Finding the Balance
The extinction was nothing short of catastrophic. Enough species met their end that the event qualifies as a major extinction event, not unlike the Cretaceous-Paleogene extinction that decimated the dinosaurs or the Permian-Triassic extinction which earned the title "The Great Dying". The various ecosystems of Earth that survived in any form have a sort of emptiness to them, a lack of diversity painfully obvious to the eyes and ears.
But with time comes healing, and with healing, change. 20,000 years after the cataclysm, the remaining biomes have largely relocated to their new homes, though the organisms themselves are still struggling to fit in. By this point, all survivors have lived in their preferred temperature zones for several generations, long enough to adapt to their changed world. As the years pass, the biosphere finds ways to tolerate its new struggles, as the species within it begin major evolutionary change.
This scene in what is now Bolivia showcases just how different things are in the new world. What was once a hot tropical rainforest is now a cool temperate grassland, dotted with shrubs and the occasional small tree. Some clouds are coming in from the east, perhaps a sign of some well-appreciated rainfall in the near future. Nearby, two members of a herd of large mammals find something to eat.
These mammals are guanacos (Lama guanicoe), the wild ancestors of llamas, though llamas obviously do not exist in this timeline. One might be surprised to learn that these animals survived; in fact, they are some of the largest terrestrial creatures to push through the extinction. Most large, endothermic animals found themselves starving to death as their ecosystems collapsed around them, since they required much more energy than their environment could provide. These guanacos, though, had some advantages. Their camelid ancestry grants them a very strong tolerance to dehydration, as well as a willingness to eat plants as tough as cacti in desperate situations. What most ensured their survival, however, was their location. Guanacos lived on and around the Andes mountains, whose high elevation provided them cool habitats even in the heat of the tropics. With temperatures falling so fast, all these animals had to do to survive was to walk down the mountains into what was previously a hot rainforest.
Most of the grasses and shrubs that they feed upon largely have the same survival story: A mix of having the right traits and living in the right place. For example, the majority of small shrubs and trees here are plants of the genus Polylepis, relatives of roses and strawberries that are very well adapted to the cold and dry conditions of high-elevation life. With a majority of plants hailing from their homeland, the guanacos have a similar diet to what their ancestors did back in the Andes, and thus haven’t changed much.
In equatorial Sundaland, a golden-spotted tiger beetle (Cicindela aurulenta) forages on a sandy plain near the beach. Some plants have begun to take root here, such as these Sambac jasmine seedlings (Jasminum sambac). Though the region is as tropical as it gets, there is no escape from the cold; even here, freezing is guaranteed during winter.
Sandy plains such as this are a common sight on the borders of Sundaland. Because so much of its area is near sea level and because most of the ocean around it is so shallow, the coastline of Sundaland is still shifting even long after the extinction. Very few inhabitants of these coastal regions are native here, and this beetle is no exception.
Golden-spotted tiger beetles are, like all tiger beetles, extremely fast pursuit predators, but this species stands out with its bright iridescent colors of unclear function. They once lived in the tropics and subtropics throughout East Asia. Though their wings have served them well, allowing them to pursue the heat toward the equator as the cold claimed their homelands, they found that even the equator was too cool for them. In at attempt to find the warmth they once knew, the surviving beetles took to the sandy beaches of the steadily growing shoreline. Here they survived, hunting in the day with the unobstructed sun warming the sand, and retreating to nooks and crannies anywhere they could to wait out the cool night.
Having squeezed by while most warmth-accustomed organisms in the area died, these beetles have few competitors and predators, and thus have grown from 1.8 cm to 3 cm in length (0.71 to 1.18 in). Though they spend their time far from the waves to avoid the desiccating salt, their newfound niche has offered them a plethora of strange new prey, all of which are slow enough to catch.
The Sambac jasmines represent one of the few species of tropical plant to pull through the extinction due to their limited cold tolerance. Ancestral Sambac jasmine plants required a rich, well-drained soil to grow, but they have undergone rapid evolution to prefer less ideal soil. Still, these two seedlings which have managed to sprout in the sand will not last long, for the soil is too poor for them.
In what is now Pakistan, where the semi-arid shrubland has long ago yielded to taiga, a pair of snow buntings (Plectrophenax nivalis) flutter above the trees. In the background, a golden eagle (Aquila chrysaetos) soars some distance away, riding the summer thermals.
The trees here are descended from mountain-living ancestors, as many survivors are. In contrast, these two species of bird once inhabited lands thousands of kilometers away. The snow buntings in particular have come a long way; their species once lived north of the Arctic Circle.
In fact, as small endotherms, it might seem a miracle that these buntings survived at all. Small endotherms consume a lot of food for their weight to keep their small bodies warm, which is a lot of energy that an ectotherm could use for other purposes. To make things even worse, many small endotherms use the tactic of hibernation to survive cold temperatures, so when the cold came, they were doomed to eternal sleep as the return of warmth never came. For these reasons, small endotherms had a very difficult time pulling through. For example, the rodents, famous for their survivability, came just a hair’s breadth from death.
The reason these buntings survived is the same reason many other small birds did: Flight. Instead of suffering in the cold, well-flighted birds could head toward the equator as their preferred temperatures moved. Generally, cold-adapted organisms with a greater ability to disperse fared better than those with a lesser ability, who instead were outrun by the glaciers.
Even considering this, the survival of the golden eagle is very strange. It is relatively large and an endotherm, but strangest of all, it is one of the precious few apex predators to survive the extinction. Their ability to fly long distances wouldn’t have been enough to save them, but unlike the snow buntings, these eagles lived nearly all over the Northern Hemisphere. Thanks to their tolerance of both hot and cold temperatures, they even lived here, in Pakistan, when it was hot and arid! With their soaring flight and adaptability to a variety of temperatures, they barely made up for their disadvantages and survived the mass extinction. They did lose their impressive size, though, having shrunk from a 2.2 meter (7 ft 3 in) wingspan to a measly 1.5 meters (4 ft 11 in).
In the southern Pacific, a large swarm of Antarctic krill (Euphausia superba) swims beneath the ice. It may be midwinter, but these animals are thriving just the same.
Krill (order Euphausiacea) are a group of free-swimming crustaceans that convergently evolved a very similar bodyplan to the unrelated shrimp. On old Earth, krill were found in all oceans, usually as small- to medium-sized omnivores or filter feeders. Because of how common they often were, they formed an integral part of most ocean ecosystems. However, their presence was never so striking as it was at the poles, especially the south pole.
Antarctic krill were the dominant species of krill around Antarctica. They thrived there, filtering the water for food and scraping algae off the ice. They often swarmed in large groups, and though they darted from threats with an extremely rapid reaction time given their ectothermy, they formed a key part of many animal’s diets. It’s difficult to gauge just how successful they were; in fact, they were so numerous that they had the largest biomass of any animal species, terrestrial or aquatic, and thus were the most successful species on Earth by that measure. When the cold came, they were ready for it.
With such a wide belt of inhabitable cold water, plus an opportunity to jump to the Northern Hemisphere, the Antarctic krill’s numbers have exploded to a staggering level. With predators now scarce, swarms of krill can cover enormous swaths of ocean, attaining a total biomass that dwarfs even their ancestors’ records. With the majority of the world’s oceans in their range and a large chunk of total biomass in their ranks, the world truly belongs to the Antarctic krill.