Monarch Butterfly's Conservation Status Reevaluated: Not Endangered but Vulnerable

In an unexpected turn of events, the International Union for Conservation of Nature (IUCN) has recently reevaluated the conservation status of North America's monarch butterfly, determining that it is no longer considered "endangered." Instead, the classification now stands as "vulnerable" to extinction. Furthermore, the IUCN has hinted at the possibility of lowering the alarm even further to "near threatened" if forthcoming census data indicates stable or growing populations.

The decision, made on September 27th, came as a response to a challenge by a researcher regarding the population models used by the IUCN team to justify the endangered status, which had been conferred just over a year ago. Ecologist Andrew Davis from the University of Georgia argued that the IUCN team had committed a "scientific injustice" by overlooking data suggesting that monarch butterflies are faring well.

The IUCN's shift in classification represents the latest chapter in an ongoing scientific debate concerning the status of these iconic black and orange insects. Monarchs, found worldwide, have gained particular significance in North America due to their awe-inspiring ecology and migrations. The North American subspecies, known as the migratory monarch (Danaus plexippus plexippus), has become a symbol of species conservation.

Migratory monarchs comprise two populations divided by the Rocky Mountains. Every autumn, the larger eastern population embarks on an epic migration spanning up to 4000 kilometers from Canada and the northern United States to central Mexico, where they overwinter. Subsequently, their descendants make a stepwise journey northward, taking multiple generations to complete the return trip. The western population, on the other hand, winters in Southern California along the Pacific coast and breeds near the Rockies.

Researchers generally agree that the number of monarchs wintering in Mexico began to decline in the 1990s or earlier but appeared to stabilize around 2014, with approximately 55 million individuals. The decline has often been attributed to logging in Mexico's forests and the adoption of herbicide-resistant crops in the U.S. and Canada, leading to increased herbicide use and the destruction of milkweed, a critical food source for monarch caterpillars.

However, an alternative perspective, presented by Davis and others, suggests that historic monarch populations may have been artificially inflated due to large-scale forest clearing for agriculture in the 1800s. This land use created open landscapes that supported milkweed and monarchs, only to revert to forests or intensive farming in later years. Davis also highlights past instances of overwintering populations crashing, only to rebound in the following summer.

In 2022, an IUCN assessment relied on annual counts of the wintering population from 1993 to 2020, coupled with two population models, to analyze long-term trends in monarch numbers. One model employed a "linear" method that assumed a consistent rate of change over time. According to this model, the population might have declined by 22% to 72% over a decade, meeting the criteria for an endangered classification. It also noted that the stability in overwintering monarch numbers observed since 2014 made "current rates of decline less concerning than they were in years past."

However, many scientists believed that the "endangered" classification was warranted, as factors such as drought along migration routes or severe winters could potentially push the population into a dangerous downward spiral. Karen Oberhauser, a conservation biologist at the University of Wisconsin–Madison and a member of the assessment team, emphasized that monarch populations were at a level considered unsustainable by most scientists.

Last week, an IUCN panel concluded that the linear model was "implausible" and suggested a more complex model, published in February 2020 by Wayne Thogmartin and his team at the U.S. Geological Survey. This model indicates that the eastern monarch population reached an inflection point around 2014, with a steep decline followed by a slower decrease or even a slight increase. These findings supported a "vulnerable" designation, with the possibility of changing to "near threatened" if forthcoming winter census data indicates continued stability.

While the IUCN's ruling is not legally binding on its member nations, including the U.S., it can influence conservation policy. In 2020, U.S. officials determined that designating the monarch as a threatened or endangered species was "warranted" but not an immediate priority.

The revised classification has garnered mixed reactions. Some experts, acknowledging the challenges of predicting future monarch populations, advocate maintaining the "endangered" listing with the option to upgrade it later as a precautionary measure. Others, like ecologist Anurag Agrawal from Cornell University, who believed that the original "endangered" listing was hastily decided, express fewer concerns. Davis himself is contemplating submitting his own assessment to the IUCN.

In the interim, Anna Walker, an entomologist at the New Mexico BioPark Society who led the IUCN panel recommending the "endangered" listing, points out that the "vulnerable" classification still implies a high level of extinction risk for these iconic butterflies.

Sources:

Study link: www.Science.org