Just about anywhere you are in the world, you are confronted constantly with invasive species.
Some are so ubiquitous we've stopped even noticing them. Here in the United States, for example, most lawn grasses are non-natives (including, amusingly, Kentucky bluegrass), as are dandelions, daisies, burdock, garlic mustard, multiflora rose, bush honeysuckle, and thistle. None of our domesticated animals are native to North America, but neither are such ridiculously common creatures as house mice, the various species of rats, Japanese beetles, pigeons, starlings, house sparrows, and goldfish.
It's tempting to lump all these species together and say "exotic = bad," but that's a vast, and inaccurate, oversimplification. Some have clearly had devastating effects on native species; feral and owned-but-outdoor cats, for example, kill an estimated two billion birds a year in the United States alone. (Yes, that's billion, not million. Cats are responsible for more bird deaths than any other single cause.) Other exotics have had far less impact; dandelions may be in every lawn in North America, for example, but they don't seem to do much in the way of outcompeting other species. (And, as I said earlier, lawn grasses are exotics themselves anyhow.)
A lot of effort by environmental agencies has been put into eradication of exotics, to varying levels of success. Rats and mice, for example, are generally a lost cause, given their fast reproductive rate and ability to survive on damn near any kind of food; but some isolated islands have done pretty well, most notably South Georgia, which wiped out their rat and mouse infestation in 2018 in order to save endangered birds that nest there.
The southeastern United States, however, has had almost zero success controlling kudzu, also called "mile-a-minute vine" because of its stupendous growth rate. Introduced in 1876, and hailed as a source of browse for cattle and starch-rich roots that could be used in place of potatoes, the vine went on to cover trees, barns, and slow-moving individuals, and to this day blankets acres during its growing season.
Where it gets interesting is the observation by one of my AP Environmental Science students a while back, who said, "But if you go back far enough, isn't everything exotic?" It's a point well taken. Species move around, and introductions happen by accident pretty much continuously. (In fact, there's a whole mathematical model called island biogeography that has to do with the effects of such factors as island size and distance from the mainland on immigration rate and stable biodiversity.) Our own deliberate and accidental introductions are only continuing a process that has been going on for a long time.
A very long time, to judge by the research of Ian Forsythe (of the University of Cincinnati) and Alycia Stigall (of the University of Tennessee - Knoxville). They've been studying the "Richmondian Invasion" -- a sudden influx of new species into the shallow sea that covered what is now northern Kentucky, southwestern Ohio, and southeastern Indiana that occurred during the Late Ordovician, 450 million years ago.
The invasion was surprisingly rapid. Due to exceptionally well-preserved strata, they were able to show that the new species were introduced from the north, as rising seas allowed them to cross what had been a low ridge of dry land, over only a few thousand years. And what Forsythe and Stigall found was despite the magnitude of the invasion, and the speed with which it occurred, it didn't have very much effect on the recipient ecosystem's pre-existing species.
The reason, Forsythe and Stigall say, is that most of the invaders were low on the trophic ladder -- they were filter-feeders and grazers on phytoplankton. It'd have been a different story if the invaders had been high-trophic-level predators.
All of this should inform our decisions on where to put our limited resources for environmental management. High-impact, high-trophic-level invaders -- feral cats, rats, and the like -- are more critical to control than low-level herbivores like pigeons and house sparrows. (It bears mention, though, that just being a herbivore doesn't mean "harmless;" here in the northeastern United States, whole forests of ash trees are being killed by the emerald ash borer, and farmers and viticulturists are rightly flipping out about the wildfire-spread of the spotted lanternfly.)
So it's a complex subject. But it's fascinating that an analysis of an exotic invasion 450 million years ago might inform our decisions about how to manage exotics today. Yet another indication of the value of pure research -- it can give us an angle on real-world problems that we wouldn't have arrived at otherwise.
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