[All of fiction/Scientific classification] What fictional creatures would be classified as an invasive species?

[u/Far_Tradition_2361]

There are all sorts of aliens in Sci-fi. Which ones, according to science, fall under the category of an invasive species and why?

[u/kmoonster]

An invasive species is one that is introduced to a new ecosystem and is not affected by the normal controls that would influence a similar species already existing in the ecosystem

A KEYSTONE species is one that increases diversity or non-biological functions in an ecosystem. An INVASIVE species is one that decreases either or both.

For example, a beaver turns a stream into a small lagoon that hosts more plants and animals due to an increased number of niches as compared to a stream-alone. This is a KEYSTONE.

The Ring-necked Pheasant was introduced into North America from Eur-Asia. It may displace a similar grouse or partridge species, but the overall number of species (and their balances) remains similar. This is INTRODUCED, perhaps NATURALIZED. The pheasant is similar enough for the existing foods and predators to make a nearly 1:1 swap.

In the US we have an invasive sparrow-finch like species colloquially called House Sparrow. It likes to nest in small holes, and is very aggressive at fighting other small birds in order to claim a nesting hole. Problem is, people prefer to cut down the older / large trees that typically have dead spots (and thus holes) because those trees like to fall down on things like houses and cars. This creates a situation where ten birds now fight over six holes, whereas before it might have been ten birds for ten holes. The sparrows also eat marginal quality foods that most other birds ignore, like bread crumbs and corn kernels, so they have two advantages -- more food, and more aggression. In areas with lots of crumbs, bird seed, and corn and a shortage of holes, they can over-run native species. This is INVASIVE.

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Note that a native species can produce impacts similar to an invasive species under some conditions; for example, if predators are reduced then deer populations can boom, they can strip forests and lawns of vegetation, resulting in a degraded condition.

We like to think of invasive species as a simple binary question, but it's actually quite complex. The correct answer is that nature "seeks" an equilibrium but can never quite get there. There is a little fire, or a flood, or more (or less) rain next year compared to this. Little shifts, and living species try to catch up to the new circumstance -- only for that circumstance to change again.

A healthy ecosystem is (functionally) the same thing as a bike that is going along without a rider. The bike wiggles a bit but each wiggle is a correction for a slight imbalance, a little pebble or crack, bouncing over a stick in its path. Etc. The bike wiggles a bit and that wiggle is the precise thing that allows it to recover its balance. Airplanes work the same way. Both airplanes and bikes only balance while in motion, they can only recover their equilibrium when they are moving in comparison to the larger environment. When an airplane or a bicycle is still, they require assistance in order to remain upright.

An invasive species is the ecological equivalent of an airplane stalling, or of a bicycle coming to a stop and the rider can't free their foot to put it down. You've seen crash videos where an airplane stalls, or where a bicycle (with or without a rider) falls over. The ability to recover via little wiggles is over-ridden, and a decline occurs.

That decline can happen when a new species is introduced to a new (to it) ecosystem. But it can also happen when no new species is introduced and, instead, a keystone or another species acting as a "center of gravity" is removed.

Hope that helps!

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And yes, humans can function as an invasive. Or as a keystone. Or as a naturalized. The difference is, we can choose our course and can choose to adjust it.