There are two different types of ecology that go along with the Catacombs in Paris: Community and Population Ecology. The limestone in the Catacombs, which falls under community ecology, was used to build the Church of Notre Dome. The Black Plague also may have begun because of the rats that lived there in the past; they would have been down in the Catacombs around the decaying bodies. They picked up the disease and spread it to living humans. The rats still live in the Catacombs, sometimes even moving the bones by disturbing and lifting them. There must have been a lot of rats to move bones because the Catacombs are one-kilometer-long tunnels lined with bones, neatly stacked, of some six million former residents of Paris.
Population ecology is related to the Catacombs because, at the time, the cemeteries were filled, so they began digging underground. Also because of the burials, human traffic was very, and still is, very popular. Moving people is the primary rationale for the underground pedestrian walkways.
Also dealing with the population, because of the Catacombs, there are certain places in Paris that cannot be built upon, because of the tunnels below.