While monarch summer population abundance has declined in some areas of the United States, notably in the Midwest, it has increased in several other areas, says a study published in the journal Global Change Biology (New window) (in English).
Ecologist Andy Davis and his colleagues at the University of Georgia have compiled some 135,000 citizen sightings of monarchs made between 1993 and 2018 and recorded in the annual surveys of the North American Butterfly Hobbyists Association.
According to the researchers, these data show that population growth during the summer compensates for the winter losses of butterflies.
millions of butterflies Danaus plexippus follow two migratory routes on the North American continent: the one west of the Rocky Mountains and the one to the east, where the butterflies are much more numerous. These corridors extend over 4,000 km between southern Canada on the one hand and southern California and central Mexico on the other.
For 20 years, the drastic decline in the number of butterflies has worried scientists, especially those who study the eastern population, which spends the winter in the forests of Mexico.
Three causes explain the decline in monarch populations, according to current knowledge:
- Climate change causes more weather events like storms and droughts during fall migration.
- The use of pesticides destroys the milkweeds that often border agricultural fields. This plant is essential to the monarch’s life cycle since it is the caterpillars’ only food source.
- In Mexico, illegal logging has led to a loss of butterfly habitats in that country.
Summer population
The work focused on the summer population of adult butterflies across the United States. Andy Davis and his team say that overall this monarch population has remained relatively stable over the past 25 years and that the abundance of the species in general may have even increased slightly during this time. According to these researchers, it would increase by nearly 1.4% per year.
Increases in summer monarch numbers in some areas, including California and the southern United States, may reflect the replacement of migratory populations by resident populationsnote the researchers in a press release.
Selective methodology
The methodology of the study and the analysis of the results by Andy Davis are not unanimous among the experts of these butterflies.
Biologist Alessandro Dieni, coordinator of the Monarch Mission at the Montreal Insectarium, says that the original idea of the study is interesting since it attempts to establish an overall portrait of the monarch summer population using of data collected by citizen observations.
However, Mr. Dieni has reservations. He wonders about the choice of the 403 observation sites used in the study. The selection criteria: butterflies had to have been recorded there over a period of at least 10 years between 1993 and 2018 and also had to have been observed for more than five years.
” Only high quality sites, certainly rich in milkweed and nectar, were kept in the study. The picture may show stability or even growth, but it does not reflect what is happening on a continental scale. “
Mr. Dieni nevertheless believes that the data on which the study is based make it possible to take a Photo of the continental situation. However, this portrait would benefit from being better framed.
I would be curious to repeat the same exercise from sites where there have been fewer observations. I would also take into account immature stages, especially those of eggs and caterpillars, which would give a more complete pictureadds Alessandro Dieni.
” Sometimes there can be a lot of adults, but if the caterpillars are rare for a certain period, there will be fewer adults who will migrate south later. “
However, the data show a decline in the number of individuals in the agricultural region of the Corn Belt in the Midwest, considered the center of the breeding range of this species.
This reality, combined with the very low winter counts observed in Mexico, does not worry Andy Davis. When the butterflies make their way north again in the spring, they can really make up for winter losses.
” A single female can lay 500 eggs, so they are able to bounce back a lot if given the right resources. This means that the decline of colonies in winter is not really representative of the entire population of the species: it is somewhat misleading. “
The situation in Mexico
Also, in recent months, some studies have reported new colonies in forest sites in Mexico. Some studies also show that colonies seem to be doing better this year.
Alessandro Dieni explains that this information must also be placed in a global context.
The scientist recalls that studies have established that the risk of extinction of the eastern migratory population is high if it covers less than six hectares of forest in Mexico.
” On the news, we hear that populations are rebounding by 35%. In reality, we went from 2.1 hectares to 2.8 hectares. This is good news, but we are still well below the target of six hectares, which would be a population size considered to be stable and less in danger. “
The researcher says monarch populations are sometimes discovered in unexplored areas of the forest or in places where the butterflies had never been seen. However, in general, even with newly discovered colonies, the numbers are still relatively low.
Participatory science
Mr. Dieni does not question the database used in the study, created thanks to a community of volunteers who, each year, publish observations there.
We may not agree with the interpretations of the article, but I find it interesting that we use citizen science datanotes Mr. Dieni. At Mission Monarque, we believe that data collection by the non-scientific community is a great way to help endangered species.
Source: Radio-Canada
