More seriously though your observation arises from speed and density, not activity. Consider chickens for example, which if you have a couple dozen chickens using a hectare (~2.5 acres) to live on their feeding and feces are completely supported in the existing cycles of that land (presuming non-desert, non-arctic). What gets it out of control is when you want 2000 hens laying in a single building on less than an acre. High density, imported feed, waste disposal becomes and issue, etc.