Local-scale factors predict wild bee richness, whereas landscape-scale factors predict butterfly richness in city parks.

Abstract

Urbanization has a growing influence on pollinator species worldwide, benefiting some species while threatening others. Urban areas have high cover of impervious surfaces, altered floral communities, and competition from non-native species. Additionally, the existing biodiversity in urban areas may not be equitably distributed, instead reflecting patterns of wealth (ie luxury effects). Despite these challenges, city parks can serve as refuges for pollinator biodiversity. We investigated how wild bee and butterfly species richness and community composition differed across city parks depending on landscape-scale (neighborhood income and land cover surrounding a park) and local-scale (floral traits and honey bee abundance) factors in Denver, Colorado, United States. We found increased wild bee richness was primarily related to local-scale factors like increases in floral species richness and non-weedy plant cover. Butterfly richness was instead primarily negatively associated with landscape-scale factors like increased cover of impervious surfaces and income. Although we did not observe broad-scale luxury effects, we found that lower-income parks had a higher proportion of non-native weedy plants and lower wild bee species richness than higher-income parks, which had primarily native plantings and higher wild bee richness. Wild bee communities differed between parks with high and low abundances of honey bees (Apis mellifera), with Bombus huntii most associated with abundant honey bee parks, and 7 solitary bees associated with lower honey bee parks. These findings suggest managers can implement species-rich native plantings in parks to support wild bee diversity regardless of landscape context. However, interventions to support butterflies may rely more on landscape-level planning.