If you are like me, you have been eagerly awaiting the change of seasons from the cold gray days of winter to the brighter, warmer days of spring. Although spring has not quite arrived in full force here in New Jersey (in fact, the first official day of spring brought us snowfall) all signs point to its eminent arrival – the days are growing longer and every week there are a few more warm days. I have begun to notice the birds chirping again and have even seen green shoots and budding trees on my daily walks to work! Soon the gray and white color scheme of winter will be replaced with a panoply of color as flowers bloom and everything greens up.
Much like leaf color changes in the fall, these natural, phenological, events are often the best indicators of which season we are experiencing. Phenology is nature’s calendar. It is the scientific term for the study of the timing of biological events1.
Phenology describes when different life stages appear. These life stages are a response to seasonal variation (like temperature changes in the spring or fall or increased rainfall during monsoon season in the desert).
Because climate2 has been relatively stable throughout the centuries organisms have evolved somewhat predictable responses to these climatic changes. This stability has allowed humans to use these natural events to track the change of seasons and to schedule critical life-sustaining activities like planting and harvesting crops as well as important cultural or religious celebrations. Today there are still many festivals related to harvest time or tied to spring flowering, like the National Cherry Blossom Festival or the Branch Brook Park Cherry Blossom Festival in Newark, NJ.
However, there is year-to-year variability in the exact timing of these events. Dr. Richard Primack and colleagues analyzed flowering data for cherry trees in Japan where records go back over 1000 years! In the image below you can see that with some variability the cherries bloom in April. Historically it was mid-April but in the past century the date of first bloom has been moving earlier. Now the cherry trees in Japan are blooming approximately two weeks earlier than they used to.
Spring is advancing, coming, on average, earlier each year. Ancient patterns are changing due to global warming (see my last post) and not just in cherry trees and not just in Japan! Dr. Ignasi Bartomeus and colleagues found that just as the average temperature in April is increasing (shown in image (b) below), apple trees in the mid-Atlantic are blooming earlier (indicated by the solid line in image (a). Pollinating insects are also emerging earlier (indicated by the dashed line in image (b).
These changes are a big deal! Timing is everything, for us, and for nature!
A common theme in many stories is the idea of how an entire life’s trajectory can change due to the timing of one crucial event. Timing changes interactions, preventing some from meeting while making others cross paths unexpectedly. These interactions can sometimes trigger a cascade of unforeseen events.
In nature, the timing of events has similar importance; it determines the interactions between organisms, which in turn determine ecosystem functioning.
So what happens if this timing is thrown off?
Let’s look at the example of almond orchards in California. California produces 80% of the world´s almonds.5 The industry was estimated to be worth $2.84 billion in 2010-2011.6 However, almond trees bloom very early in spring, before most of California’s native bees are available. This means that the almond crop is dependent upon managed pollinators, like honey bees, that can be manipulated to be active at the time needed. Ideally the timing of almond bloom and native bee availability would match up, but because there is a timing mismatch, farmers must pay to use managed honey bees and are at risk should there be a shortage of honey bees (see my first blog post).
Timing is also crucial in many food chains or food webs. For example, many birds feed their chicks caterpillars. If caterpillars are emerging earlier and are not available when the birds require them their chicks will starve.
These changes can range from partial to complete mismatches with outcomes ranging in tandem.
Mismatches are generally caused because interacting organisms use different triggers or cues for their phenology. For example, temperature changes alone seem to dictate when insects emerge from their winter burrows in the soil or vegetation . But plants respond to both increasing temperatures and changes in light availability. Since climate change is only affecting temperature, plants that are more dependent on light as a cue for their phenology will be less sensitive to spring advancement than insects that depend entirely on temperature as their cue to emerge. Other organisms like birds may only have a weak response to temperature and so will not track spring advancement as closely.
This is nicely explained in this TED-Ed video.
Scientists are trying to determine how likely phenological mismatches will be between interacting organisms and the potential consequences. However, for those interacting organisms that do become unlinked, there may be very grave consequences, like lack of pollination for crops or steep population declines. It is also likely that there may be consequences that we don’t fully understand right now. Organisms have a few ways to respond to seasonal and climatic changes. They can either adapt or they can move. Adaptations include changing the timing of their events such that they remain linked with their interaction partners or transitioning to new interactions, including changes in diet or pollinator. Instead some organisms may move, i.e. shift their home ranges, where possible, to locations where the climate is more like what they are adapted to.
However, there are limitations to both of these strategies. Some organisms like polar bears have no suitable habitat they can move to and some specialized interactions may prevent switching to new partners. On the other hand, some organisms will thrive under the new climate conditions, including pest species like ticks (as discussed in this post by fellow EcoTome blogger Jill Devine) which have lower mortality in mild winters associated with higher global temperatures. Other organisms, like polar bears and brown bears, that historically have had separate ranges, are starting to overlap more, potentially resulting in greater levels of competition or hybridization.
There is a high level of variability in individual species phenological responses to climate change and the ability of organisms to successfully adapt or shift depends on their habitat requirements and available habitat. This is why conservation of land and habitat is so important (see post from fellow EcoTome blogger Tony Cullen). Currently our understanding of climate change impacts globally are incomplete and this is why it is so important to fund and support scientific research (see post from fellow EcoTome blogger Megan Litwhiler) on climate change and its human causes .
You can also help scientists study phenology by becoming a Citizen Scientist and joining one of these (or other8) monitoring projects:
As a Citizen Science you can help create a base-line of phenological data and track future phenological changes. It is a great excuse to get outside and the observation skills gained will enrich your daily life, but don´t take my word on it, ask the Cape Cod National Seashore volunteers and Nina Leopold Bradley, daughter of Aldo Leopold.
1 National Phenology Network: https://www.usanpn.org
2 Video with Neil DeGrasse Tyson from National Geographic: https://www.youtube.com/watch?v=cBdxDFpDp_k
3 Primack, Richard B., Hiroyoshi Higuchi and Abraham J. Miller-Rushing. 2009. “The impact of climate change on cherry trees and other species in Japan.” Biological Conservation 142: 1943-1949.
4 Bartomeus, Ignasi, Mia G. Park, Jason Gibbs, Bryan N. Danforth, Alan N. Lakso and Rachael Winfree. 2013. “Biodiversity ensures plant-pollinator phenological synchrony against climate change.” Ecology Letters 16: 1331-1338.
7 Durant, Joël M., Dag Ø. Hjermann, Geir Ottersen and Nils Chr. Stenseth. 2007. “Climate and the match or mismatch between predator requirements and resource availability.”Climate Research 33: 271-283.
8 If you have other Citizen Science projects, please post them in the comments section!