New Gaia Surface
Image from Anders Sandberg

A biome is a major ecosystem that extends over a broad region, has a characteristic climate, and presents a typical common appearance due to the suite of organisms that are adapted to it. In the context of a garden world or a large megahab it is a subset of the overall ecosphere, whereas with smaller worlds or habs a biome and its overall ecosystem may be one and the same. The dominant autotrophic organisms (the vegetation or equivalent) tend to converge on a set of common morphotypes adapted to the local pattern of available temperature, moisture, nutrients, energy flux, air or water movement, and so on, so one may see biomes that are similar even if the particular organisms are unrelated. For instance, 'rainforest', 'desert', 'savanna', 'tundra', and 'reef' biomes arose independently at multiple times and in multiple locations on Old Earth, and analogues of these may be seen even on completely non-Terragen garden worlds. Many other biome types are now known that were either long extinct when humans first developed the concept or were completely unknown on Old Earth, either because that particular combination of abiotic factors never existed or because the characteristic organisms that make the biome possible never evolved.

Since biomes are bracketed by a distinct set of abiotic conditions their extent may be primarily horizontal, as with Old Earth's terrestrial biomes, or may have a vertical component, as with Old Earth's aquatic biomes or To'ul'h's aerial biomes. Worlds or habs that are small or have a narrow range of conditions might have only a few biomes. Likewise if the palette of available organisms is small there will be fewer biomes. Worlds lacking the equivalent of multicellular eukaryotic life, where no structure more complicated than a biofilm or a stromatolite exists, may have the same biome over a much broader area, as seen in the case of a very large number of xenobiont worlds. In the case of terrestrial biomes the absence of vascular plants (or the equivalent) means that only a few biomes are seen even in the face of great variety in climate. This may be seen in the case of Old Earth prior to the Devonian, on xenobiont garden worlds like Rajasekhar or Darwin, or in the extensive lichen-plains of any partially terraformed arean world. The 'felts' of New Gaia are an interesting intermediate case. A full-blown evolved garden world like Old Earth, Chorus, Trees, To'ul'h Prime, Ridgewell, or any fully terraformed garden world may have a dozen or more distinct biomes.

The major biomes known to humankind on Old Earth have been widely replicated by humans and other Terragen clades in the time since; variants on them are considered classics of ecosystem design all through the Terragen Sphere. These include:

Old Earth Terrestrial Biomes

Chaparral: regions too dry to support rapid growth in summer and too cool to support it in winter, dominated by heavy scrub adapted to seasonal fire; moderate animal and plant diversity
Desert: regions too dry for much of the year to support plant growth; dominated by xerophylic plants including succulents such as cactus or euphorbia; moderate to poor animal and plant diversity
Mangrove Swamp: tropical intertidal regions dominated by low-growing salt-tolerant forest; high abundance and moderate diversity of life
Midlatitude Forest: cool regions wet enough to support trees but not warm enough to support growth in every season of the year; moderate diversity of plant and animal life
Midlatitude Grassland: (steppe/prairie/pampa) cool dry regions covered primarily in grasses and herbs, with some scattered trees; moderate animal and plant diversity
Taiga: very cool conditions with sufficient moisture to support conifers; low diversity of plant and animal life
Tropical Grassland: warm regions that support plant growth in most but not all seasons of the year, dominated by grasses; high diversity of plant and animal life
Tropical Rainforest: warm regions wet enough to support trees that grow in every season of the year; extremely high diversity of plant and animal life
Tropical Seasonal Forest: warm regions wet enough to support trees but with a dry season in which growth slows or stops; high diversity of plant and animal life
Tundra: very cold regions that do not permit the growth of any plant more than a few inches high due to cold dry winds and frozen soil; dominated by lichens and very low-growing plants
Wetland: land covered in shallow water for most or much of the year, dominated by grasslike forms (Pantanal, Sudd, etc.)

Old Earth Marine Biomes

Benthic: sea bottoms below the photic zone; high diversity but low abundance of life
Coral Reef: warm shallow regions with high light penetration, based on coral formation; extremely high diversity of life
Estuarine: regions where fresh water and sediment enter the marine environment; high abundance and diversity of life
Kelp Forest: shallow cool littoral regions rich in macroalgae; high abundance and diversity
Littoral: intertidal zones to edge of continental shelf; high abundance and diversity of life
Pack Ice: cold regions where ice accumulates on the surface of the ocean for long enough to form a distinct environment
Pelagic: ocean surface away from upwellings and in deep water; low abundance and diversity of life; often into regions according to depth (epipelagic, mesopelagic, bathypelagic, abyssopelagic, hadopelagic) or temperature (tropical, temperate, arctic) which may be regarded by some as biomes in their own right
Upwelling: regions where nutrient-rich water rises to meet the photic zone; high abundance of life

Old Earth had other biomes in the past that were extinct by the time that baseline humans evolved, either because their characteristic organisms no longer existed or because the conditions that gave rise to them were no longer found in extensive areas of the planet's surface. These included polar gingko forests, rudistid reefs, fern savannas, lepidodendron swamp-forest, epeiric pelagic seas, and so on. These biomes have been reconstructed from lazurogened organisms with varying degrees of authenticity; some of the most extensive reconstructions are in the custody of the Jurassica Institute.

Some xenobiont biomes can often be categorized as variants of the Terragen types. The grasslands of Whisper or the rainforests of Dante are examples of this. Others have no clear analogue on Old Earth. This may be because the prerequisite abiotic conditions are not present or not widespread, as with the saprophytic surface biomes of To'ul'h or else because the necessary evolutionary innovations never arose in Terragen history, as with To'ul'h's cloudforests or Trees' equatorial skywrack forests or Orwell's 'bamboo seas'.

Some Terragen habs have biomes that cannot exist on a planetary surface. This is true of the many varieties of microgravity biomes, or of the rimwall biomes found on Bishop Rings and Banks Orbitals.

Mechosystems may show regions analogous to biomes. Most of these exist by design, since botworlds of Terragen origin have not had time to evolve the complexity that allows something comparable to a biome. However the evolved xeno botworld of Stanislaw has a number of distinct regions that might be called biomes (or mechomes), some with no clear analogue in the biological world.

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Development Notes
Text by Stephen Inniss and M. Alan Kazlev
Initially published on 08 October 2001.