The term species is a somewhat murky one; after all, if two distinctly different creatures can interbreed, are they really a different species? Everyone knows that horses and donkeys produce mules, but did you know that beefaloes (bison and cows) and ligers (lions and tigers) also exist?
Most biologists would agree that two species are distinct if they can't produce fertile offspring together. So, while a horse and donkey can breed, because a mule is sterile, the two are of similar but still distinct species.
Speciation is a much studied and debated topic, and I am not going to go into the how of speciation in this post. That is, I'm not going to try to explain the mechanism of how two species can diverge from one, as that is a hideously complex subject and I lack the requisite knowledge. Instead, I'm going to use the why of speciation to explain a speculation I have.
There is something very important to remember about all living things: they only really care about feeding and reproduction. All other concerns are secondary. Humans are somewhat of an exception, as we have been known to expend vast resources on projects of no utility. However, even for us these two desires are very strong.
Now, animals do not care about things like environmental destruction or conserving resources. They will eat as much as possible and breed as fast as possible. This is why a population of rabbits can often literally eat itself out of house and home: they consume food faster than it can grow. In the long run this crashes the population, but animals are only concerned with the short term.
These two drives, to eat as much and breed as much as possible, explain the why of speciation. Without them there would never have been any drive for life to evolve beyond algae, because evolution would have brought no benefit. However, algae are a concentrated food source, and thus primitive creatures evolved to feed on them. This constant struggle to eat everyone else produced a kind of arms race. Whenever there is a food source that is not being consumed, it is a sure bet that some creature will evolve to take advantage of it.
This leads me to my speculation, and the reason I have a seagull in this post. Most people think the Earth is going through a major extinction, and maybe that's true. However, I happen to think we are also going through a major explosion in biodiversity. The reason for both events, paradoxically, is the same: urbanization.
Consider what an urban environment is: is it large, and full of countless nooks and crannies for creatures to hide. Animals can live deep underground or high in the air, and anywhere in between. Cities have spots that are warm all year round, spots that are wet all year round, and a basically limitless food supply in the form of trash. Roads kill small animals constantly, providing food for scavengers.
Is it any wonder that formerly wild animals are now adapted to life in cities? Gulls started out as sea birds but are becoming so well adapted to city life that "urban gulls" may be a distinct species in the future. Coyotes have grown smaller and sneakier to better live in cities. Many species of raptors nest in sky scrapers and feat on the endless hordes of rodents. In dry areas like the southwest, the relatively wet cities (think sewers and sprinklers) have attracted countless reptiles. Insects, which diversify the fastest, have probably already formed unique urban species.
In the future, cities may act like islands. For example, if a fad in Saskatoon causes many people to have pet tarantulas which later escape, it is possible there will be a unique species of tarantula. While they could hide inside buildings to keep warm, they would never be able to leave the city. Over the years they would become truly unique, just as island species do.
Plants too are adapting to city life. While I do not know for certain, I would suspect trees in Manhattan or Bangkok can tolerate much higher levels of pollution than wild trees. Impermeable pavement and drainage systems shunt rainwater all over a city, which leaves some spots very wet and others very dry. A plant that grows in both spots may be so changed by the differences in water levels that it eventually diverges into two species.
As the saying goes, life finds away. If mankind is willing to provide an entirely new habitat with an huge supply of food, is it any wonder that animals will move in to take advantage of it?