Perceived biodiversity: is what we measure also what we see and hear?

Biodiversity is crucial for human health and well-being. Perceived biodiversity - the subjective experience of biodiversity - seems to be particularly relevant for mental well-being. Using photographs and audio recordings of forests that varied in levels of species richness, we conducted two sorting studies to assess how people perceive visual and acoustic diversity and whether their perceptions align with measured tree and bird species richness (actual diversity). Per study, 48 participants were asked to sort the stimuli according to any similarity-based sorting criteria they liked (open sorts) and perceived diversity (closed sorts). The main perceived visual forest characteristics identified by participants in the open visual sorts were vegetation density, light conditions, forest structural attributes and colours. The main perceived acoustic forest characteristics identified in the open acoustic sorts comprised bird song characteristics, physical properties such as volume, references to the time of day or seasonality and evoked emotions. Perceived visual and acoustic diversity were significantly correlated with actual tree and bird species richness, respectively. Notably, the relationship was twice as strong for the acoustic sense. The acoustic sense may thus be crucial to obtain a more thorough understanding of perceived biodiversity. We further computed several visual and acoustic diversity indices from the photos and audio recordings, e.g. colourfulness or acoustic complexity, and assessed their relevance for perceived and actual diversity. While all acoustic diversity indices were significantly associated with perceived acoustic diversity and bird richness, we could not identify a visual diversity index that captured perceived visual diversity and tree richness. Our results suggest that people can perceive species richness. Our identified visual and acoustic forest characteristics may help to better understand perceived diversity and how it differs from actual diversity. We present acoustic diversity indices that quantify aspects of perceived and actual acoustic diversity. These indices may serve as cost-efficient tools to manage and plan greenspaces to promote biodiversity and mental well-being. ### Competing Interest Statement This research was funded by the ERA Net BiodivERsA project Dr.FOREST that investigates links between forest biodiversity and human health and wellbeing, with the national funders German Research Foundation (DFG: 428795724, Germany), French National Research Agency (ANR, France), Research Foundation Flanders (FWO, Belgium), Austrian Science Fund (FWF, Austria) and National Science Center (NCN, Poland, project no. 2019/31/Z/NZ8/04032), as part of the 2018/2019 BiodivERsA call for research proposals. KR, RRYO and AB gratefully acknowledge the support of iDiv funded by the German Research Foundation (DFG FZT 118, 202548816). SM additionally receives financial support via the AkWamo project (2221NR050C): Feasibility study: integration of (bio)acoustic methods for quantifying biological diversity in forest monitoring, which is supported on the basis of a resolution of the German Bundestag with funds of the Federal Ministry of Food and Agriculture (BMEL) via the Agency of Renewable Resources (FNR) as project management agency of the BMEL for the funding program Renewable Resources. SMs contribution further benefited from the support of the Centre de Synthese et d'Analyse sur la Biodiversite (CESAB) at the Fondation pour la Recherche sur la Biodiversite and the inspirational discussions among the Acoucene consortium.