Greetings monarch and milkweed enthusiasts! I am writing with a year-in-review regarding scientific findings and observations about M&M. First, it was a wonderful breeding season, with most observers in the northeast reporting many caterpillars and adults. As monarchs now complete their long flight and southern migration this fall, we are hoping for abundance in the Mexican highlands! For more background on Monarchs and Milkweeds, see my book. As a reported in a post from January, the past few years have seen a rebound in monarch numbers from the lowest populations on record 2010-2014, what I called the red-zone.
Although many threats to monarchs have been identified, there continues to be scientific debate as to their relative importance. Indeed, the continental scale of the problem, and the decadal timescale on which the decline is occurring, presents substantial issues for identifying the drivers of monarch population declines. Additional data and statistical modelling are needed. Nonetheless, here is the current list:
One of the key problems has been to identify the factors that determine the number of monarchs that make it to Mexico, and thus are the ‘overwintering population’ – that which is reported by WWF Mexico (as above) and widely considered the best estimate of how monarch are doing (because this is the bottleneck that the monarch population must go through each year). Two important papers were published this past year that contribute to our understanding. Saunders et al. (published in PNAS) reported that winter abundances were predicted by variation in the input of summer breeding monarchs, in addition to variation in an index of floral nectar availability (during the southern migration) and forest patch size (at the overwintering sites). Some of this post below has been excerpted from a commentary I wrote for the same journal.
Monarchs use a wide variety of flowers for nectar, and milkweed is typically no longer flowering during the migration; thus the findings support a long-standing, but previously untested hypothesis of floral nectar limitation. The impact of overwintering forest cover at colony sites has also been long-suspected to be critically limiting to monarchs, but only recently has a quantitative link been made showing the limiting effects of degraded forests. Together it can be concluded during the decade of the steepest declines in monarchs, climate and its impacts on availability of floral nectar and continued degradation of Mexican forests were critical factors. Nonetheless, why the summer breeding population of monarchs plummeted during 2004-2015 is still being debated. Was it a delayed response to the biofuel boom and enhanced usage of herbicides, the 100-year drought experienced in Texas, which is a critical spring and fall bottleneck, or some combination of stresses? Fortunately, new research suggests that large-scale illegal logging has largely been halted in the protected monarch biosphere reserve.
The second notable study by Boyle et al. took the temporal long-view, examining monarch and milkweed dynamics over the past 116 years. The remarkable approach was to employ tens of thousands of museum records of both monarchs and milkweeds as indices of their population sizes. Although the approach was criticized for all sorts of limitations (see several replies to the study listed here), it importantly brings an entirely new set of data, and one of tremendous temporal depth, to the table. Boyle et al.’s key conclusion is that the monarch decline began over 60 years ago and was concordant with the abandonment of small-scale farms (which served as a reservoir for milkweeds). This result is concordant with the long-term decline of monarchs in California supported by observational data over 45 years. Such multi-decadal declines have important implications, the first of which is that no single recent event, such as the advent of genetically modified crops, can be implicated in monarch declines. Perhaps more importantly, the whole of land use change, including agricultural practices, chemical inputs, habitat fragmentation, pollution, development, disturbance, etc. is what has been incrementally creeping up over the last century. Which of the many aspects of this long-term environmental crumbling is responsible for monarch declines is unclear. One of the most disturbing findings of Boyle et al.’s study is the strong and persistent declines of eight milkweed species over several decades. Although the common weedy milkweeds (Asclepias syriaca and A. speciosa) have enjoyed the benefits of the agriculturalization through soil disturbance, fertilization, and creation of ditches until recently (halted by the use of more and more herbicides), the non-weedy Asclepias, numbering well over 100 species in North America, appear to be in trouble.
Several missing pieces to the puzzle of monarch declines remain. First, the role of predators and parasites in monarch populations is likely to be underestimated. Monarchs are consumed at every life stage and some studies have suggested increases these natural threats. For example, incidence of the protozoan parasite Ophryocystis elektroscirrha has increased from about 1% in the early 1990s to 10% beginning around 2010. Second, non-native milkweeds have recently been implicated in not only disrupting the monarch migration, but also increased levels of disease; if invasive species and disease synergize, there could be dire consequences. Thirds, we have relatively little quantitative insight as to the role of insecticides, especially neonicotinoids which have recently come under fire, in suppressing monarch populations. Evidence to date suggests that they can have lethal and sub-lethal effects which may impact butterfly populations at multiple stages. Insecticide use may be correlated with herbicide use, and thus teasing apart their relative roles may be difficult. Finally, there is much to be learned from the correlated declines of the semi-independent populations of monarchs in Florida and California, in addition to the many other declining migratory species including birds and bats, which together may indicate independent, common, or synergizing threats.
Saving an iconic butterfly is important and would help us sustain beauty, wonder, and majesty in nature. But for me, the concern is much larger than a single species. The warning sign I see is that the health of our continent may be at risk. Monarchs are sentinels, traversing the continent, and tasting their way as the move. Thus, to understand what is happening to environmental health more generally, monarchs may be a source of information for both our own population and biodiversity writ large.
In closing, last year marked the end of an era with the passing of legendary monarch biologist Lincoln Brower. Over the decades of his life-long passion for everything monarch, from their metamorphosis and chemistry and to their migration and conservation, his views on the demise of the monarchs evolved and swirled. He was frequently quoted in the New York Times, and here is some of what he said to their journalists: 1986: “Herbicides are a major threat to monarchs, because the butterflies need weeds and wildflowers to survive.” 1990 on deforestation in Mexico: “But until it is clear that cutting has stopped, there is danger of a catastrophe that’s going to spell the end of monarch butterflies in eastern North America”. 2002 on monarch mortality caused by a winter storm in Mexico: “It was really macabre, I’ve been going down there for 25 years, and I’ve never seen anything like it”. 2006: “The biggest threat to the migration is the steady attrition of forests because of illegal logging”. 2011 on use of herbicides in agricultural fields: “It kills everything. It’s like absolute Armageddon for biodiversity over a huge area”. And 2017: on climate change impacts on monarch butterflies: “It’s hard to know what’s going to happen, but I don’t think it will be good.”
Well, on that note, have a happy holiday season and best wishes for the new year! And thanks for considering giving the gift of Monarchs & Milkweeds! -Anurag