Slash oak, caterpillar and big dream

The story of the Great Dream begins with the tilted oak. In early spring, oak buds open and new leaves appear. This is the food of the caterpillars of the winter moth. In winter, the mother butterfly lays its eggs on oak trees. The eggs overwinter and must hatch in time so as not to miss the young leaves. “This would be accurate even today,” says Marcel Visser, head of the animal ecology group at the Netherlands Institute of Ecology (NIOO-KNAW) in Wageningen. He has been involved in the institute’s great research and winter bedding since 1993. “If the caterpillars are late, the leaf contains a lot of tannins (antibodies) and is difficult to digest.” Then the larvae have less chance of survival. But too early is also not good: “Then there will be no food and the young larvae die en masse.”

When the larvae are a little larger, it is an ideal food for larger chicks. Each spring, the large nipple raises a nest of about ten chicks, mainly by catching caterpillars from the smaller winter moth. This period with fatty larvae lasts only about three weeks. During that time, each breast chick should get one gram per day. Parents have to catch a caterpillar about a minute for this. After three weeks, the larvae themselves descend to the ground on a silk thread to molt, becoming out of reach of the bird. “The caterpillar peak is not the same every year, because even before climate change, one year was cooler than the next,” Visser says. “The timing of the caterpillar peak can vary by two weeks from year to year.”

A large size chick should gain a gram a day, mainly thanks to eating the larvae.

Signs from nature

So it should be a good time each year when you start breeding. There are 22 days between a great nipple’s decision to incubate and her eggs hatching. Visser: “So she has to anticipate when there will be plenty of food, look into the future. She does it on the basis of ‘signals’, cues from nature. The length of the day is one of the most important cues, so she knows what season it is. She also uses the temperature of Spring to adjust every year.”

This prediction went well for the great nation. “It wasn’t perfect, but peak larva and peak larva usually get along nicely.” But now there is climate change. Some times of the year it warms up more than others. It is precisely the peak period of the caterpillar (late April to early May) that it warms up faster than the period when the sparrow decides to breed. Result: The signals are no longer valid. “The old rules used by the great nipple no longer apply. As a result, the larva advances twice as fast as the nipple. Scientists call this a phenological mismatch: the timing of the life stages (phenology) of a species and its food is no longer synchronized.

Oak woodland with nesting box of large nipple, and fabrics on which larval droppings are collected for an indication of larvae number.

Marcel Visser, New Knau.

The mismatch is because the large nipple can move only slightly after the first egg is laid. “They are flexible about the date of the first egg, but the laying and incubation of eggs have a fixed duration. If the temperature rises suddenly during that time, there is not much they can do about it. Then the larvae grow faster, while reproduction takes the same amount of time.”

Nature in motion

The great thing is just one example of the mismatch of climate change, which affects animals and plants around the world. From reindeer and deer to fish, amphibians, flowers and their pollinators. Migratory birds also have difficulty predicting the rapidly changing climate in their breeding areas from a remote wintering location. These mismatches can disrupt ecosystems and threaten their functionality for humans. Incompatibility between flowers and pollinators can put pressure on food production; Pest insects can outgrow their natural enemies and cause damage to agriculture and forests. So it’s important to know how nature responds to climate change, and how quickly it can happen.

Caterpillar never too early?

The fact that the caterpillar hatches so early is not good news for the caterpillar itself. Visser: “The winter moth is very sensitive to temperature.” Due to the high temperature sensitivity of the winter moth, the eggs hatch early and before spring. Indeed, very early on, the oak was still in its infancy. “At the low point in 1995, there was a difference of more than ten days between eggs hatching and the appearance of oak leaves,” says Natalie van Dis. She is a doctoral student in Visser’s group and studies the temperature sensitivity of winter moths. The mismatch resulted in massive mortality among the larvae. Van Dis saw in an experiment that half of the newly hatched larvae starved after a day without food. But the winter moth was not doomed to starvation. Visser explains that the curse has become a blessing. “Death is a difficult choice.” Sensitive caterpillars starve, the less sensitive they survive.

Something special happened: the winter butterfly has evolved. With each new generation after 1995, the egg became less sensitive to temperature. The Visser group, including Van Des, hunts the wingless females of the winter moth every five years on oak trees in November. Each female’s eggs hatch at different temperatures. This gives them an idea of ​​the sensitivity of her offspring to each female butterfly. That sensitivity has diminished over the years. “It’s one of the few species that we’ve shown a rapid evolutionary response to climate change in the wild,” van Dis said. “A guide to adapting to climate change.” Thus the winter butterfly of disaster became a success story.

A winter moth embryo in an egg, photographed under a fluorescent microscope. Winter moth eggs have become less sensitive to temperature over generations.

Natalie Van Dis, NIOO-KNAW.

tit families

Like any species, the big tit can adapt to a warmer climate in two ways: within the individual and from generation to generation. The first is called plasticity, and the second is evolution. Researchers at NIOO-KNAW have been observing large-breasted breeding pairs at four sites in the Netherlands since 1955 (Hoge Veluwe, Oosterhout, Vlieland and Liesbos). Using this data, they can paint a picture of the nipple’s great adaptability. “Climate change was not widely accepted in the late 1990s,” Visser recalls. “When it was said that temperatures were going up, we looked at our data series and suddenly we saw that there was a clear trend in temperature and fitting history.” The temperature rose, and it was early to install.

To see if temperature really determines the date of delivery, the researchers conducted an experiment in cages with a pair of breasts each. Climate can be set for each cage. “We gave a set of low temperature, and the other high. The birds with the higher temperature started breeding earlier, with the temperature lowering later. So it is not just a correlation in the field, but a causal relationship between temperature and the date of first egg laying.”

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slow evolution

So the date of placing an individual large nipple depends on the temperature. By looking at the pedigree of all large breasts since 1955, Visser’s group can see if this sensitivity is also passed down from parents to offspring – inheriting the history of the birth. Scholars (grandparents) define the fathers, brothers, and sisters of every great breast. “The Great Dream lives and breeds for about two years. So we’ve had several generations since 1955, with the date and temperature of each nipple set per year.” If you plot the date of extension against the temperature for each individual nipple in a graph, you get a line with height (early or late stratification) and slope (temperature sensitivity) for each nipple. The more similar the bloodlines of the relatives, the higher the heritability in the history of the situation. Extension date was found to be 16 percent heritable (the remainder of the variance in status history is due to other factors). “It’s not much,” Visser concludes.

besides the number of young in each breast that were found as breeding birds in subsequent years (Fitness), ecologists can calculate the evolutionary response to selection. A strong response indicates evolution: populations of large breasts become more sensitive to temperature with each generation. This evolutionary response turned out to be very small relative to the history of nipple placement. “This means that evolution in the setting of history is very slow. This is normal in nature, but now the pace of climate change may be a factor too fast by a hundred that a great person cannot keep up with.”

Big nipple back

However, the large nipple is catching up with the caterpillar. Since 2010, the warming of the caterpillar season has stopped, while the period when the great tit picks up its signals to breed is getting warmer. So the tit breeds earlier and earlier, while the height of the larva remains the same. In the long term, however, Visser expects that warming will continue into the larval period. “Then the great nipple will definitely lag behind.”

Catching up with large breasts is therefore short-lived, and the bird does not owe this to its ability to adapt. Both the intra-individual adaptation and the evolutionary response are extremely slow. This is bad news. “We see the number of young people surviving is decreasing every year,” Visser says. However, he doesn’t think we’ll be getting lower breasts anytime soon. “There is also a dependence on density. In good years, breasts raise more young, in bad years less. But the few breasts that get bigger have a better chance of surviving through the year, than in a good year many young ones grow up. It’s like a lottery.” : If few people buy a ticket, they will have a better chance of winning.” This insulating effect ensures that the number of large breasts remains approximately the same in winter.

“The great nipple is definitely going to start late,” says ecologist Marcel Visser.

Fred Brin, NIOO-KNAW.

normal bird

The buffer has a limit. In the long run, Visser predicts that in the bad years, so few young people will grow up that there is, so to speak, no great idea to “catch the prize” (in reference to the lottery). Then the population will shrink. “But this buffer has a strong effect, and it will last for a long time.”

You might wonder who grieves over a world without large breasts, but the great nipple isn’t the only bird with this problem. “It’s an ordinary bird, found everywhere in the Netherlands,” Visser says. “This mismatch effect is true for many bird species.” So the winter feeding rack may look empty a lot in the future.

Sources

  • United Nations Environment Program (2022). Frontiers 2022: Noise, Fire, and Mismatch – Emerging Issues of Environmental Concern. Nairobi.
  • Reed, T. E.; Jenab, B., Visser, M. E. (2016). Testing of selection bias on reproductive traits of birds and stratification of direct and indirect selection using quantitative genetic models. Evolution 7-10:2211-2225.
  • Visser, M. E., Lindner, M., Gienapp, P., Long, M. C., Jenouvrier S. (2021). Recent natural variability in global warming has weakened the phenological mismatch and selection by seasonal timing in the large breast. Proceedings of the Royal Society of London B 288: 20211337.

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