Population Dynamics

Population dynamics is the study of how populations change over time. The composition of the population, including the age of the population, the birth and death rates, and the amount of immigration and emigration are all components of population dynamics. In the case of monarch butterflies, the timeframe of study can be anything from a breeding generation to an entire migration cycle, or even longer. The Monarch Larva Monitoring Project (MLMP) is essentially a study of monarch population dynamics.

The eastern population of monarchs is usually largest at the end of summer, after several generations of breeding have taken place. These "last-generation" monarchs migrate to Mexico for the winter. Because of resource availability, predation, and the potential for poor weather, the population shrinks during the winter months (November-March). During a particularly harsh winter storm in 2002, it rained in the overwintering sites, and then the temperature dropped below freezing, resulting in hundreds of thousands of dead monarchs. An event like this can have a lasting effect on the population, especially if the population is small when the event occurs.

On the monarchs' return to their summer habitat range, the butterflies lay eggs in the southern U.S. where there is milkweed. Their host plant is crucial to the dispersal of the population. If there is no milkweed present in parts of this southern breeding range, or if that milkweed is in poor condition, there will be few, if any, monarchs that come from these areas. The offspring of the monarchs that were in Mexico for the winter continue north, where they and their offspring breed throughout the summer over multiple generations. It is during this time - when resources are plentiful and the weather is suitable - that the population increases. Suitable weather during the summer is warm, but not too hot, and wet, but not too wet.

A group of student monitors in New London, MN used data they collected for MLMP to see if their site produced a greater percentage of monarchs than other sites throughout the state. They also wanted to know which year, 2004 or 2005, had more monarchs at their site. Using the two years of data they collected, the students determined that the percentage of monarchs varied from 2004 to 2005 both statewide and in their site, so they concluded that they need more data to come up with an answer to their first question. They did learn that more monarchs were at their site in 2004, but there were some interesting environmental factors that may have affected their outcome.

Another group of students in Sparta, WI monitored for MLMP for six years. Amanda Swain-Sheppard used that data to help determine the correlation between temperature and monarch egg and larvae abundance. Amanda hypothesized that warmer weather would correlate with more monarchs because monarchs develop more quickly in warm weather. Her results may surprise you though!

One winter storm in 2002 had an impact on the monarch population for multiple subsequent generations. Imagine what the cumulative effect of global climate change could have on the population. University of Minnesota graduate student Rebecca "Reba" Batalden used monarch presence data collected by MLMP citizen scientists to determine habitat needs. Then, using computer modeling and climate change predictions, she projected where those habitat needs would be in 50 years. Reba’s poster includes maps of where monarch habitat currently is as well as where it is projected to be in 2055.