Calculating the diversity of species communities

The “Diversity of Species Communities” indicator describes how the composition of the various species groups develops within individual habitats and in the various regions of Switzerland. This calculation is based on the presence and absence data for individual species from the two BDM monitoring networks.

An important addition to the number of species

The data from the BDM monitorings can be used to determine the average number of species in habitats and landscapes. But is this information sufficient to describe biodiversity in Switzerland in great detail? No, it's not! Relatively long species lists may be compiled for a particular habitat type or a particular region, but these may differ only slightly between the individual sampling areas. This indicates that such species communities are not particularly diverse. This fact is taken into account by the “Diversity of Species Communities” indicator, which compares the species lists of the sampling areas. Particularly high diversity is only achieved when as many species as possible occur per plot and the overlap of species lists between the sampling areas is low.

Example of a simple calculation

The number of species is counted separately for each area. The indicator value is calculated from the mean value of these species numbers. To calculate the diversity of species communities, however, the species lists (presence-absence data) of two sampling sites are compared with each other. The proportion of different species is calculated using a diversity index commonly used in ecology, known as the Simpson Index.

  • In the diagram on the left, two circles each represent an area. One has four flowers, the other three. One light blue flower can be found in both circles.
  • In the right-hand diagram, the two circles are superimposed, the light blue flower is in the intersection.
    In the diagram on the right, the two circles are superimposed; the light blue flower is located at the intersection.

The same procedure is used for all possible combinations of two sampling sites. The average of all calculated index values gives the indicator value. An indicator value of 1 means maximum diversity of the species communities. On a purely hypothetical level, this would occur if no species occurred in two compared sites. An indicator value close to 0 means that the species communities are very uniform. Although the calculation is simple in principle, on a computational level it is complex due to the various comparisons involved.

So what happens if the species composition changes over time? Two scenarios are set out below.

  • In the left-hand diagram, two superimposed circles each represent an area. There are four flowers in both. The light blue flower occurs in both circles. The additional purple flower only occurs in one area.
    In this scenario, a rare species becomes more common. In the example, the species circled with a dotted line has recently appeared on the right-hand site. Compared to the initial state, the mean species diversity has increased from 3.5 to 4.0 species. At the same time, the species communities are becoming more diverse, with the Simpson Index rising from 0.67 to 0.75.
  • In the graphic on the right, the two circles with four flowers each are superimposed. The orange-coloured flower now appears in both areas and is therefore in the intersection.
    In this case, a common species is becoming more frequent. The species circled with a dotted line migrates into the right-hand site and continues to occur in the left-hand site as it did in the initial state. In this scenario, the mean species diversity also increases from 3.5 to 4.0 species. However, this makes the species communities more similar and the Simpson Index falls from 0.67 to 0.5.

Watercourse monitoring network

Field workers survey aquatic invertebrates over a section of 5 to 100 metres in length, depending on the width of the watercourse. The animals are collected for identification in specialised laboratories.

Landscapes monitoring network

Field workers record the plants, butterflies and breeding birds over an area of one square kilometre. They generally follow a precisely defined route along paths and roads.

Terrestrial habitats monitoring network

Field workers monitor the selected species groups over a circular area of 10 square metres. They collect the vascular plants directly on-site. They take samples of moss types and snails for analysis in specialised laboratories.