"Biotic pollution, the introduction of a foreign species into an area where it is not native, often upsets the balance among the organisms living in that area and interferes with the ecosystem's normal functioning. Unlike other forms of pollution, which may be cleaned up, biotic pollution is usually permanent." –Solomon, Berg, Martin. Biology.
Removal of a native species or addition of an exotic species usually has drastic consequences on the local ecosystem. When it comes to additions, Phragmites, Starlings, Russian Olives, Salt Cedar or Tamarisk, Zebra Mussels, and Cheatgrass all come to mind. When it comes to removals, I think of the Gray Wolf.
Now that the Gray Wolf is absent in the Southwest, ungulate herds runs unchecked; when Elk and Deer numbers are too high, they destroy native vegetation. Since the Gray Wolf has been eradicated, coyote populations have increased, which decreases the population of small game such as rabbits, voles, etc. When there isn't enough small game in an area, it affects Red-Tailed Hawks, Golden Eagles, and other birds of prey. It's been known for a long time that the removal of an ecosystems top predator will greatly affect that ecosystem.
The addition of a species to an ecosystem can also have dire consequences. The Great Basin ecosystem has been evolving to its present state for millions of years. The organisms here have been coevolving with each other for hundreds of thousands of years. Everything worked pretty well together before we started shuffling the deck.
Recently I've been worried about the Tamarisk Beetle. The Tamarisk Beetle is a Eurasian species of beetle that eats (you guessed it) Tamarisk. So what's the big deal? The beetle eats and destroys Tamarisk, which is a non-native, exotic species; when they kill all the Tamarisk, the beetle dies off too, right? What worries me is that the Tamarisk beetle could evolve to exploit other food sources. Many beetles in the history of the world have shown such an ability to evolve in short periods of time. What would happen if the Tamarisk Beetle suddenly evolved into the Fremont Cottonwood Beetle?
So why would it hurt to introduce a Starling (I have to tie this into birds somehow)? Birds have been coevolving with their environments for the past 66 million years. In a way, birds are genetically programmed to kind of know what to expect throughout their lives. They were anyway. We introduced European Starlings to the United States in 1890; that's 120 years ago. Our native species have been coevolving with their environment for the past 66 million years, and 120 years ago we threw them a huge curveball.
One thing that impresses people about Starlings is their ability to outcompete other species and survive in a hostile environment in which they are not endemic. To me, that's what is scary about Starlings: They outcompete native bird populations. Starlings are cavity nesters that outcompete native species of birds. I think it's safe to state that most of our cavity nesters in the state of Utah have been affected by the arrival of European Starlings. It's not about one species displacing another. This very well could be about 1 species displacing 20 or 30 other species. Here are just a few, off the top of my head, which I think could be affected by Starlings: Lucy's Warbler, Hairy Woodpecker, Downy Woodpecker, Northern Flicker, Western Screech-Owl, Northern Saw-Whet Owl, Wood Duck, American Kestrel, Purple Martin (not in Utah), House Wren, etc. I'm sure there are many more. I also read a study on Starlings displacing Gila Woodpeckers in Arizona.
Starlings are terrible for our native bird populations, and I think they should be managed a little more intensively. Should we all go buy bb guns and take out a handful on a weekly basis? Maybe. Should we destroy their nests when they set up shop in our Kestrel or Bluebird boxes? Probably. Should we knock their nests out of our roofs, trees, buildings, and bridges? Probably.
Exotics don't belong in Utah. The Utah ecosystem has shown that it isn't fit to deal with exotic, introduced species. The more that we introduce, and the less that we control the ones we already have, the worse it will get.