Summer Solstice, Shifting Spring
The life history cycles of the bay checkerspot butterfly and its host plant, Plantago, don’t match up anymore. When the butterfly eggs hatch, the plant is no longer edible. Photo: kqedquest.
Tomorrow is our summer solstice—the longest day of the year here in the Northern Hemisphere. For folks in the Southern Hemisphere, tomorrow is the winter solstice, the shortest day of the year. The solstices occur thanks to the tilt of the earth. Humans have been recognizing and celebrating the solstices throughout history; Stonehenge is just one example. But we humans are not the only creatures that pay attention to day length. The life cycles of myriad plants and animals are controlled by the length of the day.
Many plants and animals are sensitive to the photoperiod, or day length. As day length grows longer throughout the springtime, many species of plants begin to flower. Other plants are triggered to reproduce when the day length becomes shorter. In these plants, a protein is actually responding to the number of hours of darkness, not to the hours of light. Many animals respond to day length, too. For many bird species, a critical day length initiates their reproductive maturation and is their cue to begin migrating. Decreasing day length also prompts hibernation in many animals. In all of these examples, photoperiod is controlling organisms’ phenology—the timing of life events, like plant flowering and bird egg laying. Phenology is often tied to the seasons, because of organisms’ responses to day length.
Phenology can also be controlled by other factors, like temperature and the amount of rainfall. As the days grow warmer because of climate change, the timing of organisms’ life cycles is shifting. Spring happens earlier than it used to, and many springtime life events are happening earlier too. In major 2003 study of nearly 700 species, including birds, insects, frogs, flowering plants, and trees, 62% of species’ life cycles had shifted over an average of 45 years. Birds and frogs bred earlier, migrating birds and butterflies arrived sooner, and plants flowered and buds burst earlier.
This is likely leading to a widespread phenological mismatch; while some organisms are responding to earlier springtime temperatures, other organisms are still tracking day length. This means that insects emerge ready to feed on particular plants, but the plants are not yet edible. The insects don’t get their food, and the plants don’t get pollinated. Or migrating birds arrive hungry, and their food source has not yet ripened.
It is difficult to know to what extent phonological mismatches are taking place. A proper study of phenology requires a lot of data—many widespread observations of when a particular plant is flowering, or when and where a particular migratory bird is present. This is where you come in. The National Phenology Network has a citizen science program that allows people across the country to record their observations of plants and animals. This crowd-sourced data will be used to determine the extent and effects of shifts in the timing of organisms’ life cycles.