It'd be a nice essay...if he had any clue what he was talking about. The entire thing reeks of lack of understanding and simple lack of research.
First, the issue of dispersal and range expansions in nature vs those induced by man. In spite of the author's ill-informed claims to the opposite, there are three main differences: distance, speed, and continuity.
Natural range expansions are slow, short-distance, and continuous (at least in the sense that a species cannot colonize habitat farther away than an individual can travel). This can be due to a variety of factors, but it means that the new species come with some important hitch-hikers: parasites. These keep the new species from enjoying too much of an advantage. In contrast, human-induced range expansions are discontinuous, covering vast distances in short periods of time, and consequently breaking the life-cycle of any parasites the exotics carry, leaving them free of a major natural check.
Second, the consequences. Introduction of a new species will often cause extinction, and it doesn't matter whether humans or nature is responsible. Google "The Great American Interchange". When the Panama isthmus rose, it allowed large mammals from north and south america a chance to invade, and they did. When did you last see a litopteran or a toxodont? You haven't outside of a museum; this natural invasion wiped out much of south america's distinct fauna. Ecosystems recover and re-diversify, but the unique species lost will never be recovered.
Additionally, lumping 'exotic' with 'invasive' is fallacious. Not all exotics are invasive; some simply cannot meet the demands of the habitat, or reproduce, or manage to accumulate sufficient numbers. My boa is exotic, but could not be invasive; if he escaped, one Ohio winter would kill him. Additionally, few enough boas would escape in Ohio in any year that there could be no wild babies; the snakes would never find each other (though this is not always true, see the Burmese pythons in Florida). There are snapping turtles in the wild in England, and in good numbers, and they're breeding...but it never gets warm enough for the eggs, so they never hatch. Only some species can become invasive, and identifying these is an area of great current research.
The most egregious error of the essay, however, is the outright lie that invasive exotics have never caused problems. In australia, cane toads eat everything that moves, endangered or not, and have no predator that can withstand their toxins. In the USA, zebra mussels choke out native species, replacing the diversity of many species with a single one.
This is also something I have direct personal experience with, as I conducted fieldwork last summer on an invasive species in Guam, the brown tree snake.
For those unfamiliar, Guam is a small pacific island in the middle of nowhere. Like most such islands, it had a wide variety of amazing birds. A few introduced species showed up with the polynesian colonists, and a few more from the Spaniards who took it over. Then, in WW2, Japan took it over. Eventually, the US won it back and, due to the location, established major naval and air bases there (to this day, the entire northern 3rd of the island is Anderson AFB). And that's where the trouble started.
Sometime around 1950, several brown tree snakes (Boiga irregularis) stowed away in cargo containers and arrived in Guam. This is a species of mildly-venomous, constricting snake which is an excellent climber, swimmer and crawler, can reach over 10 feet long, and reproduces extremely rapidly. By 1970, the island's bird population (which had evolved in the absence of snakes, and thus did not have any aversion to them as predators) was in free-fall. By the 90's, almost a dozen species of birds had been eliminated from the island, some now extinct in the wild as a result. Faced with a lack of food, the snakes began turning to the lizards and anything else. Large adults have snuck into houses and attempted to kill and eat *human* babies. They climb into powerline transformers, causing island-wide blackouts to the cost of millions of dollars.
From personal obervations: I've caught snakes in florida and the amazon, and *nowhere* had abundances like Guam. In the Amazon the entire group found a dozen snakes in a week. In Guam, I found 15 in *two hours* just walking along the side of the road with a headlamp. I literally almost tripped over one. (The offical estimate of their density is 14,000 snakes per square mile. 14,000!) And there are almost *NO* birds. Even the marine birds are infrequent, and if you don't count those, I saw 6 birds in 3 weeks (two sparrows, a pigeon, and 3 of a native crow).
A formerly complex and diverse ecosystem has been reduced to a handful of species, almost all of which are eaten by this snake.
Oh, but wait, there's more!
You know Hawaii? Think it'd be a nice place to visit, with pretty tropical birds and plants to see? Go there soon, because it'll be like Guam within 50 years. Guam is a major air and sea hub to Hawaii, and the snakes aren't content to stay in Guam. They've been found in boxes in Hawaii, the Phillipines, Japan, Texas, and *Spain*. The snake-sniffing dogs recently found one in a box that had been on Guam for 15 minutes, and most boxes don't get inspected. If they get to Hawaii, you can kiss every bird species there goodbye, along with all the plants they polinate and disperse the seeds of.
Will the ecosystem fix itself over time? Sure. But in the meantime, we severely damaged it through carelessness. And nothing will ever bring those species back. Not to mention the economic costs. How popular do you think Hawaii will be when it's covered in 10-foot, aggressive, mildly-venomous snakes?
The entire essay you linked to is nothing but one man with an axe to grind undergoing excruciating logical contortions in a vain effort to make his perspective seem supported.
Many years ago, the modern environmental movement was kicked into gear with the publication of 'Silent Spring'. This book laid out evidence that pollution, especially with insecticides, was destroying the bird and amphibian populations, and if unchecked, it would lead to a 'Silent Spring'.
I've stood in the forests of Guam, with nothing but nature for miles, and heard that horrible silence. Such is the power of invasive species, more than our most toxic chemicals.
Mokele