Introduction to the guidance manual
14. Species assemblages

 

 

 

 
14.1 What is an assemblage ?
ASSIs/SSSIs, SPAs and Ramsar sites may each be notified because of the presence of important assemblages of species. This might seem straightforward, but in the context of species features two situations can be envisaged:
 
  1. A colony of different species all occurring / living together, where the total number of individuals is the key aspect of the interest on the site (e.g. more than 20,000 seabirds on a SPA site).
  2. A number of characteristic species which together form the feature and usually share similar ecological or habitat requirements (e.g. the co-occurrence of woodland or upland bird species, or heathland invertebrates).

 

The term 'assemblage' can also be used in a third, functional, way; when there are a number of features which co-exist, yet are individually notified (i.e. they are features in their own right). While it may be possible to assess them using the same or very similar attributes, these species must be assessed as individual features independently of any assemblage of which they may also form a part (e.g. under scenario 1).
 
14.2 Scoring systems
For some taxonomic groups, the selection of SSSIs for species assemblages is based on a simple scoring system with clearly-defined thresholds for sites considered to be of SSSI quality (e.g. vascular plants, breeding birds, amphibians and dragonflies). Quantitative thresholds are also used to identify SPAs and Ramsar sites for waterfowl assemblages, and SPAs for seabird assemblages. In other cases (e.g. most invertebrate assemblages), SSSI selection guidelines are less clearly defined.
 
There are several possible approaches to monitoring species assemblages. Where there are quantitative selection thresholds an acceptable approach might be to evaluate the assemblage against the selection criteria. For example, by recording presence/absence of the constituent species and comparing the scores with the SSSI selection guidelines and/or the assessment at the time of notification. However, it needs to be recognised that not all species may be recorded during one site visit, while the original selection of the feature may have been made after several visits using a compilation of data. It may be sensible, therefore, to set a favourable condition threshold which takes account of such circumstances. Recording relative abundance of the components of the assemblage on a scale of common to rare, could provide additional information for subsequent comparisons.
 
14.3 Indicator species
An alternative approach is to use indicator species to assess the condition of the whole assemblage. Suitable indicators would typically include species characteristic of a range of different micro-habitats, and, if possible, conspicuous species that could be recorded by a non-specialist. Both approaches should usually be complemented by setting target values for habitat attributes which relate to the features required for the maintenance of the assemblage.