Climate change might not be to blame for all cases of species - like pollinators and the flowers that depend on them - being out of sync.
Mike Singer was a graduate student at Stanford in the 1960s when he first marveled at the reproductive moxie of the Edith’s checkerspot butterfly.
Although the butterflies he observed were laying their eggs on plants that were green and edible, an awful lot of the plants dried out between when the eggs were laid and when they hatched.
“Those little larvae hatched out, looked around, and died,” remembers Singer, “and that's a very strange thing to see.”
As climate change wasn’t even on the radar yet, Singer approached the phenomenon from a more basic Darwinian perspective. Why, he asked, might a species evolve to the point where so many of its young were dying so quickly?
“Eventually, I figured out what I thought was happening,” says Singer, professor of biology. “If you think about the situation of the female caterpillar, the longer she stays a caterpillar and grows, the more eggs she can lay when she finally becomes a butterfly. The later she lays her eggs, though, the more of them will die. So she has a choice between growing late, being big and fat and fecund and killing most of her kids, or becoming a butterfly earlier, where she’s sort of svelte and slim and has fewer kids but more of them survive. In practice they tend to take the time to get big and full of eggs, and thereby kill most of their kids. That's how they deal with the tradeoff between their own fecundity and their offspring’s mortality.”
Having thought that he understood the riddle of this butterfly life cycle decades ago, it came as a surprise to Singer when he began encountering articles in the scientific literature, in recent years, that were blaming these kinds of high mortality rates entirely on climate change.
It seemed to Singer and to Camille Parmesan—his wife and fellow biologist in the College of Natural Sciences—that some scientists were attributing too much, too soon, to climate change. And that in so doing, they risked not only getting the frame wrong, but exposing the already vulnerable discipline of climate science to further attacks.
“We don’t have the luxury of publishing first and then going back and forth in the literature,” says Parmesan, an associate professor of biology whose work on the climate change-induced range shifts of the Edith’s checkerspot butterfly were pioneering. “It doesn't do us any good to publish things that are not extremely well documented because the climate deniers, if they catch something with holes in it, might use that to characterize all of the responses that we've been documenting as being not due to climate change. So we have to be much more rigorous than other fields. We've got to be more careful.”
One specific target of Singer and Parmesan’s critique, which they’ve just published in Philosophical Transactions of the Royal Society, is a team of Dutch biologists who in 2007 used data on winter moths to identify “another example of climate change induced asynchrony.”
For Singer, the winter moth was a particularly striking example. He remembered attending lectures during his undergraduate days at Oxford in the 1960s, given by a biologist doing work on the winter moth. He remembered learning that the winter moth made a reproductive gamble similar to what he later observed in Edith’s checkerspot butterflies.
Winter moths fed on various trees and laid their eggs on twigs in December. Many of the eggs hatched before buds burst in spring, and the newly hatched caterpillars starved. Other eggs hatched late, after budburst, and became tiny, wimpy moths with few eggs because they ate older leaves that had accumulated defenses as they matured. But a few eggs hatched at just the right moment, when the host plants had just become edible. Those lucky few survived better than early-hatching individuals and laid more eggs than late-hatchers. So, once again there was a trade-off between death rate and fecundity.
“So the butterflies that we study gamble with their kids’ lives by becoming big and fecund,” says Singer, “and the winter moth egg gambles with its own life, by hatching early, but in each case it's a life history trade-off, and the fact that the insect is out of sync with the plants is not a maladaptation, and should not be ascribed to climate change knocking things out of whack.”
Climate change may not be the root cause of asynchrony in these species, but it is, almost certainly, changing the odds of the asynchronous bet. The natural baseline asynchrony varies from year to year, rendering populations vulnerable to extinction in years when it reaches extreme levels. In the case of the Edith’s checkerspot butterfly, for instance, climate change has been implicated in the extinction of entire populations over the last decade or two, likely including those studied by singer in the 1960’s.
“We know that asynchrony is a life history strategy that the butterfly has evolved toward,” says Parmesan, “but now if you put climate change on top of that, what are the consequences? The consequences are that it takes that asynchrony, where 95 percent die, and turns it into 100 percent dying. So it's not that climate change is unimportant at all. It’s that those species that have a baseline asynchrony are the most sensitive, most negatively impacted.”
For Singer and Parmesan, this kind of nuance is part of what makes climate science so fascinating, and often so frustrating. The larger warming trends are, by this point, very clear, but identifying the local effects of the broader patterns can be tricky, particularly when you’re trying to tease out relationships between species and within complex ecosystems. It can be tempting for scientists to try to close the loop too soon, and to fail, therefore, to peer far enough into the past to get a proper perspective.
“Everyone wants to say they're innovative, and cutting edge, and of course it's good to be innovative,” says Parmesan. “It's good to be cutting edge, but what we can’t forget is how important historical literature can be. Not just as a history lesson, but as an element in our research right now.”
Says Singer: “You need to know some very old people, like me, who made measurements before anthropogenic climate change set in.”
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