Thorn Atoll

Far out to sea in the hothouse age, there lies a lonely and forgotten land where life is as harsh as it ever was in the ice age, and the bountiful world beyond might as well be nothing but a distant dream. This is the Thorn Atoll, the harshest habitat of the hothouse.

A traveling handstander, a member of a widespread new lineage of tribbfishers, finds itself one day over a strange, tiny island in the middle of the vast Unbroken Ocean, hundreds of miles northwest of Serinaustra. Yet it will find little respite here; the island is inhospitable, a desert at sea, situated between continental weather systems and so left excruciatingly dry. Only the hardiest plants survive here; primarily the last remnant species of cactaiga that dominate the bare stone substrate of the small island chain, and which in turn support a bare-bones food chain to nourish the large, grazing megamurd, a far-flung relative of the jirdbird which lucklessly washed ashore here a million years earlier and has since grown far larger to better digest its terrible, thorny diet. A small surrounding reef supports a tiny population of the even tinier pretenguin, the waif, another castaway that tries to make the best of the harsh conditions. It nests in the shaded crevices of the island's rocks, and brings scarce nutrients from the sea onto land via its droppings, making the cactaiga's survival possible. But it is a delicate balance, and a precarious existence for all the players in this insular ecosystem, which may well be among the least habitable environments of the entire hothouse world.

The tribbfisher might stop for a night to rest, but this is no place for anything to live if it has the option to leave. Tomorrow, it will resume its endless oceanic migrations and continue a journey onward to richer places: it is a privilege the atoll's endemic species, far less able to cross the vast and barren open seas, are not afforded.

Yet the endemic life of Thorn Atoll won't give up and die just because their habitat is unforgiving. As hardened by the unrelenting scarcity of the island as the rocks they live on, two endemic animals cling to life here through incredible tenacity and persistence.

Murds, and burdles in general, are excellent rafters, their metabolisms being able to slow down into a semi-dormant state during times of stress, and the wet, flooded nature of coastal Serinaustra's habitats lends well to occasional wash-outs of coastal sediment, tangled up with roots and live plants, forming rafts that are carried out to sea. In this way murds have also reached mainland Serinarcta and the Anstevan archipelago - places the ancestral burdles initially didn't - and in the other way, multiple lineages of skuorcs - animals with similar metabolisms - reached Serinaustra. Rafting to the shore of a large continent is itself a very rare occurrence; washing upon a small isolated island in the ocean that is only a few miles across is extraordinarily unlikely - but over millions of years of deep time, such unlikely events do occur with regularity.

Thorn Atoll, the home of the megamurd, is series of small island rising from a small seamount that has been formed by the top of a limestone snail reef, raised above the water by a decrease in sea level over the last couple million years. Only ten miles long and eight wide at its extreme, the atoll and the reef it originates from is located in an oceanic desert. Though reefs still surround the island, home to colorful tropical fish and other sea life, they form only a narrow ring in a vast ocean, and any distance beyond has extremely low populations of animal life. There are no permanent nesting colonies of tribbats and only small numbers of native pretenguins which breed on the atoll. The ground there has little soil (for on such islands it is usually produced from animal droppings) and is instead predominately made up of limestone karst covered in deposits of salt. There is also no freshwater upon the island, and scarce rain drains through the rock into the sea without pooling substantially. These factors combine to make Thorn Atoll one of the most inhospitable environments anywhere on Serina during the hothouse age. Yet these factors also make the tiny island one of the most unique habitats in this age; the closest thing Serina now has to a desert, Thorn Atoll is a refuge for an ancient organism and its associated biome that survives almost nowhere outside similar such seaside places: cactaiga.

The megamurd survives here by feeding on the water-storing, spiny stems of low-growing cactaiga plants which can survive only in marginal, coastal habitat where where few other plants are present and able to out-compete them - not even the rockroots have reached this far-flung landscape, and this may be why the cactaiga still persists. A group of living fossils, there are only a handful of species of this once dominant plant group left, and all of them are associated with rocky sea coasts and far-flung islands such as this. Unable to tolerate saturated soil and to grow quickly enough to avoid being shaded by trees and grasses in such a wet climate, these desert specialists from a bygone time have eked out a fragile and precarious existence on the fringes of the hothouse world, taking root widely spread apart on salt-sprayed and inhospitable sea cliffs, though even here facing competition with rockroots.  It is only on islands like thorn atoll, too far out to sea to be reached by insects carrying seedlings, that the cactaiga can still grow in large numbers and form low, prickly thickets across the rocky landscape. Originally spread here in the droppings of wandering birds, which visit the island but rarely stay long - for little food exists - they are now grazed upon by the megamurd, which has grown to its huge size partly to facilitate the easier chewing and digestion of the plant in the absence of its natural seed diet. Its favorite food is the plants' small pink berries - which it eats before any birds can arrive and get any, so that this particular cactaiga is now entirely restricted here and not found elsewhere. But the plants fruit irregularly, so that most of the murd's diet must be the highly unpleasant spiny stems that it crushes in its large bill, getting both food and water from this diet.

Colored the same chalky grey as the island's salt-covered surface, the megamurd avoids absorbing too much heat from the baking sun. It rests by day beneath the thickets or in crags within the rocks, feeding mostly after dark. The island's limited resources and very few enemies - no large predator birds, and only rare-visiting larger tribbfishers, regularly come this far into the ocean desert  - have greatly reduced its reproductive output to just one or two young annually. The total population is stable but very small at under a thousand living animals, only about 150 being mature adults, with most mortality being starvation in the first few years before young are large enough to most effectively digest the very limited diet available to them. Juveniles often exist in a state of chronic malnutrition, barely surviving and growing exceedingly slowly, and require some meat in the diet to survive. They try to catch fish off the shores, but are slow swimmers, and will resort to scraping algae from rocks for extra nutrition, even though this can significantly deform and wear down their beaks. Mothers tend their young for at least a year, and will seek out the eggs and chicks of the few pretenguin colonies which nest sporadically in rocky crags upon the atoll to help feed to her chick during this most critical period. Huge hooked claws are used to dig out nests from stony shelters and to pry chicks out of their hiding places. But these visitors are seasonal, and nests are only around for a few months and not every year, so that nearly all births are timed to coincide with this brief window of increased food availability as best as they possibly can be, though predicting just when the pretenguins will breed is difficult, and sometimes the megamurds time it wrong, losing a whole generation of chicks. When the pretenguins aren't nesting, in the midsummer, the desolate island is left to its only full-time inhabitants - the giant murd, and the bygone relic, together living in their own insular world unlike anywhere else. It is a home ideally suited to neither, but with their lives shaped by forces outside their control and now intrinsically connected, they will remain stuck there together as long as it is possible to do so. 

The pretenguin that ultimately allows both the cactaiga and the megamurd to survive here, linking the meager nutrients of the surrounding sea to the land, is the waif - a bird weighing just 3 lbs. It is the smallest of all pretenguins, an example of insular dwarfism - the opposite phenomenon that has made the megamurd so large. For while the megamurd could afford to be bigger without enemies on the atoll, and could more easily digest the poor quality food there by growing larger, the waif has had to adapt to limited food resources compared to its ancestors. Thorn Atoll is a tiny island in a vast, usually barren tropical ocean and supports only a small fringing reef. There is no annual seasonal change in currents to bring nutrients, meaning the reef must be self-sufficient most of the time; energy used there is deposited there again as animals excrete waste and eventually die, without significant outside input. This closed system is delicate and highly vulnerable to fall out of balance. To survive, the waif has had to become small enough that its caloric needs can be provided for by the small endemic fish of the reef without overexploiting them. It hatches just one chick at a time, in a deep crag in the rocks of the atoll, and the waste products deposited in and around the nest are the only source of nitrogen available to the cactaiga here. After each breeding season, the whole population molts and ceases breeding again for many months in a highly irregular cycle depending entirely on the population of their food. Breeding is usually annual for a few seasons, occurring every 9-15 months, but as the fish become more scarce, it slows to every couple years and may cease for as long as six years. Eventually, however, usually once a decade, a rare change in oceanic conditions brings a surge of cool surface water to the atoll from an upwelling in the Equinoctial ocean. Similar to the El Nino-Southern Oscillation in Earth's Pacific ocean, but less regular, this temporarily condition brings a surge of plankton that results in a great increase in available food for the waif, allowing a very occasional summer of plenty and letting the birds raise several young successively; the numbers of the megamurd also rise at this time, as they prey on the chicks of the waif.

But the good times on Thorn Atoll are temporary. Within 12 to 18 months, the oceans become warm and stagnant again, and the reef's productivity slowly declines. Like the cactaiga and the megamurd, the waif only can survive here, but rarely thrives. A member of a common genus to coastal waters along both continents, the waif descends from a species of larger bird that followed the temporary food-rich current from over a thousand miles to the east and then became marooned there when the current ceased, trapping them along the small reef with no way to escape, about half a million years ago. Its ancestor species, like that of the megamurd, still exists; it is four times heavier, with brighter markings, and nests in huge aggregations along the southern coast of Serinarcta. During surge years, when the island is briefly connected back to the mainlands by a band of food-rich water, some of these birds occasionally follow their prey and reach the atoll like the waif's ancestors did, usually perishing of starvation if they remain too long, for they are simply too large to find enough food there once the surge has diminished. Even more rarely, a single waif has shown up at the Serinarctan nesting colonies among its bigger cousins, having taken advantage of the brief opportunity to flee its isolation. While the megamurd is forever trapped on Thorn Atoll, without any currents in the right direction to take it anywhere but further out to sea, there is evidence a few waifs may manage to escape every surge year, and go on to hybridize with their close relatives. They almost certainly never return home again on purpose once they find themselves in the outside world - though there may be more enemies off the island, it is made up for by never facing starvation. A barren rock in the sea is no-one's first choice of a home, and that anything survives there at all is a sheer testament to the hardiness and adaptability of life to persist through unforgiving circumstances beyond their control.