Phenology and Climate Change

You have probably seen it, maybe in one newspaper or another.  Or maybe you heard it on the radio, or noticed it yourself.  It’s happening everywhere: timing of natural events is changing.  Many studies have documented the phenomenon: Miller-Rushing and Primack (2008) [used historical data on 43 plant species in Massachusetts to show that they are currently flowering an average of 7 days earlier compared to the second half of the 1800’s.  Parmesan (2006) wrote a comprehensive account of such changes in various organisms, and also changes in species ranges and in important ecological interactions of species, all of which are linked to climate change.

The periodic cycles through which plants and animals go is called phenology.  Over thousands of years, organisms have had their phenology defined and established through natural processes and environmental cues such as temperature changes.  Many of the organisms dependent on temperature changes to guide their phenology also developed some close relationships with other organisms essential for their survival, such as food plants or prey.  If the timing is off, due to factors such as climate change, those relationships will be affected, and the organisms may experience stress.  The Edith butterfly is a well-known example where climate change was a factor contributing to a density decrease and eventual local extinction of populations (although it was not the only factor at play).  According to Parmesan:

“The relationship between climate and survival of E. editha is typically mediated not by direct effects of temperature or precipitation on the insect, but by their indirect effects on timing of the butterfly’s life cycle relative to that of their host and nectar plants. […] The gradual warming and drying trend in southern California has likely led to a steady shortening of the window of time in which the host is edible, causing increased larval mortality in these southernmost populations.”

Of course, there has always been variation in timing for many natural events that are brought about by cues other than sunlight or photoperiod.  Temperature and humidity vary on a yearly basis, as exemplified by a warm winter or an unusually cold one.  However, there has been a conspicuous and proven pattern towards warmer temperatures in our recent past, and in spite of year-to-year variability, it has become clear that it is getting warmer earlier than it used to.  Maybe you and I don’t notice it, but those ultra-sensitive organisms, which perfected their phenological timing over thousands of years, certainly do.  And if they happen to be dependent on another organism whose phenology has not responded to increased temperatures in the same way, well, you get the picture. 

Unlike phenotypic changes, whereby organisms change their physical and/or structural appearance in response to climate change – and which can help a species adapt to climate change in the long run, because they are genetically based – phenological changes are just an immediate response with no adaptation value to be carried from generation to generation.  They can lead a species to endangerment, and eventually even to local extinction, if essential interactions are not kept correspondingly.  Therefore, we see that climate change not only can affect species directly, but also indirectly, in many and complex ways. 

It’s happening everywhere, and it is due to climate change.  There are many ways to detect important phenological changes, and there are various efforts underway that involve local communities.  See how you can help at the USA National Phenology Network website.

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- who has written 12 posts on dotWild.

Astrid Caldas is the Climate Change & Wildlife Science Fellow for Defenders of Wildlife. Astrid provides scientific support for Defenders, including providing technical assistance for integrating climate adaptation into programs, doing synthetic research, and publishing papers and reports on climate and wildlife issues.

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