Periodically in the spring, I have the pleasure of teaching Plant Taxonomy to students at a small college in Asheville, North Carolina. Among other things, I love the way that teaching this class forces me to pay close attention to what is coming out of the ground, leafing out, or flowering at any particular point of the season in the Blue Ridge Mountains where our campus is nestled. Each week, I fill the classroom with clippings from plants for my students to examine, up close and personal, as they learn to recognize different families of plants and how they compare with one another: how trilliums differ from jack-in-the-pulpits, or spring beauty differ from rue anemone.
But a couple weeks into the semester this spring, it became abundantly clear that I was going to need to scrap my syllabus and completely rearrange my labs. A very warm and short winter followed by an early spring meant that many of the plants I depend on appeared to be blooming weeks earlier than usual. While I initially doubted my intuition, based solely on passing observations, I then pulled out my collection notes for lab on March 6 and found it was dated April 6, 2013. My intuition was right on target. The flowering period was three to four weeks earlier than when I last taught the class, just four years ago.
In my research, too, the early spring was evident and influential. I study pollination and floral biology in sweetshrub, Calycanthus floridus, which has wine-red-colored flowers with the scent of overripe, rotting fruit that attracts their pollinators, little sap beetles that crawl into the flowers and feed there. I’ve been following the timing of flowering and fruiting in this plant since 2007, and the data so far show that in years with an early, warm spring, the plant flowers earlier…and the beetles are nowhere to be found. The flowers are there in their glory, flooding the area with their intoxicating sweet aroma, but they are holding a party with no guests—and this does not bode well for their future. The plants depend on the beetles for pollination and subsequent seed production, and in years when the beetles don’t visit, their reproductive success drops to almost nothing.
Phenology and climate change
Timing of biological events—such as flowering and leaf-out in plants or egg-laying in insects—is called phenology, and increasing attention has been given to the study of phenology as we face a changing climate. Many organisms depend on climatic signals such as temperature as cues for their timing during the season, and so as the planet warms, their response to these cues will cause them to leaf out, bloom, mate or lay eggs earlier.
But here’s the rub: many organisms, like the sweetshrub, depend on relationships with other species…and not all species use the same cues. One may use mean daily temperature as its phenological cue while another uses day length. If two species that depend on their interaction with one another use different cues in a changing environment, or respond differently to similar cues, they may end up missing each other entirely—what is likely happening with the beetles and the sweetshrub.
Scientists keeping watch over phenology are accumulating more and more evidence that our changing climate is affecting many diverse species and potentially disrupting the interactions among them. For example, a study of bumblebees and the plants they visit in the Rocky Mountains has found that the timing of both has shifted earlier, but not by the same amount. The shift in flowering has been greater than the shift in bumblebee timing, resulting in decreased synchrony—and both plants and pollinators may suffer as a result. In Japan, biologists have followed a spring wildflower (Corydalis ambigua) and its bumblebee pollinators and similarly found that the plants were more sensitive than the bumblebees to early onset of spring. Reduced synchrony of bees and flowers resulted in lower availability of pollinators for the plants, and potentially also lower availability of food for the pollinators.
As the planet warms, plants and pollinators alike may adjust to the changes in different ways, leading to mismatches between these symbiotic partners. This impact of climate change on phenology compounds all the other challenges facing pollinators today, like the loss and fragmentation of habitat, disease, pesticide use, and the spread of invasive species.
Consequences for agriculture
So why should we care about such disruptions in phenology? Being forced to scrap my syllabus is a very minor consequence compared to the potential impacts on agricultural production. By some estimates, 35% of all crop species worldwide depend on or benefit from pollination by animals (including bees and other insects). Some 16% of all vertebrate pollinator species (such as hummingbirds and bats) are threatened with extinction, while at least 9% of all insect pollinators are threatened as well. Pollinators are essential partners with farmers who grow fruit, vegetables and nuts; without them, our own species faces loss of an important component of its food source. Similar mismatches may also change and disrupt relationships between crop plants and pest species, creating new challenges to agriculture or enhancing existing threats.
Farmers see the changes in phenology in their own fields, and they are already concerned about the future of agriculture in a changing climate. But we all need to be aware of the impact of climate change on the web of interactions that make up the world around us, so that we can support lawmakers and others who are ready to stop the human activities impacting our planet’s climate. Many biologists are out there watching, accumulating evidence with the systematic eye of science. We must support their efforts—and listen to their messages about our impacts on the planet and our future.