Living in the cold
Three commonalities, many differencesThe polar regions’ special geographic and climatic conditions present the fauna and flora of the Arctic and Antarctic with particular challenges. For millions of years now, organisms in both regions had to:
- overcome cold to extremely cold ambient temperatures,
- deal with the presence of snow and ice in its various forms, and
- endure extreme seasonal fluctuations of sunlight and temperatures.
Nonetheless, both the Arctic and Antarctic environments have given rise to a great diversity of life. In the north polar region more than 21,500 species of fauna and flora have now been identified, all of which have adapted to the extreme conditions – from bacteria and viruses living on glaciers and fish that spend the first years of their lives hidden under the marine ice, to the well-known species such as the Arctic fox or the polar bear. There are approximately 14,000 Arctic terrestrial species; a further 7600 species live in the Arctic Ocean.
In contrast, the terrestrial life of Antarctica is relatively species-poor. A mere 1600 animal and plant species occur in the few ice-free terrestrial areas of the Antarctic. The ocean, however, is teeming with life – some 10,630 species have been identified, with the majority displaying special adaptation mechanisms that can be found nowhere else on Earth.
A matter of geographic locationA comparison of the ecosystems of the two polar regions highlights the significant differences between them. While there are large-scale, glacier-free areas of tundra and extensive river systems in the Arctic, both of which produce sufficient biomass during the summer to feed even large herbivore species such as caribou and musk oxen, 98 per cent of the Antarctic landmass is still covered in ice. This means that lichens, mosses and higher plants can hardly find substrates on which to grow. Therefore, the main food source for all the animals native to the Antarctic is the ocean which completely surrounds the continent.
The resultant isolation of the Antarctic from the rest of the world has had a similarly lasting impact on the development of life in the south polar region as its glacial history. For more than 34 million years now, oceanic basins more than 3000 metres in depth have separated Antarctica from the surrounding land masses of South America, Africa and Australia. Even at its narrowest point, at the Drake Passage, the Southern Ocean is still 810 kilometres wide. Terrestrial species of fauna intent on migrating to Antarctica from temperate latitudes would have had to be capable of long-distance flights or swims even in the past. Once they had overcome that obstacle, the Antarctic immediately presented them with the next challenge – several times over the past millions of years the southern continent became completely covered in ice, sometimes even beyond its coastline. The Antarctic terrestrial species were left with the choice to either migrate or to move out onto the marine ice. Otherwise they faced extinction.
- 4.1 > A three-dimensional model of the Southern Ocean. With its deep basins and circulating water masses, it continues to form a barrier that many animal and plant species from more northern latitudes can scarcely overcome. Similarly, it has prevented species native to the Antarctic shelf sea areas from migrating northwards.
- In comparison, settling in the Arctic was much easier as it is directly connected to large continental land masses stretching far south and into warmer climatic zones. Eurasian and North American species of fauna and flora adapted to the cold were therefore able to colonize the north polar region by land. When during the last glacia-tion large ice shields formed in the Arctic, this did not generally spell the end of terrestrial life in the way it did in Antarctica. For one thing, the Arctic species had the opportunity to shift their ranges toward the south and thus to flee from the ice. Moreover, during this glaciation the Arctic was never entirely covered by glaciers and ice shields. Regions such as Beringia and eastern Siberia remained ice-free and served as a refuge for many organisms. As a result, the north-eastern tundra regions of Russia are among the most species-rich terrestrial areas of the Arctic to this day. Given that the Arctic is not separated from more southern climes by oceans or high mountain ranges it is not surprising that the north polar region hosts many terrestrial predators such as polar bears, wolves and Arctic foxes while there is not a single four-legged predator species in mainland Antarctica. Instead, millions of penguins breed in the Antarctic – birds that cannot fly and that know no enemies outside of oceanic waters. If penguins were resettled in the Arctic, they would easily be picked off by polar bears and other predators, given that these large birds have no intuitive sense of danger when on land. The only bird species resembling penguins which ever lived in the Arctic was the flightless great auk (Pinguinus impennis). It lived on remote rocky islands in the North Atlantic were there were no polar bears, wolves or foxes. However, in the early 19th century European seafarers discovered the birds’ colonies. They hunted the defenceless auks to extinction in a mere four decades. The last great auks were killed in June 1844 on the Icelandic island of Eldey.
- 4.2 > Four endemic species of true seals are at home on and underneath the jagged Antarctic pack ice. These include the crabeater seal, Ross seal, Weddell seal and leopard seal.
A “biodiversity pump”The geographic conditions in the Arctic and Antarctic have also had a decisive impact on the species diversity of the polar seas. The ring-shaped Southern Ocean made it possible for many of its inhabitants to establish ranges encircling the entire continent. At the same time, the Circumpolar Current, which reaches great depths, and the rapidly decreasing water temperatures at the top 200 metres of its water column impede the migration of species from more northern climes. Moreover, water temperatures have decreased since the Drake Passage opened 34 million years ago – at first there was only episodic cooling and interim warming phases but in the past 15 million years temperatures have continuously declined. The Southern Ocean is on average ten to twelve degrees Celsius colder today than it was 40 million years ago.
Sea ice conditions in the Antarctic changed in step with the cooling of the Southern Ocean, with far-reaching consequences for life in and underneath the sea ice, in the water column, and on the sea floor. The more ice formed in the Antarctic in the course of a cold period and the further glacial and shelf ice masses expanded out into the ocean, the less space there was for residents of the shelf, such as sea-floor dwelling sponges or starfish. Many shallow-water areas became completely uninhabitable and formerly conjoined marine regions were separated by the glacial advance. Scientists believe that many of the marine organisms inhabiting the continental shelf seas at that time were forced to migrate down the continental slope or into the deep sea. At the same time, however, the current assumption is that the isolation of habitats of the continental shelf gave rise to new species. Since the Antarctic ice masses have repeatedly expanded and contracted over the past 2.1 million years, biologists speak of a “biodiversity pump”. The premise here is that the repeated isolation of biocoenoses (cold period, growth of ice mass) and the subsequent opportunity for expansion (warm periods, retreating ice) provides perfect conditions for the evolution of a unique and highly differentiated species diversity which, moreover, includes a high proportion of endemic species, i.e. species of fauna and flora that can only be found in the Antarctic. Approximately 50 per cent of Antarctic sea squirts, anemones, bryozoans, mussels and sea spiders are endemic species; roughly 75 per cent of sea snail species are endemics and for Gammaridea, a suborder of amphipods, as well as for octopuses the proportion of endemic species is as high as 80 per cent. In this way the Southern Ocean has given rise to a much greater and more colourful level of biodiversity than one would expect at first sight. Biologists have identified more than 8000 different species of invertebrates in the Antarctic, and some regions have not even been properly studied as yet.
The fish fauna of the Antarctic continental shelf seas is dominated by a group of Antarctic fish termed Notothenioidei. They constitute more than 70 per cent of species diversity and more than 90 per cent of fish biomass in the continental shelf seas. However, there are also faunal groups for which to date there are only occasional sightings in the Antarctic continental shelf seas, the red king crab for example, or which as yet do not occur in the Antarctic. The latter include lobster and hermit crabs, which also explains why the benthic fauna of the continental shelf has not developed defence mechanisms against clawed predators.
Southward migrationGeologically speaking the Arctic Ocean is younger than the Southern Ocean which means that species of high northern latitudes had less time to adapt to polar conditions than the southern fauna. But they, too, had to survive periods of large-scale ice formation, for example some 140,000 years ago when major ice shields covered North America and northern Europe and pushed their up to 1000 metres thick shelf ice onto the entire Arctic Ocean, which presumably became completely frozen over.
At that time, the biocoenoses of the Arctic Ocean either withdrew to greater depths or they migrated to more southern latitudes along the Atlantic and Pacific coastlines. When the ice masses slowly disappeared it took some time for the marine organisms to recolonize the Arctic ecosystems. Biologists therefore consider the diverse biocoenoses of the Arctic marginal seas to be not much older than 125,000 years. Moreover, in the Arctic Ocean scientists distinguish between the Atlantic and the Pacific sector respectively. Their inhabitants migrated from the respective neighbouring ocean and separately adapted to the polar conditions. It is for this reason that to this day different species play the exact same role in the two sectors’ ecosystems.
- 4.3 > Antarctic white-blooded fish such as this juvenile blackfin icefish inhabit the world’s coldest marine regions and have no haemoglobin in their blood.
- In the Arctic, furthermore, it has been and still is much easier than in the Antarctic for inhabitants of the continental shelf seas to migrate from one continent to the next, given that the northern coastal areas of Europe, Asia and North America share contiguous offshore shelf areas. Antarctica, in contrast, lacks such a shallow water connection to neighbouring continents. In the Antarctic, therefore, the pressure to adapt has always been much higher than in the Arctic. During cold periods, marine organisms of the Southern Ocean had significantly fewer refuges at their disposal than species of the far north. The organisms of the Southern Ocean had only two options – they either adapted or they became extinct. It is for this reason that Antarctic marine life developed significantly more sophisticated adaptation mechanisms than the inhabitants of the Arctic Ocean.
Survival tactics of terrestrial animals in the polar regionsConditions in the Arctic and Antarctic are characterized by extreme fluctuations over the seasons. In summer there is sunlight, warmth, ice-free terrestrial and water surfaces and an overabundance of food resources; in winter, however, conditions are the exact opposite. In the Arctic, for example, winter surface temperatures drop to around minus 40 degrees Celsius for weeks, and minimum temperatures of minus 50 to minus 60 degrees Celsius are not uncommon. There are also major temperature differences between north and south as well as between coastal regions and more inland areas respectively. Such contrasts can only be survived by species that adopt one or more of the following survival tactics:
- fleeing the cold by migrating to warmer areas (migration),
- surviving the winter in a protected location (dormancy or hibernation),
- optimizing body heat regulation, and
- provisioning by means of accumulating large body fat reserves.
Fleeing from hunger and coldThe flight from low temperatures and food scarcity is a tactic used primarily by the many seabirds occurring in the polar regions. The Arctic hosts a total of 200 bird species, the majority of which are geese, ducks, shorebirds and seabirds. Compared to temperate regions there are few songbirds. Most of the Arctic bird species spend only a few summer months in the far north. As winter approaches, 93 per cent of the species migrate to warmer regions. Their migration routes lead to regions all around the world. While many of the geese, passerines, owls, birds of prey, auks and gulls overwinter in adjacent temperate latitudes, some of the shorebirds, phalaropes, and the Sabine’s gull (Xema sabini) migrate as far as to the tropics and Australia. The bar-tailed godwit (Limosa lapponica), for example, flies 12,000 kilometres from its breeding area in Alaska over the Pacific to New Zealand. Long-distance migrants such as the Arctic tern and skuas even target Antarctica where they overwinter on the edge of the Antarctic pack ice zone. This means that on their way from the Arctic breeding areas to the Antarctic overwintering areas and back the birds cover a distance of up to 80,000 kilometres per year. But this effort is worth it as both polar regions provide the birds with an abundance of food during the summer. And as the Arctic terns rely primarily on their eyesight for hunting they benefit significantly from the fact that in their chosen habitats the sun does not set for a total of eight months, enabling them to theoretically hunt for prey around-the-clock.
- 4.5 > A herd of caribou moves through the Arctic National Wildlife Refuge in October in search of food. At this time the region has already seen snowfall which means that the animals need to scrape away the snow from potential grazing areas.
- However, there are also bird species in the Arctic that do not migrate to warmer areas. Among the terrestrial birds, these include the common raven (Corvus corax), rock ptarmigan (Lagopus muta), snowy owl (Bubo scandiaca) and Arctic redpoll (Acanthis hornemanni). Among the seabirds that spend winter in the far north are the black guillemot (Cepphus grylle), thick-billed murre (Uria lomvia), ivory gull (Pagophila eburnea), Ross’s gull (Rhodostethia rosea) and common eider (Somateria mollissima).
Mammals also undertake seasonal migrations – baleen whales for example or reindeer (Rangifer tarandus), known as caribou in North America. In eastern Alaska as well as in the Canadian Yukon Territory, for example, every spring a herd of between 100,000 and 200,000 of the so-called Porcupine caribou undertakes a 1300 kilometre northward migration to the coastal plains of the Arctic National Wildlife Refuge where the females give birth to their calves. The kindergarten on the coast of the Arctic Ocean offers many benefits to the wild herd.
The landscape is flat and without forest cover, allowing the caribou to spot potential predators such as bears or wolves from afar. The fresh ocean breeze keeps the annoying mosquitoes in check, and there is a plentiful supply of food and water. At the end of the summer the caribou start on the return journey to their winter territories in the more southern Ogilvie Mountains. Other herds migrate even further south and overwinter in the subarctic boreal forests. But a few herds spend the winter in the tundra.
ThermoregulationJust like all other homoeothermic animals in the polar regions, these caribou face the challenge of maintaining their body core temperature at a level of between 37 and 41 degrees Celsius despite the air around them being up to 100 degrees Celsius colder. The only way to achieve this is to prevent the loss of body heat to the environment. This is a difficult task as body heat can be lost in three different ways:
- by heat conduction,
- by heat radiation and
- by evaporation.
- curling up into a ball (reducing the body surface to volume ratio),
- huddling together in a group for mutual warmth,
- withdrawing to a protected location,
- accumulating a warming layer of fat or a double layer winter coat or plumage, and
- cooling down their breath and extremities.
- 4.6 > Polar bears are very good swimmers and, as this large male demonstrates, they are also good divers. However, the animals get cold very quickly in the water. On long-distance swims mature bears with a thick layer of body fat for insulation have better chances of survival than juvenile bears.
- Animals living in groups, herds or colonies often stand closely together in order to warm each other and thus to minimize their own heat loss. The most well-known example is the circular “huddles” of emperor penguins in Antarctica. In winter when ambient air temperatures can be as low as minus 50 degrees Celsius and the males must stay on the ice to incubate the eggs, the birds huddle together by the thousands and so closely together that up to ten penguins may be squeezed up on a square metre of ground – back to belly, side-by-side and with the head placed onto the shoulder of the penguin in front. In the middle of this giant incubator the air warms to up to 24 degrees Celsius. This is however too warm for the birds at the centre who gradually seek to escape the heat. The birds on the margins meanwhile are cold and slowly push towards the centre. This is why the penguins continuously change their position and why the huddle is constantly moving with each bird at some point enjoying the warmth. In this manner these large birds are able to reduce their heat loss by half even during the harshest of winter storms. Arctic musk oxen display similar behaviour. On cold days the members of the herd form a tight circle, allowing the animals to warm each other and collectively remain relatively unimpacted by icy winds.
A third strategy employed to reduce heat loss is to withdraw to a protected area. This could be a cave or else the animals may curl up and let themselves get snowed in. Polar bears, wolves, foxes, hares and ptarmigans are known to at least temporarily seek shelter in snow dens during the winter. Smaller Arctic species such as lemmings or stoats must even spend most of the winter underneath the insulating snow cover due to their small size and the associated heat loss. Dependent on the thickness of the snow cover, temperatures may be as high as zero degrees Celsius, allowing these small mammals to survive.
Birds and mammals overwintering in the polar regions also protect themselves from the freezing cold by means of a dense winter coat or plumage. In mammals and birds which need to enter the water in search of food, or in whales which spend their entire lives in the ocean, a thick layer of fat (blubber) generally takes on this insulating function. Just how well feathers or fur can conserve body heat depends on two factors, one being the individual thermal conductivity of each individual hair or feather, the other being the degree to which the coat or plumage is able to trap an insulating layer of air near to the body, as the thermal conductivity of air is only half that of hairs or feathers. Presumably this is the reason why the guard hairs of caribou are hollow and internally sectioned into thousands of tiny air cells, each separated from the next by a thin wall. In this manner, the animals’ guard hair does not only protect them from external influences such as snow or rain, it also forms a second and very effective layer of insulation in addition to the underfur.
- The fur’s insulating characteristics differ significantly between species, with the ability to retain heat generally increasing with the thickness of the layer of fur. The insulating effect of the fur or plumage can be further increased by fluffing up the plumage or erecting the hairs, thus trapping a greater amount of insulating air near to the body. Small furry mammals such as lemmings or stoats are clearly at a disadvantage when it comes to keeping themselves warm by means of their body hair. They need a short-haired coat that still allows them to move.
But the large mammals with rather thick coats must also pay attention to a number of factors so as to avoid dying of hypothermia. The polar bears’ long guard hair for example provides superb insulation as long as the coat is dry. However, when the bear jumps into the sea, for example in order to swim from one ice floe to the next, water reaches the skin, and water conducts heat away from the body 25 times faster than air. At moments like that fully grown bears trust the insulation provided by their thick blubber which reaches a thickness of up to 11.4 centimetres. For bear cubs, however, a swim like that can be very dangerous as they lose heat very rapidly. They are similarly at risk when it rains as both rain and sleet considerably impair the functional characteristics of fur or plumage.
Icy winds can also result in significant heat loss. When wind passes through fur or plumage it swirls the layer of air close to the body, thus reducing its insulating function. Snowy owls, for example, that are exposed to 27 kilometre per hour winds at an ambient temperature of minus 30 degrees Celsius lose heat so quickly that in order to not freeze to death they need to generate twice as much heat as would be required if the air was still. In contrast, the guard hair and underfur of reindeer and musk oxen provides such complete insulation that the animals loose little or no heat even during winter storms.
Whales, polar bears and seals protect themselves from the cold by means of thick blubber. While the insulating capacity of this layer of fat is not as great as that of fur, it is also functional in the water where fur generally fails as a means of protection. This layer of fat can be impressively thick. In bowhead whales (Balaena mysticetus) it can reach a thickness of up to 30 centimetres. And just like the coats of reindeer and musk oxen, blubber also changes with the seasons, at least in seals. Their blubber is at its thinnest in summer when the animals’ moult forces them to stay on land and fast. In the run-up to winter they fatten up again and blubber thickness increases.
Penguins such as Adélie and emperor penguins protect themselves from the icy cold by means of a plumage that offers superb insulation. However, when they are diving in the sea the feathers are compressed and the trapped air is expelled which means that the plumage loses its insulating properties. The birds’ blubber then protects them to some extent from heat loss.
Physiological protective mechanisms against heat lossAnimals can also prevent the loss of body heat by conduction if they cool down external body parts or their limbs while maintaining a constant body core temperature. This type of behaviour is displayed by, for example, reindeer, emperor penguins and gulls. Under certain circumstances they are able to lower the temperature of their feet to close to freezing while their body core temperature remains at a normal level.
The often badly insulated extremities can be cooled down to this extent as the blood vessels in legs, wings or flippers are located so closely together that heat can be exchanged between arteries and veins. Warm arterial blood originating in the centre of the body passes on its heat to venous blood which had previously cooled down in feet or fins and is being transported back towards the body core.
In this way, only blood already cooled down reaches the extremities, thus greatly reducing heat loss from feet, flippers or wings.
Reindeer have such long legs that the close proximity of veins and arteries alone is sufficient for heat exchange. In the seals’ short flippers, however, the heat exchange is amplified by the veins branching into blood vessels surrounding the centrally located artery which conducts heat to the veins. Moreover, the animals can regulate their blood flow and thus also the heat supply to their extremities – they may want to reduce heat loss in a cold environment, or they may want to quickly cool down, for example following major exertion or when they are at risk of overheating.
- 4.8 > When a reindeer inhales it moistens and warms the incoming cold air by means of tissues in its nose that are richly supplied with blood vessels. These tissues are supported by curled, thin bone structures, clearly visible in this photo of a reindeer skull.
- Surprisingly, the animals do not lose sensation in their wings, flippers or paws even when these have become very cold. Impulse transmission in nerves and muscles of the ball of the foot of Arctic wolves and foxes continues to function even when the animals stand on cold surfaces with temperatures down to minus 50 degrees Celsius and their paws have cooled down to freezing point. Studies have shown that the muscles and nerves in poorly insulated extremities still function when the tissue has reached a minimum temperature of minus six degrees Celsius – an adaptive mechanism that appears to be widespread among mammals and birds in high and medium latitudes.
A similarly sophisticated system helps animals to not unnecessarily lose heat and water vapour to the environment when breathing. When a human exhales at an ambient temperature of minus 30 degrees Celsius, one can see the roughly 32 degree Celsius warm and moist breath as it exits the nose in the form of a light cloud of vapour. Reindeer, in contrast, do not produce such a cloud. The air they exhale is dry and cooled down to 21 degrees Celsius, thus reducing water and heat loss to a minimum. Once again, the secret of these energy savings is effective heat exchange which in this case happens in the nose. In contrast to the human nose, the nasal cavity of reindeer contains numerous convoluted muscous and other membranes that are richly supplied with blood. This nasal structure is highly beneficial in two ways: Firstly it increases the surface area of muscous membranes along which inhaled or exhaled air passes. This gives the reindeer sufficient opportunity to expel or retain heat and water in its breath. Secondly, the complex nasal anatomy divides the breath into numerous thin layers of air, thus further optimizing heat exchange.
When a reindeer inhales, the icy cold polar air passes over the well-perfused nasal membranes. In less than a second it is moistened and its temperature is raised to the animal’s body temperature. The air reaching the lungs has a temperature of 38 degrees Celsius and is sufficiently moist to ensure optimum oxygen uptake. As a result of the heat transfer to the inhaled air, the membranes briefly cool down. When the animal exhales, its warm breath once again passes the now cooled nasal membranes and transfers back some of the heat. This cools down the breath to 21 degree Celsius and most of the water vapour it contains condenses. This mechanism ensures that reindeer exhale only cool and dry air, thus saving a great deal of body heat and moisture. The latter is critical in particular when all ponds, rivers and lakes are frozen during the winter and the animals are forced to consume snow in order to obtain water.
- 4.9 > A thick layer of blubber and warming underfur protects polar bears from losing body heat to their environment. Moreover, the transparent guard hairs allow for solar radiation to reach the skin which means that the bears can warm up in good sunny weather.
- Despite their thick winter coat and their sophisticated heat-conserving mechanisms it is possible for the animals to lose heat and for their body temperature to drop to dangerous levels. When this happens, most of the animals increase their metabolism and begin to shiver, generating heat by means of muscle contractions. Wind and moisture generally accelerate heat loss while sunshine can help the animals to maintain their body temperature. Harp seals, for example, bask in the sun when they are cold, a strategy also employed by polar bears. Their long transparent guard hair is particularly suited to letting solar radiation pass through, allowing for its optimum absorption by the bears’ black skin.
The polar bears’ guard hair also has another special characteristic. It absorbs the longwave heat radiated by the bears themselves. Simply put, it re-absorbs much of the heat radiated by the bears despite their thick underfur. This means that the animals lose very little heat from their body surface. However, this can also be disadvantageous, for example when the bears move swiftly. It can quickly put them at risk of overheating. This is the reason why most of the time polar bears move at a rather leisurely pace. And if they ever get too hot after all, these largest of all terrestrial Arctic predators cool down by jumping into the water.
That option is not generally available to reindeer, even though they often overheat especially in winter under conditions of great exertion. At such moments, reindeer cool down the most critical parts of their brain by directing cold blood from their nasal membranes through a facial vein towards their brain. Just before reaching the brain, a heat exchange takes place with the blood flowing through the carotid artery. This mechanism ensures that only blood at normal temperature circulates in the brain while the surplus heat is distributed to the rest of the body until such time as the strain has subsided and the heat can once again be exchanged by the nasal membranes.
Thermoregulation in young animalsIn the polar regions, animal offspring is born at very different times and under a variety of conditions. Nonetheless, all young animals have one thing in common – their ratio of body surface to body mass is significantly worse than that of their parents, which means that young animals suffer relatively greater heat loss. Most of them are born without fur or plumage, or if they are, then its insulating powers are not nearly as good as their parents’ coat. This is a particularly perilous situation if the young birds or mammals are wet at birth.
Polar birds and mammals have developed special behaviours to ensure that their offspring have a chance at survival. Altricial species the young of which require a lot of parental support at the start, such as polar bears or lemmings, generally give birth at a protected location, such as a snow cave, den or nest. While a polar bear female is forced to fast for the first three months after giving birth because she never lets her cubs out of her sight, lemming females must leave their young at times in search of food while their pups stay behind in the burrow on their own. During this period the baby lemmings’ body temperature drops to well below 20 degrees Celsius but this does not kill them. During the first days of their lives they are surprisingly immune to cold. The older the pups are, the better they get at regulating their own body temperature. The strategies they employ include muscular heat generation (shivering), a thicker fur, or the burning of fatty acids from their brown fat, a process often described in the special literature as nonshivering or biochemical thermogenesis.
Brown fatty tissue can be found in almost all newborn mammals. Its cells are significantly smaller than those of the white, insulating fatty tissue. It contains many small lipid droplets and a particularly large number of mitochondria, the cells’ power plants. The breakdown of lipids in mitochondria generates heat which enables a variety of polar mammal species to survive.
- 4.10 > As can clearly be seen in this infrared image, polar bears primarily lose body heat from their noses which are only sparsely covered in fur.
- In contrast, newly hatched birds are dependent on being kept warm by their parents. Antarctic procellariids (a group of seabirds including petrels and shearwaters), for example, hatch at an average temperature of minus 25 degrees Celsius on bare rock. Once the chicks are hatched, their parents must keep them warm for at least eleven days. The chicks of emperor penguins hide in their parents’ brood pouch for up to 50 days – initially that of the male, and subsequently in the female’s brood pouch when she returns from the ocean and for the first time feeds the chick food sourced at sea.
Reindeer calves and ptarmigan chicks must stand on their own feet from day one. They are precocial species. Unlike penguin chicks they are born with their own protection against the cold. Ptarmigan chicks hatch with warming plumage, are strong enough to walk long distances even on their first day, and are able to maintain their body temperature by means of breast muscle shivering. Nonetheless, the little ptarmigans seek their mother’s warmth when their body temperature drops to below 35 degrees Celsius. Young reindeer and musk oxen get cold in particular when there is wind, rain or sleet. At such times their coat loses its warming traits much faster than that of their parents. The offspring primarily resorts to the burning of lipids from their brown fatty tissue in order to stay warm. Most seal pups in the polar regions must also avoid the water. They are born with a woolly and normally white covering of lanugo which only keeps them warm as long as it stays dry.
Body heat generation requires energy which the offspring of mammals obtain from their mothers’ milk. The milk of species that are at home in the polar regions is particularly high in fats. In whales, seals and other marine mammals the milk’s fat content is between 40 and 50 per cent, while the milk of terrestrial species contains between ten and 20 per cent fat. (For comparison: normal cows’ milk has a fat content of roughly four per cent.) The young of different species are suckled for different lengths of time. While hooded seals nurse their pups for only two to four days, walrus calves suckle for more than a year.
When food becomes scarceAnimals in the polar regions must not only deal with extreme air and water temperatures. They are also faced with the challenge that they can only find sufficient amounts of food at certain times of the year. Different species solve this problem in very different ways. Musk oxen, for example, can lower their metabolism by 30 per cent. Similar observations have been made in Arctic foxes, Arctic hares and ptarmigans. The animals also limit their movement radius in order to save energy. Reindeer on Spitsbergen spend up to 80 per cent of the day in a standing or lying position during the winter as any amount of exertion and any additional step in the snowy terrain has a price. If the animals begin to trot their energy consumption quadruples even if the herd moves at only a moderate pace of seven kilometres per hour.
For this reason, most of the animals build up major fat reserves in times of plenty as something of an “insurance policy”. As early as in August, ptarmigans on Spitsbergen begin to eat anything and everything they can find. By November the birds will have gained so much weight that their layer of fat comprises 30 per cent of their bodyweight. They do however need this amount of reserves as the birds need to draw on these whenever winter weather makes it impossible for them to search for food, for example when there are heavy storms. By February the birds have generally exhausted their fat reserves. In reindeer and musk oxen the quantity of fat reserves also determines whether a female is fertile and able to produce offspring.
The Arctic ground squirrel (Urocitellus parryii) is among the few polar species that sleep through the winter. Despite the fact that the squirrels’ body temperature drops down to as low as minus three degrees Celsius, their blood does not freeze and their organs and tissues are not damaged by ice crystals. To avoid death by hypothermia the animals wake up every three weeks from their state of torpor and begin to shiver for one or two days which raises their body temperature back up to 34 to 36 degrees Celsius. In the course of this process the squirrels burn a lot of fat which they had accumulated during the short summer. They then fall back into hibernation.
- 4.11 > An Arctic ground squirrel is curled up in its den on a bed of moss and sleeps through the winter which can last for seven to nine months. The animals are active only during the short Arctic summer.
- In polar bears, only pregnant females spend the winter in a snow den where they also give birth to their cubs. Juvenile bears and adult males are more or less active throughout the winter; after all they need to accumulate a great deal of body fat as long as the sea ice allows them access to the seal territories.
Since seals moult once a year they too face a regular period of fasting. During this time the animals stop searching for food and reduce their metabolism by half. They generate body heat and kinetic energy solely by drawing on their fat reserves. In contrast, Arctic foxes and stoats do not solely rely on their accumulated body fat. They also hoard food, a task that keeps them busy from September to November. Some animals hide their kills in many different places while others store them all in one place. The biggest known hoard of an Arctic fox contained 136 seabirds which the predator had apparently taken at a breeding colony. For stoats there are reports of individual animals accumulating as many as 150 killed lemmings in winter stores.
Adaptations to light conditionsOne of the polar regions’ unique characteristics is the change between long periods of daylight in summer (polar day) and long periods of darkness during the winter (polar night). In the interim periods, light conditions change so fast that in places such as Spitsbergen or in northern Greenland day-length increases or decreases by 30 minutes per day. These changes require constant behavioural adaptations on the part of the animals, as the available light not only determines the animals’ daily rhythm but also their annual calendar and thus the timing of important events such as mating, hibernation or moulting. This is true not only for organisms residing in the southern and northern polar regions but also for the animals in the rest of the world.
The animals’ internal clock is regulated by means of biochemical processes which commence when information on light conditions is received by special light-sensitive neurons in the eyes’ retina. The signals are transmitted along neural pathways, first to the suprachiasmatic nucleus and subsequently to the pineal gland. The former is a nucleus within the brain; it is situated in the hypothalamus and is, just like an internal clock, responsible for controlling the circadian rhythms of mammals. The pineal gland is located at the back of the midbrain. Only during periods of darkness does it produce the hormone melatonin which is then released into the blood and the cerebrospinal fluid. This means that with decreasing night length, the amount of melatonin in the body also decreases and in turn so does its process-inhibiting impact.
Simply put, melatonin synchronizes all the processes taking place in an animal’s body and adapts its internal clock to the current time of day and season. However, polar species display a special characteristic in this respect. While most animals outside of the Arctic and Antarctic are active during the day and rest at night, Arctic and Antarctic species adapt their behaviour to the current light phase.
- Arctic ptarmigans are a good example. During spring and autumn when the sun rises and sets they search for food in the morning and evening, just like many other bird species. However, during the phases of constant darkness and constant brightness respectively the birds are basically searching for food around the clock except for some breaks. This same pattern of behaviour has been observed in reindeer on Spitsbergen and in Adélie penguins. Similarly, it is known that male emperor penguins do not have strikingly high levels of melatonin even during the polar night. The animals thus do not display typical diurnal rhythms during the polar day and polar night.
It is easier for reindeer than for other animals to search for food even during long periods of darkness as they are able to detect light in the ultraviolet spectrum. This ability provides them with a crucial advantage. Since snow and ice largely reflect incoming ultraviolet light, the animals see the landscape as a light-coloured surface. In contrast, anything that absorbs UV light appears black to them. This includes lichens, the reindeer’s main food source during the winter. But white fur (polar bears) and the fur of wolves also only reflect a small portion of UV light. The reindeer can therefore detect potential attackers at an early stage which greatly increases their chances of survival.
Scientists also assume that the UV light allows the animals to detect the texture of a snow surface, since the proportion of reflected UV light changes with the snow cover’s physical characteristics. Presumably the herds are able to see at first glance whether it is worth searching for food in a particular place, or whether they would be better off taking a little detour because the snow in a particular location is too harsh or too soft to cross.
Changing colour at the start of winterThe changing light conditions also signal the start of the typical moult which gives Arctic foxes, Arctic hares, stoats and other animals their mostly grey or brown summer coat and their white winter coat. In the temperate and polar latitudes of the northern hemisphere there are 21 species of mammals and birds at present that change colour with the seasons. This means that the animals have to grow an entirely new coat or plumage twice a year. While the evolution of seasonal colour changes is not yet fully understood, presumably the species developed this ability independently of each other. Interestingly, however, different species in a region change their colour almost at the same time and hold onto their winter plumage or winter coat for a similar period, in alignment with the general local timing of the first snowfalls and the length of time the snow cover tends to persist. Species living in areas with highly variable or patchy snow cover have also adapted their coat colour to these conditions. Their winter coat or plumage contains a number of pigmented hairs or feathers and generally appears speckled whitish-brown or whitish-grey.
Scientific research has been conducted into the purpose of the colour change. The results indicate that it primarily serves camouflage and thermoregulation. A white coat or plumage in winter is highly advantageous for both predators and prey. If the landscape is covered in snow, both groups are harder to be spotted by their respective adversaries. The former has greater prospects of catching food while the latter has a greater chance of survival. For this reason, scientists consider a species’ ability to camouflage themselves as being one of the primary drivers in the evolution of mammalian coat colour.
- 4.12 > Rock ptarmigan, Arctic hare, stoat and Arctic fox are among the world’s 21 animal species that change the colour of their coat or plumage with the seasons. This makes it harder for both predators and prey to be spotted and increases their chances of survival. Often the winter coat or winter plumage also has better insulating properties compared to the summer coat or plumage.
- However, an animal can only optically become one with its environment if the moult and the onset of snowfall or snowmelt take place more or less at the same time. If the onset of winter is delayed or if the snowmelt starts much too early in the year, the animals have the wrong coat colour and their evolutionary advantages turn into disadvantages. It is for this reason that species which change their coat colour face a greater threat to their existence from climate change than animals that maintain their coat colour.
Scientists consider birds to be an exception to this rule as often their self-awareness is so strong that they notice the discrepancies between the colour of their environment and their plumage respectively and adapt their behaviour accordingly. Rock ptarmigan and white-tailed ptarmigan, for example, only rest in locations where the dominating ground colour matches that of their plumage. And researchers in Canada observed ptarmigans that deliberately dirtied their plumage when the snowmelt began too early and the birds in their clean white winter plumage were at risk of being detected too easily.
Another effect of the change from summer to winter coat is that the animals improve their furs’ insulating properties. Colourless or unpigmented hair tends to be somewhat broader than pigmented hair or it contains a greater number of air-filled chambers, thus improving its insulating qualities. Additionally, the white winter coat is often longer and denser than the summer coat. This is true for the Arctic fox, the northern collared lemming (Dicrostonyx groenlandicus) and the Djungarian hamster (Phodopus sungorus).
The term vascular plants is applied to all ferns and seed-producing plants, as these have internal vascular tissues which distribute resources through the plant.
- Just like many other processes, the moult is triggered by changes in melatonin concentrations. When melatonin increases in the autumn, signals are transmitted to the pituitary gland which produces the growth hormone prolactin, among others. This hormone in turn regulates hair growth and other functions. When the prolactin concentration rises in the spring, collared lemmings and Arctic hares lose their winter coat and commence their search for partners. However, if the production of this hormone is suppressed, Arctic foxes, lemmings and other animals produce their light-coloured winter coat. In experimental studies, mammals whose prolactin production was suppressed kept their winter coat throughout the entire year, independent of day-length.
But melatonin also inhibits the production of the pigment melanin which gives skin, feathers and eyes their colour. In animals with seasonal coat colours, a high melatonin concentration thus directly results in the growth of a white winter coat. There is less of an understanding as to how day-length and hormones regulate the moult and change of plumage in birds. In part this is due to the fact that birds have at least three “internal clocks”. Information regarding changing day-length is processed not only in the pineal gland, but also in the hypothalamus and in the eyes themselves. Moult and reproduction are coordinated such that the change in plumage does not commence before the breeding period has finished.
In contrast to day-length, environmental factors such as temperature and snowfall only have a limited impact on the change in coat or plumage. Studies have shown that low autumn temperatures accelerate the growth of winter coats or plumage in mammals and birds respectively. Moreover, ptarmigans were shown in experiments to produce a darker winter plumage if they were kept at higher temperatures. In contrast, a cold spring with plenty of snow slowed down the change from winter to summer colour. The moult, however, is solely triggered by day-length.
The flora of the polar regionsDespite their extreme climate, the north and south polar regions host a remarkably rich flora in places. For example, in the Arctic researchers have counted almost 100 different species of vascular plants, mosses and lichens in an area of 25 square metres, making the site examined roughly as species-rich as the most species-rich grasslands of the temperate and subtropical latitudes. Compared to tropical rainforests, however, the polar regions are indeed species-poor. This is primarily due to the low temperatures, the short growing season, the lack of nutrients, the difficulty of rooting in permafrost soils, and extreme weather events in the Arctic such as the typical spring floods. Moreover, growing conditions for plants in the polar regions are often greatly divergent between locations.
On the Siberian Taymyr Peninsula, for example, a mere 500 kilometres separate the sub-Arctic with its relatively lush growth from the polar desert of the High Arctic in which only few plant species survive.
The vegetated lowlands of the Low Arctic are also termed tundra. This term is derived from the word t˜undar which, in the language of the Saami, the original inhabitants of northern Scandinavia, means “a plain devoid of trees”. While in addition to grasses and vascular plants willow, birch and alder, all of which have tree relatives further south, do indeed grow in the tundra, they do not grow up high in the classic tree shape but form creeping scrub or mats just above the ground, not least in order to escape the icy winds. In the northernmost areas of Siberia, on the eastern and western coasts of Greenland, in the Canadian Arctic Archipelago and in the north of Alaska the areas of tundra grade into the High Arctic with its thin vegetation cover dominated by lichens, mosses and dwarf vascular plants. To its south, the tundra is in many areas bordered by the subarctic krummholz zone consisting of climatically stunted and distorted trees.
- 4.14 > Vascular plant species diversity is highest in the tropics and declines with increasing latitude. In the Arctic, the regions relatively species-rich are primarily those that were not glaciated during the past ice ages.
Angiosperms and gymnosperms
Angiosperms are flowering plants and are characterized by the enclosed ovary, which contains and protects the developing seeds. In contrast, gymnosperms are characterized by the unenclosed condition of their seeds. Gymnosperm seeds develop lying unattached on the surface of individual carpels. European larch and Scots pine are well-known gymnosperms.
- Vascular plant species diversity in the polar regions declines with increasing proximity to the poles. In the Arctic, the current vegetation of which has only developed over the past three million years, an estimated 900 species of mosses and 2218 species of vascular plants have been identified. Almost all of the vascular plants are flowering plants (angiosperms). Gymnosperms, in contrast, are rare in the Arctic and where they occur their species diversity tends to be low.
The majority of Arctic plants are considered to have a circumpolar distribution. Nonetheless there are major differences between different regions in terms of their species diversity and composition. While a mere 102 species occur in the northernmost part, the High Arctic, the southern tundra regions host more than 20 times that number of species. Approximately five per cent of Arctic vascular plants are endemic species, which means that they occur nowhere else but in the Arctic. Those species are mainly forbes and grasses.
The diversity of the Arctic flora is also supported by herbivores. When researchers excluded grazing animals such as geese, lemmings, musk oxen and reindeer from certain areas as part of a study, large amounts of plant litter accumulated, insulated the soil and led to the soil not thawing to a sufficient depth in the summer. Vascular plants could no longer develop a sufficient root network and disappeared. Mosses now grew in their place. Moreover, the herbivores’ faeces provide badly needed nutrients, as nitrogen and phosphates are scarce in Arctic soils.
- 4.15 > Biologists differentiate three vegetation zones in the terrestrial north polar region. The High Arctic is the northernmost zone. It borders on the tundra of the Arctic lowlands, and the tundra in turn borders on the northern margins of the boreal zone.
- Compared to the Arctic, the Antarctic flora is truly species-poor. In its continental zone, defined by biologists to include the few ice-free areas of continental Antarctica and the eastern side of the Antarctic Peninsula, only a small number of 40 to 50 different species of lichens and mosses thrive. These generally grow in rock crevices or depressions between stones and mainly on dark rocky ground which absorbs most of the incoming solar energy and radiates heat. Most of these lichens are truly extreme survivalists. Even at a temperature of minus ten degrees Celsius they can still photosynthesise and survive even under conditions of strong and persistent desiccation and extreme cold. Some of the species occur even in the ice-free Antarctic dry valleys of Victoria Land.
The western side of the Antarctic Peninsula and the nearby islands offer a warmer and moister climate and thus more favourable conditions for plants. In this zone, termed the maritime Antarctic, two vascular plant species can be found – Antarctic hair grass (Deschampsia antarctica) and Antarctic pearlwort (Colobanthus quitensis). The bulk of the Antarctic vegetation, however, is composed of cryptogams. Some 100 species of mosses have been recorded as well as 750 species of lichens and an estimated 700 species of terrestrial and oceanic algae. The number of fungus species has not been determined.
Fighting the coldWith increasing proximity to the poles, conditions for plants deteriorate, or to put it differently, physical and chemical factors which limit plant dispersal have increasingly greater impact. These factors include, for example, the length of the growing season, the duration and intensity of frost periods, and the degree to which the plants are exposed to wind. However, the plants’ chances of survival are also linked to available resources. Whether they are in the tropics or in the polar regions, plants can only exist if their carbon budget is positive, which means they must be able to sufficiently photosynthesise in order to grow and store energy reserves in the form of glucose or starch. To this end the plants require sufficient amounts of heat, water, light, carbon dioxide and nutrients as well as oxygen. The latter is required in particular by plants growing in wetlands or swamps.
The polar regions rarely offer ideal conditions for plant growth. The Arctic flora has therefore developed a range of adaptation mechanisms that allow them to tolerate conditions of nutrient deficiency, cold and darkness and to survive with little or no harm extreme events such as prolonged snowfall or spring floods. These adaptations include the following:
- slow, resource-conserving growth,
- a more brown than green coloration,
- a squat stature,
- heat-optimizing characteristics such as fine hairs or special flower shapes,
- mechanisms to protect cells from frost damage,
- a large number of important enzymes enabling photosynthesis even in adverse light conditions,
- nutrient recycling,
- major energy reserves in the root system, and
- the opportunity of asexual reproduction at locations where conditions are such that sexual reproduction does not work.
- 4.16 > Mosses colonizing a lava field in Iceland. The Arctic is home to some 900 species of mosses. They can mostly be found in Arctic wetlands and on snowbanks.
Small is beautifulPolar plants particularly like to settle in sheltered locations where they are not exposed to the full forces of the wind, ice and cold. A second important survival strategy is to grow slowly and reduce energy consumption especially at times of low resource availability. This approach is known as the Montgomery effect, named after Edward Gerrard Montgomery, a scientist at the University of Nebraska Agricultural Experiment Station (USA). When conducting experiments involving a variety of cereal cultivars he found that in locations offering low environmental resources slow growth does indeed confer ecological benefits onto plants. In the Arctic, for example, the summer and therefore the growth phase is so short that plants such as the Arctic wintergreen (Pyrola grandiflora) growing in Iceland and Greenland take several years to grow from the initial sprout to a mature plant capable of seed production. This also explains the longevity of many plants in the polar regions.
- 4.17 > Alpine bistort (Polygonum viviparum) is one of the Arctic plant species that can persist underneath a snow cover for periods of more than two years.
- The tiny pygmy buttercup (Ranunculus pygmaeus) is a species that has perfected prudent resource use. It often grows surrounded by mosses in the vicinity of glaciers, streams or snow drifts and survives even if it is occasionally covered by so much snow in the winter that this snow does not melt in the course of the following summer, resulting in the plant missing out on an entire cycle of growth and reproduction. Other species are so thrifty in their resource use that they can persist for even two or three years in series underneath a snow cover. These include Alpine bistort (Polygonum viviparum), mountain sorrel (Oxyria digyna) and polar willow (Salix polaris).
The small and squat stature of many polar plants is not only a result of their drawn-out growth. Plants forming thick ground-covering carpets instead of having their leaves and flowers shoot upwards will escape the icy Arctic winds. The air held inside these carpets or cushions is swirled to a lesser degree and is more easily warmed by the sun. In this manner the carpeting plants create their own microclimate, the temperature of which may reach 25 to 30 degrees Celsius on summer days when the ambient temperature at a height of two metres is a mere eight degrees Celsius. The plants inside the carpet thus enjoy optimum metabolic conditions at such times.
In order to grow and flower during the short and cool summer, polar plants also employ strategies which in warmer regions would lead to immediate death from heat stress. One of the strategies is coloration. Darker colours absorb a greater amount of solar radiation than lighter colours. This explains why the vegetative cover in many of the Arctic areas appears predominantly brown instead of green. This is particularly true for plant communities on Arctic beaches where the growing season is particularly short.
- 4.18 > Antarctic hair grass (Deschampsia antarctica) is one of the two vascular plant species that are native to the Antarctic continent.
- Moreover, plants like the glacier buttercup (Ranunculus glacialis) are able to align their leaves and flowers at an optimum angle to the sun. Its initially white flowers then function like little parabolic dishes which direct the incoming sunlight directly to the reproductive organs at the centre of the flower. This increases the air temperature inside the flower which in turn results in the reproductive organs developing at a faster rate and in the flowers attracting a greater number of insects. Following pollination the glacial buttercup closes its flowers and the petals turn red, allowing the flower to absorb a greater amount of solar radiation, the heat content of which in turn protects the seeds developing inside the flower.
Other Arctic plants create their own “greenhouse”. Female polar willows (Salix arctica), for example, grow fine downy hairs on their leaves and along their inflorescences. This downy cover traps an insulating layer of air close to the leaf surface. The hairs also reduce the leaf surface area which normally would be subject to heat loss as a result of evaporative cooling. The downy hairs so efficiently protect the little willows that the temperature of the leaves may be up to eleven degrees Celsius higher than the ambient temperature.
- 4.19 > The white petals of the flowers of glacier buttercups (Ranunculus glacialis) reflect sunlight towards the centre of the flowers which makes them very attractive to insects.
- 4.20 > The white cotton-like plumes of cottongrass are a familiar sight in Arctic wetlands. However, these are not the flowers but only develop along with the seeds. The long perianth bristles form white tufts which also protect the seeds from the cold.
- Northern plants also avoid growing their roots deep into soils where much of the ground stays frozen throughout the year and where meltwater accumulates. Instead they take root in the shallow layer of topsoil which is the first to melt in springtime and generally tends to be waterlogged only for short periods. At the end of the summer, trees and shrubs drop their needles and leaves and overwinter in a dormant bud stage. Before they go dormant, however, they cover their buds in a wool-like substance in order to protect them from the frost.
Many Arctic plant species defy the freezing cold winter temperatures by moving water, among other substances, from their cells into intercellular spaces. In this manner the plants reduce the risk of ice crystals forming inside of cells and damaging these. The plants simultaneously strengthen the cell membranes with certain types of sugars and proteins; the membrane’s lipid composition also changes. Special enzymes prevent the cells from suffering damage due to dehydration. However, these cellular frost protection mechanisms are not activated year-round. They only play a role when temperatures drop at the end of summer and the plants are acclimatizing. At the height of winter most plants are so well protected from frost damage that some even survived laboratory trials as part of which they were briefly dipped into liquid nitrogen at a temperature of minus 196 degrees Celsius.
However, problems arise when unusually warm periods and severe frost alternate during the winter or when normally snow-covered areas suddenly become free of snow. These kinds of conditions can damage even the hardiest of Arctic plants. Nonetheless, in most cases the plants will be able to compensate for such damage by growing new leaves and shoots in the spring.
Making the most of the short summerPlants need active enzymes in order to take up carbon dioxide, to photosynthesise and to generate energy reserves in the form of glucose and starch. Cold-adapted plants of the polar regions contain a particularly high level of active enzymes. Large quantities of the enzyme RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase) allow the Arctic flora to uphold metabolic activity even at lower temperatures. But even polar vascular plants cannot grow at subzero temperatures. The plants must wait for the short summer in order to develop leaves and flowers and they must be able to make optimum use of this short period. The cells of cold-adapted plants contain particularly high numbers of mitochondria which, as the cells’ “power plants”, are responsible for energy generation. With their help the plants increase their metabolism to maximum levels during the summer. Not only do they make optimum use of the 24 hours of daylight but they are also able to photosynthesise in unfavourable light conditions.
This strength, however, makes the cold-adapted plants susceptible to heat stress. If the ambient temperature rises to greater than average levels, both metabolism and cellular respiration increase well beyond healthy levels. The plants then swiftly use up all their energy reserves and suffer damage. This explains why polar plants of the Arctic do not spread further south. It also highlights one of the mechanisms by which climate change poses a risk to polar plants. Rising temperatures increase the probability of cold-adapted plants succumbing to exhaustion.
Searching for nutrientsSome plants actively seek out resources so as to have sufficient amounts of nutrients and light at their disposal during the short growing period. They develop small shoots or runners above or below ground which they use to tap into light and nutrient sources away from their original location that are crucial to their survival. This strategy can offer the plants clear locational advantages, as a comparison between two closely related cottongrass species has shown, both of which grow in Arctic wetlands.
Common cottongrass (Eriophorum angustifolium) develops runners and actively searches for minerals, an ability that allows the plants to survive in the very wet parts of the marshes. Their runners tolerate stagnant water and allow the species to spread into flooded areas. In contrast, the hare’s-tail cottongrass (Eriophorum vaginatum) does not send out shoots into its nearby environment. It grows instead as tussocky grasses and thrives in particular in the drier locations where there might be significant water level fluctuations.
The energy reserves produced by the plants by means of photosynthesis during the short summers tend not to be invested into the development of new leaves but are mostly put into subterranean storage in the form of starches in the plants’ roots. Therefore the root systems of Arctic plants are generally larger than those of plants of temperate or tropical latitudes. It makes sense for the plants to accumulate significant reserves in the north polar region with its highly variable weather conditions; there is always a possibility that they might miss out on one or even two growing periods while buried under snow, a time during which they must live on their reserves. It is for this reason that plants of the tundra such as bog-rosemary (Andromeda polifolia) store up to 75 per cent of their energy reserves in their roots.
Nutrients such as nitrogen, phosphorus and potassium are also particularly valuable to polar plants. Plants such as the dwarf birch (Bitula nana) have therefore found ways to recycle them once they have taken up and processed such elements. Shortly before the birch drops its leaves at the end of summer the plant withdraws a major proportion of the nutrients stored in these leaves back into the more permanent plant body. Hare’s-tail cottongrass employs the same mechanism; it can recycle 90 per cent of the phosphorus contained in its leaves, which means that in the springtime the plant only needs to newly take up ten per cent of its phosphorus requirements from the soil.
Two ways to reproduceMost animal species rely on sexual reproduction for their species’ survival. In contrast, plants often have the option of asexual reproduction. They may form runners, branches or even seeds, with the latter being produced in the absence of classic pollination (agamospermy). These strategies have allowed several plant species at home in the Arctic to persist for centuries or even millennia, one example being Arctic sedges such as Carex ensifolia.
Sexual reproduction in vascular plants may fail either because of a failure to develop flowers or because pollination could not take place. In some species the latter may be caused by even just a brief cold spell. In the northern range of the American dwarf birch (Betula glandulosa), for example, only 0.5 per cent of birch seeds germinate. In order to survive in these regions the species has no choice but to resort to asexual reproduction.
In the springtime shortly after the snowmelt the tundra suddenly bursts into bloom. This spectacle is primarily caused by perennial plants. With very few exceptions there basically are no annual plant species in the polar regions. In order to develop flowers in such a short timeframe, Arctic plants need flower buds which have already been initiated in the autumn and which can immediately kick into action following the snowmelt.
- 4.21 > Autumn in the Arctic: A yellowish-orange carpet of shrubby willows and dwarf birches covers this headland in the Canadian Arctic. This far north the American dwarf birch (Betula glandulosa) primarily reproduces asexually.
- The many flowering plant species of the Arctic are primarily pollinated by flies, which is not very surprising as there are hardly any bees north of the polar circle. When scientists in Greenland took a closer look at the insects responsible for pollinating mountain avens (Dryas octopetala), a characteristic plant of the Arctic, they counted a total of 117 different insect species which visited the plant. However, pollination was primarily performed by a single species, a small relative of the housefly called Spilogona sanctipauli.
To spend the winter in the form of a seed in the soil is a globally widespread and highly successful survival strategy of plants – this is no different in the polar regions. When scientists studied the flora of Spitsbergen they found that 71 of the 161 native plant species produced seeds in order to ensure the survival of the species. The same strategy is employed by the only two Antarctic flowering plant species. Plant seed longevity varies around the globe. While the seeds of some species persist in the soil for less than a year, some Arctic plant seeds display surprising levels of resilience. As part of scientific studies, seeds of the sedge species Carex bigelowii which were approximately 200 years old were still able to germinate; and in Alaska seeds of the small-flowered woodrush (Luzula parviflora) germinated after an estimated 175 years in the ground. If one day environmental conditions were to rapidly deteriorate, these species would therefore be in a position to persist as seeds in the soil for several decades or even centuries, and to germinate once conditions have become more favourable.
Over the past two to three million years, the flora of the polar regions has displayed a remarkable capacity to survive and adapt, and especially in the Arctic a rich diversity of species has emerged. Global warming will now pose new challenges for the cold-adapted flora, and the degree to which the polar biodiversity will be able to persist is uncertain.