According to FiveThirtyEight, if you don’t pick up your dog’s poop, the results can be pretty awful (at least, they could be in New York City).
Besides the aesthetic problems — or improvement, if we’re talking Times Square — there’s a public health issue. Dog poop has 23,000,000 fecal coliform bacteria per gram, so that’s an additional 2 quadrillion coliform bacteria added to the sidewalks each day, give or take, which can’t be healthy. Studies of three midsize cities found that 10 to 50 percent of bacteria in air samples was derived from dog waste.
When you leave your dog’s poop behind, you’re endangering the health of everyone else in the area who uses those sidewalks.
Not to mention the affects on the environment itself:
There might be environmental effects, too. Not that the Hudson River is exactly a paragon of cleanliness, but microbiologists suggest that dog poop can be a big polluter. The Environment Protection Agency says domestic animals — which includes cats, which at least have the decency to pick a place and stick with it — contribute nitrogen and phosphorous to ground and surface waters, and poop from pets can contribute to eutrophication and closure of shellfish beds.
[So] what would happen if people didn’t pick up their dog’s crap? Pestilence, plague, pollution, declining property values (I assume) and possible environmental damage. In reality, if 41 percent of New Yorkers didn’t clean up after their dog (as was found in the Chesapeake Bay study), 39.3 tons of dog crap gets left on the sidewalks each day — a little less than the weight of a subway car on the F train.
(Picture taken from reddit – a thread entitled “So majestic“)