Vegetation complexity and nesting resource availability predict bee diversity and functional traits in community gardens

Bees in the city: Findings from a scoping review and recommendations for urban planning

Over the last decades, bee biodiversity has dropped sharply due to land use change, including urbanization. To contrast this, recent research has pointed to cities as a hotspot for bees. Because of this ambiguity, a scoping review has been conducted to examine the urban characteristics that impact bees and how bees are impacted. A total of 276 articles were analyzed against landscape and local habitat characteristics. The key findings include first that natural areas are more valuable for bees since biodiversity levels are higher. Second, urban areas generally score better than agricultural and rural areas. Third, plant biodiversity positively influences bee biodiversity. Fourth, the urban environment strongly affects some bee traits and the proportion of native bees. For making cities bee friendly and bee inclusive, we recommend to maintain natural areas, connect natural areas to urban ecosystems, encourage floral abundance and diversity and increasing the size of urban green areas overall.

Local and landscape factors shape alpha and beta trophic interaction diversity in urban gardens

Promoting urban green spaces is an effective strategy to increase biodiversity in cities. However, our understanding of how local and landscape factors influence trophic interactions in these urban contexts remains limited. Here, we sampled cavity-nesting bees and wasps and their natural enemies within 85 urban gardens in Zurich (Switzerland) to identify factors associated with the diversity and dissimilarity of antagonistic interactions in these communities. The proportions of built-up area and urban green area at small landscape scales (50 m radius), as well as the management intensity, sun exposure, plant richness and proportion of agricultural land at the landscape scale (250 m radius), were key drivers of interaction diversity. This increased interaction diversity resulted not only from the higher richness of host and natural enemy species, but also from species participating in more interactions. Furthermore, dissimilarity in community structure and interactions across gardens (beta-diversity) were primarily influenced by differences in built-up areas and urban green areas at the landscape scale, as well as by management intensity. Our study offers crucial insights for urban planning and conservation strategies, supporting sustainability goals by helping to understand the factors that shape insect communities and their trophic interactions in urban gardens.

Response of pollinator taxa to fire is consistent with historic fire regimes in the Sierra Nevada and mediated through floral richness†

Many fire-prone forests are experiencing wildfires that burn outside the historical range of variation in extent and severity. These fires impact pollinators and the ecosystem services they provide, but how the effects of fire are mediated by burn severity in different habitats is not well understood. We used generalized linear mixed models in a Bayesian framework to model the abundance of pollinators as a function of burn severity, habitat, and floral resources in post-fire, mid-elevation, conifer forest, and meadow in the Sierra Nevada, California. Although most species-level effects were not significant, we found highly consistent negative impacts of burn severity in meadows where pollinators were most abundant, with only hummingbirds and some butterfly families responding positively to burn severity in meadows. Moderate-severity fire tended to increase the abundance of most pollinator taxa in upland forest habitat, indicating that even in large fires that burn primarily at high- and moderate-severity patches may be associated with improved habitat conditions for pollinator species in upland forest. Nearly all pollinator taxa responded positively to floral richness but not necessarily to floral abundance. Given that much of the Sierra Nevada is predicted to burn at high severity, limiting high-severity effects in meadow and upland habitats may help conserve pollinator communities whereas low- to moderate-severity fire may be needed in both systems.

Effects of forest management on native bee biodiversity under the tallest trees in the world

It is not clear if mature secondary growth coniferous forests can support similar pollinator communities as old growth coniferous forests, or how active management (e.g., retention forestry) in mature secondary growth forests may affect pollinator communities. We compare the native bee community and plant-bee interaction networks of old growth, naturally regenerating and actively managed (retention forestry) mature secondary growth forests of similar stand age. Old growth forests had a higher bee species richness and Shannon's diversity index, but not Simpson's diversity index, than both actively managed and naturally regenerating mature secondary forests. Forest type (old-growth, naturally regenerating mature secondary growth, and actively managed mature secondary growth) had a significant effect on bee community composition. Redwood forest bee-plant interaction networks were small in size and had lower complexity than expected and few connector species. While studies suggest that small-scale timber harvest may increase bee biodiversity in the short-term in other coniferous forest habitats, our study suggests that there may be long-term negative effects of clear-cutting that lower bee biodiversity in mature secondary growth forests as compared to mature old-growth forests.