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Jeff Mitton: Natural Selections: Late fall pollination of rabbitbrush by sulphur butterflies reflects changing climate

A clouded sulphur butterfly sips nectar from a sunflower in November. (Jeff Mitton / For the Camera)
A clouded sulphur butterfly sips nectar from a sunflower in November. (Jeff Mitton / For the Camera)

On Nov. 4, 5 and 6 I went to Rabbit Valley in the McInnis Canyons National Conservation Area in western Colorado to enjoy camping, hiking in canyons, and photographing scenery.

Balmy days and chilly nights were predicted, and I was resigned to find dried flowers and no insects. But I was pleasantly surprised to find butterflies attending persistent flowers.

Jeff Mitton. Natural Selections

Driving into Rabbit Valley, I kept scanning the area for yellow leaves, for Fremont’s cottonwoods would still be displaying fall foliage, but the yellow that I found first, and most pervasively, was the lacy last blooms on rabbitbrush, Chrysothamnus nauseosus. Rabbitbrush, also called chamisa, turns lower-elevation western valleys yellow in late summer and early fall, but I did not expect to find lingering blooms in November. I walked to a small cluster of these shrubs and was surprised again to find what I think were clouded sulphur butterflies, Colias philodice (I cannot exclude orange sulphurs, Colias eurytheme), nectaring on the blooms. I did not expect to see pollination continuing into November.

Clouded sulphurs have an immense range, from Alaska to Nova Scotia and south to Mexico. In the northern portions of their range they have three generations from May to October, while in southern portions of the range they have four or five generations from March to November. Could it be that the life history typical in southern populations is migrating northward?

The balmy weather I enjoyed in early November was not a freak occurrence, but another part of documented trends in the seasons. A study of temperature records from 1952 to 2011 for locations in the northern hemisphere between 30 and 60 degrees of latitude reported that all four seasons were changing. Summer had lengthened from 78 to 95 days, while spring, fall and winter shrank from 124 to 115, 87 to 82 and 76 to 73 days, respectively. Spring comes earlier, and fall comes later than when I was a kid. Analyses of 54 years of data in Korea and Japan found that leaf senescence and leaf fall were occurring later in the majority of plant species examined. Other observations consistent with changes in season lengths follow.

Japan has historically treasured its cherry trees and celebrated cherry blossoms each spring. In 2021 cherry trees in Kyoto bloomed on the earliest date in the 1,209 years that cherry blossom records have been kept. In spring 1912, Japan gave America 3,020 cherry trees of 12 different varieties, which were planted around the tidal basin in Washington, D.C. In 2021, cherry blossom peak occurred on the third earliest date in 99 years of blooming in Washington.

Sailboat racing attracts many boats on Lake Mendota in Madison, Wisconsin. Because the sailboats are built for speed, their hulls are fragile, and would be damaged if a boat were to hit a piece of ice. Consequently, each year they announce when Lake Mendota is clear of ice, and data on ice melt have been kept from 1850 to present. I compared the averages of the first 10 years and the most recent 10 years. Nowadays, ice disappears two weeks earlier than it did in the 1850s.

Following 20 years of temperatures above average and eight years with abnormally low precipitation, 186-mile-long Lake Powell is now at its lowest ebb since the lake first filled on June 22, 1980. It is now 156 feet below its level when full and contains less than one-third of its full capacity.

Since 1985, wildfires in the West have become more numerous and larger. Prior to 1983, wildfires consumed 3.5 million acres per year, but more and larger fires burned 8.5 and 6.5 million acres in 2020 and 2021, respectively. In short-term studies of a few years and in long-term studies covering thousands of years, fires consume more forest acres in warm, dry years.

My observation that fall temperatures and pollination seemed to last longer this year is anecdotal. But the anecdote seems to fit into a documented pattern of climate change altering the onsets and durations of the seasons. But why is all of this happening?

Atmospheric CO2 drives climate change, and atmospheric CO2 started rising during the Industrial Revolution (1760-1840). But since my birthday in 1947 the human population has tripled. Furthermore, in my lifetime the number of vehicles in the U.S. increased by eightfold and atmospheric CO2 rose by one-third. Human population growth and human production of CO2 are driving climate change.

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