Ponds, Pools and Lochans

3.5 Assessing species 'richness' (the number of species), occurrence of uncommon species and calculating Species Rarity Indices


To assess species richness simply add up the total number of species recorded and compare to the value in Table 8.

To assess the number of local, nationally uncommon or Red Data Book species, note the number of these species occurring in the pond and compare to the values in Table 8. Species rarity is assessed by allocating a numerical rarity score to each plant and invertebrate species. The scores used for plants and invertebrates and their definition is given in Tables 9 and 10. The Species Rarity Index (SRI) is simply the average rarity value of the species at a site. It is calculated in the following way:

  • All species present are given a numerical value depending on their national rarity status as shown in Tables 9 or 10.
  • The values of all the species present are added together (to give a total rarity score).
  • The total rarity score is divided by the number of species present at the site to give the SRI.

Once the calculations of species richness, occurrence of uncommon species or a Species Rarity Index value have been made, compare the values with those given in Table 8. This allows ponds to be placed in one of four conservation value categories (Very High, High, Moderate and Low).

When assessing conservation value put the pond into the highest conservation value category it can go into using any of the measures. In other words if a plant assemblage had only six species but a SRI of 1.2 (because it had a rare plant), it would have a HIGH conservation value. Note that in Scotland many sites may naturally have a low number of species; care needs to be taken to ensure that such sites are not assumed to be of low conservation interest.

Table 9.
Invertebrate species rarity terms and scores
Status Score Distribution
Common 1 Species generally regarded as common
Local 2 Species either (a) confined to limited geographical area, or (b) of widespread distribution but relatively low population levels.
Nationally scarce 4 Recorded from 16-100 10 x 10 kn grid squares in Britain.
RDB3 8 Red Data Book: Category 3 (Rare).
RDB2 16 Red Data Book: Category 2 (Vulnerable).
RDB1 32 Red Data Book: Category 1 (Endangered).
Invertebrate RDB categories are not yet based on the new IUCN (IUCN, 1994) categories. It is likely that they will be modified as invertebrate Red Data Books are updated
 

 

Table 10.
Wetland plant species rarity terms and scores
Status Score Distribution
Common 1 Species generally regarded as common. For wetland plants, these are species recorded from more than 700 10 x 10 km grid squares in Britain.
Local 2 Local species recorded from between 101 and 700 10 x 10 kn grid squares in Britain.
Nationally notable B 4 Nationally scarce. Recorded from 31-100 10 x 10 kn grid squares in Britain.
Nationally notable A 8 Nationally scarce. Recorded from 16-30 10 x 10 kn grid squares in Britain.
RDB3 16 Red Data Book: Category 3 (Lower Risk).
RDB2 32 Red Data Book: Category 2 (Vulnerable).
RDB1 64 Red Data Book: Category 1 (Endangered and Critically Endangered).
Note: exotic species are given a score of 1, as are uncommon native species (e.g. Water Solder, Stratiotes aloides) which are known to have been introduced to a site.
 

 

Ponds_fig13.jpg (46098 bytes)

Figure 13. A standard NPS plant survey will take a skilled botanist 1 to 2 hours on a medium sized pond; large sites may take up to a day to survey. Graham Burns

 

Ponds_fig14.jpg (21763 bytes)
Figure 14. The horny orb mussel (Sphaerium corneum) is locally distributed in Scottish ponds, lochs and rivers Pond Action
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