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I am building a fantasy world roughly the same size, elemental makeup, and atmospheric composition as Earth. In this world, there is an island (about the size of Australia) known by outsiders as the Cloistered Realm for its extremely high rocky walls 100km in height which bar entry from all sides, save for two cavernous passages.

At one point, it was a normal island whose southern coast is about 60° south of the equator. After a mysterious, cataclysmic event ~5000 years before the start of the story, it is now sparsely populated by humans, mineral rich, has oodles of volcanic activity (similar level and type of volcanic activity to Yellowstone Park), and an everlasting night (the sun, mysteriously, never rises within the walls, but the moon does appear normally - the length of time the moon stays in the sky varies on the time of the year).

How might the wildlife adapt to such an area? What might it take for large life forms (dense forests, grassy hills, predators the size of large bears, etc) to survive/evolve?

EDIT: Magic of six varieties (light, shadow, fire, water, wind, and earth) is readily available, even to wild animals, plants, and fungi. It is "fueled" by the same biological energy that these creatures use for their own physical processes and all creatures are limited to one of these six varieties.

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    $\begingroup$ Just use water and earth magic if you want trees over 32 feet tall. There. Solved. $\endgroup$ – Serban Tanasa Aug 4 '15 at 0:06
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Not much of anything will live there.

The walls are three times higher than the atmosphere. They're in danger of hitting satellites. As such, the area will be in the shade except for a certain few days in the summer. This means it's going to be very cold (it's already very far away from the equator) and very unlikely for any large plants to grow there. Hopefully there wasn't much fire during the mysterious event because, without large plants or a mixing of atmosphere, there won't be much oxygen for any animals that survived the event. The volcanic activity will likely burn up quite a lot of the oxygen produced by small plants.

So, the area will be fairly dead and it will be deadly to any explorers. If humans do live there, it's very near the entrances. As the magic you're using relies on the energy from the animals, it won't really exist there, because there won't be much energy. There wouldn't be enough time for any large creatures to evolve to use the volcanic energy, it's a fairly high entropy energy source, so is not actually very useful to life.

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If there is no light, there are no plants. There can be plant shaped things, fungus, bacterial mats, even possibly lichen. There could be plenty of energy being released into the environment from the volcanic events, both as heat and chemical, to help life.

Some of these forms of life might even have bioluminescence. Which could be used for both attracting prey and pollinators or animals to eat fruit to spread seeds. Like watering holes, predators will hang around light sources to catch the unwary.

Most animals will likely have enhanced hearing to try and hear predators and prey, like wise smell will become much more important too, you can't hear something that isn't making any sounds, but it will still have a scent.

The diversity that exists, will depend a lot on the available food sources and biomass available to be exchanged. How much energy and nutrients come from the vents? Mix in a little magic and it helps out a lot. There might be some 'plants' or animals that are efficient at turning inorganic material, (sand, rocks, etc) into living flesh that others will eat.

BTW you will need magic because 100KM wall can't exist without it. Mount Everest is about the tallest something can be on Earth. Olympus Mons is only 25KM and at 1/4 the height is almost maxed out what Mars can support, with regular physics.

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Photosynthesis for plants, and solar protection for animals (melanin, dark fur etc.) are both useless in this region due to the lack of sunlight. I doubt that 5000 years is enough to evolve out any now-unnecessary traits so that surviving humans are likely to be fairer than their ancestors but not albinos. Unlike these, green plants might be extinct, replaced by fungi or lichens etc. that can subsist on the energy from frequent geothermal activity.

Large herbivorous animals switch over to feeding on lichens or algae, but the great reduction in food (green plants mostly dead) would also cause great losses among them due to lethal famine. Carnivores are impacted worse higher up on the food pyramid, as there is insufficient prey which means having to hunt for longer to find less energy/food.

Reptiles would be dominant over warm blooded mammals due to less expenditure of energy to regulate temperature. Possibly the largest creatures would be giant snakes that can eat algae when needed and hibernate most of the time.

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