Chtorrans are a collective group of alien species who invaded Earth in the 21st Century. They are an Alien Ecology, who invades planets using their advanced ecological tactics.
Set primarily in a devastated early 21st Century America with logical expected advances in current technology such as a fledgling Moonbase, this series of science-fiction novels describe the invasion of Earth by an alien ecology. The story is unusual in that the tactics used by the aliens eschew the usual direct attack in favor of terraforming the ecosystem.
A series of devastating plagues sweeps the world, killing 80% of humanity. As the survivors struggle to rebuild civilization, they discover that hundreds of alien plant and animal species have begun establishing themselves. They are almost universally far superior to the native organisms which occupy the same ecological niches. As a result, Earth's entire ecology is being rapidly supplanted (or "chtorraformed"). The invaders are called Chtorrans after the sound made by the most deadly predator encountered so far.
There is no sign of sentient aliens, but humans presume the invasion to be deliberate, either "seeded" from space or brought by undetected spacecraft. Many of the Chtorran organisms (see below) exhibit behaviors that are quasi-sentient (building structures, creating and using tools, farming/herding, setting traps, singing), yet the central question of whether they are doing so out of sentience or collective and programmed behavior is unanswered. With each new layer of organisms, a bit more hierarchy to the Chtorran "societal" structure is revealed, allowing the possibility that all these organisms will transform the Earth in support of some worse, higher form of Chtorran life. The presumed goal of these off-stage aliens appears to be nothing less than the complete replacement of Earth's entire ecology and the resultant extinction of all native life, at which time they can claim the planet without a single shot needing to be fired. Another possibility is that collectively the Chtorr are the alien, the invasion has begun and gained major footholds, and humanity has yet to figure out who the true enemy is (let alone how to successfully fight it).
Chtorran ecology was designed in large part by British reproductive biologist Jack Cohen. It is quite complete and consistent, making it hard science fiction.If there are two things that all Chtorran life forms have in common, it is that they are hungry, and are normally a bright shade of red. Some of the most notable plants/animals that are encountered are:
Chtorran Gastropedes (commonly known as "Worms"): Range in size from as small as a horse, to as large as a bus. Normally bright red, but have been seen in other colors ranging from white to green. They have two "arms" with claws, and are very dexterous. Their bodies are covered with symbiotic "fur", each strand of which is a distinct lifeform and acts as a sensory input. Though capable of remarkable acts of problem solving and interaction, both their intelligence and their communication abilities have yet to be fully understood.
Bunnydogs/Bunnymen: Bipedal herbivores about a meter in height possessing roughly the same intelligence as chimps. Bunnydogs are covered in the same "fur" as the worms and resemble pink or red rabbit-eared teddybears. Some Bunnydogs eventually become Bunnymen—a variant of bunnydogs that lack that fur and are about half again as tall, making them resemble bipedal rats—with a disposition to match. All bunnydogs/bunnymen are male.
Libbits: These quadrupeds resemble pigs. Short and squat, they are covered with red fur and are very docile. All Libbits are female. It was later discovered that Libbits and Bunnydogs/Bunnymen are actually male and female of the same species. Their reproductive process is strange—if a single Bunnyman mates with a Libbit she will give birth to a litter of Libbits, but if multiple Bunnymen mate with a Libbit, she will produce a litter of Bunnydogs.
Gorps: These large creatures resemble sloths, but stand 10–15 feet tall. They emit a horrible smell (capable of destroying non-Chtorran life), and are mostly carrion eaters, though they do not always wait for their meals to die before they eat them.
Finger Babies: These small creatures are from one to two inches in length, and closely resemble humans. Their ecological niche appears to be that of an insect.
Enterprise Fish: These are the largest Chtorran creatures known. These creatures go through the oceans, eating anything in their path. The upper size is unknown, but they have been blamed for the near-sinking of the USS Nimitz. The largest one killed massed over one million tons, and far larger ones have been sighted.
Millipedes: These bear no relation to terrestrial millipedes. Though similar in appearance, they tend to have hard black shells and bellies of varied colors. Their bite is very powerful, and they are capable of devouring and digesting wood, leather, rubber, plastic—anything even remotely organic save their own (or worm) waste. Their bite is also toxic, introducing various diseases and micro-organisms into the bloodstream. The worms keep them in "corrals" near their huts like chicken farms—and eat them like popcorn.
Shambler Trees: Chtorran trees that act as hosts to countless separate species of predatory creatures identified as "tenants". The trees are mobile, able to cover up to a half mile a day and leaving a root network of "tickler nerves" in their path. The "tickler nerves" alert a tree to approaching prey, which triggers the tenants to swarm and feed. Once the tenants eat their fill, the prey's remains will fertilize the root network. Ultimately , the trees become a Clonal colony or "Shambler Grove". They are a complex and key element of Chtorran ecology on Earth.
Red Kudzu: Similar to the terrestrial plant, but red in color. This plant ranges from jet black to pink, red, violet, and orange. It grows extremely quickly and can cover an entire town in weeks. Its greatest danger is in its ability to shelter Chtorran predatory life—attempts to control its spread through napalm strikes are negated in days by further growth, but massive numbers of human remains have been found before it reclaims the scorched ground. All attempts to destroy it have failed, and it appears to be resistant to all known poisons.
Meeps: These are red furballs, ranging in size from tennis ball to bowling ball in diameter. Like another alien fuzzball envisioned by Gerrold, they reproduce very quickly, and possess many traits which induce adoration in other species. In the wild, mother animals of many species will actually abandon their young in order to take care of meeps—a potent adaptation indeed.