Check out the pictures here to see what happens to discarded plastic. It doesn't quickly dissolve, break down, or disperse into tiny little harmless particles. It kills baby birds (and other animals, too).
There is a place out in the ocean called the Great Pacific Garbage Patch that is composed almost entirely of plastic waste products -- bottles, caps, fishing line, etc. But don't worry, most of the trash in it is small and already starting to fall apart. Of course, when plastic disintegrates, it can also release lots of nasty, poisonous and nefarious chemicals into the water.
But these baby albatrosses didn't get the chance to fly over the Garbage Patch and see the damage to their environment for themselves. Their parents inadvertently fed them junk food -- literally. You see, birds can't readily distinguish a colorful piece of plastic from a colorful bit of edible marine life, so many of these young birds were force-fed trash until their digestive systems clogged and they died. It's not the mama bird's fault -- it's ours.
All plastic is human produced. All plastic trash is human waste. So all those dead birds (and turtles, otters, fish, etc) are our responsibility. So, how much wildlife have you killed today?