I recently read a claim that the domestic cat is an invasive species worldwide. Such a thing never occurred to me before, and it both feels completely wrong and makes perfect sense. Are there living beings on this planet that are considered nonnative to, and harm, every ecosystem that exists? If so, where do such species “belong”?
(And yes, I have considered humans; Discourse actually suggested a thread from 23 years ago discussing it, but no conclusion or consensus was really reached.)
I guess there are ecosystems that cats are native to, but not in the numbers of domestic cats. And it isn’t the same when it’s a domesticated species. I mean, domestic cats are pretty close to their wild ancestors, as I understand it, but we feed them and keep them healthy and safe, in a way that puts them in a different position in the ecosystem.
Almost all invasive species were brought by humans. I recall a story of a cat owned by a man working in a lighthouse in New Zealand that made a species of bird extinct (his cat killed the last ones).
Bilge water from ships is another big problem.
The list is long.
If one assumes life started at a single location (or a limited few locations), then every species is or is descended from an invasive species. It’s really a matter of choosing some sort of defining characteristic that makes something “invasive”.
I guess you could say a species is invasive if it didn’t evolve in its current location. But some species have been around for millions of years. Are they really invasive if they have been somewhere for 100,000 years?
Alternatively you might say a species is invasive if it is a relatively recent arrival that is (as a consequence) causing change in a certain area. This is, I think, the only really meaningful definition.
Humans and the animals they have spread would be the key players if you apply this definition and think of animals that are invasive in most places. Pigs, cats, and rats for a start.
This SMBC cartoon is pretty funny but it does raise a serious point – the dog was intentionally domesticated for work and companionship, and while it’s genetically very close to the wolf, it isn’t identical. A dog’s bond with its human is a direct result of millennia of selective breeding. It’s not clear (to me, anyway) what the domestic cat’s genetic relationship is to its wild ancestors, or whether selective breeding has been for any purpose other than appearance.
The Fire ant.
At least cats bring something to the game. They purr, they’re funny and highly pet-able(all except the Siamese*).
Fire ants, as far as I can tell do nothing but harm. Man or beast, we’re tormented by them. They can actually kill.
Farmers fields suffer from them.
I know of a house that was deemed uninhabitable because of fire ant hills in the crawl space.
Invasive is not even enough of a word for them.
FWIW it seems cats domesticated themselves:
A comprehensive survey of cat genes suggests that even after felines wandered into our lives, they remained largely unchanged for thousands of years.
< snip >
In a new comprehensive study of the spread of domesticated cats, DNA analysis suggests that cats lived for thousands of years alongside humans before they were domesticated. During that time, their genes have changed little from those of wildcats, apart from picking up one recent tweak: the distinctive stripes and dots of the tabby cat. - SOURCE
They don’t fit the OP’s requirement of universal spread, though.
I think for something to be considered harmful everywhere, that would have to be from the specific perspective of humans (that is, something that humans consider verminous or pestilent) - everything came from somewhere and without humans, nature just does a combination of sometimes balance and sometimes ebb and flow - that is, without our intervention, species would still seize opportunities at the expense of other species.
for example if a new volcanic island emerges and life gradually takes hold there, that initial colonising life is invasive only of an empty niche; if, later, other species drift in and overwhelm some of the initial colonisers, those species would be ‘invasive’, but in a natural sense. If, later still, humans arrive and accidentally or deliberately introduce further immigrant species, and those overwhelm the existing ecosystem, those would be invasive in the common modern sense of the word.
Rats might be an example of universally invasive species, but only because we tend to regard them as unwanted in any and every place we see them.
I’m surprised the tabby markings are described as a recent tweak (although I suppose it depends what is meant by ‘recent’) - Scottish wildcats look almost exactly like big tabby cats
Rats have voracious appetites, can and will eat nearly anything, and quite famously have an ungodly fertility rate. They have wiped out many species of ground-nesting birds, for example.
And that’s coming from someone who likes rats!
I’m not at all sure that the dogs didn’t also.
It often seems to be phrased as something that humans did to the wolf, all one sided. I suspect it was a cooperative endeavor, and may well have changed both species.
Other have noted we need to define or else quibble about what “global” and “invasive” mean.
I’ll add that we also need to define or quibble about what “species” means. The concept is vague once you look at it closely enough. As a quick spelunk through wiki will tell you. How different is different enough arising over what timeframe from what causes?
Big picture I’ll argue that the Trifecta of “globally invasive species” pretty well means:
Humans, or critters introduced by humans into nearly every human-inhabited area on Earth, that:
- are successfully established as a wild population there, AND
- are out-competing at least some other species that predated human arrival / importation.
Under that criteria I’ll suggest that rats, cats, and dogs (in that order) seem like leading contenders after humans themselves.
My theory is that it was more that cats conquered us, as mostly benign invaders.
Details: Everyone knows that cats are comfort loving creatures. Everyone knows that the natural environment is actually not full of comfort, at least what cats consider comfort. It doesn’t offer plentiful occupation sites that are safe, totally protected from rain and snow, maintained at a good temperature year round, with potable water available within a few yards, and occupied with ample supplies of the meal sized vermin for the taking.
As soon as humanity started farming, we did. Even the most primitive hut generally has a roof that keeps out rain, a fireplace, furniture that can be lain upon or hidden under. Plus human servants that maintain a fresh water supply and do our best to keep predators out. And endless supplies of rats and mice which we don’t even utilize!
Of course cats saw this valuable new niche and moved in, why wouldn’t they? At worst they have to put up with being petted sometimes, and even that’s not all bad since our laps are even warmer than the rest of the huts generally speaking.
You can still see this process going on today. An ‘unhomed’ cat will often start coming around the house of a human, evaluating it as a possible abode. Does this human behave ‘properly’, as in not yelling at the cat or attempting to attack it? Does it not freak out when the cat starts lounging brazenly on its porch or patio? Are there no dangerous other occupants like dogs? Does the human start leaving food out for it, supplying a water bowl?
After a few weeks, if the evaluation criteria are met, the cat will simply stroll into the house and claim ownership.
Trust me on this. I’ve been conquered three times in my life.
Well, ok.
Down here in the south they’re everywhere.
This page has a map showing the distribution of fire ants in the US. They’re pretty much contained to the southeast, between Texas and North Carolina. Thankfully. Although I’m concerned to see a couple of reported sightings in southern California.
At least some petting is definitely on the positive side from their point of view. Mine show plenty of indication that they’re strongly in favor of it – at the times and in the manners of their choosing, of course. (As, I note, is generally also true of humans.)
Absolutely not true! Everything else on the planet is an invasive species that cat’s just have to tolerate 
The cat who lives on my mom’s porch literally will not let you feed her, until after you’ve petted her.
Not global, but for North America, the goddam Canada Geese!