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9.4 Discussion
This chapter identifies, summarises and synthesises research on the impact of biodiversity 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 studies 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 investigates 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 relationships 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 relationships. However, 17 of these studies reported one or more non-significant findings. 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 investigation, not assessed in any of the 24 studies, is participants’ perception of the abundance of a specific taxonomic group on mental health and/or mental well-being.
M. R. Marselle et al.
9.4 Discussion
This chapter identifies, summarises and synthesises research on the impact of biodiversity 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 studies 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 investigates 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 relationships 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 relationships. However, 17 of these studies reported one or more non-significant findings. 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 investigation, not assessed in any of the 24 studies, is participants’ perception of the abundance of a specific taxonomic group on mental health and/or mental well-being.
M. R. Marselle et al.