A Monarchy Deposed: The Demise of the Monarch Butterfly
Monarch butterflies (scientific name: Danaus plexippus) migrate annually to forests in central Mexico from Canada and California. Those surviving the 1200 - 2800 mile migration overwinter in Mexico. In this activity, students will learn about the conservation biology of monarch butterflies, threats to their survival, the implications of their potential extinction, the sustainability of natural ecosystems, and ways to protect the species.
Lessons learned from this activity could be used by students to protect species in their own geographic locations.
Engages students in civil discourse/communications that lead to more effective decisions.
Provides students with tools for engagement at all levels.
Encourages self-reflection and promotes creative visioning around sustainable futures.
Encourages critical thinking and data analysis.
Context for Use
Description and Teaching Materials
A Monarchy Deposed: The Demise of the Monarch Butterfly
Daniel C. Abel
Dept. of Marine Science
Coastal Carolina University
2.94 acres (1.19 ha) is about the size of 2.2 U.S. football fields, or about 1.5 international soccer fields. Imagine sitting high in the stands, looking out over the field below and thinking that the entire fate of a species relies on an ecosystem only slightly larger.
The question is not hypothetical. The species in question is the monarch butterfly (scientific name: Danaus plexippus; Figure 1), and the latest scientific assessment found that the 9 remaining hibernating colonies in Mexico occupied just 2.94 acres, down from 7.14 acres a year earlier.
What is happening to this species? Is the decline natural, or a result of human impact? Can the species be saved? Is it worth saving? You will answer these and other questions in this activity.
Question 1: What is the percentage decrease in area occupied by overwintering monarch butterflies in Mexico from 2012 – 2013?
Question 2: Previous surveys (about 20 years earlier) found a maximum 50 acres of overwintering habitat. What is the percentage decrease from 50 to 2.94 acres?
Question 3: Explain whether you think declines of this magnitude are cause for alarm. In other words, explain whether you think the species may be endangered by this decrease in population.
Recall the concept of doubling time (or halving time): When a population grows exponentially (by a percentage of the original number), the time it takes for the population to double, called doubling time(symbol "t"), can be approximately calculated using the following formula:
where t is the doubling time (usually in years) and r is the growth rate expressed as the decimal increase or decrease ×100 (for example, you would enter 7 for a 7% increase).
Question 4: Use the doubling time formula to calculate how long it will take for the overwintering area to be halved (to 1.47 ac).
Scientists calculate the overwintering area instead of the actual population size as an indicator of the health of the species because determining the latter is too difficult. Thus, overwintering area is a proxy for population size.
To determine the health of the species, in addition to knowing the population size (or the proxy of overwintering area), scientists need to know the monarch's critical number.For any species or population of organisms, there exists a population size below which that group is doomed to extinction. This is known as a critical number. When the number of adult individuals drops to this critical level, there is simply not a large enough reservoir of genetic variability and potential for mating to allow the population or species to propagate
and successfully face the rigors of its environment, including competition with other species. Genetic variation is central to the survival of a species because it is the raw material for natural selection. When a population's environment changes, genetic variation can
produce some individuals that have the characteristics necessary for survival. Critical numbers are difficult to determine, and efforts to do so are often not undertaken until a species is near extinction. Thus, it is unclear whether monarchs have reached the critical level, but given the magnitude and trajectory of the decline, it would not be unreasonable to conclude that the population is critically stressed.
Question 5: What kind of threats do you think migrating monarch butterflies might face? List as many as you can.
According to conservation biologists, the most recent decline owes to higher-than-normal temperatures and drought in parts of their range in North America. The higher temperatures resulted in monarchs arriving at their breeding sites in North America earlier than usual. The monarchs also nested farther north than they have done in the past. The result: The timing of breeding was affected, and eggs desiccated in the face of the heat and dryness. Moreover, the nectar content of their main food – milkweed, declined as a result of the weather and the female monarchs were weakened and laid fewer eggs.
The milkweed also suffered from increased use if herbicides on corn and soybean crops. Declines in the past have been attributed to logging in forests in Mexico.
According to Chip Taylor, director of the conservation group Monarch Watchat the University of Kansas, "That habitat is virtually gone. We've lost well over 120 million acres, and probably closer to 150 million acres."
Question 6: Is it worth saving the monarch butterfly? Justify your answer.
Question 7: Do you think there may be other organisms like the monarch whose populations have declined but of which you are unaware?
Question 8: Do monarch butterflies live or migrate through your area? If so, what actions could you take to protect monarchs in your area?
If not, what responsibility do you have to protect monarch butterflies?
Question 9: Summarize the issues in this activity.
Teaching Notes and Tips
References and Resources
McConnell, R.L. & D.C. Abel. 2013. Environmental Issues: Looking Towards a Sustainable Future. Prentice Hall.
Also: http://www.nytimes.com/2013/03/14/science/earth/monarch-migration-plunges-to-lowest-level-in-decades.html (accessed 3/16/13)
https://worldwildlife.org/stories/monarch-butterfly-survey-indicates-lowest-numbers-in-20-years (accessed 3/16/13)
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