Seite - 169 - in Biodiversity and Health in the Face of Climate Change

169 8.2.8 Mediators, Confounders and  Covariates The level of biodiversity, in terms of species diversity, may go hand in hand with that of other characteristics of a natural area, such as its perceived naturalness. From studies on landscape appreciation it is well known that the perceived naturalness of an area tends to have a positive impact on its scenic beauty (Gobster et  al. 2007), and therefore may be a relevant concept in itself. Although perceived naturalness is not a very well-defined concept, it is almost by definition negatively affected by the presence of buildings and other human artefacts, while this presence does not neces- sarily lower the level of biodiversity of an area. Also, a park may seem highly natu- ral to a lay person, while it is completely artificial from an ecological perspective. Thus, although the two concepts are likely to be correlated, they are definitely not the same. This brings up the following question: if, whether by observation or experimentation, the level of biodiversity has been shown to be associated with mental health, is it really the level of biodiversity that is instrumental in these asso- ciations, either directly or indirectly, by way of its effect on perceived naturalness? Or does the level of biodiversity tend to co-vary with perceived naturalness, without actually influencing it? In other words, is perceived naturalness to be considered a mediator, or a confounder, when researching the effect of the level of biodiversity on mental health? Besides perceived naturalness, there are other characteristics that might be con- sidered, for example visual complexity in terms of the richness and diversity of elements in the landscape, including their shapes, and how these are arranged in space (Ode et  al. 2010; see also Marselle, Chap. 7, this volume). This is also not the same as, but likely to co-vary with, the level of biodiversity, while at the same time it may be relevant for mental health in itself. Similar conceptual questions can be asked as those for perceived naturalness. Moreover, a specific causal path may involve more than one mediator, complicating matters further (Dzhambov et  al. 2018). Especially in epidemiological research, there are also confounders that are less directly linked to the level of biodiversity, but are likely to co-vary with it in real-life situations, even more so when it comes to availability and access. These are to a large extent the same variables that are also important covariates in research on the amount of nature, rather than on its variety in terms of the number of species. For example, one could think of noise level, air quality, socio-economic position and population density. In research focusing on biodiversity, it should be noted that an additional covariate is the amount of nature: one would like to make sure that the variety makes an independent contribution, and it is not solely the amount of nature that is present, or the size of the nature area, that drives the association or the effect. 8 Biodiversity in  the  Context of  ‘Biodiversity  – Mental Health’ Research