As of: 01/13/2023 6:06 p.m
Winter is exceptionally mild. But what is pleasant for humans confuses the animal world: Many species spawn, incubate or hatch earlier – and then find no food.
Double-digit temperatures in January: Most people should find such mild winters comfortable. But the spring-like conditions can throw the animal and plant world quite a mess. Especially when it comes to reproduction. Because of global warming, animals breed, hatch or spawn earlier – and that can lead to major problems.
Herring stocks are declining
An example of this is the herring off the German Baltic Sea coast. The Greifswalder Bodden is something like their nursery: they spawn there in spring and attach their eggs to aquatic plants – an ancient biological rhythm.
But the number of young herring has been declining sharply for the past 15 years. “On average, the number of herring larvae has fallen by around 90 percent,” says Christopher Zimmermann from the Thünen Institute. According to the marine biologist, the main reason for this is that the larvae hatch earlier and earlier due to the warming of the sea – according to studies, almost three weeks earlier than 30 years ago.
The problem: After a few days, the herring larvae have consumed their yolk sac and then need the tiny offspring of zooplankton, i.e. small crustaceans, as food. However, according to Zimmermann, it does not seem to be available in sufficient quantities at this early stage because – unlike the herring larvae – it is produced by light. The result: the larvae starve.
And as a result, the number of adult herring that are ready to spawn is declining from year to year. A downward spiral.
“Mismatch” as a problem
In other animals, too, processes that have developed in the course of evolution become lopsided within a few decades. For example with some sea turtles: With them, the temperature influences the sex of the offspring. Males hatch below 27.7 degrees Celsius, females from 31 degrees, both sexes are possible in between. In Florida and on the Great Barrier Reef, conservationists report that hardly any male animals hatched.
Biologists call the phenomenon “mismatch”. Translated this means: hunters miss their prey. Flowers and their pollinating insects live side by side. In short: nature is losing its complex rhythm that has been coordinated over a long period of time. Because food webs are built on top of one another, a problem in one species can trigger a series of changes.
“Waken from hibernation prematurely”
This also applies to animal species right on our doorstep. In many places, blackbirds and great tits are already singing like in spring: “The rising temperatures since Christmas with a heat record on New Year’s Eve have roused nature from hibernation prematurely,” says NABU ornithologist Stefan Bosch.
But what may please walkers can cause problems for animals that prefer colder temperatures – such as the rock ptarmigan or the yellowhammer. They have to retreat to higher altitudes and more northerly areas, which reduces their habitat and also their breeding areas. That could have a negative effect on the number of offspring, says Bosch.
Migratory birds can no longer find food
Migratory birds can also be confused by the mild winter or an early spring break. This applies to the pied flycatcher, for example: “When the songbird returns from Africa in April and May, there is less food for it and its chicks because insects hatch earlier than usual in this country. And the competition for nesting sites is greater, since local species come with them earlier can start breeding business,” explains the NABU specialist.
In addition, predators such as the edible dormice are on the move earlier. In addition to plant-based food, bird eggs and chicks are also on the menu. In some regions of Europe, the population of the pied flycatcher has already declined by 90 percent.
Butterflies react to temperatures
Double-digit temperatures in the middle of winter can also set off a chain reaction for insects. The peacock butterfly, for example, a species of butterfly, hibernates during the cold months. If it becomes active as early as January, there is not yet enough food for the insects, says NABU expert Bosch: “They are already using up a lot of their energy reserves and therefore have less of a chance of reproducing.”
In addition, they lack suitable host plants in spring to lay their eggs, such as the stinging nettle. Other plants are out of the question as the caterpillars feed exclusively on stinging nettles.
Other species benefit from the warmer temperatures. For example, the pigeon tail, a butterfly whose appearance and flight behavior resembles a hummingbird. The insect’s reproductive conditions have improved with milder temperatures. Instead of only in the Mediterranean region and in southern Germany, it is now also successfully multiplying in the north of the country.
Fragile ecosystems
It is not yet possible to assess the long-term consequences of the asynchronicity of biological processes. Because ecosystems or food webs are complex and very fragile structures. When a change occurs at one level, it can affect many other ways.
Some species can, within certain limits, adapt to changing temperatures. But that can take a while – for the herring in the German Baltic Sea, marine biologist Zimmermann expects at least 25 years. This means that until then there will be very little offspring – not a good perspective for species protection and also for fishermen on the German Baltic Sea. Or as Zimmermann puts it: “Until then we can only stand on the beach and loudly ‘Crap!’ call.”
“Fighting Climate Change”
That’s why rapid countermeasures are now needed, says Zimmermann, so as not to further confuse the reproductive behavior of animals and plants: “For the herring, this means in concrete terms that we have to reduce other stressors, such as over-fertilization of the sea.” Then the stocks could recover within a few years despite the few offspring.
But it is also important to address the fundamental problem. “Of course we have to slow down global warming and climate change.” Because with every tenth of a degree more, new faults threatened, which could throw well-established processes out of rhythm for hundreds of thousands of years.