Weed does not exist, as noted earlier, but David Quammen's 1998 article The Planet of Weeds' asks for closer thinking on the subject. As he is finding out about predictions for expected loss of species in the coming decades he also learns about the natural world we will end up with. As many specialized animals and plants will vanish a few robust and opportunitic species will step in and do even more damage: the weed species.
Listen to David Quamman:
What do fire ants, zebra mussels, Asian gypsy moths, tamarisk trees, maleleuca trees, kudzu, Mediterranean fruit flies, boll weevils and water hyacinths have in common with crab-eating macaques or Nile perch? Answer: They're weedy species, in the sense that animals as well as plants can be weedy. What that implies is a constellation of characteristics: They reproduce quickly, disperse widely when given a chance, tolerate a fairly broad range of habitat conditions, take hold in strange places, succeed especially in disturbed ecosystems, and resist eradication once they're established. They are scrappers, generalists, opportunists. They tend to thrive in human-dominated terrain because in crucial ways they resemble Homo sapiens: aggressive, versatile, prolific, and ready to travel. The city pigeon, a cosmopolitan creature derived from wild ancestry as a Eurasian rock dove (Columba livia) by way of centuries of pigeon fanciers whose coop-bred birds occasionally went AWOL, is a weed. So are those species that, benefiting from human impacts upon landscape, have increased grossly in abundance or expanded in their geographical scope without having to cross an ocean by plane or by boat--for instance, the coyote in New York, the raccoon in Montana, the white-tailed deer in northern Wisconsin or western Connecticut. The brown-headed cowbird, also weedy, has enlarged its range from the eastern United States into the agricultural Midwest at the expense of migratory songbirds. In gardening usage the word "weed" may be utterly subjective, indicating any plant you don't happen to like, but in ecological usage it has these firmer meanings. Biologists frequently talk of weedy species, meaning animals as well as plants.
Regarding impoverishment, let's note another dark, interesting irony: that the two converse trends I've described--partitioning the world's landscape by habitat fragmentation, and unifying the world's landscape by global transport of weedy species--produce not converse results but one redoubled result, the further loss of biological diversity.
Wildlife will consist of the pigeons and the coyotes and the white-tails, the black rats (Rattus rattus) and the brown rats (Rattus norvegicus) and a few other species of worldly rodent, the crab-eating macaques and the cockroaches (though, as with the rats, not every species--some are narrowly endemic, like the giant Madagascar hissing cockroach) and the mongooses, the house sparrows and the house geckos and the houseflies and the barn cats and the skinny brown feral dogs and a short list of additional species that play by our rules. Forests will be tiny insular patches existing on bare sufferance, much of their biological diversity (the big predators, the migratory birds, the shy creatures that can't tolerate edges, and many other species linked inextricably with those) long since decayed away. They'll essentially be tall woody gardens, not forests in the richer sense. Elsewhere the landscape will have its strips and swatches of green, but except on much-poisoned lawns and golf courses the foliage will be infested with cheatgrass and European buckthorn and spotted knapweed and Russian thistle and leafy spurge and salt meadow cordgrass and Bruce Babbitt's purple loosestrife.
Earth will be a different sort of place--soon, in just five or six human generations. My label for that place, that time, that apparently unavoidable prospect, is the Planet of Weeds. Its main consoling felicity, as far as I can imagine, is that there will be no shortage of crows.