By Chip Taylor, of Monarch Watch
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Sometimes it’s too hot, sometimes too cold and sometimes it’s just right. That’s the story of Goldilocks and the Three Bears, but it is also the story of weather. Weather represents the physical conditions, temperature, windspeed and direction, incident radiation, etc., at any one time period for any specific place. Climate on the other hand represents a long-term record of weather events, usually a 30-year interval, and it is within those records that we can see patterns and trends. Climate change is all about those trends. It’s getting hotter, the intervals between rainfall events are becoming longer, droughts are more severe in some places, rainfall more abundant in others, hurricanes are now more frequent, and ocean temperatures are rising along with sea level. The list goes on.
When it comes to monarchs and pollinators, the challenge is to identify how weather affects the physiology and demography of these organisms. Once such affects are identified, the task shifts to understanding these responses in the context of the trends represented in the recent and projected climatic records. For example, if we note that every year the mean summer temperature is >2.4F above average that the population the following winter is lower, we might hypothesize that the elevated temperatures had an impact on the growth of the population. That leads to questions about how the physiology and demography are affected by temperature. It follows that answers to those questions lead to a general theory or understanding about how a particular species responds to the too hot, too cold and just right range of weather factors. Those understandings allow us to predict outcomes given the observed and expected changes in the climate. We can also look back at the recent climatic record to see if it helps us understand recent changes in the abundance and distribution of a particular species. The goal is to predict population increases or decreases.
To understand how monarchs respond to temperature, it is useful to consider the link between temperature, metabolism, energetic needs, activity, longevity and realized fecundity (the number of eggs actually laid vs the potential total). Monarchs are an enzyme, which is to say that they respond to temperature similarly to an enzyme activation curve. Enzymes are usually proteins that catalyze chemical interactions within living systems. With enzymes, there are temperatures below and above which there is no activity. There is also a temperature at which the interaction is optimal. Monarchs also have a developmental zero below (52.7F) and above (91.2F) which development stops. The optimal operating temperature for monarchs is about 84F. Temperatures well above 84F result in increased respiration/metabolism, greater activity, higher demand for nectar and ultimately shorter lifespans and reduced reproductive output (realized fecundity). The increased demand for nectar is compounded since plants have shorter flowering periods, produce less nectar and can shut down nectar production entirely during periods with high temperatures under drought conditions. The monarch story gets more complicated when we recognize that even though the optimal temperature may be 84F, various activities such as egg laying and migrating are conducted over a range of temperatures with optima that are lower than 84F. We need to understand the role of windspeed and direction in combination with temperature as well. In the spring, returning monarchs have responded to warm March and April temperatures and strong southwesterly winds by moving too far north too soon (2010, 2012, 2017). The populations declined in the winters that followed, in part, because these females had moved into areas with little or no milkweed, in some cases freezing conditions and lower temperatures that slowed the development of the surviving larvae. In other words, the response to the physical conditions had effectively reduced the collective reproductive success of the monarchs returning from Mexico.
There is much more to say on climate, but I will finish with two observations based on my awareness of weather/climate patterns of the past and those projected for the future. First, the good news. While temperatures are increasing in all coastal states, the rate of increase in the heartland, i.e., the Upper Midwest, at least as far south as Texas, is the lowest in the country. This area produces the greatest number of monarchs that overwinter in Mexico. That says, unless the climate changes rapidly, or there are a series of negative events over a period of a year or more, monarchs will be with us for a while. The bad news is that there are signals in the year-to-year records indicating that the growth of the monarch population is negatively affected by increasing March temperatures in Texas, summer temperatures in the Upper Midwest that are more the 2.4F above the long-term averages and high September temperatures that delay the migration. Increasing frequency or severity of droughts could also be a factor in the future.
We often hear the refrain, “Well, you can’t change the weather.” Well, we have. Our use of fossil fuels since the end of the Little Ice Age in the 1850s has changed the level of greenhouse gasses in the atmosphere resulting in elevated temperatures world-wide. These changes have changed weather patterns, ocean temperatures and currents, etc. Temperature increases threaten the existence of monarchs and pollinators and much, much more. We have a choice. We can reduce greenhouse gases to sustain life as we know it, or we can let events overwhelm us.
Mitigating the effects of climate change is just one example of how the work we do for monarchs can make a difference in many ways. What are the co-benefits of monarch conservation that matter most to you? Keep following our “More than Monarchs” series to hear more stories of what monarchs can do for us, our communities and our world.
Article written by Dr. Chip Taylor of Monarch Watch for the Monarch Joint Venture Communications Working Group and NAPPC Monarch Taskforce’s More than Monarchs Series. The Monarch Joint Venture is a 501c3 nonprofit organization and a national partnership of federal and state agencies, non-governmental organizations, businesses and academic programs working together to conserve the monarch butterfly migration. The content in this article does not necessarily reflect the positions of all Monarch Joint Venture partners. Header photo by Wendy Caldwell.