How Anthropogenic Climate Change Can Harm Our Friends, the Monarch Butterfly
Author: Peyton Kopp, Edited By: Autumn Berlied
Figure 1: a monarch butterfly sitting on a flower (10). Credit: https://butterfly-conservation.org/butterflies/monarch.
The Monarch Butterfly
Monarch butterflies are well known insects with distinguishable colors in both their larval and adult forms. The larval caterpillar has a brightly colored body pattern of yellow, black and white stripes. As an adult they show off a black body and wings that are colored brightly orange and black, typically with a few white spots and this coloration is indicative of the toxicity of the insect due to their diet of milkweed (1). They are distinguishable from imitators of their colors by their larger size compared to most butterflies, with the wingspan averaging about 10cm (12). Males are typically larger than females but there is much variability.
Like most butterflies, there are multiple life stages before metamorphosis allows the life cycle to reach completion, yielding the familiar butterfly commonly seen in the Northeastern United States. They start as eggs laid by a female butterfly on the food for the caterpillar form that will hatch. The food for the Monarch caterpillar, as stated before, is milkweed plants. The seeds are often “glued” in place when the female butterfly lays them to the underside of a milkweed leaf. After, the caterpillar sheds its skin for the final time in growth, it becomes a pupa hanging upside down encased in a self-grown silk to further develop and will then hatch to become a butterfly (4).
Migration patterns in Monarchs
These butterflies exhibit a migration pattern that runs in two directions. They migrate from north to south, and then back from the south to the north. This pattern is similar to the bimodal migration of birds (2). Monarch butterflies that migrate from the Northeastern United States make the trip down to a mountainous range in central Mexico (11). The butterflies move here during winter months when temperatures get too cold in northern United States and pose a risk of freezing to death. After the winter is over the butterflies migrate back from the south to Northern U.S.A. areas.
Figure 2: Migration patterns of the monarch butterfly (9). Image from the Conserve Wildlife Foundation of New Jersey and website.
The Monarch butterflies migrate when they have gone into reproductive diapause, and conserve energy by keeping metabolic needs low. When they migrate and rest at low metabolic function, this is called overwintering and is a survival tactic. They do this because temperatures in the north are going to become unbearably cold, resulting in the death of many. If the monarchs choose to stay, they will surely freeze. Thus, they stop at night on their journey, resting in trees so that they may reach their new home for the months of November-March, among the Mexican firs.
Figure 3: Monarch butterflies roosting on a tree (5).
How do they know where they’re going?
It has been found that the antennas play a key role in directionality of migration, while the changing temperature directs the timing of migration. Losing both of them shows that the capability to get back home is significantly decreased and direction of travel is scattered (7). However, losing one antenna shows no significant difference in directional awareness, as indicated in Figure 4. Monarchs only travel in daylight hours and are said to have a sun compass used for timing related to light-driven circadian clocks in these antennae (13).
Figure 4: This image shows (6) A. the general direction of spring northern migration in monarch butterflies, red dots show orientation collected from remigration in 2011, and the grey dots represent that information from 2012. B. shows scattered and not clearly orientated directionality for remigration in butterflies that had both antennas amputated. C. shows similar data to that in A, but this is for butterflies which have only one of their antennas, left or right is not a factor that matters (7).
Why does climate matter?
The temperature is another indicator of when and where to go. When the butterflies fly south, they repeatedly navigate to the same spot, then migrate back north by the guide of these temperatures. It has been found that cold temperatures indicate to the butterflies to move back north in March. If those temperature cues come early in flight down south, the butterfly will switch flight directions to head north again (6). What if it is no longer cold enough to give that direction?
Impacts on the species from climate change.
The continued increase of global temperatures in current times can therefore affect the monarch butterflies dramatically. The Monarch butterflies roost in trees equipped for housing them and protecting them from extreme temperatures. The migrate to the Mexican firs where it is cold enough to suppress reproduction still but not so cold that they die (3). If there is such an increase in temperatures, there are a few results that are alarming. First, colder weather in the southern migration location doesn’t occur to trigger the migration back north, the butterflies would continue south and that would disrupt the natural species diversity of the invaded areas and the places they were expected to return to. This could then also have a negative effect on the species that depend on monarch butterflies for food. Secondly, with increasing temperatures, the butterflies would have gonads become active, which could trigger earlier migration north to lay eggs on milkweed plants that they may not find. Another consequence of early migration north is the eggs could still be killed by starvation from frost that occurs and damages their food source when temperatures in the north aren’t quite up to par (7).
Figure 5: A male monarch butterfly on a red flower (11).
Another thing to keep in mind is that the trees the monarchs roost in for overwintering are required to be full enough to shield the butterflies from extreme temperatures by supplying shade and wind cover. If the trees are reduced by deforestation that is a habitat loss that would change where the monarchs go when they migrate. Reducing the amount of covering they get from the trees puts the butterflies at risk for freezing or drying out, and that results in death.
Because these butterflies are so sensitive to the environment and the cues given to them by sunlight and temperature, human induced climate change must be a major focus for protecting the gentle lives of hundreds of thousands of creatures. If there is such an impact on butterflies, imagine the hardships/changes for thousands of other helpless creatures.
Citations
(1) Agrawal, Anurag A. Monarchs And Milkweed. Princeton University Press, 2017, pp. 21-38.
(2) Åkesson, Susanne, et al. “Bimodal Orientation and the Occurrence of Temporary Reverse Bird Migration during Autumn in South Scandinavia.” Behavioral Ecology and Sociobiology, vol. 38, no. 5, 1996, pp. 293–302. JSTOR, www.jstor.org/stable/4601207. Accessed 7 Mar. 2020.
(3) Brower, Lincoln, et al. "Oyamel Fir Forest Trunks Provide Thermal Advantages For Overwintering Monarch Butterflies In Mexico". Insect Conservation And Diversity, vol 2, no. 3, 2009, pp. 163-175. Wiley, doi:10.1111/j.1752-4598.2009.00052.x.
(4) Fuentes, Tracy. "Monarch Life Cycle | Ask A Biologist". Askabiologist.Asu.Edu, 2009, https://askabiologist.asu.edu/monarch-life-cycle.
(5) Ford, Ray. "How To Help Monarch Butterflies Survive | Cottage Life". Cottage Life, 2020, https://cottagelife.com/outdoors/how-to-help-monarch-butterflies-survive/.
(6) Guerra, Patrick A., and Steven M. Reppert. "Coldness Triggers Northward Flight In Remigrant Monarch Butterflies". Current Biology, vol 23, no. 5, 2013, pp. 419-423. Elsevier BV, doi:10.1016/j.cub.2013.01.052.
(7) Knight, A., & Brower, L. P. (2009). The influence of eastern North American autumnal migrant monarch butterflies (Danaus plexippus L.) on continuously breeding resident monarch populations in southern Florida. Journal of chemical ecology, 35(7), 816-823.
(8) Merlin, Christine et al. “Antennal circadian clocks coordinate sun compass orientation in migratory monarch butterflies.” Science (New York, N.Y.) vol. 325,5948 (2009): 1700-4. doi:10.1126/science.1176221
(9) Miller, Kendall. "New Jersey’s Key Role In The Monarch Migration". Conserve Wildlife Foundation Of New Jersey, 2020, http://www.conservewildlifenj.org/blog/2016/10/04/new-jerseys-role-in-the-monarch-migration/.
(10) "Monarch". Butterfly-Conservation.Org, 2020, https://butterfly-conservation.org/butterflies/monarch.
(11) Negley, Erin. "9 Things You Can Do To Save Monarch Butterflies". Lancaster Online, 2018, https://lancasteronline.com/features/home_garden/things-you-can-do-to-save-monarch-butterflies/article_02b20a4c-32b5-11e8-a857-574dda28f641.html. Accessed 14 Feb 2020.\
(12) Oberhauser, Karen S et al. Monarchs In A Changing World. Comstock Publishing Associates, 2015.
(13) "Petition to protect the Monarch butterfly (Danaus plexippus plexippus) under the endangered species act" (PDF). Xerces Society, 2014
(14) Reppert, Steven M et al. “Navigational mechanisms of migrating monarch butterflies.” Trends in neurosciences vol. 33,9 (2010): 399-406. doi:10.1016/j.tins.2010.04.004