Species don’t exist in a vacuum. That is, if you go nearly anywhere on this planet, you’re not going to find just one form of life. You’re going to find several, all filling different niches and frequently interacting with each other. (I say nearly because I don’t know whether the extremophile bacteria in the Earth’s crust are one species or several. I’m betting there are a range of them in any given location, given how resilient and diverse life is.)
The most familiar relationship between species is probably that of predator and prey. There’s the lion and gazelle, the wolf and the caribou, the anteater and the ant. We’re familiar with parasitism too—one species feeding off another without killing them first. It’s as easy to cast Earth parasites as villains as it is to cast predators. Parasites are often widely known, affect humans, have historical impact, or get handy evil-sounding names. Examples of the first three categories are fleas, mosquitoes, intestinal worms, ringworm, lice, and insect-born diseases such as malaria. Examples of the last one include strangler figs, vampire bats, and the zombie ant fungi that have been in the news lately.
Stargate’s Goa’uld, Spider-man’s Venom, and the xenomorph from Alien are examples of fictional parasitic antagonists. There’s a list of other made-up parasites on Wikipedia, though it’s probably incomplete. That said, I think we could go further. I’m not sure I’ve seen a bio-apocalypse or bio-thriller with protozoa or insects as a vector, though I’ve seen them with bacteria and viruses. And we shouldn’t forget the parasites that don’t affect humans. Some insects, such as wasps, lay eggs in other animals. A number of vines choke the life out of their supporting plants. Who knows what other kinds of parasites might evolve on other planets? Or if an alien parasite could use the strangling or egg-seeding techniques on humans?
Discussion of parasites leads us into other types of symbiotic relationships. (Yes, parasitism is a form of symbiosis.) There’s mutualism, where both species benefit. There’s commensalism, where one species benefits and the other is neither helped nor harmed. There’s also amensalism, where one species inhibits or kills off another but isn’t affected itself. Penicillium mold does this with some bacteria, for instance, and some plants produce substances that kill off competing plant life.
Mutualism can involve trading resources (think of nitrogen-fixing bacteria in plants), trading a service for a resource (pollinators, remora and sharks, animals dispersing seeds), or trading services (clownfish and anemones, ants nesting in trees). Humans are in mutually beneficial relationships with the bacteria in their intestines, and with domesticated animals.
Examples of commensalism include the cattle egret, which feeds off the insects stirred up by grazing cattle; barnacles, which attach to animals and plants as well as rocks and ships; plants that use other plants for support, such as orchids or moss; and hermit crabs, which use shells as housing.
Of course, the plants, animals, fungi, protozoa, and bacteria that engage in symbiotic relationships continue to evolve. They become better parasites, or better killers of parasites, or better nitrogen producers, or better protectors of their symbiotes. They’ll change size or shape or color or biochemistry. A change in one often means a change in the other. Symbiotes may even suffer a disability if their counterpart is removed. Lichen wouldn’t even exist if you separated the fungi and algae that form it, and removing a symbiote from an ecosystem could cause a cascade of species deaths and ultimately destroy the ecosystem.
Some questions that may spark a story or two:
- What happens if you introduce an alien (let’s say, truly alien) species that becomes a parasite or other symbiote to a native organism?
- Could you bioengineer a lifeform to enter into a symbiotic relationship with a plant/animal that needed a boost? Could you turn parasitism into mutualism?
- Could you alter a symbiote’s genes to give it freedom? Would there ever be a circumstance where you’d want to do that?
- What if one intelligent species was oppressing its equally intelligent symbiote for, say, eating insects instead of plant matter or having a strange physiology?
- Since symbiotes tend to co-evolve, pick a possible resource or service that a species could provide, the crazier the better, then create a species that would make use of it. Remember that it will likely also be providing a service or resource for the other species. (E.g., a mollusk that feeds off electricity produced by electric eels; a plant that grows on a herbivore’s head and acts as a sound amplifier; an insect that cultivates a particular plant so that its eggs can hitch a ride on the seeds)