Keystone species are the species present in a particular ecosystem that plays an important role as it directly influences the diversity of a community. It does not get affected by the abundance or the numbers of the community. Without the keystone species, ecosystems could not be able to exist altogether or change dramatically.
An indicator species is a species whose number or absence or presence reflects or indicates a specific environmental condition. It can help in determining a change in the biological condition of a particular ecosystem.
Loss of a keystone species or indicator species help in indications for some type of environmental condition, however, the loss of the indicator species takes place due to some change whereas the keystone species is itself affects the particular ecosystem.
A dam that could allow fish to still pass through is a fishway/fish pass its a "fish ladder" that is structured on or around artificial or natural barriers to allow for fishes natural migration and the movement for potamodromous species.
