butterfly population in Cape May, New Jersey, found that there has been a shift in the monarch migration time window by 16-19 days over a 29-year period (1992 – 2020) with an average of a six-day per decade shift (Culbertson et al., 2021). They found that this shift in migration timing is correlated with higher-than-average temperatures in September and October. Similar research done in Ontario, Canada, off the Long Point Peninsula in the Great Lakes region for a 25-year period (1995 – 2020) found no significant shift in migration time; researchers hypothesized that the Great Lakes could be creating climate change refugia (Ethier and Mitchell, 2022). Increasing temperatures could lead to a mismatch in resource availability, such as nectar, in southern migratory locations (Ethier and Mitchell, 2022). Increased temperatures in temperate regions have been shown to increase the growth rates of Monarch caterpillars, allowing them to pupate faster (Lemoine et al., 2015). However, some studies have shown that increased temperatures during rearing have not affected body mass or larval survival rate (Lemoine et al., 2015; Soule et al., 2020)

Changes to Morphology or Physiology

Research has found that Monarch caterpillars in warm locations or lower latitudes have thinner black-pigmented lines than caterpillars in colder temperatures or higher latitudes (Tseng et al., 2022). The coloration of Monarch caterpillars is linked to larval development and adult pigmentation, which is correlated with mating success and flight distance (Davis et al., 2012; Hanley et al., 2013; Davis et al., 2007; Tseng et al., 2022). The results from this research could be used to investigate monarch population dynamics and temperature-mediated shifts (Tseng et al., 2022). Migrating Monarch butterflies have elongated wings compared to their non-migrating counterparts (Li et al., 2016; Soule et al., 2020). Their elongated forewings aid them in their ability to fly for a long period of time and over a great distance. Monarch caterpillars reared in increased temperatures showed reduced flight ability, resulting in shorter distances and duration of flight (Faldyn et al., 2018; Soule et al., 2020). These studies found that increased rearing temperatures directly impacted the aspect ratio of the forewing, making it less elongated and similar to non-migratory Monarchs. The decrease in forewing size with elevated temperatures could reduce the ability to complete the Monarch’s annual migration by limiting the duration and distance of flight

Changes in Population

Monarch butterflies migrate over long distances with multiple generations within an annual cycle. Studying the effects of climate change on the entire annual life cycle of the species and across its entire range is important for understanding the entire Monarch population and the potential threats to them. Crossley et al. (2022) found a decline in the overwintering abundance of the Monarch butterfly in California from data collected by citizen scientists through the North American Butterfly Association network across Canada and the United States. This decline in overwintering monarch population abundance is thought to potentially be caused by exposure to agricultural glyphosate and higher-than-average ambient temperatures during migration (Crossley et al., 2022). Research on the population abundance of the Monarch butterfly has shown a difference in abundance in areas with increased precipitation and limited changes in temperature (e.g., northern Ohio) to have a positive effect on abundance compared to areas with higher ambient temperature (e.g. the northern limit of their range; Zylstra et al., 2022). Alignment with host plant emergence and resource availability due to climate change and habitat loss could also greatly impact the abundance of overwintering populations as they migrate between overwintering and breeding locations (Zystra et al., 2022). In the northern regions of the species range, Thogmartin et al. (2017) found that annual stochastic changes in population size could be related to climatic changes in the early breeding period in northern parts of the range. They also noted a negative association between population size and an increase in warm temperatures during the early season (May 1 to 20) and a positive association between warm temperatures later (May 31 to June 9) on abundance, and as minimum temperatures increased during June, they saw a negative effect on population size (Thogmartin et al., 2017). Looking at populations in Canada, Crewe et al. (2019) found that overwintering population sizes were affected by weather conditions during spring migration and recolonization. Weather conditions during the spring and summer seasons are extremely important when it comes to determining the size of summer populations, which is a strong indicator of the size of the winter population for that year (Zylstra, 2021).

Indirect Effects

The concern for phenological asynchrony between milkweed species and the Monarch butterfly has little research, but Gilmour and Kharouba (2022) found that Monarch larvae may not be negatively affected if asynchrony were to occur between Monarchs and milkweed. Their research found that milkweed plants were available throughout the Monarch’s breeding season for ovipositing, and mature larger plants did not have a negative effect on larval performance (Gilmour and Kharouba, 2022). Although Monarch larvae have shown neutral success rates when feeding on native Asclepias species, the asynchrony of flight and the flowering window of the milkweed plants can become mismatched with increasing temperatures (Howard, 2018). This mismatch could lead to fewer milkweed plants being pollinated and the decline of their host plant populations. Increasing temperatures speed up the growth rate of milkweed species, negatively affecting the host plant's quality. One of these negative effects is increased cardenolide concentrations in the invasive Asclepias curassavica (Faldyn et al., 2018). The spread of A. curassavica or the presence of the species in extended population ranges in future climate change scenarios could result in lower success rates due to increased cardenolide concentrations. Larval success was compared to individuals who fed on native milkweed species under the same climate conditions. Native milkweed had a lower cardenolide concentration when reared in warmer temperatures. Mismatched hatch and host plant emergence have been shown to affect narrow-leaf milkweed in plant size, seed production, flowering time, and more (Rasmussan and Yang, 2022). It is projected that the overwintering habitat of the Monarch butterfly in Mexico will disappear within this century, with climate change being a contributing factor (Hellmann, 2016). The decline of Monarch overwintering habitat is attributed to the stress of drought and habitat loss (Gómez-Pineda et al., 2021). Under the RCP 6.0 scenario, these crucial montane forests could increase altitude by 300 – 400 meters (Gómez-Pineda et al., 2019). Translocation of two of the most important coniferous species in these forests indicated that these species could be successful in higher altitude locations as temperatures increased and precipitation declined to establish more overwinter habitat for Monarchs. This shift in overwintering habitat would lead to a 46-77% habitat loss (Gómez-Pineda et al., 2019). The Monarch butterfly's fall migration takes them across Texas as they fly to Mexico during hurricane season (Ries et al., 2018). Ries et al. (2018) examined the potential effects of hurricanes on Monarch butterflies along their migratory pathway and found that, like birds, Monarchs are capable of avoiding the direct effects of hurricanes but potentially would be affected by the indirect effects and the loss of available resources, like nectar, during their journey.