Concerning the basic functioning of mangrove ecosystems, three groups of key factors are examined and explained. Several parameters external to these ecosystems have a strong influence; i.e. - the regular and abundant input from fresh water sources; - provision of nutrients; - the stabiity of the substrata. Each of these groups of factors is related to a number of parameters which do not lend themselves to in situ studies. For example, the stability of the substrata depends on the sedimentation load which itself is linked to the geological nature of the basin, land utilization, local climate, etc. The principal cause of the decline in mangroves is analyzed.