Why there will always be monarchs

Last year, the International Union for Conservation of Nature (IUCN) added the monarch butterfly (Danaus plexippus) to its Red List of endangered species. The simplified headlines and accounts that followed this announcement led many people to assume that the monarch is on a path to extinction. What that decision actually targeted was the eastern migratory population that overwinters in Mexico and not the species itself. The assumption that the eastern migratory population is endangered is based largely on the premise that the monarch populations of the mid-1990s (1994-1996) represented the average numbers that could be expected at the overwintering sites each winter. Based on that standard, due to the declines in monarchs in the following decades, many observers declared that the population has declined by 85%.

While the monarch decline from the late 1990s can be attributed to the loss of habitat following the adoption of herbicide tolerant (HT) crop lines as well as the implementation of the renewable fuel standard (RFS) (Pleasants and Oberhauser, 2013, Zuckerman, 2014, Lark, et al 2015, Pleasants, 2017), the loss of habitat due to these changes in agriculture declined after 2012 (Taylor, in prep). Nevertheless, there seems to be the assumption that the population is continuing to decline and that the monarch migration is threatened, possibly endangered and could be lost forever. That premise has been challenged by Meehan and Crossley (2023) who have shown that the population has not declined in the last decade. Indeed, multi-year running averages since 2012 show that the population averages between 2-3ha and is not continuing to decline (Taylor, in prep).

So, what does this mean for the United States? Should the Fish and Wildlife Service declare the monarch threatened or endangered or, given the present numbers and examples of representation, resilience and spatial redundancy, determine that regulatory protection is unwarranted? I favor the latter. In my view, there is nothing to be gained by declaring monarchs threatened or endangered at this time (Taylor, 2023). The “at this time” is key. In the near term, the numbers will certainly vary from year to year, and the prospect of losing the eastern migration may loom from time to time, but the monarch population is remarkably resilient, and the migration will be with us for decades. In the long term (>50 years), due to climate change, the migration will be lost. However, as a species, monarchs will be with us forever. That said, many species are less likely to survive during the coming decades than monarchs. In the following paragraphs, I will outline how the prospect of extinction is related to reproductive rate, specialization and resilience.

There are millions of species on this planet. Some complete their lives in hours while others live thousands of years. For each of these species, continuation from generation to generation depends on continuous successful reproduction by enough individuals to sustain the species. Failure to do so results in extinction. Continuation depends on the reproductive success of individuals that have the capacity to survive and reproduce – adaptations that have been shaped by selection through time. In the simplest terms, the goal for each individual is to replace itself. Some are unfit to do so while others are exceptionally successful. Reproductive rate is the result of long-term selection for numerous traits such as age to first reproduction, the number and size of offspring and more. And all of those traits are influenced by the rate of death prior to reproduction. What selection has produced – for the species that are still with us – is a reproductive rate that offsets the death rates with most species being able to produce an excess of potential reproductive individuals each generation. It is the ups and downs in these excess numbers that we track when trying to assess the status of species of interest. In spite of the shaping of reproductive rates by evolution, there have been intervals in evolutionary history during which change has out-paced the ability of species to evolve or adapt leading to mass extinctions. We are at the beginning of another such period of extinction known as the Anthropocene. Climate change and related human activities during this period seem certain to lead to the extinction of a large number of species that will not sustain their numbers in the face of higher death rates or lower birth rates, or both, associated with these changes.

So, what species will survive, and what species will be lost? We can’t be sure, but it already seems clear that species that are highly specialized, those that live in isolated and unique habitats, and those with extremely low reproductive rates are likely to become extinct in the coming decades. Indeed, some species appear to have already done so, e.g., the golden toad in Costa Rica (Note 1.) Specialization can be adaptive. Selection can favor traits that improve survivorship in a manner that narrows the resource base and habitats in which species reside. It can even result in reproductive modes that rely on the presence of other species, e.g., the emu/quinine/ant/seed dispersal in Australia (Note 2). These specializations can be maladaptive in the face of rapid environmental change. There are a variety of species with these specializations including many butterflies, other insects and even plants. In the case of plants, we have no idea how many involve a relationship with a specific pollinator such that, if the pollinator were to disappear, it would also lead to the loss of the plants themselves due to their inability to produce seeds. There are indications that this may already be happening. Specialists can be left behind during periods of change (Note 3).

As to the pollinators, many, especially social and solitary bees, and many wasps, have low reproductive rates. Bumble bees (Bombus) are especially vulnerable. Colonies are started in the spring by a queen that has survived the winter. If the colony grows well during the summer, reproductive males (drones) and females (queens) are produced toward the end of summer. These mate and the mated queens overwinter. However, during the winter losses can be high due to weather conditions and predation by skunks and other predators leaving few to start the population in some years. Nest failure is common following winter and in other cases, especially when resources are scarce, developing colonies produce only a few reproductives or none at all. After two to three years of low queen production and survival, bumble bees can become locally scarce or even locally and regionally extinct. Pockets may remain, and slow recoveries may be possible in some cases if habitats are stable, but that is less likely if resources decline or weather conditions continue to limit reproductive success. And then there is disease. If disease is the main driver of the decline, recovery may not occur with extinction to follow. Solitary bees, most of which are single brooded, also have low reproductive rates and are probably vulnerable to climate change although we know very little about their demography and year to year survival. Solitary bees are responsible for the pollination of many species of plants that are not visited by honey bees and bumble bees. These bees have an important roll in maintaining the integrity of the ecosystems in which they reside. Given the changing conditions, it is likely that many bee species will decline rapidly in the coming decades with significant numbers becoming extinct. Should that happen, there are indications that plant diversity will decline along with insect species that specialize on these plants. Those declines could be followed by declines in birds and other species that depend on the insects and seeds of the plants that are lost. The long-term result of such a negative cascade would be habitats and sometimes entire ecosystems dominated by a small number of species with high reproductive rates and the ability to survive in a broad range of environments. This has already happened in many areas where humans have extensively modified the landscape.

So, what does all of this say about monarchs? In contrast to the species described above, monarchs have a high reproductive rate and they are highly vagile (wide ranging). They are resilient in that they can quickly recover from conditions that led to sharp declines in numbers as we have seen during the recovery from 2013 (0.67ha, the all-time low) to 2015 (4.01) in the East and in the West from 2019 (30K) to 2022 (247K). Whether monarchs will be resilient enough to sustain the migration for the next 50 years is an open question. That will depend on the pace and extent of climate change, but surely, the migration will continue for several decades. However, even if the migration is lost, as a species, monarchs will always be with us (Note 4).

References

Lark, T. J., Salmon, J. M., and Gibbs, H. K. (2015). Cropland expansion outpaces agricultural and biofuel policies in the United States. Environ. Res. Lett. 10:044003. doi: 10.1088/1748-9326/10/4/044003

Meehan, T.D. & Crossley, M.S. (2023) Change in monarch winter abundance over the past decade: A Red List perspective. Insect Conservation and Diversity, 1–8. doi.org/10.1111/icad.12646

Pleasants, J. (2017). Milkweed restoration in the Midwest for monarch butterfly recovery: estimates of milkweeds lost, milkweeds remaining and milkweeds that must be added to increase the monarch population. Insect Conserv. Divers. 10, 42–53. doi: 10.1111/icad.12198

Pleasants, J. M., and Oberhauser, K. S. (2013). Milkweed loss in agricultural fields because of herbicide use: effect on the monarch butterfly population. Insect Conserv. Divers. 6, 135–144. doi: 10.1111/j.1752-4598.2012.00196.x

Taylor, O. R., (2023). The pending decision: Will monarchs be designated as threatened or endangered? Monarch Watch Blog.
https://monarchwatch.org/blog/2023/06/14/the-pending-decision-will-monarchs-be-designated-as-threatened-or-endangered/

Zuckerman, J., 2014 Plowed Under. https://prospect.org/power/plow

Notes

1. Golden toad – This toad, first reported in 1964 from a small rainforest site in Costa Rica was last seen in 1989. It has now been listed as extinct by the IUCN. https://en.wikipedia.org/wiki/Golden_toad

2. There are many species whose reproductive success is dependent on the presence and specific response of another species – sometimes two. One of the more complex three species interactions involve seed dispersal aided by emus and ants in Australia. https://www.indefenseofplants.com/blog/2019/9/30/emus-ants-one-heck-of-a-seed-dispersal-strategy

3. The Emerald Ash Borer, an introduced species from Asia, is decimating 18 species of ash trees over much of the United States. Ash trees are known to be the hosts for 300 insects with about 100 species being Lepidoptera. Among these are species, such as the seed weevil (Thysanocnemis bischoffi ), that specialize on ash trees. Since, there seems to be no natural resistance to the ash borer among most of the ash species, it is likely that the insects that specialize on ash trees, and lack alternative hosts, will become extinct along with their host trees. https://entomologytoday.org/2023/02/07/ash-trees-insects-alternative-host-plants/

4. Monarchs are genetically programed to migrate, and would continue to migrate in the spring and fall as long as populations persist through the winter in southern states.