Everyone is talking about the record low count of monarchs at their overwintering site in Mexico, but what does the science say is happening to them and why does it matter?
Monarch butterflies have a special place in the North American imagination. They are beautiful, plentiful, and have a legendary predator-repelling capacity.
With a range that covers the United States, watching monarchs was once a shared cultural experience, but that is changing fast.
No scientists are arguing that monarchs will go extinct, but numbers are dwindling at an alarming rate.
The most recent overwintering population in Mexico covered about 0.67 hectares of forest – a record low. That’s down approximately 44% from the 2012-2013 overwintering population, which was already very low (Rendon-Salinas 2014).
The record high was in 1996-1997, when the monarchs covered about 18.19 hectares of forest (Rendon-Salinas 2014).
Why are the numbers so low?
The short answer: Monarchs need two things, milkweed and very specific overwintering conditions. Both of these are at risk.
Overwintering and Oyamel Fir Trees
There are two large North American populations of monarch butterflies. The eastern population is the largest. These butterflies come from Canada and various US states, but they all (or very nearly all) gather in Mexico to overwinter, primarily in the Monarch Butterfly Biosphere Reserve (MBBR).
Overwintering butterflies go into a state called diapause – it’s not exactly hibernation but they’re not fully active either.
During diapause they can preserve calories without eating, but they can’t easily fly and they require a stable temperature range to stay in this state.
If it gets too cold, these butterflies freeze, but if it gets too hot, they become active and have to search for nectar. The most devastating recorded freezing event in 2002 killed off 75% of the butterflies in the observed area and up to 90% estimated in other areas (Barve 2012, Brower 2004).
It turns out that dense forests of oyamel fir trees provide the most stable temperature for monarchs to overwinter.
Deforestation in the oyamel forests where monarchs overwinter can be a major threat to the species (Barve 2012, Brower 2012, Lopez-Garcia 2012, Vidal 2013).
Take out even a single tree and the monarchs near the open area are more likely to freeze (Brower 2011).
Increased protections and monitoring by the Mexican government and conservation organizations has all but stopped illegal logging in the MBBR (Vidal 2012), making deforestation less of a threat.
Unfortunately, the overwintering sites are still threatened by extreme weather events and climate change.
The places with the highest densities of butterflies now are predicted to have more extreme cold/wet events by 2050, making them unsuitable for the butterflies (Barve 2012, Brower 2012).
In addition, the trees that monarchs seem to favor and perhaps rely on when overwintering, the oyamel firs, are threatened by predicted warming trends in the region (Saenz-Romero 2012).
Starting from the prediction that there will be a 1.5C rise in temp by 2030 (a prediction questioned by Chip Taylor of Monarch Watch on Yale E360) a study by Saenz-Romero suggests that suitable habitat for these trees will quickly disappear in its current range until there is no suitable habitat left in 2090.
A surprising threat to monarchs from climate change has to do with the way that they navigate.
Monarchs’ decisions about which way to fly are affected by temperature. Exposure to colder than usual temperature in the fall causes monarchs that were headed south to turn around and fly north (Guerra 2013, Kyriacou 2013).
That’s right: if they hit colder temperatures, they might fly north for the winter!
It is also possible that if temperatures in their overwintering grounds were not cold enough, they would continue to fly south until they found a colder area (Guerra 2013, Kyriacou 2013).
Milkweed and Changes in Agriculture
Despite the improvements in protecting the MBBR, monarch numbers have continued to plummet.
This turned the attention of scientists to another phase of monarch life. In the spring and summer, monarchs of the eastern population spend their time in Canada and the United States, breeding and laying eggs on milkweed.
Monarch caterpillars are uniquely adapted to eating milkweed. It is likely that they need milkweed (any species of milkweed) to survive (Luna 2013).
Milkweed used to be abundant in the United States, especially in agricultural fields. That changed with the introduction of glyphosate-resistant crops. Milkweed all but disappeared from agricultural lands (Luna 2013, Pleasants 2013).
At the same time, because of increased demand for corn and soybeans in biofuels, more land has been planted with these glyphosate-resistant crops, leading to an estimated 58% decline in milkweed habitat in the Midwest (Pleasants 2013).
There is strong evidence to show that the Midwest is an important area for monarch production. Even though monarchs are typically only counted at their overwintering grounds in Mexico, stable isotope measurements can be used to tell where butterflies were hatched. The data accumulated thus far shows that most of them are from the Midwest (Flockhart 2013, Miller 2012, Pleasants 2013).
Furthermore, a study by Pleasants that estimated monarch production based on availability of milkweed (taking into account loss of agricultural habitat) revealed a positive correlation between estimated monarch production in the Midwest and the subsequent size of the overwintering population in Mexico (Pleasants 2013).
That is to say – as milkweed friendly habitat in the Midwest disappeared, the number of monarchs overwintering in Mexico dropped.
Going forward, climate change may also play a role in the suitability of monarch habitat in the US, but there are many variables that make it difficult to determine what effects it will have (Zipkin 2012).
Where are the butterflies going?
The drop in butterflies counted in the overwintering grounds is disturbing, but it does not necessarily match up to the drop in monarch butterfly numbers overall.
The butterflies could be going somewhere else.
As mentioned above, there are two major populations of monarchs in North America. They are called the eastern and western populations, but they are not genetically distinct, indicating that they continue to interbreed (Lyons 2012).
In recent years, the western population has grown.
Furthermore, some North American monarchs stay in Florida year-round or travel to the Caribbean (Flockhart 2013). In Mexico, there are some small clusters of butterflies farther east than the main overwintering population (Barve 2012).
It could be that as conditions in the MBBR have become increasingly unsuitable for overwintering, more monarchs have chosen these alternative locations.
Beyond the North American populations, there are also introduced populations of monarch butterflies in Hawaii, New Zealand, and Australia. These colonies have shown that monarch migration is highly adaptable.
Why is the overwintering phenomenon important?
Even if it is possible for butterflies to survive elsewhere, it is important to try to save the overwintering phenomenon.
Not only is it an amazing sight that we should want to preserve for its own sake and for future generations, it contributes to monarch health, to the economy, and to the preservation of pristine forests and biodiversity.
Lincoln Brower told Science Now that when butterflies stay in the same place for many generations, as they might if the overwintering phenomenon ends, they are more susceptible to the deadly Ophryocystis elektroscirrha parasite.
Ecotourism is a significant source of income for people living in and around the MBBR (Lopez-Garcia 2012, Lopez-Hoffman 2010, Vidal 2013).
Money from ecotourism is an important incentive for people to protect the forests. Without this many people in the area would have little choice but to turn to logging as a source of income (Vidal 2013).
In the United States, butterflies may be a “canary in the cornfield,” as Brower calls them in the aforementioned Science Now article. The loss of monarchs could be the first sign that the widespread planting of glyphosate-resistant crops is irrevocably disrupting food webs in the Midwest.
What can we do?
See Matt Miller’s previous Cool Green Science post for some of the most important ways that we can help keep monarch butterflies going.
Other things you can do to protect monarchs include:
- Carefully planned, controlled burning of milkweed patches can help make sure that have new growth at the times when monarchs are coming through and laying eggs (Baum 2012).
- Be careful what kind of milkweed you plant. Be sure that it is a species that grows locally. Karen Oberhauser warns Science Now that some tropical species grow year round, which encourages butterflies to stay in one place instead of migrating.
- Continue to protect the MBBR by tightening logging restrictions (Brower 2011, Navarrete 2011) and paying people there more for ecosystem services (Vidal 2013).
- Relocate oyamel firs to habitats that will be more suitable as the climate changes (Saenz-Romero).