Announcing! Listing the Monarch Butterfly under the Endangered Species Act? No.


The Endangered Species Act of 1973 (ESA), enacted under President Richard Nixon, is perhaps one of the greatest pieces of environmental legislation in the world.

Six years ago, in 2014, four entities petitioned the US Fish & Wildlife Service (F&WS) to list monarch butterflies as threatened under the ESA. The groups were the Xerxes Society for Invertebrate Conservation, the late Professor Lincoln Brower, the Center for Biological Diversity, and the Center for Food Safety. The CFS is an anti-GMO organization. In any case, when the F&WS dragged its feet in making the decision, in part because it is so overwhelmed by petitions and protection of endangered species and chronically underfunded), this group sued the F&WS and the result was that they were required to make their determination by December 15th 2020.

This morning, the F&WS announced that the monarch will not be listed under the Endangered Species Act, although listing as threatened was “warranted but precluded by work on higher-priority listing actions” according to the F&WS.

It is important to note that no credible scientists thinks that the monarch butterfly, Danaus plexippus, itself as a species is going extinct. In other words, there are hundreds of millions of butterflies, and the species as a whole is not at significant risk. Although native to the Americas, monarchs have been introduced and maintain self-sustaining non-native populations in places as far-flung as Spain, Australia, and Hawaii. Nonetheless, the petition to list monarchs was based on the “migratory phenomenon”, the annual migration that takes monarch butterflies four generations to go from Canada through the United States, to overwinter in Mexico for four months, and then to re-migrate back step-wise, beginning in the spring. It is a magical, unprecedented, and absolutely stunning natural event!

A male monarch on resting on Milkweed

And, the ESA seeks to protect not only species, but also the ecosystems that allow species to maintain sustainable populations, and coverage of unique phenomenon such as the monarch butterfly’s annual cycle. Thus, the F&WS entertained this petition.

And to be clear, the monarch butterflies population has been steadily declining over the past 25 years, at least where we have the best data, which is in the bottleneck of their overwintering grounds in the highlands of Central Mexico. There, the forest area covered by monarchs is censused annually as an estimate of their population size. although this only represents one generation of the four that the monarchs make annually, because most butterflies must pass through this in Mexico, it represents an important pinch point in the annual migratory cycle.

Analysis of the declining monarch population, based on the WWF-Mexico Annual censuses

With 27 years of data, there are various ways to plot and assess the trends.  Above I plotted four year averages for seven periods working backwards. Any way you slice it folks, the trend has been negative, and the population is not what it was. Nonetheless, the extreme downward trend seems to have bumped up in the last period of four years. Is this the new norm, a winter population hovering between 2 and 4 hectares of dense butterflies, perhaps representing 50 million butterflies?  How dangerously low are these numbers? And what can be done to continue to reverse the trend and buffer the population?

High density of monarch butterflies (orange on trees, thousands per square meter) on Oyamel Fir trees in the Monarch Biosphere Reserve (photo by Lincoln Brower)

So, what does this all mean? The listing decision was a highly controversial one. Many serious scientists that support monarch conservation efforts were against the listing. There are many reasons behind that, including the extreme difficulty of implementing protections of an animal that covers nearly every square mile of non-forested land east of the Rocky mountains! In addition, the tremendously high abundance of the butterfly in general, even if recently declining, puts it in a decidedly different category than other threatened plant and animal species.  And, the F&WS is committed to continue to evaluate this listing decision into the future.

Understanding the mechanisms of the monarch’s decline has also been debated. I have written extensively about it in two publications, one published in Science in 2018 and the other in PNAS from 2019. Other topics on the biology and conservation of monarchs are covered in my popular book, Monarchs and Milkweed (Princeton University Press),  newly available as an audiobook. As with many things on this iconic insect, a symbol of nature, a sentinel of the health of our continent, and with staggering momentum in the public’s eye, this decision will echo far and wide. Nonetheless, we must stand firm in our trust of the scientists studying the monarch butterfly, and studying conservation of our wildlands and species much more broadly.

Best wishes, -Anurag

Anurag investigating introduced populations of common milkweed, Asclepias syriaca, in Hungary

8 Replies to “Announcing! Listing the Monarch Butterfly under the Endangered Species Act? No.”

  1. Thanks for the explanation, Professor, and thanks for the photo. I was wondering what you look like, after reading your book two years ago. I’ve been a monarch tagger for over ten years here in Manhattan. Doyou speak Spanish. I painted a mural in Guadajajara in 1984 dedicated to the monarch migration. If you and your assistants need a translator on your next post-COVID field trip to Mexico, I could be your man. I work for free.

  2. I am a master gardener in Southern New Jersey. For the past 5 years I have been very interested in the conservation of the monarch butterfly. I still don’t understand why putting the monarch butterfly on the list would be a detrimental action. I know that this year even though I have milkweed and nectar plants in my garden I saw a dramatic decrease in the amount Of monarchs

  3. Hi Professor Agrawal,
    It was enjoyable and to hear your recent talk for the New England Botanical Club and to read your book Monarchs and Milkweeds. I had plans to teach a class next summer on New England milkweeds at Native Plant Trust and your talk was well timed to further propel my curiosity. I observed the fact that the plants and flowers that provide adult butterflies nectar during their migration back north such as goldenrods are very different to those that sustain the larval stage. How important are these plants? Is there a wide enough variety of nectar plants that it is not much of an issue?

    I read this article in the conservative biweekly The National Review and wondered if you could comment on their perspective

    • There is growing evidence that aspects of butterfly ecology during the fall migration in particular, may be a problem for the monarch population. So, yes, nectar limitation driven by many factors could well be a problem. The authors of this article you point to seem to be somewhat biased (aren’t we all?) — their position that the ESA is disastrous conflicts with what I know… and their opinion that regulation is not the answer seems short-sighted. Nonetheless, there are lots of opinions, and they are entitled to theirs. Thanks for writing!

  4. Thanks for your reply. Thanks too for all the heavy lifting on this topic by you and your colleagues. The evolution of milkweed diversity and the complexity of the monarch link, other insects involved as foragers, pollinators, yet more plants as nectar sources is mind boggling. This is just as good or better than Darwin’s finches. We can see it right here in our backyard and don’t have to go to the Galapagos!
    Perhaps you will all be successful with ESA in the future – or new legislation created such as the one on wild and scenic rivers, or migratory birds which can figure out how to protect a whole system of plants and animals and places. Plants have been shortchanged by the ESA. We are all grateful!

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