ONE HOME, ONE PLANT AT A TIME
Behind the cause
Why The Monarchs are endangered
A Drastic Decline
The 4,500-kilometer migration of the monarch butterfly may become a tale of the past.
Each fall, monarchs travel from their summer homes in the northern United States and Canada to winter habitats in California and Mexico.
In 2018 the Western Monarch Thanksgiving Count dropped 86% from the previous year, to 20,456 butterflies.
According to the National Wildlife Federation, the number of eastern monarchs that wintered in Mexico this year has decreased by more than 80 percent in the last 20 years.
These counts are just some of the many bad news updates for the charismatic butterfly that makes one of the longest migrations known among insects.
The combined forces of man-made climate change and habitat loss are now putting North American monarch butterflies in danger of extinction.
Rising levels of carbon dioxide can cause milkweed (milkweed), the only food monarch caterpillars eat, to become too toxic for monarchs.
Higher temperatures have been displacing summer breeding grounds further north, lengthening and complicating the Monarchs migration.
Although monarchs are on the brink of extinction, since their numbers are too low to recover the species, a new habitat for monarchs can be created by planting indigenous milkweed species that provide the staple food and rest stops for traveling butterflies, as well as taking more steps to address climate change.
cifras
Where is the milkweed?
A 2004 email from a Midwestern farmer first alerted Chip Taylor, an entomologist at the University of Kansas, to the monarch's apocalypse. By using herbicide-resistant corn and soybeans, farmers can eradicate weeds and other understory plants, such as milkweed, that were competing with their crops.
Fear washed over Taylor. He had spent years studying monarchs and knew that they depended on milkweed for their migratory journey through the Midwest. The arrival of these new varieties of crops meant the death of milkweed.
The data for the following years only confirmed Taylor's worst fears: the number of monarchs began to plummet. "In a very short period of time, the monarchs received a terrible blow, with tremendous consequences," says Taylor.
In addition to the loss of milkweed in the fields, drought is also another detrimental factor for this plant. In 2013 in Texas, there was a major drought that reduced the amount of milkweed, and therefore the number of monarchs that year.
Increased levels of carbon dioxide from the burning of fossil fuels is the main source of climate change, and this increase in carbon can alter the way plants like milkweed build certain molecules, explains ecologist Leslie Decker, researcher postdoctoral at Stanford University.
Carddenolides
Milkweed produces toxic steroids called cardenolides. Monarchs have evolved in such a way that they can tolerate low levels of this poison, storing it in their bodies to use it, due to its bitter taste, as a deterrent to predators
The cardenolides also help butterflies by preventing the growth of a parasite called Ophryocystis elektroscirrha.
The single-celled parasite can infect newly hatched caterpillars by poking holes in their guts to replicate. If the caterpillars survive, the resulting butterflies have misshapen wings and reduced stamina. Cardenolides allow monarchs to tolerate the parasite without harming them.
Toxicity zone tolerable
On the other hand, not all milkweed originate equal. When people started planting milkweed in their yard, the large nurseries eagerly supplied.
The plants they used to offer, however, were a hardy and easy-to-grow milkweed species native to Mexico, the Asclepias curassavica. Like its other North American relatives, A. curassavica produces toxic cardenolides, but at significantly higher levels than the United States milkweed species. "These levels are too high and monarchs cannot tolerate them," explains Matt Faldyn, a doctoral student in ecology at Louisiana State University.
In a paper published in Ecology, Faldyn reports that high temperatures further increased the levels of cardenolides in A. curassavica, making them extremely toxic to butterflies.
"There is a tolerable zone where these toxins are not too toxic but not too weak either. With climate change, milkweed can get past this tipping point and out of the tolerable zone," explains Faldyn.
According to Oberhauser, because both studies on milkweed toxicity were conducted in the laboratory, scientists still don't know for sure what effects changing levels of cardenolides will have on monarchs. However, Faldyn suggests looking for and planting indigenous milkweed species, as they are better adapted to local environments and less likely to become ultra-toxic.