The Brown Rat (“Norwegian rat”) might be a good contender. Probably native to China or Mongolia, it is invasive and generally outcompetes other native rats worldwide (apart from Antarctica).
Speaking of rodents the house mouse (Mus musculus) has spread to every place humans have.
The globally invasive species that has caused the most destruction of ecosystems is Homo sapiens. There is no other contender. We are the worst thing that’s happened to planet earth since Chicxulub.
However, cats are extremely destructive. Domestic cats derive from a species native to the Middle East, Felis sylvestris lybica (they were domesticated in ancient Egypt). Pigs and goats are also environmental scourges but they don’t touch cats. Rats and mice mostly affect humans, not ecosystems. Cats kill more than two billion birds annually in the US alone. Up to maybe 20 billion mammals. I’ve never met a cat lover who would admit this is a real statistic, and that they are direct contributors to this devastation. They are invariably in denial.
There are plenty of other introduced species which have displaced or destroyed the native biota, whether plants or animals. For example, the Spanish introduced (probably not even on purpose, but mixed with seed grains) the annual wild oat grass to California in the late 1700’s. Combined with overgrazing by their cattle, which killed the primarily perennial grass species, oat grass took over all the grasslands of California. That’s why California turns yellow in summer. It’s practically a monocrop, displacing perhaps hundreds of species.
I’m a cat owner and I don’t dispute the statistics. I can tell my cat wants to murder every living thing he sees, including me if he were big enough. However, I don’t agree to being a “direct contributor to this devastation” because all my cats were abandoned/strays and I never let them outside. No denial there, so please reconsider your stereotyping.
^^^
This. Mine never go outside for the very reasons you named. They are all rescues as well.
Just read 1493, about the Columbian exchange. The author describes a host of such “invasions.” One huge one was mosquitos - and the malaria and yellow fever they carried. Not sure mosquitoes and malaria are good anywhere, but both readily transferred to the new world.
Another groups included funguses such as the one that caused potato blight.
The book makes clear that some of the transferred species became more problematic with changing agricultural/social conditions.
We lived in Italy’s Po Valley in the mid 80s. We rented a 100+ y/o farmhouse complete with feral cats. I found a litter in the barn and when they were about 6 weeks old set out to catch and re-home them. -Bad plan. Those were some nasty little buggers even at such a young age.
The locals despised feral cats. You would often see them as roadkill. We even witnessed a car in front of us swerve off the road to hit one.
Wiki says that Italy made the killing of feral cats illegal in 1991.
Domestic cats fill niches once taken up by predators that we have displaced. Far fewer mountain lions, foxes, wolves, eagles, etc than there used to be. It isn’t like pre-Colombian America was a small-animal paradise.
In addition to what @Darren_Garrison just said:
The cats have been all over the Americas for well over a century by now, during most of which time nearly everybody let them outdoors; and the devastation is far worse now than it was. The primary problem is habitat destruction: which is on the humans, not the cats.
There are certainly specific situations in which cats are a major contributor; in particular on small islands, or in areas that didn’t originally have other mammalian predators filling the niches.
My hat is off to you and everyone who does this.
None of the large predators you name ate songbirds for a living. Cats are more of the weasel family niche – and weasels are equally deadly to birds when released on islands. I knew a woman who once worked for the Hawaiian park service trapping and gassing African mongooses which were decimating the fragile native bird populations. (she said it was the worst job she’d ever had).
Large predators are always rare, because they are at the top of a food chain that loses energy as it goes up. Cats are not apex predators, although they might function that way in a suburban neighborhood. Given a healthy population of apex predators, cats unprotected by humans will disappear rapidly. They are exploiting the disruption in ecology caused by human beings. If they were replacing large predators they’d be eating deer, not songbirds.
I think the brown rat is the species which is usually found in the subantarctic islands, including those where massive baiting programs appear, we can all hope, to be capable of killing them off again.
Yes, the New Zealand Department of Conservation has rat elimination/monitoring programs on the sub-antarctic Auckland Islands, as well as several island sanctuaries close to New Zealand. There are even a few pest-free sanctuaries on the mainland - small peninsulas with extensive fencing and trapping.
While the govt aim is pest-free by 2050, I don’t believe that there is a good way to achieve that without the use of genetic engineering - a gene drive to extinction. Even if a suitable gene drive is developed (I think Australian researchers have identified a candidate in a species of mouse), releasing genetically modified rats, stoats, weasels and rabbits into the wild will be contentious with some groups, I suspect. Then we are looking at larger pests - goats, pigs and deer that will upset the hunting lobby.
Then of course we exterminate the hunting lobby. Certainly a species and certainly invasive, amirite? A modest proposal of course.
Agreed - and in situations like introductions to an island with flightless birds, they can be a devastating and obvious invasive species - I was thinking more of contexts where other technically-invading species might be less obvious - for example in urban settings - rats are still considered pests by humans (thus qualifying more as 'universally invasive) where as other invading species might not even be noticed that much - like scurvy grass - normally a strictly coastal plant, but which has found a perfect niche along urban roadsides (due to roads being constructed of free draining materials, and salted to melt ice)
I don’t know what they displace, but house sparrows are everywhere. Humans are, of course, the #1 globally invasive species, but house sparrows are part of the coterie who followed us around the world.
Earthworms are likely another invasive species everywhere. Well, except where it’s native to, of course. They’re certainly not native to the Americas, for example.
Lionfish have invaded much of the suitable habitat (Caribbean, West Atlantic, Mediterranean) outside their native range of the Indian and South Pacific Oceans. Mainly due to humans (as usual) transporting them around the world as aquarium fish.
They displace other seed eating birds.
I think a case can be made for the idea that what’s really harmful is when species from a very large united ecosystem- the Eurasian continent for example- get introduced into smaller more restrictive habitats. That’s because when a species’ habitat stretches from Britain to China, it is evolutionarily advantageous to evolve traits to take advantage of that roominess: higher fertility, more rapacious eating habits, mobility. Both predators and diseases will also be adapted to these traits, which helps limit the rapacity of a given species. If a local area becomes denuded and suffers collapse there is usually somewhere else to move to, or survivors elsewhere that would eventually recolonize an area once it has recovered. A “Mongol” strategy if you will; plunder and move on.
By contrast the long-term survival traits for a very limited habitat such as an island are virtually the opposite: breed slowly so as to not overrun one’s environment. So since the European expansion of the sixteenth century, Eurasian species of plants, animals and diseases have been introduced to progressively smaller habitats with increasingly drastic results. The Americas suffered somewhat from exposure to Eurasian species, Australia more so, and scattered small islands around the globe most severely.