Classification development - approach and methods used

 

In addition to a review of classification schemes, an extensive review of the literature describing marine habitats was also undertaken. This helped formulate the initial lists of types which might form the basis of the classification. For this the scientific literature was of considerable help for sediment habitats (a traditional area for marine studies) but relatively poor for rocky habitats (which, in the subtidal, attracted attention only relatively recently through use of SCUBA diving techniques). These initial lists of types were then refined on the basis of new dedicated field surveys, data analyses and field trials.

Consultation and testing

 

The advice of external consultees has been important in two key areas:

 

Field surveys and other data acquisition

 

The data comprise information on the nature of each site (such as substratum, wave exposure and height or depth surveyed), the type of sampling undertaken, the site's location and the species present (together with an indication of their abundance) within discrete habitats at the site. MNCR field recording techniques are described in Hiscock (1996), with Appendix 8 providing the guidance on how to complete MNCR field recording forms (the forms can be downloaded from here).The terminology relating to field survey methods is described further below, and should help users of the classification interpret the habitat information contained in the biotope descriptions. Procedural Guidelines for a wide range of field sampling techniques are given in the Marine Monitoring Handbook (Davies et al. 2001).

 

In total, data for over 16,000 sites comprising more than 36,000 habitat records from around Britain and Ireland were collated and entered onto the MNCR database (as described by Hiscock, 1996). The database includes a module which holds definitions of each classification type, linked to a national dictionary of marine species and to the field survey data. The field survey data have been made widely accessible via the web-based MERMAID application, and more recently, via the National Biodiversity Network from an MS Access-based 'relational' database, Marine Recorder. The Marine Recorder database application has been specifically developed to accept marine biological data from a wide range of survey techniques, including the data held originally in the MNCR database. The application can be downloaded here, and includes a dictionary of the habitat classification types.

 

Data analysis

using the TWINSPAN and DECORANA clustering and ordination techniques were employed to help define the types. The analytical processes adopted are described in Mills (1994).

 

The 1997 version was revised and refined to develop the present version. Extensive re-analyses of the data were carried out using  the analytical techniques available in PRIMER (Clarke & Warwick, 2001). The data were initially divided into the five broad habitat types shown in the primary habitat matrix, i.e. Littoral Rock, Littoral Sediment, Infralittoral Rock, Circalittoral Rock and Sublittoral Sediment. Due to the large size of the datasets within each broad habitat, some further a priori divisions of the data within broad habitats were necessary before analysis was possible. Additional analyses were carried out on data from "borderline" habitats to ensure these a priori splits did not force artificial divisions into the classification where this was not supported by differences in the survey data. Analysis within each broad habitat was led by a specialist for that habitat type. Figure 2 shows the data analysis process for the littoral sediment section. The following paragraphs describe the analyses

Littoral rock

(Connor et al., 1997 a, b) were generally considered satisfactory, analysis focused on clarifying the boundaries between closely related types and confirming the validity of certain less-well defined types. This included attention to the inter-relationship of fucoid-dominated types regarding the bedrock/boulder/mixed substrata and fully marine/variable salinity transitions and examination of the various red algal-dominated types. Additionally new data from intertidal caves enabled substantial development of the classification here. On the basis of these analyses, some restructuring at biotope complex level was necessary.

Littoral sediment

(>4000 records), some a priori division was necessary to provide datasets that could be managed within PRIMER (Clarke & Warwick, 2001). Data were divided based on the sediment type categories at habitat complex level in the 97.06 classification (Connor et al., 1997a, b): gravels and sands, muddy sands, sandy muds, muds and mixed sediments. Semi-quantitative epifaunal data were considered to be of less value than quantitative infaunal data for the purposes of the analysis and were thus excluded. Epifaunal data were however used to define types where a significant proportion of species would be sampled in epibiota sampling techniques, and/or where few infaunal samples were available, e.g. for mussel beds.

 

Cluster analysis was carried out based on species matrices listing individual counts per m² in each sample, using the PRIMER software package (Clarke & Warwick, 2001). The data were divided into small clusters of biologically similar records, based on the resulting dendrograms. Comparative tables were produced to compare the species data and physical data between each of the small clusters. Where there were no notable differences between the physical and biological characteristics of the small clusters, they were amalgamated into larger groups which would form the preliminary basis for biotopes and sub-biotopes. Where similar biological and physical profiles appeared from clusters derived from different datasets, those data were joined and re-analysed. In particular, there was some overlap between the 'gravels and sands' and the 'muddy sands', and between the 'muddy sands' and 'mud' datasets. This re-analysis was carried out to ensure that the a priori divisions of the data did not artificially force divisions of otherwise coherent clusters. The resulting preliminary biotope and sub-biotope groups of records were then checked to ensure cohesion of both the environmental and species data. Individual records which differed significantly from the average profile for the group (in terms of biology or physical habitat characteristics) were removed, resulting in a group of records which formed the basis of the biotope descriptions (core biotope records). The physical and biological profiles from the core biotope records were then used to group biotopes of similar character into biotope complexes, and these in turn were assigned to habitat complexes and broad habitats. Note that, in addition to the habitat complexes defined on sediment character, two additional categories were created based on epifaunal characteristics (littoral sediments dominated by macrophytes, and littoral biogenic reefs).