The magical marvelous Monarch Butterfly is surging – they are ramping up – populations numbers are skyrocketing! That is to say, according to Monarch censuses, the numbers of migrating Monarchs overwintering in both the Western Migration and the Eastern Migration have vastly improved over last year.
Monarch butterflies (Danaus plexippus) and their annual North American migration represent one of the mysteries of the animal kingdom, with their so-far inexplicable ability to produce one generation each year capable of traveling up to 2,500 miles and then, even after such a long journey for such a small insect, overwintering, without any eating, until Spring, at which time they fly north again eating and mating along the way.
Monarchs feed on flower nectar, like other butterflies, but in order to breed, they require milkweed plants, on which they lay their eggs. Although milkweed is slightly poisonous, monarch caterpillars will only eat milkweed leaves. It is believed that changing agricultural practices to suppress weeds and the widespread mowing of highway verges has greatly reduced the available milkweed for monarchs and contributed to their declining numbers.
And this last winter?
The Western Migration (and see map above) showed a fabulous comeback. According to the Xerces Society, the western migration for the 2021-2022 season was a great success with upwards of 250,000 monarchs found overwintering known sites along the southern California coast from Monterey south to San Luis Obispo. Xerces says this is “an over 100-fold increase from the previous year’s total of less than 2,000 monarchs and the highest total since 2016.”
We went to see the Monarch overwintering in Pismo Beach many years ago. It was pretty awesome. A grove of Eucalyptus trees (note, non native), literally covered with Monarchs and shimmering as they moved.
How can this be so? No one is really sure. For any closely watched annual natural phenomena to increase by 100 times in a single year is more than a little unusual.
However, it is not strange at all to those who are familiar with real-world population dynamics. It is possible that the Western Monarch population may be acting like an “island species” in which local abundance or scarcity of sources and intra-species competition control species population size. In these conditions, the mathematical formulas of population dynamics show definite chaotic features, including population crashes and booms (see the graphic of May Island Squirrel Population). The actuality of this type of chaotic behavior has been confirmed in the natural world many times.
Monarchs, however, are capable of living year-around in the southern parts of California and the northern parts of Mexico and are found quite commonly living and breeding at all times of the year. This means that not all the monarch west of the Rockies take part in the annual migration. Many just do what humans often do, they move to southern California for the winter and get on with their normal lives. For monarch, that means mating, laying eggs, dying, and the new generation hatches as caterpillars which eat milkweed and pupate to become new monarchs. Tagging efforts have shown that some few Western Monarchs may even migrate to the same sites in Central Mexico as the Eastern Monarchs.
Extinction fears for the Western Monarch are not about a real extinction of Monarch butterflies west of the Rocky Mountains, but rather the fear that the Western Monarch Migration will cease to exist: “…in 1983, the IUCN took the unprecedented step of creating a new category in the Invertebrate Red Data Book, in order to list the monarch migration as a Threatened Phenomenon. This is because the numbers of American migrants are falling sharply. Figures for 1997-2016 show a 74 percent decline in California’s overwintering monarchs.” And last year, the numbers for the western migration were vanishing small….almost nonexistent.
The usual sources of census data for the Eastern Monarch Migration have been silent the last 2 years – figures are usually published in February by Mexico’s CONANP – Comisión National de Áreas Naturales Protegidas. Last year there was not official announcement (that I could find). The Monarch Sanctuaries in Mexico were officially closed to visitors because of Covid. I am not sure how this prevented the rangers and researchers from performing their usual census, but either they did not do so, or it is running late.
The latest population data for the eastern migration is from the winter of 2019-2020:
I am attempting to get an official number for the 2020-2021 season – but have had no luck so far. The best I have found is a comment made by Monarch Watch’s Chip Taylor in his post on the 6th January 2022:
“Last year my estimate for the hectare total was almost spot on – 2.0 hectares vs a measured 2.01 hectares. It was more of a guess than a data-based prediction, but I’ll take credit for being close. There are reasons to think I will be close again this year and other reasons to predict that the number will be higher.” (said while predicting another drop in population for 2021-2022).
I have information from local Monarch researchers in Mexico who report that CONANP may release a census soon (it has not as of 20 March 2022).
I can only offer this good news from an eye-witness account:
“But in our estimation, in the Cerro Pelón Sanctuary, there was doubling of the butterflies from last season.” (personal communication — paraphrased for language differences)
I would be guardedly optimistic about their population. From personal observation, Monarchs seemed a little less common here over the last couple years than they had before.
For what it's worth, "butterfly season" has started in Southern Maryland, I've identified 3 species so far, all three relatively cold tolerant species, but as our weather starts to warm more, I expect more shortly.
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