Species diversity enhances perceptions of urban coastlines at multiple scales.
Biodiversity is increasingly understood as an important mediator of human aesthetic appreciation of scenes and landscapes, with implications for cultural services and well-being. However, the generality of biodiversity effects across affective emotions, scales and habitats remains unclear. Urban coastal intertidal habitats on seawalls and other artificial structures are expanding worldwide. Despite growing calls to prioritise biodiversity in urban coastal planning and management, the potential co-benefits determined by people's responses to biodiversity in these novel intertidal communities are unexplored. We investigated, using image-based questionnaires, how several facets of biodiversity influence how people perceive urban coastal structures at both landscape and close-up scales. Species richness strongly enhanced people's ratings of images for aesthetic appeal, interest and calming potential at both scales, but was more pronounced at the close-up scale. Species evenness also increased ratings at the close-up scale, while functional diversity (Rao's Q) was associated with a decline in aesthetic appeal and interest at the close-up scale, indicating that people can disfavour scenes dominated by species with contrasting traits. Analysis of free-text assessments showed that people strongly and positively valued scenes that were perceived to be 'diverse', a response that was much more common when viewing scenes with high species richness. The underlying structure type also clearly affected appraisals, with more obviously engineered structures being perceived to be less natural and thus less desirable. Our results show that biodiversity's effects on aesthetic appreciation extend to multiple affective emotions and to unfamiliar urban intertidal habitats, suggesting that managing these environments for biodiversity may simultaneously support aesthetic, educational and well-being benefits. Nevertheless, the sensitivity of responses to the facet of biodiversity and viewing scale in our results underlines the context dependency and complexity of people's perceptions of urban environments.