Article 9(1) of the NEC-Directive requires that Member States conduct monitoring on the basis of: ‘a network of monitoring sites that is representative of their freshwater, natural and semi-natural habitats and forest ecosystem types, taking a cost-effective and risk-based approach’.
There is a large number of ecosystem types distributed throughout Europe  with a significant variation in the number of ecosystem types per Member State. While the network coverage must be representative of the ecosystems existing in their territories, Member States should take a cost-effective and risk-based approach as provided in Article 9(1) of the NECD when choosing the number and locations of the sites and the kind of indicators monitored. 

A starting point for identifying a representative number of ecosystems and their habitats to be monitored is the number of biogeographical regions in each Member State. The latest classification of the EU's biogeographical regions comprises eleven areas (Alpine, Anatolian, Arctic, Atlantic, Black Sea, Boreal, Continental, Macaronesian, Mediterranean, Pannonian and Steppic).Ideally, at least one monitoring site should be established for each ecosystem type in a biogeographical region.

Source: EURO-Lex

Ecosystems can be classified according to various criteria: from the point of view of energy source, two major types of ecosystems can be distinguished. Autotrophic ecosystems have primary producers as a principal component and sunlight has the major initial energy source; etherotrophic ecosystems depend upon preformed organic matter that is imported from autotrophic ecosystems elsewhere. Ecosystems can also be classified in terrestrial, marine and freshwater.

Source: EEA Glossary

Ecosystems can be classified according to various criteria: e.g. from the point of view of energy source, two major types of ecosystems can be distinguished. Autotrophic ecosystems have primary producers as a principal component and sunlight has the major initial energy source; etherotrophic ecosystems depend upon preformed organic matter that is imported from autotrophic ecosystems elsewhere. Ecosystems can also be classified in terrestrial, marine and freshwater. In European nature policy a further classification into habitats is widely used.

Source: EEA Glossary