I was fascinated to read research confirming my butterfly abundance/diversity observations, although I’d rather have been wrong about the drought reducing butterfly populations. But the findings were more complex than just fewer butterflies in my area. I’ve commented many times about seeing few or no butterflies during California’s 2011-2015 drought years* and speculated it might be due to the lack of rain limiting host and nectar plant growth.
The fascinating part isn’t that my anecdotal info and maybe my speculative cause were scientifically validated. It’s that I didn’t see many butterflies because of where I was looking in the Sierra Nevada foothills. If I’d traveled downslope to the lowlands, I’d have seen more than were usually present. At low elevations, butterfly population numbers have been decreasing since the 1990s, but the drought and record-high temperatures brought a temporary increase. Surprisingly the data also showed the relatively stable montane populations decreased during 2011-2015. Another interesting point is that what happened to these butterfly populations was the opposite of what biologists guesstimated.
A study published this summer looked at nearly 45 years of butterfly observations at 10 longterm study sites from California’s Pacific coast into the Sierra Nevada Mountains.
“Counter to intuition, butterfly faunas near sea level apparently benefited from the drought, temporarily reversing long-term declines, while montane Sierran faunas were severely harmed,” said Distinguished Professor Arthur Shapiro, Department of Evolution and Ecology [UC-Davis] . [...]
During the so-called millennium drought, butterflies at the low elevation sites experienced some of the most productive years in nearly two decades, increasing in both number of individuals and number of species. At high elevation sites, the effect was opposite, with number of species and individuals declining during the drought.
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That the study was published in Climate Change Responses illustrates what biologists were guessing — declining populations at low elevation sites were the result of climate change increasing local temps. The study results, however, indicated the need to examine longterm data to verify the validity of what scientists considered most logical. Natural conditions altered by climate change might not follow human logic.
“What our study demonstrates is that our imaginations are not very good in terms of projecting how things are going to react,” Shapiro said.
Shapiro speculates that the low elevation population boom was due to winter weather affecting conditions close to the ground. Many of the butterfly species overwinter in the ground debris and are more susceptible to diseases and insect predation when ground level conditions are warmer and wetter. Bacteria and fungi flourish and predaceous insects are more abundant. During the drought, these low elevation areas had desert-like winter conditions with warm sunny days and cold, low humidity nights. More overwintering butterflies probably survived to mature and breed in spring, boosting the summer population size. In the mountains, the winter snowpack usually acts as insulation that protects overwintering butterflies. During the drought, the snowpack was reduced or absent in many areas. Ground temperatures fluctuated with more extreme highs and lows.
Another consequence of the drought and high temps was earlier timing of first spring flight for low elevation butterflies. This opened up a longer time window for them to mate, find host plants, and lay eggs. The opposite happened at higher elevations.
...the total flight window expanded at lower elevations, while in the mountains the flight window shifted and compressed towards the start of the season, a change that is reflected in fewer overall flight days at higher sites….
Shapiro notes that the population boom at low elevations during 2011-2015 doesn’t mean that these butterfly species are rebounding long-term. Lowland butterfly population sizes and species diversity have decreased due to land changes (habitat loss) and warming temperatures. “We have lost species on a regional basis and these didn’t come back, but the species that were still around ended up doing much better in the drought,” Shapiro said.
Another reason for the drought years population increase of low elevation butterflies may be their access to agricultural lands supporting host and nectar plants on the crop margins. Irrigated agriculture increases the likelihood that more of these plants are available during drought. Thus any phenological mismatch between spring emergence of butterflies and growth of their plant associates is ameliorated more in lowlands compared to montane areas. The response of lowland butterfly populations to drought suggests some species might have adaptive tricks not included in models and guesses. While that could be a pleasant surprise for us, undoubtedly nature will display a variety of other surprises.
The scientific paper concludes with this observation about the assumption that plants and animals can move to cooler zones upslope to offset warming temperatures at low elevation. The newest surprise contests the assumption that higher elevations are more resistant to climate change induced temperature increases.
It has been known for some time that high latitude environments are warming faster than the rest of the planet, with negative consequences for many high latitude species [36] but positive or neutral effects for others [37]. It is only recently that climatologists have become aware that higher elevations may also be experiencing a disproportionate share of warming [38], which raises the issue of how cold-adapted, montane ecosystems will respond. [...]
The results reported here suggest that we have much yet to learn about organismal responses to extreme weather, and the extent to which different habitat types might or might not buffer populations against climate change.
We are constantly learning more about global warming affects on weather and local climates, and how species will respond to these changes. Sometimes we discover nature is doing the opposite of what our logic says is most likely. The take-away message here isn’t that I can trust that my incidental observations are validated by scientific findings, but that scientists’ guesses and computer models can’t necessarily predict what what is happening, let alone what will happen. We’re experiencing situations we’ve never seen before. It is, as Shapiro says, “a totally novel climate regime.”
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*I saw no butterflies in my foothill area from 2012 to 2015. In 2016, I saw several California buckeyes working the flowers of my deck container garden and one monarch flying through my yard. In 2017 (after a winter of catastrophic rainfall), I saw buckeyes and California sisters in my yard, along with cabbage whites. In comparison, 2018 was an extravaganza of butterflies. From early May through September throughout my region, I’ve seen many different swallowtail species, California buckeyes and sisters, checkerspots, gray hairstreaks, orange sulphur and many more.
I’ve not seen a monarch since 2017.
Dr. Shapiro confirms this observation, too, and says 2018 is the worst monarch season he’s seen in California, but it’s also the best milkweed season. Throughout the western states, the monarch population that overwinters in California has been absent or seen in exceptionally low numbers all summer.
Butterfly guru Shapiro, distinguished professor of evolution and ecology at UC Davis, has monitored butterfly population trends on a transect across central California for 46 years, from the Sacramento River Delta through the Sacramento Valley and Sierra Nevada mountains to the high desert of the Western Great Basin.
It’s been a troubling year.
“I have not seen a wild egg or caterpillar of the monarch this entire calendar year at low elevations,” he said Sept. 6 during an interview on the “Insight with Beth Ruyak” program on Capital Public Radio in Sacramento.
“Not one.”