Dear probably Concealocanth,
How would Earth's species (collectively or individually, whatever is interesting) be different if humanity had never existed? Obvious issues include extinction and invasive species, which I'm happy to hear about, but I'm more particularly interested in species' evolution. Much thanks!
-probably not yayfulness
Dear probably not,
Concealocanth offered some great feedback to guide and inform this question.
Without humans, 'Canth says, "there would be no corn, no cows, no chickens, no dogs, no cats... etc. just teosinte, wild buffalo, red junglefowl, wolves, desert cats, etc..." These species changed because humans kept breeding the animals and plants they liked best, who had traits they wanted to encourage--e.g. a submissive and gregarious wolf would be chosen over a wolf that constantly snapped and growled at the humans harboring it.
Selecting for specific traits like docility can change the genetics of an animal in a relatively short period of time, as in the case of foxes that developed floppier ears and curlier tails in the USSR's domesticated fox experiment.
Indirect Effects Through Niche Creation
"Humans alter habitats. Crazy, right?" Humans tend to drag other things in tow with them. A vertebrate researcher I met over the summer informed me raccoons were once limited just to woodlands in the southeastern United States. It turns out these critters do a great job living off of human waste and thrive in habitats disturbed by people, so as Western settlers infiltrated the continent, so did our masked amigos.
Coyotes are another example of an animal that expanded its range from the Southwest and Great Plains to most of North America as humans thoughtfully paved the way for them by removing some weird animal no one's ever hear of called wolves.
Mass Extinction (the main effect)
Humans are really good at killing things they like to eat, like passenger pigeons and dodos. They'r even better at changing habitats to better suit them (at least, for short-term benefit). Did you know the eastern half of North America used to be an enormous deciduous (you know, leaves that fall off each year) forest? Notes the University of Michigan in an informative piece on global deforestation:
"It is impossible to overstate the importance of humankind's clearing of the forests. The transformation of forested lands by human actions represents one of the great forces in global environmental change and one of the great drivers of biodiversity loss. The impact of people has been and continues to be profound. Forests are cleared, degraded and fragmented by timber harvest, conversion to agriculture, road-building, human-caused fire, and in myriad other ways. The effort to use and subdue the forest has been a constant theme in the transformation of the earth, in many societies, in many lands, and at most times. Deforestation has important implications for life on this planet.
Just think, originally, almost half of the United States, three-quarters of Canada, almost all of Europe, the plains of the Levant, and much of the rest of the world were forested. The forests have been mostly removed for fuel, building materials and to clear land for farming. The clearing of the forests has been one of the most historic and prodigious feats of humanity."
The University of Michigan acknowledges "much forest re-growth has occurred in the eastern USA during the 20th Century, although these second-growth forests differ in structure and composition from their predecessors." This is to say that while much life has returned, it doesn't necessarily have the same wide range of species it once had.
You're probably aware that deforestation is a serious problem in the tropics worldwide. The tropics are unusual in the regard that they hold a far greater number of species on an acre-to-acre basis than anywhere else on earth. For example, the particularly rich Yasuni region of the Ecuadorian Amazon contains "an extraordinary abundance of birds, primates, reptiles, and amphibians. The park contains more tree and insect species in a single hectare (2.47 acres) than in all the U.S. and Canada combined." Pick a different hectare (that's a little less than two football fields), get an entirely new set of species. Coral reefs are also staggeringly diverse.
You've mentioned you already know about a lot of this, so I'll cut myself short and hope you'll understand me when I say there would be many more species around if humans hadn't shown up with a chainsaw to the party.
Species outside of their natural habitat can cause some big problems. In the absence of the predators and ecological pressures they're accustomed to, they can spread, proliferate, and cause significant monetary damages to agricultural efforts, urban areas and wild regions. Rather than enumerating each of these species, though, I'd prefer you just sit down, grab a 128 oz. Diet Croak and ask Wikipedia at your leisure.
So why all the words?
Humans are more influential than we realize, which is both terrible and pretty cool. Check out individual entries on Wikipedia's lists of domesticated plants and animals if you want to learn more!
P.S. Interestingly enough, domestication is a two-way street. Michael Pollan notes in The Botany of Desire that you could say many of our domesticated plants have actually domesticated us.