Saturday, October 5, 2013

When did "Common Milkweed" Become Common?

Monarch on common milkweed. © Beatriz Moisset. 2010

The prairies of this continent used to be rich in biodiversity before they were plowed under and turned into cropland. We can only guess at the structure of those lost plant communities by studying the remaining plots of prairie. A couple dozen species of milkweed (Asclepias) prospered in the Midwest two centuries ago, each adapted to its own habitat. Certain species preferred high moisture, others, drier spots. Some showed a preference for coarse, loose, damp, or undisturbed soils; still others did well on almost any soil type. Some needed more sunlight than others. A few survived drought or fire better than others.

Among this variety of habitat preferences, common milkweed, Asclepias syriaca, a "weedy" or pioneer species, does best in disturbed areas, patches that, for a variety of reasons, have lost their established vegetation. In a few years, it is displaced by other species when the ecological succession continues. Two hundred years ago, disturbed spots must have been rather uncommon. Everything changed when the prairie became farmland and was annually plowed. Many species of these plants lost ground under the new treatment. However, plowing created ideal conditions for the common milkweed. It can grow between rows of plantings and along edges, and its deep rhizomes, underground stems, allow it to survive year after year despite plowing. Is this when common milkweed earned its name?

For many years, farming practices in the Midwest continued to disrupt the land creating the conditions that this plant prefers. Farmers dislike this plant for its tenacity. It is said that farmers rejoice at the sight of big fat yellow and black caterpillars devouring the cursed weed. By now, you probably guessed that those caterpillars are monarchs.

USDA map of corn production

Monarch butterflies feed on most types of Asclepias, but Asclepias syriaca is by far their main food source. Perhaps, when the prairie became our breadbasket, it also became the monarch's breadbasket, who took advantage of the spreading weed.

In the meantime, all along the Eastern United States, forests were being cut down. A tree-covered area is not the right habitat for any types of milkweeds. When forests were cleared, the conditions favored by common milkweed emerged. Populations of monarchs must have grown along with the expansion of this plant.

If all this is true, and much of it may be just speculation, then these butterflies must have benefited from these man-made changes. We must ask ourselves: what were the monarch's populations like before the expansion of common milkweed numbers? Were they as abundant before the early 1800s as in recent times? What were the population sizes in their overwintering sites in Mexico? We became aware of the monarch butterfly migration in the past one hundred years. The whole story of their incredible trip to Mexico became known only in recent years. Curiously, no references to monarchs in Mexico can be found until 1890. By then, the transformation of the Midwest with its expansion of common milkweed was well underway. Were their numbers in Mexico so low before this time to escape notice?

Present day discussions about monarchs and their preferred food plant, common milkweed, seem to accept the numbers reported around the 1950s as the norm. Those were the highest numbers ever recorded, and it is assumed that these had remained the same for a long time, perhaps from the days when glaciers receded, tens of thousands of years ago. However, we must consider the possibility that milkweeds and monarchs were never as abundant as in the twentieth century; that this is largely a man-made phenomenon. The numbers of both, plant and butterfly have been going down steadily in recent years. Is the new normal similar to the old normal of hundreds of years ago?

Monarch caterpillar on common milkweed. © Beatriz Moisset. 2010

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© Beatriz Moisset. 2013