Biodiversity at the Tipping Point?

Abstract

Rising temperatures worldwide are already causing significant shifts in the distribution ranges of plant and animal species (range-shifts). Paleontology also knows this phenomenon from earlier phases of global warming. Migration and dispersal of warmth-loving species that have been carried away by the movement of human goods will give rise to communities of organisms of hitherto unknown composition (novel communities), about whose stability little is known today. At the same time, the temporal sequences are also shifting. The formerly synchronized occurrence of plant and animal species increasingly no longer matches (mismatch). The effects of climate change first show up in individuals, depending on their genetic characteristics, and then propagate, bottom-up, through populations to ecosystems in which many individuals of different species interact with each other. During extreme climatic events such as marine or terrestrial heat waves, mass mortality events can occur, as recently in the Pacific Northwest. Climate change is not the cause of the biodiversity crisis we are currently experiencing but threatens to dramatically exacerbate it in future.

The creatures that inhabit this planet on land and sea are closely and in many ways connected to its atmosphere and the climate that prevails in it. Many hundreds of millions of years ago, marine unicellular organisms, the cyanobacteria, ensured that the oxygen produced as a by-product of their photosynthesis accumulated in the atmosphere over a long period of time, thus creating the conditions for the development of more complex organisms. By breathing this oxygen and using it to produce energy, living organisms are also significant producers of carbon dioxide. Methane is also to a large extent a product of living organisms, because it is produced, among other things, during the microbial decomposition of organic matter. Thus, the most important greenhouse gases are not least a product of biological processes. In view of the major problems caused by an anthropogenic increase in greenhouse gas concentrations, it is often forgotten that it is precisely thanks to this greenhouse effect caused by CO₂, water vapor, methane and other gases that tolerable temperatures prevail on Earth at all.

If it were not for this effect, our planet would be hurtling through space as a cold ball of rock or ice and would probably be inhabited by cold-resistant microbes at best.

The climate determines the distribution of living organisms on the planet, on the continents, above all through the distribution of water. If the climate changes, this has a direct impact on the composition, nature, temporal organization and spatial distribution of the biotic communities that exist on Earth. These processes are studied by a still young discipline within biology.

“Climate Change Biology”, defines Lee Hannah in his book of the same name, “is the study of the effects of climatic change on natural systems” (Hannah, 2011, p. 3). Of course, the main focus is on the effects of the current climate change, which is almost certainly caused by humans. However, in order to understand and correctly classify the changes that will affect us and the organisms of the Earth, Climate Change Biology also looks far back into the Earth’s historical past, examines the changes in the world of organisms today and attempts to model future developments with the help of the most modern computer techniques (Kegel, 2021).

The prominent palaeobotanist John W. Williams has described the options that plants and animals have in a changing climate as “move, adapt, persist, or die,” (Williams & Burke, 2019, p. 129) four options that will help us in what follows to classify the variety of responses that Climate Change Biology is investigating in nature.

Shifts in the Distribution Areas: “Move”

For years, biologists all over the world have been observing that the distribution ranges of plant and animal species have been on the move. These so-called range shifts accompanying rising or falling temperatures are also known from earlier times and are well documented in the fossil record (McInerney & Wing, 2011). In a warming world, living creatures move polewards in order to be able to continue living in their usual temperature range, i.e., northwards in the northern hemisphere and southwards in the southern hemisphere. The resulting shifts are already substantial. In Great Britain, of almost 330 animal species studied, 275 have migrated northwards at a rate of 14–25 km per decade. These include representatives of a wide range of animal groups, from mammals, birds, and fish to spiders, butterflies, dragonflies, and millipedes. Within a few decades, the poleward boundaries of their ranges have shifted up to 60 km to the north. The more the temperatures within the old distribution boundaries have risen, the more pronounced the changes have been (Hickling et al., 2006; Chen et al., 2011).

Of course, these are not migration or migration-related issues. In contrast to the migration of animals between summer and winter habitats, as practiced by many animal species, it is more a case of slow movement in one direction. Living creatures always try to gain a foothold outside their traditional distribution areas, but then it was too cold (or too warm) for them for a long time, their eggs did not develop, the young froze to death or they were displaced by climatically better adapted competitors—there are many reasons why expansion attempts fail. If they succeeded in single warm years, cold years pushed the species back into its old boundaries. Viewed in fast motion over longer periods of time, their distribution areas would have pulsated, as it were, without ultimately shifting. Now, with permanently increased temperatures, living beings can survive beyond their old boundaries. They reproduce and future generations can venture even farther—until they come up against new boundaries.

Studies from all over the world show that the distribution patterns of species are shifting in the mountains as well. Many animals and plants now live at higher altitudes than they did 20 or 30 years ago. On the slopes of the Antisana volcano in Ecuador, the plant species identified by Alexander von Humboldt and Aimé Bonpland at the beginning of the nineteenth century and located in their altitude profile are now found up to 266 m further uphill (Fig. 1) (Moret et al., 2019). This corresponds to a shift of 10–12 m of elevation gain per decade. New studies from Switzerland show that this value has been exceeded manifold by plant and animal species in the Alps during the last 50 years. The development has apparently accelerated considerably. Since 1970, average temperatures in the Swiss Alps have risen by 0.36 °C per decade, while at the same time the upper edge of the occurrence of various animal species has moved upwards by 47 to a maximum of 91 m per decade. For plants, it is 17–40 m. However, as isotherms have shifted by up to 71 m, these considerable shifts are not enough for most plant and animal species to keep pace with rising temperatures (Fig. 2) (Vitasse et al., 2021).

Fig. 1

Alexander von Humboldt’s famous “Tableau Physique” (1807) schematically shows the altitude profile of the Andes. The plant species entered there grow up to around 250 m higher today. (Moret et al., 2019)

Fig. 2

The altitudinal migration of species is an excellent indicator of climate change. For the Swiss Alps, shifts of up to 90 m have been documented. (Data from Vitasse et al., 2021)

In the oceans, the distances bridged are even greater, averaging 72 km per decade, because the temperature gradient in the water is shallower. To continue living in the same ambient temperature, a fish at the same latitude has to move much farther polewards than a mammal on the nearby coast (Parmesan, 2019). No wonder, then, that European scientists and fishermen all the way up to the south-west coast of Norway are increasingly catching fish species that were once native only to the waters off the coast of Portugal or in the Bay of Biscay. The North Sea is now home to several squid species that for a long time had been found there only sporadically (van der Kooij et al., 2016).

Hundreds of studies show that these shifts are happening in the same way all over the world, a uniformity that surprised even the experts (Newman et al., 2011).

In many cases, plant and animal species have increased the area they inhabit because (in the northern hemisphere) the southern edge of their range has not shifted as fast. This was the conclusion of a study of 80 British breeding bird species, for example. In terms of the size of their range, they have tended to benefit from rising temperatures in recent years (Massimino et al., 2015). As new species migrate into the existing communities, biodiversity in the temperate climate zones could even increase in the medium term. However, it is doubtful whether this will continue in the future if warming continues. There will be winners and losers among the plant and animal species at various stages of this long-lasting, perhaps centuries-long process of change. And today’s beneficiaries are not necessarily tomorrow’s winners.

It is also noticeable that about half the investigated species have not shifted their range: they persisted. Some find thermal refuges within their old distribution limits where they can survive. For others, there has been no need to move to cooler regions because the climatic changes were still tolerable or even beneficial for them. For example, a longer growing season due to warming enables some bird species to raise several generations per year. The same applies to bark beetles, which attack drought-weakened forests in Europe and North America and could thus become an even bigger problem.

However, the persistence of plants and animals in their old distribution limits means in many cases that these species cannot escape the rising temperatures. In mountainous regions, the peak regions are reached at some point, and in shallow shelf seas like the North Sea, there are no cooler depths. On land, humans have changed nature so massively that range shifts have been made difficult or almost impossible. What palaeobotanists have been able to document, for example, in the Bighorn Basin, a plateau in the U.S. state of Wyoming, would hardly be possible in today’s man-made world.

The Bighorn Basin yields numerous fossils, including some from 56 million years ago, which is of great interest to palaeobiologists in connection with current climate change. With the greenhouse gas content of the atmosphere rising sharply, the temperature at that time rose by about 6–8 degrees within a few thousand years, albeit from a much higher level than today. In the Bighorn Basin, there were drastic changes in vegetation, among other things (Wing et al., 2005; McInerney & Wing, 2011). Before the temperature rise, birch, elm, walnut, laurel, and cypress trees grew there in a river landscape. Almost nothing of this remained during this so-called PETM, the Paleocene/Eocene temperature maximum. Only two plant species persisted, 27 disappeared and were replaced by 46 species from the tropics and subtropics. This new flora, adapted to heat and drought, dominated the Bighorn Basin for several tens of thousands of years, until temperatures slowly dropped back to their baseline. Afterwards, it was almost the same sight as before the temperature maximum: 22 plant species that used to be native here returned, only five were new immigrants. It was almost as if the dramatic climate change during the PETM had never happened.

This example impressively shows how living organisms react to climate change, so long as conditions are not turned upside down by catastrophes such as the impact of an asteroid or large-scale volcanic activity. They evade rising or falling temperatures, survive the unfavorable conditions in refuges and then, after the changes have subsided and if circumstances permit, return to their original settlement area.

In the present, however, the situation is fundamentally different—circumstances today prevent and hinder what has proven itself over Earth’s history in climatic crisis situations. 50–70% of the mainland has been altered to a greater or lesser extent by humans and no longer supports natural vegetation (Barnosky et al., 2012). Huge, often pesticide-treated monocultures, roads, canals, and progressive urbanization have created insurmountable obstacles for many creatures to spread.

If temperatures continue to rise, species that cannot either move, or adapt on site will therefore sooner or later run into difficulties.

^“A General Redistribution of Life on Earth^”

Even if the observed range-shifts do not keep pace with climate change and many species do not or not yet shift their ranges, the range-shifts of the other half of the animal and plant world will reach a magnitude that scientists are concerned about: “the largest climate-driven redistribution of species since the last glacial maximum” 24,500–18,000 years ago. This process, as Australian marine ecologist Gretta Pecl and more than 40 scientists from around the world explain, “is a substantial challenge for human society” because range-shifts can amplify climate change in a positive feedback loop because altered vegetation also changes its reflectance and evaporation behavior (Pecl et al., 2017, p. eaai9214). Range shifts are affecting organisms such as the tiger mosquito (Aedes albopictus), a vector of dengue fever and other viral diseases, which has already reached southern Germany. In Africa and South America in particular, millions of additional people will be threatened by malaria, as the vector, the Anopheles mosquito, spreads into the previously malaria-free highlands with the rising temperatures. Food production is also affected, not only by the relocation of cultivation areas, as in the case of coffee, but also by the migration of fish shoals in the oceans, the loss of pollinators, and the spread of pests on land.

Neobiota

The loss, decline or migration of native animal and plant species as well as the immigration of warmth-loving species will change the existing biotic communities and are already doing so today. But these two will be joined by a third group, the so-called neobiota. These are thousands of species of organisms that have been deliberately brought into the country by humans or introduced with the movement of goods and travel. The spectrum ranges from flatworms to hippos, from grasses to sequoias.

In Germany, about 400 neophyte species, mainly from Asia, North America, or the Mediterranean region are already considered established, i.e., they grow spontaneously without horticultural help from humans, reproduce and have already gone through several multiplication cycles. Most of them are rare and many are still limited in their occurrence to heat islands such as large cities or river valleys.

However, model studies (Kleinbauer et al., 2010) show that a large proportion of these alien plants have by no means exhausted their potential. They will spread beyond their current range, especially if warming continues. As the globalized movement of goods continues to distribute alien species, species will be added all over the world that we do not even know about now. A human-induced redistribution of species has already been in full swing for centuries, even without climate change (Kegel, 2013) (Fig. 3).

Fig. 3

Range shift of Ambrosia artemisiifolia: the highly allergenic mugwort ambrosia is increasingly spreading northwards. (Data from Cunze et al., 2013)

In general, aggressively spreading neobiota, so-called invasive species, are considered to be one of the most important reasons for the worldwide crisis of biodiversity, even if the problems caused by them are very unevenly distributed globally (Simberloff, 2013). Central Europe has so far come off rather lightly in this context, unlike New Zealand or Australia, for example, which have lost many native species to introduced rats, foxes and cats. On the other hand, there are more and more voices that see hope in foreign animal and plant species that could help fill the gaps in the native species population caused by climate change and take over their ecological functions (Pearce, 2015).

New Communities

The communities of the future will be made up of representatives of these three groups of species, the native species that tolerate the changed climate, the heat-loving immigrants and the neobiota. In many cases, especially in the tropics and subtropics, where temperatures are expected that Homo sapiens has never experienced before in its history, these will be so-called novel or no-analog communities, i.e., animal and plant communities that are completely new in their composition and for which there are no equivalents anywhere in the world today (Williams & Jackson, 2007).

This, of course, makes it difficult to predict how these new communities might behave and develop, how stable and resilient they will be, and to what extent they might replace or maintain the functions of current ecosystems. In the absence of empirical data, ecological surprises are almost certain. There are many examples of animal and plant species revealing very different characteristics and preferences from those in their native habitats when released on other continents. As recently as the middle of the twentieth century, no one would have thought it possible that tropical bird species could become breeding birds in temperate Central Europe. The collared parakeet has proved the opposite, and other parrot species could follow. South American nandus feel at home in Mecklenburg-Vorpommern, and Asian water buffalo graze in the Oderbruch. Apparently, at least some animal and plant species show only one of several possible faces in the native species structure of competitors, predators, partners, and rivals.

A Brief Look at the History of the Earth

The formation of such no-analog communities is also known from earlier epochs of Earth history, as transitional phenomena in times of climate change. A look into the geological past is helpful anyway when dealing with the possible effects of climate change on fauna and flora. It shows not what will or could happen under certain assumptions, but what actually happened. However, you will look in vain for an event that is similar in every respect to what is happening today.

Of particular importance is the aforementioned PETM, the temperature maximum at the Paleocene-Eocene boundary 56 million years ago, which was discovered in 1991 on the basis of isotope anomalies in Antarctic deep-sea cores (Kennett & Scott, 1991). It was triggered by large amounts of carbon (as CO₂ and/or CH₄) entering the atmosphere relatively quickly—in Earth-historical terms—and thus belongs to a series of rather rare phases in Earth history called hyperthermals, because global temperatures rose sharply during these times as a result of high greenhouse gas concentrations. During the PETM, the temperature increase was at least 6–8 °C, a value that would make large parts of the Earth uninhabitable for humans. Where this carbon came from in the case of the PETM, whether from thawing permafrost regions in Antarctica, volcanic activity, destabilized methane hydrate deposits on the ocean floor or even a meteorite impact, cannot be answered today.

There is also uncertainty about the speed of the release. It probably occurred at a rate of 1–2 Pg (10¹⁵ g) per year over several millennia, a rate not much below the average annual emissions of the last 150 years—though annual emissions have recently reached many times this amount. The total amount that entered the atmosphere at that time is estimated at 10,000 Pg. This is roughly equivalent to the total supply of fossil fuels believed to exist on Earth today.

Because warming is always slow in comparison to the input, it took 60,000 years until temperatures reached their maximum during the PETM (A. Sluijs cited by Dunne, 2017). Ocean acidification was also slow. Nevertheless, according to palaeobotanists Francesca McInerney and Scott Wing, the temperature rise had considerable biological consequences (McInerney & Wing, 2011). The oceans experienced oxygen depletion and massive algal blooms, and in the deep sea, which experienced a 5-degree temperature jump, nearly half of the foraminifera species, a widespread group of single-celled organisms with chambered calcareous shells, became extinct.

Climate-driven migratory movements began, the beginnings of which we are also experiencing today, and massive changes in fauna and flora occurred on the continents. In North America and Europe, the first representatives of cloven-hoofed and uncloven-hoofed animals appeared relatively suddenly, presumably migrating from Asia. The first modern apes emerged in America and Asia. Since the so-called meridional temperature gradient, the temperature difference between the equator and the poles, was only about 6 degrees during the PETM—today it is 22 degrees—and the continental masses in the northern hemisphere were even closer together, it was also possible for more sensitive species to move from one continent to the other (Sluijs et al., 2007).

As drastic as these changes were, leading scientists such as John Williams and Richard Zeebe fear that they could be surpassed by the current climate change, because the release of climate-altering carbon compounds now takes place in a much shorter time span than at the beginning of the PETM. Williams and his colleagues conclude that the expectation that the future will bring effects like those of the PETM “should be considered conservative.” It is probably going to be worse. Richard Zeebe, a prominent oceanographer working in Hawaii, also sees catastrophic developments coming for the oceans, which will would be “completely without precedent” (Williams & Burke, 2019; R. Zeebe cited by Dunne, 2017).

In terms of Earth history, we are still in the Ice Age, or more precisely, in an interglacial, an intermediate warm period that we call the Holocene. We know from drilling in the ice sheet of Greenland, which began in the 1990s, that after the last retreat of the glaciers there were some abrupt temperature jumps that had a global impact on flora and fauna. They coincide with rising CO₂ concentrations in the atmosphere but were also triggered by changes in air mass circulation and disturbances in global ocean currents. The cause of the latter was, for example, the input of huge amounts of sweet glacial melt water 8200 years ago, which led to the so-called Misox oscillation, a drop in temperature of up to 5 degrees within a few years (Williams & Burke, 2019).

Investigations of post-glacial climate changes have the great advantage that they can be dated precisely to decades or even years with the help of ice cores and lake sediment deposits. In addition, numerous fossils are found in layers of lake sediments, especially pollen from plants growing in the lake environment. From this, palaeobotanists can infer the composition of the vegetation and follow its change through the increase or decrease of individual species.

A worrying result of these studies concerns the speed with which the plant communities in the vicinity of the lakes reacted to the temperature changes (Fig. 4).

Fig. 4

The PETM at the Paleocene-Eocene boundary 56 million years ago stands out as a needle-pointed jag in the paleotemperature curve just before the Eocene temperature peak was reached. It lasted 200,000 years. (Data from All_palaeotemps_G2.svg)

Whether at Lake Gerzen in Switzerland or Meerfelder Maar in the Eifel, the researchers observe “near-immediate effects.” “Forest response time”, John Williams and Kevin Burke summarize the state of knowledge, “were consistently less than 20–40 years, and often had no detectable time lag” (Williams & Burke, 2019, p. 139).

The temperature drop during the Misox Oscillation 8200 years ago also led to a “pronounced and immediate response of terrestrial vegetation” in Central Europe (Tinner & Lotter, 2001, p. 551). This can be seen, for example, in the stratigraphy of the sediments of the Swiss Soppensee in the canton of Lucerne. It took only about one tree generation, which corresponds to about one human generation, to collapse the predominant hazel stand there, from which 40% of all pollen grains originated before the temperature drop, and to replace it with a completely different forest of pines, birches and lime trees. Scientists speak of a regime shift in the case of such profound, more or less abrupt, but lasting changes in ecosystems.

Forests

It remains to be seen whether the damage from recent drought years, which became known as “forest dieback 2.0,” is already the prelude to such a regime shift. The tree death caused by the drought and bark beetle infestation destroyed a forest area the size of Saarland throughout Germany (app. 2.500 km²). The damage was mainly to spruce, but pine and important deciduous tree species were also affected. In the past 25 years, mortality in European forests has shown a “worrying upward trend.” Especially endangered are conifers on “productive sites” (George et al., 2021).

Regime shifts are already occurring in Arctic habitats. The thawing of permafrost is reshaping the landscape in vast areas, shrubs are encroaching on the tundra, and the disappearance of ice is threatening marine communities (Kegel, 2021). Because their ice cover opens earlier and light can penetrate the water for longer, the character of many Arctic lakes has changed (Smol et al., 2005). Regime shifts are also looming in the boreal coniferous and tropical rainforests, which have been suffering from increasing drought for years. The latter even threaten to lose their important role as carbon sinks. The carbon uptake measured in the 1990s has never been reached again since then and is declining. Since 2010, this has also been the case for the long-stable African rainforests. Fire clearance and the resulting soot particles exacerbate the situation (Hubau et al., 2020).

Stressed by drought, shorter and milder winters, and the resulting bark beetle invasions, the largest contiguous forest area on Earth, the boreal coniferous forest, is also getting into trouble. In North America, the beetles killed 30 billion trees and the timber industry which, through its disastrous forest management, made problems of this magnitude possible felled another 30 billion (Hannah, 2011; Nikiforuk, 2011).

Bottom Up

Although climate change can alter entire ecosystems and drive them into regime shifts, it affects individuals first, every single plant and animal. The response of organisms to rising temperatures is “largely, if not entirely,” a bottom-up process (Newman et al., 2011, p. 73). How tolerant an individual is to fluctuating environmental parameters and at what thresholds this tolerance ends depends primarily on its genetic make-up, possibly also on environmental experiences it has had. It is the individual animal that moves beyond the old distribution limits and tries to survive there. When many do this, the range-shifts discussed occur. On the way bottom-up, “up” through the food chains and communities, these effects will “combine, amplify, weaken and generally interact” (Newman et al., 2011, p. 73).

Since practically all life processes, especially biochemical reactions in the cells, are temperature-dependent, living organisms are directly affected by rising temperatures. Numerous studies already show that many animal individuals even change their body proportions by way of adapting to a warmer environment. These so-called shape-shifts follow two rules postulated over a hundred years ago, Allen’s rule and Bergmann’s rule, according to which the size of body appendages such as extremities, tails, and ears decreases towards the poles, i.e., with falling temperatures, while body size increases. Both have to do with the exchange of heat between the body and the environment. So, in a warmer world, body size should decrease and limb size should increase.

A recently published study of 77 bird species in the Brazilian rainforest shows that their body proportions actually change in step with rising temperatures. Since the 1980s, the animals have lost body mass and developed larger wings (Jirinec et al., 2021). Australian parrots have seen their beak area increase by 4–10% since 1871 (Campbell et al., 2015). American bison were 37% larger 40,000 years ago than they are today. The average annual temperature increased by 6 degrees during this period. If this trend were to continue at the same rate, bison would lose another 46% by the end of this century with a 4 degree warming compared to today’s average mass of 665 kg, and would weigh only 357 kg on average (Martin et al., 2018). In the aforementioned Bighorn Basin, fossil finds prove that the prehistoric horses of the genus Sifrhippus living there shrank by 30% in the course of warming during the PETM, only to gain 76% in body mass again during the recovery phase when temperatures fell. Insects and worms even lost almost half of their body mass (Smith et al., 2009; Secord et al., 2012).

The biological effects of a warming world show themselves especially in polar habitats and the tropics because plants and animals living there are used to only small temperature fluctuations and do not tolerate larger deviations. The eggs of the Arctic cod, for example, begin to die at temperatures above 3 degrees. The marine species of the Antarctic have lived since time immemorial in a range between −1.9 degrees, the freezing point of salt water, and +1.8 degrees, the highest water temperature ever measured there. Many have to surrender at temperatures as low as 3 degrees, and even after a long acclimatization period, 6 degrees is the absolute maximum. Crocodile icefish have no haemoglobin, so they get into trouble when oxygen content drops with rising temperatures (Somero, 2010, 2012).

Tropical organisms generally live closer to the maximum temperatures they can tolerate than animals and plants in temperate zones. Therefore, a small increase of a few degrees can also be fatal for them. The distance between these maximum tolerable temperatures and the highest temperatures to which living organisms are actually exposed in their environment is called the “thermal safety margin”. Its size depends largely on whether there are refuges in the habitat where organisms can avoid dangerously high temperatures. This can be a shady spot under trees or a body of water, but often a crevice in the rock or a stone under which one can hide is enough. If such refuges are missing or no longer offer protection, the thermal safety distance can drop to zero. This can lead to life-threatening hyperthermia, and the animals usually die of heart failure (Pinsky et al., 2019).

For mussels, which live in water at 21 degrees, the lethal limit is around 28 degrees. No wonder, then, that the animals have expanded their range on the US East Coast by 350 kilometers to the north. At the southern edge of their range, water temperatures have risen so high that hardly any mussels can survive (Somero, 2010, 2012).

However, living creatures are not completely defenseless against rising temperatures. Mussels, too, have such defensive means, which are quite old in phylogenetic terms. Organisms have always been confronted with dangerously high temperatures and have developed astonishingly effective repair mechanisms against them. Modern methods of molecular biology show that damage to protein molecules occurs in mussels even far below critical temperature values, to which the cells react immediately by activating the genes of so-called heat protection proteins (Hsp). They help the damaged protein molecules to regain their correct three-dimensional structure, without which they cannot fulfil their function. If temperatures continue to rise, proteolytic enzymes are produced. So many proteins are now damaged that these enzymes can only break them down and dispose of them. If it remains warm, the cell cycle is stopped as a last resort. Cells can then no longer divide and the organism stops growing.

Mass Mortality Events

What happens beyond this threshold can now be experienced with sad regularity in Australia. For flying foxes, large fruit-eating bats, the critical temperature is 42 °C, a value that has been significantly exceeded several times during extreme heat waves in recent years. The flying foxes, which now live in large numbers in the Australian metropolises because their natural habitat had to make way for plantations, fell dead by the thousands from the treetops, where they spend the days hanging close together on the branches. The high point was 4 February 2014, when hot winds from the outback drove temperatures to 43 degrees and higher. In south-east Queensland, 45,000 animals died in a single day. The tropical black flying foxes were hit hardest. Half of the animals living in the area perished. Justin Welbergen, who has been researching these mass mortality events for years, dates the first documented event of this kind to 1791, when it hit Sydney (Welbergen et al., 2008). So these die-offs have always existed. It is difficult to say whether they have increased, because in the past they usually took place in remote areas and remained undetected.

Welbergen and his group thus suffer from a problem that affects many areas of climate change biology: the lack of old data. Scientific studies that meet today’s standards could not be carried out 50 let alone a hundred years ago, or only in rare and exceptional cases. Today’s long-term studies, however, usually only focus on the last few decades, or at most 50 or 60 years. Data documenting the state of biotic communities a hundred years ago or even before the beginning of industrialization, such as the botanical surveys of Humboldt and Bonpland at the Antisana, are rare strokes of luck.

Mass mortality events (MMEs) such as that of Australian fruit bats can occur in all habitats and affect all types of animals, but they are especially common in aquatic habitats. However, due to a lack of old data, research cannot prove beyond doubt that mass mortality events have become more frequent. Before 1940, they were documented only sporadically. Since then, they have clearly increased in birds, fish, and marine invertebrates. However, it cannot be ruled out that we are just looking more closely today and registering MMEs that would have remained undetected in the past (Fey et al., 2015).

Marine Heatwaves

The data situation for heat waves in the oceans is somewhat better, although here too experts complain that “the scientific understanding of marine heat waves is still in its infancy” (Holbrook et al., 2019, p. 2). The conditions under which marine heat waves actually occur and the factors that determine their duration and intensity are still insufficiently understood. Since the beginning of the twentieth century, their frequency has increased by 36% and their duration by 17%. The North-East Pacific stood out with exceptionally long heat waves (Oliver et al., 2018, 2019).

An especially dramatic event of this kind took place there between 2013 and 2016 (Fig. 5). This heat wave was named “The Blob” by climatologists after a voracious and incessantly growing alien, the villain of a science fiction film of the same name. It formed south of the Aleutian Islands in the winter of 2013, reached massive proportions in 2014 and continued to grow.

Fig. 5

Temperature distribution during The Blob thermal anomaly: clearly two regions with water warmer than 2 degrees. (Data from Bond, 2015; NOAA ESRL, 2015)

In the summer months of 2014 and 2015, these warm waters, two-thirds the size of the United States, reached the west coast of North America and caused thousands of animal deaths. California beaches were covered with countless orange-red red-tuna crabs, which normally live much farther south, as well as the tropical hammerhead sharks, marlins, and dolphinfish, which reached further north than ever before.

As far as is known, the sequence of events followed a classic bottom-up pattern. (Piatt et al., 2020) The trigger was apparently the widespread absence of winter storms, which normally ensure the mixing of warm surface water with cold and nutrient-rich water masses from deeper layers. The resulting nutrient deficiency first became apparent in 2014 in the form of an exceptionally low chlorophyll content, the lowest since measurements began at the end of the twentieth century. The lack of phytoplankton was followed by a collapse in zooplankton, which was dominated by nutrient-poor small crustaceans that had recently migrated from the south. Starvation continued through the prey fish species (sand eels and capelins) to the top predators of the ecosystem. Tens of thousands of dead seabirds washed up along six thousand kilometers of coastline all the way down to California. Experts estimate that up to 1.2 million guillemots died, 10–20% of the total population in the North Pacific, plus other seabirds and thousands of sea lions and fur seals (Fig. 6).

Fig. 6

Victims at the top of the food chain: anomalously warm water has led to the starvation of thousands of guillemots on many coasts around the Gulf of Alaska. (Data from Piatt et al., 2020)

Cod and pollock stocks collapsed by 70%. Off Hawaii, sightings of mother humpback whales with calves dropped by more than 70%, “unexplained” whale strandings shocked people in Canada and Alaska. The animals all appeared healthy, but like the lost birds and seals, were severely malnourished.

In addition to the bottom-up process, there was also a top-down effect, because the predatory fish at the top of the food chain develop an enormous appetite when water temperatures rise. Just 2 degrees more and their food requirements increase by 63%. Not much is left for seabirds. The fauna of the Northeast Pacific suffered as a result of the blob starvation.

It was not until the winter of 2015/2016 that the heat wave began to dissipate. The water masses had warmed by a maximum of 2.5 degrees above the long-term average during the blob, to temperatures that are expected to be normal in the North Pacific for the second half of the twenty-first century in the worst case (ICCP scenario RCP 8.5) according to computer modelling.

The frequency, duration, extent, and intensity of heat waves will continue to increase with climate change. The statements of marine biologists and oceanographers sound anything but confident: “Many ocean areas will reach a state of almost permanent marine heat waves in the late 21st century. […] We have moved away from the conditions under which marine heat waves naturally occurred, from a condition that has shaped the distribution of marine species and the structure and function of ecosystems for millennia” (Oliver et al., 2019).

Shifted Phenologies (Mismatch)

For many years, the beginning and end of the phenological seasons have been determined in Germany using certain indicator plants. Thus, the phenological early spring begins with the hazel blossom, and the summer starts with the blossom of the black elder. For Bavaria, these long-term records (Bayerisches Landesamt für Umwelt, 2014) show that the hazel blossom has come 23 days earlier as a result of the rising temperatures between 1961 and 2010, while the start of summer is now 17 days earlier. These shifts, which can be observed all over the world, naturally have a significant impact on the organism world, which depends on certain processes in nature being synchronized in time. This ensures that predators meet their prey and young hatch or are born when they find optimal food in nature. Plants must flower when their pollinators are active, parasites must meet their hosts at the right time (Fig. 7).

Fig. 7

Things are happening earlier: life cycle changes are occurring across a broad spectrum of species. In the Alps reptiles, for example, became active 6 days earlier on average per decade (*long-distance/**short-distance migrants). (Data from Vitasse et al., 2021)

However, studies from Switzerland now show that living beings react very differently to the shifts (Vitasse et al., 2021). In the meantime, many cases have been documented worldwide in which temporal synchrony is increasingly lost—mismatch occurs. Greenlandic caribou start a long migration, triggered by the length of the day, to arrive in the tundra exactly when the especially nutritious fresh green appears there. However, the plants now sprout much earlier due to the drastic rise in temperatures in the Arctic, the caribou arrive too late, and this impacts the quality and quantity of milk produced by the dams. Since 1993, the number of their calves has dropped to a quarter (Post & Forchhammer, 2008). The deer in the Champagne region of France are suffering a similar fate (Plard et al., 2014). In the Baltic Sea, hatchling young herring increasingly miss the early onset of the algal bloom.

Migratory birds adapt by shortening their migratory routes or becoming sedentary birds in order to arrive at their breeding grounds earlier in spring. This is impossible for long-distance migrants. Their numbers are declining because they find the best breeding sites already occupied by birds that were there earlier, a trend that can only be discerned after 1990. Apparently, the development has accelerated since then (Møller et al., 2008).

In the Netherlands, populations of the pied flycatcher, which winters in tropical Africa, have plummeted by a catastrophic 90% because caterpillars, their preferred prey, now hatch too early for them. Moreover, they are in fatal competition with great tits, which fiercely defend their nesting holes. More and more often, dead pied flycatchers are found in them, mostly inexperienced males that have arrived very late after their long flight (Both et al., 2006; Samplonius & Both, 2019).

Cuckoos are also long-distance migrants and have problems because their offspring have to hatch before the chicks of the host birds. The animals that have specialized in sedentary birds or short-distance migrants are increasingly arriving too late and will therefore probably become extinct. Since the breeding parasites amazingly match the coloration of their eggs to that of their hosts, they cannot simply change bird species. Cuckoos, which lay their eggs in the nests of long-distance migrants, fare better. It can already be observed that early-breeding species such as robins and wagtails are increasingly relieved of the burden of cuckoos, while late-arriving host species are increasingly burdened. The presence of breeding parasites in the nests of reed warblers has more than doubled in the last 30 years (Saino et al., 2009).

Extinction and Defaunation

Climate change and the range and regime shifts, mismatches, heat waves and other extreme weather events it triggers will massively increase the pressure on flora and fauna. However, the global crisis of biodiversity that we are currently experiencing is not an effect of climate change but caused by massive habitat destruction due to agriculture and urbanization. In addition, there is the large-scale use of pesticides, the introduction of invasive species, as well as hunting, fishing, and poaching. Currently, between 11,000 and 58,000 species die out each year, a loss that exceeds the natural extinction rate by orders of magnitude (Dirzo et al., 2014).

In recent years, nature conservation has focused primarily on rare and endangered species. However, recent data shows that the dramatic nature of the crisis has been vastly underestimated in the process, as wildlife is dying across the board. A recent study by British and Czech researchers puts the loss of bird individuals in the European Union at 17–19% of the total population since 1980. Yet, there are also winners such as blackcaps with a population increase of 55 million birds, followed by chiffchaffs, blackbirds, and wrens (Fig. 8). Overall, however, the total number of birds has shrunk by 560–620 million, with species of agricultural landscapes being especially affected (Burns et al., 2021). The decline primarily affects common species such as Yellow Wagtail, Starling, Skylark, and House Sparrow, whose populations have halved. The so-called insect mortality, which was discovered by a group of entomologists in Krefeld and which has since been confirmed for other countries, is certainly connected to this (Hallmann et al., 2017, 2019; Powney et al., 2019).

Fig. 8

Loss of bird individuals in the EU: profiteers such as the blackcap stand in contrast to dramatic declines in once “commonplace species” such as the starling and sparrow. (Data from Burns et al., 2021)

Other studies show a collapse of the global seabird population by almost 70% since 1950. (Paleczny et al., 2015) As a result of hunting and poaching, more than half of the remaining tropical forests are practically without larger mammals. The horror image of the empty forests is making the rounds (Benítez-López et al., 2019).

The Living Planet Report of the World Wildlife Federation and the Zoological Society of London, based on more than 20,000 monitored populations of vertebrates around the world, also laments a 68% decline since 1970—in individuals, mind you, not in species (WWF, 2020). To describe the full extent of this historically unprecedented faunal destruction, terms such as defaunation or biological annihilation have been coined (Dirzo et al., 2014).

So far, climate change has contributed little to this development. In the future, however, it will intensify it and, in doing so, will encounter an animal and plant world that is already under enormous stress and, through the loss of countless individuals, in danger of losing the one thing that could ensure its survival in a rapidly changing environment: its genetic diversity.

Nature conservation has therefore never been as important as it is today. The IPBES (Inter-governmental Science-Policy Platform on Biodiversity and Ecosystem Services) also demands that biodiversity and its threats must play a more important role in the future. Climate protection and nature conservation should go hand in hand. More than 100 countries, including Brazil and Russia, have just decided in Glasgow to stop the global destruction of forests by 2030. Colombia, Ecuador, Costa Rica, and Panama are creating a huge protected area off their coasts that also includes the Galapagos Archipelago, a “marine corridor in the eastern tropical Pacific.” These are important signals that hopefully will finally be implemented and must be followed by others. One of the most prominent biologists of our time, the recently deceased ant and biodiversity researcher E. O. Wilson, called for half of the earth to be placed under protection; the UN is aiming for 30% by 2030 (Wilson, 2016). The destruction and fragmentation of habitats must be stopped, and the rise in the average global temperature limited, otherwise there is a risk of losing countless animal and plant species and vital ecosystem functions, the end of the biosphere as we know it.