The warming of the planet has advanced the date when plants leaf and flower in the spring by up to 6 days in some locations, creating a potential mismatch between the hatching of butterflies, bees, and other animals, and the availability of the food sources they depend on. If these conditions prevail in the future, some species could suffer great population loss, or even disappear altogether.
An example is a species of Rocky Mountain butterfly, which has been studied by biologist Carol Boggs of Stanford for the last 40 years. The earlier flowering of a variety of alpine wildflower that the butterfly depends on puts the plant at greater risk of frost damage, which can leave the butterfly without the food it needs to sustain it, endangering the species’ population.
Most of us think of the first day of spring as March 20, the vernal equinox, the day when daylight and darkness are equal in length. But to phenologists — scientists who study the life cycle of plants and animals — the first day of spring is the first day that leaves appear on plants.
According to studies conducted by Dr. Mark D. Schwartz of the University of Wisconsin, “first leaf” spring in the contiguous 48 U.S. states is now appearing an average of 3 days earlier than in the recent past: going from March 20 (1950-1980 average) to March 17 (1981-2000 average).
The difference ranges from 5-6 days early in northern states where winters are colder, to 1-2 days early in California, Texas, and some southeastern states where winter weather is more moderate.
A study by Elizabeth Wolkovich of UC San Diego and Benjamin Cook of NASA’s Goddard Space Flight Center arrives at a similar conclusion. They compared an archive of worldwide long-term observations of 1,158 species of wild plants on four continents with results of their plant warming experiment. They varied the temperature around small plots of plants to gauge how these plants responded to higher temperatures. However, an analysis of historical records showed that leafing and flowering advanced even more than indicated by their laboratory experiments. The archives show an average of 5-6 days per degree of Celsius increase in temperature, which corresponds with the approximate average global surface temperature increase since 1900.
As planet earth gets warmer, more and more plant and animal populations will be under increased stress. Some will adapt, some will collapse and disappear. Whatever we can do to slow the progress of global warming will give these threatened species more time to adapt to the new conditions, and prevent their vanishing completely.