Which pet has a lower impact on the environment; a dog or a cat?

I've noticed a dramatic increase in the number of dog owners in the city where I live. In addition to the fact that the dogs don't have much space to roam (I live downtown), I've noticed that there is a lot of feces on the sidewalks, and that so many dogs are peeing on neighborhood bushes that the... show more I've noticed a dramatic increase in the number of dog owners in the city where I live. In addition to the fact that the dogs don't have much space to roam (I live downtown), I've noticed that there is a lot of feces on the sidewalks, and that so many dogs are peeing on neighborhood bushes that the plants are dying. Since the feces and urine ultimately end up in my local body of water, there must be some environmental impact to this dog ownership surge. On the other hand, cats use litter boxes, and those get sent to the landfill, so I'm wondering which pet people think is a more environmentally responsible choice.
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