ESA listing decision for the monarch

In a press release today, the U.S. Fish and Wildlife Service announced its decision with respect to the petition to declare the monarch a “threatened” species under the Endangered Species Act:

“After a thorough assessment of the monarch butterfly’s status, the U.S. Fish and Wildlife Service (Service) has found that adding the monarch butterfly to the list of threatened and endangered species is warranted but precluded by work on higher-priority listing actions. With this decision, the monarch becomes a candidate for listing under the Endangered Species Act (ESA), and its status will be reviewed each year until it is no longer a candidate.”

When asked to respond to this decision, my immediate response was the following:

“The warranted but precluded decision for monarchs is the right one at this time. It acknowledges the need for continued vigilance due to the numerous threats to the population while emphasizing the need to continue support for programs that create and sustain habitats for monarchs.”

“Warranted” in the above means that monarchs should be considered threatened due to the near- and long-term threats to the population. Stated another way, monarchs are vulnerable to falling below what biologists have identified as an extinction threshold, a low number from which the population is unable to recover. The term “precluded” means that among the long list of species whose populations are threatened or endangered, monarchs are less at risk than others at this time. Effectively, this places monarchs on a watch list in which favorable outcomes in the form of an increase in population numbers could result in removal from threatened status or negative trends that could result in regulations and further measures to protect the species. This ruling also means that monarchs are not subject to regulations at this time and that funding is insufficient to support monarch conservation. It follows that, in the near future at least, habitat restoration will have to be funded by the private sector.

There are many threats to the monarch population, including habitat loss, fragmentation of habitats, loss of nectar resources, widespread use of herbicides and pesticides, excessive mowing and climate change. From my point of view, the most serious near-term and long-term threats to the monarch population involve climate change. That effect can already be seen in the West. Temperatures have been increasing in the West at an alarming rate with significant consequences for the western monarchs that appear to have fallen below the extinction threshold for that region. In an earlier Blog post, I presented a preliminary analysis of the temperatures and monarch numbers along the California coast during the winter months (see “Monarchs and climate in the West“). Briefly, that analysis shows that overwintering sites are disappearing in the southern counties with an increasing proportion of the overwintering population being found in more northerly counties. These changes appear to be due to increasing temperatures during January–February along the coast. The changes in the mean temperatures during the growing season since 1901 to 2020 are shown in Figure 1 and the overall pattern of temperature changes per decade for the entire United States is shown in Figure 2. (I will provide further explanation for Figure 2 in another Blog article). Extreme temperatures, mean temperatures that are well above the long-term average, together with drought stress, appear to account for the rapid monarch decline in the West.

A rapid decline could occur in the East as well, if the overwintering population is decimated by a winter storm and followed by unfavorable conditions during the growing season such as those in 2012. I’m concerned about the winter storms in Mexico. These storms are a real and immediate threat to the longevity of the monarch migration. Temperatures in the mid-Pacific have increased, resulting in warm, moisture-rich weather systems that sweep eastward toward the Americas in the winter months. In the last two decades those weather systems have entered central Mexico in mid-winter which, when they hit the mountains, result in heavy rains, sleet, snowfall and freezing temperatures that have killed 50–70% of the overwintering monarchs. There have been 4 such events in recent years: 2002, 2004, 2010 and 2016. Fortunately, all of these events have occurred when the population was robust enough such that the numbers of survivors were sufficient to reestablish a substantial population in the spring. Still, you can see the impact of the 2004 and 2016 winter kills on the size of the population the following year. If there is a winter kill at the 70% level that hits an overwintering population of less than two hectares, it will be years before the population recovers – and it may not, if other unfavorable events such as an extremely warm March and summer, e.g. 2012, follow.

Long-term, we will lose the monarch migration due to increases in March temperatures in Texas, increasing summer temperatures in the Upper Midwest, warmer than average temperatures that delay the fall migrations, an increase in drought conditions in the South Region during the fall, and higher than average temperatures at the overwintering sites in Mexico UNLESS we significantly reduce greenhouse gasses. That said, there will be monarchs in the future, but the populations will be local, limited to southern latitudes and mostly non-migratory.

Yes, I know, the above scenario is pretty grim, but it is a reality we have to deal with. So, the question becomes: what are we going to do to sustain the monarch migration? To me, it’s clear. We have to do all we can to restore and sustain monarch habitats, and we have to support initiatives that favor the reduction of greenhouse gases.

Figure 1. Growing season mean temperatures for California in 30-year intervals starting in 1901. Note that while mean temperatures have increased from 68.4°F to 70.9°F, most of that increase (1.4°F) has occurred in the last 30 years.

Figure 2. Rate of temperature increase (°F) per decade during the growing season, 1975–2020. The rate remains unchanged for those areas with growing seasons that start earlier.

ADDITIONAL INFORMATION
Press Release: U.S. Fish and Wildlife Service Finds Endangered Species Act Listing for Monarch Butterfly Warranted but Precluded

Full report: U.S. Fish and Wildlife Service. 2020. Monarch (Danaus plexippus) Species Status Assessment Report. V2.1 96 pp + appendices.

USFWS: Assessing the status of the monarch butterfly