The Cactaiga and the Ice Age Thorngrazer

As the climate cools on Serina, climate change extinguishes what were once life-rich tropical forests as more northerly ecosystems are squeezed toward the equator in flight of advancing polar ice. Not every change of environment over the previous ten million years has resulted from ecosystems merely migrating to warmer ground, though - in some cases entirely new ones have popped up in response to the changes. With temperate sunflower-dominated forests moving to the equator, particularly along the southern coasts of Serinarcta, and tundra claiming ground where once was this forest, lying between these two is an entirely novel forest community known as the cactaiga. This cold-hardy, low-growing forest is comprised of cactus-like spiny plants very distantly related to the sunflower trees, but whose ancestors diverged from them virtually as soon as they were introduced to Serina. With photosynthetic tissues and few or no leaves, the trees of the cactaiga are descended from desert-adapted scrubland sunflower species that have found success in an increasingly cold climate using the same adaptations that served them well in Serina's deserts; thick layers of sharp, protective hairs and thorns that completely cover their tissues to protect from both water loss and herbivore attack. These defenses which also served to protect against sunburn and insulate against the cold desert night now provide protection against cutting winter winds. Growing in stands so thick that a man could not even crawl through the tangles of cutting spines and vicious thorns, the individually small trees grow together into a virtually impenetrable matrix, relying on one another for support and so functioning as a single enormous plant with millions of roots to survive winter storms in the harshest of areas that would snap broad-leafed trees like toothpicks.

The cactaiga first began spreading from isolated deserts across Serina about seven million years ago, following the trend of a cooling, drying climate, and today covers most habitable land - almost everywhere that the ground is freely-draining, as these desert-descended plants cannot survive waterlogged roots. This means that they dominate the middle third of Serinarcta almost entirely, giving way only to the low grassy tundra in the far north where the permafrost beneath the ground means water cannot drain and so the soil remains waterlogged throughout the summer. Broad-leafed forest, requiring milder conditions to compete, becomes restricted to primarily coastal regions along the Icebox Seaway where the influence of ocean currents keeps the land warm enough for their survival.

Being dense and impenetrable, the cactaiga alone supports few herbivores. Though their seeds, born in the summer inside edible berries, are eaten by birds and molodonts, few leaf-eaters can consume the plants themselves. Spreading into every crevice and out-competing other plants almost everywhere in the cooling climate, it is not the cold that would directly cause the extinction of many megafauna but a loss of food and habitat as cactaiga replaces more edible food plants and sheltering environments. The herbivorous softbilled birds which have survived the loss of the tropical forests, such as the terries, find the environment completely lacking in food or shelter and so species become isolated from one another; many become trapped along the narrow band of northern tundra, between expanding glaciers and the vicious spiny forest that offers them nothing, where for now they can survive on the surviving softer vegetation, but where their future there is very uncertain. Circuagodonts, adapted to trimming woody trees, could persist along the margins, but would have great difficulty moving deep into the dense growth to migrate south or to find shelter from winter storms.

But the cactaiga is not without its own checks and balances. One species of tribbethere, previously seen as quite aberrant, localized, and specialized in its niche, has since evolved in tandem with the thorny sunflower trees in Serina's desert regions. The thorngrazer has diversified into more than ten species and followed their spreading food plants, continuing the evolutionary arms race between them that has now gone on for tens of millions of years. With enormous gnashing mouths armored with protective tooth-derived spines they are the only animals that can truly limit the growth of the cactaiga, browsing back the forest and eating away paths through the bramble that provide shelter to less powerful animals that could not otherwise survive here. Once fairly solitary, now large herds of thorngrazer descendants move through these cold forests and browse the hostile, cutting plants with seeming effortless ease, pulverizing the most ferocious plant defenses with their grinding enormous tooth plates. They open clearings in the brush to bed down for the night, and over many years these resting places become worn-down depressions with a very compacted soil; water fills them, and they become too damp for the cactaiga to grow back. Instead grasses and herbs take root - food for the terries and many other animals that cannot subsist off the cactaiga. In this way the thorngrazers become ecosystem engineers, actively building new habitats and increasing the biodiversity of the forest drastically, albeit inadvertently. What would otherwise be a harsh monoculture of plants few animals could utilize becomes a fairly varied forest community. Selectively trimmed by the feeding thorngrazers into a maze of clear-cut paths, the cactaiga - so potentially destructive - becomes a series of sheltering hedges: windblocks to shelter plants and animals alike from winter snowstorms. In this way the thorn forest becomes a series of thorn-walled gardens, intermixed with abundant small clearings and low-lying wetlands that provide a safe, sheltered oasis for many species in this rapidly changing world.

As their food plants spread across the continent, the thorngrazer followed. Over time populations became separated, without gene flow between them, and new species formed adapted to more specific regions or ways of feeding. As the climate cooled, all of them had to grow thicker fur coats, losing their brightly colored exposed skin at least in winter, though sometimes molting their hair from their faces in summer to reveal bright patterns in the mating season. Some grew taller, feeding upon the forest from the top down, while others shrunk to more easily move through the forest down to the ground, maintaining grazed-out tunnels in the thick vegetation to hide from predators. Many have begun to evolve new, more specialized facial horns from the bone-like spikes along their mouths, switching the function from defense to offense. One species, the sextacorn thorngrazer, has developed six prominent horns, two on either side of the mouth and four on top of the snout, which the male uses in combat for mates and females use to defend their calves from newly-evolved ice age predators such as the saber-toothed circuagodog.

As Serina changes, many plants and animals alike will lose the game of life. Yet as long as something can be exploited, there are always those which will make the most of a new opportunity.

A male sextacorn thorngrazer, Sentisophagous sextops, in a sheltered clearing its herd has grazed out of the cactaiga.