Seite - 164 - in Biodiversity and Health in the Face of Climate Change

164 8.2 Measurement of  Biodiversity The choice of definition, in this case of biodiversity, has implications for (a) how to (objectively) measure the level of biodiversity and, as a consequence, (b) which environments will be considered high, and which ones will be considered low in biodiversity. For example, an arboretum may be considered an area with an extremely high biodiversity per acre, or it may be discarded completely, as not con- stituting an ecosystem. 8.2.1 Characteristics of  Nature in  General It seems fair to say that most of the epidemiological research on nature and human health until now has focussed on access to or availability of nature, and has not paid much attention to its characteristics, including the level of biodiversity (Hartig et  al. 2014). Moreover, in such studies nature usually translates to green space, greenery or vegetation, without much consideration for whether or not it may be considered a part of an ecosystem. For example, studies have been conducted looking at the amount of green space, including everything from urban parks to agricultural areas to forests (de Vries et  al. 2003), the amount of greenery (Cohen-Cline et  al. 2015), that of streetscape greenery (van Dillen et  al. 2012) and even the number of street trees per kilometre of road (Taylor et  al. 2015). Characterising the nature included in these amounts in meaningful ways with regard to its mental health impact may be considered an important next step in the research agenda (Hartig et  al. 2014; Shanahan et  al. 2015). 8.2.2 The Object to  Be  Assessed: The  Biodiversity of  What? Another issue is the definition of the area or object of which the biodiversity is to be assessed. In experimental research on nature and human health, this area or object is usually well-defined, for example the biodiversity present in the landscape that is depicted on a screen (Wolf et  al. 2017) or that is present in a large aquarium (Cracknell et  al. 2016). In intervention studies, the focus is usually on a single green area, such as an urban park. For example, such a study may be about evaluating the impact of the redevelopment of a park or woodland (see e.g. Ward Thompson et  al. 2013). In large-scale epidemiological studies, the area of choice is often the residen- tial environment. Note that from an ecological perspective, the area that is assessed may not constitute an ecosystem in itself, but be a part of a larger ecosystem.3 If so, 3 This could be linked to the discussion on what constitutes the unit that provides a certain ecosys- tem service (see Andersson et  al. 2015). S. de  Vries and R. Snep