Where does your tire tread go?

From the expert at AskPablo, pretty much everywhere:

Well, some of it ends up in the air as PM10 (particulate matter that is 10 microns in diameter) which we might breath in. Some of it ends up as that dark grime on your rims (although some of that is brake pad dust) and windshield. The rest waits by the side of the road for the next rain storm to wash it away. Once in the storm drain the little rubber bits probably make their way to the ocean where they happily float around.

Unfortunately for the little critters in the ocean these little bits look a lot like food. In fact in some areas of the oceans man-made particles outnumber plankton and other microscopic critters by an order of magnitude! When these bits make their way into an animals belly they don't break down and can't be digested. This means that they accumulate, leading to eventual starvation. If a larger fish eats it first the rubber may than accumulate in its belly until it is eaten by a seabird or mammal, and so on...