Sinks in long-lived spp, fossil fuels. N2 unusable, convert to ammonia or nitrates, plants make proteins, animals eat. Not much goes back into air. Sea is sink. Cars and coal also release SO2.
Similar environments contain similar communities since former constrains latter.
Means of dispersal differently limited in different spp. N and S. America were connected for the first time 5 million yrs ago. As a result of the land bridge placental mammals have crossed into S. Amer and a large number of marsupial mammals became extinct, only the opossum has had significant success in moving North. Starlings replaced bluebirds in NA, feral goats replaced tortoises in Galapagos.