Manure Animal manure, as well as human sewage contains bacteria and microscopic organisms that cause disease. Methane CH4 is a by-product of the breakdown of molecules that contain carbon by living things. Most organisms, including humans produce it. Microfiltration Water under pressure passes into pipes constructed of filter membranes with tiny pores too small to admit suspended solids and some pathogens. In most cases this means that water has to be piped from many kilometres away.
Pipes are expensive to build so water becomes more expensive as a city grows.
The trees are the obvious ones. Mammals and birds live in the trees. Ants collect the seeds. Mosses and fungi live on the forest floor. Lizards eat the ants or the fungi.