Home » 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.