Page - 204 - in Biodiversity and Health in the Face of Climate Change

204 9.4 Discussion This chapter identifies, summarises and synthesises research on the impact of biodi- versity on mental health and mental well-being. This was done by identifying and describing the body of evidence, published since Lovell et  al.’s (2014) systematic review, relating biodiversity to mental health and well-being, and by synthesising results from the studies identified by both Lovell et  al. (2014) and in this chapter. Sixteen primary research studies met our inclusion criteria. The assessment of biodiversity in these recently published studies has improved, compared to the stud- ies reviewed in Lovell et  al. (2014). Four studies in Lovell et  al.’s (2014) review did not directly assess biodiversity (Barton et  al. 2009; Curtin 2009; Lemieux et  al. 2012; Pereira et  al. 2005). The growing availability of biodiversity-focused studies meant that all 16 studies identified for our updated review considered the diversity of the environment in some way. Additionally, the recent body of literature investi- gates a greater variation of the biodiversity at the species community and single species levels. Further, the number of studies investigating mental health has grown since Lovell et  al. (2014). Our synthesis of the combined set of 24 studies (nine from Lovell et  al. (2014) and 15 identified in this Chapter) was conducted to describe the body of literature focused on mental health and well-being as an outcome. There is some evidence to suggest that biodiverse natural environments may be associated with good mental health and well-being. Fourteen of these studies showed one or more positive rela- tionships manifested as either better mental health or mental well-being. Positive relationships were found across all, but one, study designs. Positive relationships were most evident when assessing species abundance and mental well-being rela- tionships. However, 17 of these studies reported one or more non-significant find- ings. Non-significant effects were found across all study designs, and were most evident when assessing impact of biodiversity at the ecosystem/habitat level on mental health. There was some evidence of negative relationships (in 2 of the 24 quantitative studies). Overall, the body of evidence across these 24 studies is not yet of the extent necessary to characterise the role of biodiversity in relation to mental health and/or mental well-being. Variation in the evidence may relate to the level at which biodiversity is investigated, how the biodiversity data are collected, and which taxonomic groups are explored. These raise issues for cross-study comparability. The synthesis of results suggests that abundance of specific taxonomic groups may be an important variable. Abundance of a taxonomic group may be more noticeable by people than the number of species (Dallimer et  al. 2012). As such, it may not be the number of different species (i.e. species richness) that matter, but the total number of animals, plants or birds (i.e. abundance). Indeed, Cracknell et  al. (2017) found differential results between species richness and abundance on mental well-being; only abundance was related to happiness, but not species richness. Similar results were found elsewhere (Hedblom et  al. 2017). Clear gaps in the research were also found. None of the 24 studies investigated the effect of perceived species richness on mental health. Another possible area of inves- tigation, not assessed in any of the 24 studies, is participants’ perception of the abun- dance of a specific taxonomic group on mental health and/or mental well-being. M. R. Marselle et al.