Descriptor 1 of the MSFD is providing a definition of Good Environmental Status in relation to biological diversity. This equates to a state where there is no further loss of diversity, the deteriorated attributes of biological diversity are restored and the use of the marine environment is sustainable. The assessment of state is required at three main ecological levels: species, habitats and ecosystems. 

The definition is in line with the objectives of the EU biodiversity strategy to 2020 and its implementation is directly linked to the Birds and Habitats Directives. Indirect links exist with other environmental policies tackling pollution from various sources (e.g. Water Framework Directive, the Habitats Directive, Birds Directive), the Common Fisheries Policy (CFP) and the new EU regulation on Invasive Alien Species (IAS).
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According to the Marine Strategy Framework Directive (MSFD), biological diversity should be maintained. Specifically, the quality and occurrence of habitats and the distribution and abundance of species are in line with prevailing physiographic, geographic and climatic conditions. The Directive covers the whole range of species, habitats and associated pressures in all European marine regions (from coastal waters to open seas). The COM Decision sets certain criteria and indicators to define Good Environmental Status (GES) on species, population, habitat and ecosystem level. It is widely recognized that there are links between D1 (biodiversity per se), D4 (food-webs) and D6 (sea-floor integrity) and these are frequently referred together as the 'biodiversity theme' since the data and information requirements for these Descriptors overlap to a considerable degree, although there are separate description of what GES is for each one of them. It is also recognized that although aspects of these Descriptors are, to some extent, already addressed by other EU pieces of legislation the MSFD implementation requires further scientific and technical developments to better set the conceptual frame of biodiversity, define GES, set meaningful targets and achieve an operational capacity for a meaningful monitoring and assessment. 
D1 Commission Decision 2010/477/EU

Biological diversity is maintained. The quality and occurrence of habitats and the distribution and abundance of species are in line with prevailing physiographic, geographic and climate conditions.

Assessment is required at several ecological levels: ecosystems, habitats (including their associated communities, in the sense of biotopes) and species, which are reflected in the structure of this section, taking into account point 2 of Part A. For certain aspects of this descriptor, additional scientific and technical support is required ( 5 ). To address the broad scope of the descriptor, it is necessary, having regard to Annex III to Directive 2008/56/EC, to prioritise among biodiversity features at the level of species, habitats and ecosystems. This enables the identification of those biodiversity features and those areas where impacts and threats arise and also supports the identification of appropriate indicators among the selected criteria, adequate to the areas and the features concerned ( 6 ). The obligation of regional cooperation contained in Articles 5 and 6 of Directive 2008/56/EC is directly relevant to the process of selection of biodiversity features within regions, sub-regions and subdivisions, including for the establishment, where appropriate, of reference conditions pursuant to Annex IV to Directive 2008/56/EC. Modelling using a geographic information system platform may provide a useful basis for mapping a range of biodiversity features and human activities and their pressures, provided that any errors involved are properly assessed and described when applying the results. This type of data is a prerequisite for ecosystem-based management of human activities and for developing related spatial tools ( 7 ). 

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According to the TG1 report: 

Descriptor 1: "Biological diversity is maintained. The quality and occurrence of habitats and the distribution and abundance of species are in line with prevailing physiographic, geographic and climatic conditions". See Sections 0 and 2.1. 

Biological diversity, in accordance with the Convention on Biological Diversity (CBD, 1992), is defined as 'the variability among living organisms from all sources including, inter alia, [terrestrial,] marine [and other aquatic ecosystems] and the ecological complexes of which they are part; this includes diversity within species, between species and of ecosystems'. 

Maintained equates to: 

a) no further loss of the diversity within species, between species and of habitats/communities and ecosystems at ecologically relevant scales, 

b) any deteriorated attributes of biological diversity are restored to and maintained at or above target levels, where intrinsic conditions allow (cf. Art. 1.2a) and 

c) where the use of the marine environment is sustainable. 

The term 'habitat' in this Descriptor addresses both the abiotic characteristics and the associated biological community, treating both elements together in the sense of the term biotope (COM DEC 2010/477/EU), whereas 'quality', 'occurrence', 'distribution', 'extent' and 'abundance' form the basis of the criteria standards used to assess GES.