On Becoming a Monarch

Environmental & Science Education
Sustainability
Edward Hessler

A new paper from the Oberhauser Lab at the University of Minnesota-Twin Cities, reports on an analysis of 18 years of data from the Monarch Larva Monitoring Project (MLMP).

Two factors have been implicated in the decline of the eastern North American population of the monarch butterfly (Danaus plexippus).  One is an area effect, the decreasing availability of overwintering habitat occupied by adults. The other is also an area effect. There is less and less breeding habitat available.

Carrie Benham, MLMP

In this paper, the authors consider an alternative explanation: declining survival.  It makes use of a trove of data and laboratory work.

The goals of this research on monarchs were twofold: 1) to "estimate immature survival over broad spatial and temporal scales, and to determine what local- and landscape-level site characteristics, as well as spatial and temporal factors, are correlated with egg and larval survival" and 2) to "determine if there are temporal trends in monarch survival, and thus if immature survival rates could be drivers of the observed decline in monarch numbers."

I highlight three key findings but add a caveat.  These short summaries are not without their dangers. So, check the paper for yourself to explore the richness of the research findings. This is where the science is found--in the details--as well as the methods used.

Variation in the survival of immature monarchs is large.  There are two implications. One is for modeling monarch populations and the other is on taking monarch conservation actions.  Neither has included immature monarch mortality.

There was a tendency for survival to be higher in areas with more milkweed (Asclepias syriaca) plants.

Finally, because the research was based on a collection of long-term data, the researchers were able to make an estimate of how many milkweed plants it takes to make a monarch butterfly. This is an adult which will become a fall migrant. Thanks to this work there is now a number...an estimate...a starting point...a minimum for monarch butterfly conservationists to use in planning and management.

Barbara Powers, MLMP

Make a guess and then check the paper for the answer. There are two places it is mentioned, at the end of the abstract and near the end of the discussion of the results.  I was surprised and it made me consider in a much more concrete way what it takes to produce a monarch and then millions and millions of them.

One of the advantages of having a long-term data set is in determining whether there is sufficient data or whether there are holes and, if so, where they are. This study revealed some data gaps if the journey monarch butterflies make to Mexico in the fall (4000 km, a distance that deserves a "Wow!" And some wonder, too.) is to be understood and appropriate conservation actions taken.  The mid-latitudes and the south-east did not provide enough data for what the authors refer to as a "robust analysis."

This study is a wonderful example of the value of long-term monitoring projects done by volunteers. Karen Oberhauser has referred to citizen scientists as an "army for conservation." The need for such information cannot be underestimated as this paper shows. 

h/t to the Minnesota Academy of Science for calling attention to this paper.

P. S. Daniel Ashe, Director of the United States Fish and Wildlife Service (USFWS) visited the Minnesota Valley National Wildlife Refuge, August 20.  He was interviewed by Tom Weber, Minnesota Public Radio where he talked about the just announced 20 million dollar plan to protect the monarch.

During the interview, Ashe characterized the "midwest in the United States as Ground Zero" for monarchs. One way to think about this area is as the corn belt. This is the area where monarch conservation will be won or lost. It produces some 50 percent of the monarchs which migrate to Mexico.