Seite - 167 - in Biodiversity and Health in the Face of Climate Change

167 opportunity approach. As for the merits and (implicit) assumptions behind the two types of access metrics, the reader is referred to Ekkel and de Vries (2017). 8.2.6 Actual Versus Perceived Biodiversity Information on the level of biodiversity, in terms of species diversity, is not always readily available. Sometimes data are gathered on perceived biodiversity, as for example in terms of how survey respondents rate the species richness of a specific site, or the number of species present in their residential environment. It is not clear to what extent perceived biodiversity coincides with actual biodiversity, not even if the latter is defined in terms of species richness (see e.g. Fuller et  al. 2007 and Dallimer et  al. 2012 for contradictory findings).5 Perceived biodiversity is likely to depend strongly on the visibility of the different species, and on the extent to which they are perceived as being different. For example, biodiversity in the aquatic domain may go largely unnoticed (with the exception of aquaria). The same may be the case for the variety in the insect world, and even more so for that of micro- organisms. On the other hand, the biodiversity as perceived may be more likely to influence mental health than the objectively defined actual biodiversity (Dallimer et  al. 2012). To the extent that the two do not coincide, different things are measured. Furthermore, there is the methodological issue of a potential single-source bias when both biodiversity and mental health information are provided by the same source. Actually, when people rate the level of biodiversity of the same area, and subsequently how this is associated with their mental health is analysed, it is solely the co-variation of individual differences in perception and those in mental health that is studied, and not that of the actual level of biodiversity, which in that case is the same for everyone. A potential solution for the single-source problem is not using perceptions at the individual level, but aggregating the ratings regarding the same object to an average score for that object. A more sophisticated method of aggregating individual level data to characterise an environment is the ecometric approach introduced by Raudenbush and Sampson (1999). In this approach, the number of informants sampled, as well as the intersubjective agreement among informants, is statistically taken into account. This ecometric approach does not seem to have been applied for perceived biodiversity specifically thus far (but see de Jong et  al. 2011). 5 Fuller et  al. (2007) provide an example of a study in which objectively assessed and perceived species richness for three categories of species/taxonomic groups are compared. It may be pointed out that they selected rather easy to perceive species: plants, birds and butterflies. Moreover, they aggregated individual perceptions per site. This may have helped them to arrive at the conclusion that greenspace users can more or less accurately perceive species richness. Even so, Dallimer et  al. (2012), using the same approach, did not observe a positive association between perceived and actual species richness for any of the three taxonomic groups. See Marselle et  al. Chap. 9, this volume. 8 Biodiversity in  the  Context of  ‘Biodiversity  – Mental Health’ Research