Why is a Flower Bed Like a Pandemic?

 

Pink Evening Primrose (Oenothera speciosa) flower.
Rowan County, North Carolina.

We have a small flower bed at the corner of our garage.  It is entirely enclosed by the driveway, a curving sidewalk and the garage.  In fall we plant pansies (Viola sp.) that flower through the winter and spring.  In spring, Pink Evening Primrose, Oenothera speciosa puts on a flowering show.  Early summer brings Bee Balm (Monarda didyma) with its purple flowers and the red, pink and white blooms of the exotic Impatiens balsamina.   So, this small flower bed blooms throughout the year.

 

Our flower bed with Pink Evening Primrose and Pansies. in flower.
Rowan County, North Carolina.

  

Time lapse video of Primrose flowers, mid-April to mid-June

Rowan County, North Carolina. 

I have always been impressed at how the number of Pink Evening Primrose flowers increase, reach a peak then fall off.  This spring I counted the Primrose flowers and photographed the bed nearly every day for almost two months.  By the middle of June, the blooms were tailing off and I graphed the number of flowers for each day. 

 

Graph of Pink Evening Primrose flowers per day 
from mid-April to mid-June.

The graph shows a rapid increase in the number of flowers with the peak flower number coming in the middle of May.  Around the peak, more than 300 flowers were open each day in this small bed.  The number of flowers started a gradual decrease as we got into June.  Flower numbers fluctuated from day-to-day after the peak. This was probably because of the natural variation in flower bud production by the Primrose and may have also been influenced by environmental factors like day length and rain fall.

I was struck by how this graph of flower numbers over time resembled the pattern of cases in an epidemic.  When a new disease enters a human population the number of cases rapidly increases.  The rate of increase depends on many factors like transmissibility of the infectious agent, the density of the human population and preventative measures. The numbers eventually reach a maximum then decline. 

Idealized graph of an epidemic.  
https://www2.palomar.edu/anthro/medical/med_3.htm

The ongoing COVID-19 tragedy is a more complicated situation.  A pandemic is a world-wide, long-term epidemic.  COVID-19 started in China in late 2019 and reached the United States and many other countries in the winter of 2020.  By March 2020, US case numbers began to increase.  The case numbers peaked in April and began a gradual decline as preventative measures like closing locations where people congregate, working from home and mask wearing was required.  During the summer of 2020 cases began to increase again as people traveled for business and pleasure.  The numbers began to decline again in late summer but as the weather cooled, people gathered inside and traveled for the holidays.  This caused another spike in the number of COVID-19 cases that reached their peak in January 2021.  Two vaccines were released to the American public in early 2021 and the number of COVID-19 cases began to rapidly decline.  This decrease has continued into the summer of 2021.

Number of COVID-19 cases in the United States, February 2020-June 2021.
New York Times. 

Both Primrose flowers and COVID-19 cases showed a similar pattern. Pink Evening Primroses in the flower bed next to our garage had a rapid increase in number, peaked, then as the days got longer flower production tapered off.  The United States population has suffered through several peaks of COVID-19.  Millions were sickened, hundreds of thousands died and while the virus is still circulating, it is now on the decline.  This welcome decrease in cases was caused by a massive public health response, natural immunity in recovered COVID-19 patients and the vaccination of millions of susceptible people. In many countries the pandemic continues to rage as their people await effective public health responses.  

So, why is a flower bed like a pandemic?  Well, it is not, except in a superficial way.  The dynamics of biological systems allow for exponential growth.  At some point, the growth is limited by intrinsic or environmental factors and the numbers decline. The real connection is that both the virus and the flower are governed by similar biological laws.