Page - 206 - in Biodiversity and Health in the Face of Climate Change

206 literature. The synthesis presented here indicates that different metrics of biodiver- sity (e.g. species richness; abundance) could play a role and should, therefore, have their relationships with mental health and well-being assessed separately. Functional aspects of biodiversity, such as phenotypic diversity (colour of fish, height of trees) (Botzat et  al. 2016) and charismatic species (Dallimer et  al. 2012) could also be usefully explored. Further, studies should also measure the biodiversity that is expe- rienced by people, as opposed to the objectively measured diversity in an environ- ment. The bird hiding in a bush, or the nocturnal mammal, that is not seen nor heard, is unlikely to be experienced by humans, and unlikely to influence mental health or well-being (Bell et  al. 2014; Cox et  al. 2017). Assessments of the biodiversity that people perceive or experience can be captured with Global Positioning System (GPS) trackers, eye-tracking technology and mobile electroencephalography (EEG) devices. We also recognise that one’s perception of biodiversity is important for health and well-being. The synthesis presented here demonstrated that perceived species richness is associated with mental well-being. Future studies could investi- gate perceived species richness-mental health relationships. Further, whilst not investigated in any of the studies reviewed here, perceived biodiversity could also be investigated to assess whether it mediates the effect of objectively measured biodi- versity on mental health and/or well-being. See de Vries and Snep Chap. 8, this volume, for further discussion on biodiversity measurement considerations. Mental Health and Well-being Assessment To facilitate cross study comparison, we encourage future research to use validated scales of mental health and well-being that have been used previously in psychol- ogy and health.2 As such, researchers may wish to consider the reliability of using a mental health or well-being measure for understanding the biodiversity-health rela- tionship. When developing new measures, theoretically grounded outcome mea- sures are essential. Theory Future studies should articulate the theoretical framework(s) they are using to hypothesise about biodiversity-health relationships (see also Marselle Chap. 7, this volume). Researchers should use theory to drive the selection of outcome measures and identify mediators, moderators and confounders. To our knowledge, no study has investigated the effect of biodiversity on attention  restoration, and more studies could investigate stress as an  outcome measure; both of which explicitly test theo- ries of restorative environments. Additionally, theories on the relationship of natural environments on health, such as Attention Restoration Theory could be developed further, e.g. by differentiating general effects of natural environments, and specific aspects of biodiversity, on health aspects. Mechanisms Future studies should continue to investigate the mediators of biodiversity and men- tal health and well-being using the pathways identified in nature-health frameworks (Hartig et  al. 2014; Markevych et  al. 2017). 2 Researchers may wish to see Linton et  al. (2016) for a list of such measures. M. R. Marselle et al.