Page - 166 - in Biodiversity and Health in the Face of Climate Change

166 8.2.4 Aggregating Biodiversity Across Different Areas With regard to the level of biodiversity, an additional issue is how to arrive at an aggregated measure for the residential environment as a whole. One way might be to look at the biodiversity of each green area separately, and to calculate an average biodiversity level. This would allow for conclusions such as ‘the green areas in this environment are highly biodiverse on average’. Another approach is to assess biodi- versity at the level of the residential environment as a whole. That is, to pool all the species from the different green areas in the residential environment (and perhaps include isolated natural elements as well), and base the biodiversity score on the variety in this total pool. This would allow for conclusions such as ‘there is a lot of biodiversity in this residential environment’. Note that in extreme cases the two approaches may lead to quite different rank- ings of residential environments. A residential area with few urban parks, each with a rather high level of biodiversity in itself, but very similar to each other in species composition, may score high in the first approach. However, in the second approach it may be outscored by a residential environment with a larger number of smaller urban parks that each in themselves are not very biodiverse, but are complementary to each other in species composition. In the latter case there is more variety in the residential environment as a whole, but less variety in each individual park. Note that from an ecological perspective, one might also want to look at the functional links between the different green areas and natural elements that the inventoried area contains or their contribution to the larger ecosystem of which they are a part. In ecological studies, the Shannon Diversity index, which combines number of spe- cies and abundance of each species, is sometimes used to indicate functional diver- sity within a taxonomic group (Krebs 1989).4 8.2.5 Type of  Access Metric In the above, we focused on access to nature in terms of the availability or presence of nature within a certain area. Ekkel and de Vries (2017) have termed this a cumu- lative opportunity access metric, given that it takes all nature within that area into account. They distinguish the cumulative opportunity metric from another type of access metric, based on the distance to the nearest qualifying natural area. ‘Qualifying’ here refers to the area having at least a certain size and usually being open to the public as well. A minimum level of biodiversity could be added as another criterion in such an approach. A second option is not to use it as an addi- tional criterion, but to look at it as a quality aspect of the otherwise qualifying natu- ral area. The latter is more similar to the way access is handled in the cumulative 4 Required abundance across taxonomic groups may differ by group, e.g. lower numbers for top predators than for prey animals. S. de  Vries and R. Snep