Rarity begets rarity: Social and environmental drivers of rare organisms in cities

Background/Question/Methods

Urbanization can have detrimental effects on biodiversity, leading to the loss of important ecosystem functions and services. However, recent studies suggest that biodiversity measures such as species richness and abundance may be inadequate for assessing ecosystem services. Cities in particular are thought to host homogenous species assemblages composed of both abundant generalist species and introduced species with similar ecological functions. Therefore, rare species that provide a wide range of less common ecosystem functions have particularly high conservation potential in cities. We ask what species are rare in urban environments, what drives increased rarity in urban biodiversity, and what urban management practices support rarity in an increasingly urban world. To answer these questions, we collected plant, bird, and bee data from 18 urban community gardens in three counties (Santa Clara, Santa Cruz, and Monterey) in the central coast region of California, USA. We also conducted gardener surveys, collecting sociodemographic information as well as garden use and experience data. Using general linear mixed models, we examined which garden and gardener characteristics predicted rare plants, bird and bee species and how well rare taxa predicted each other.

Results/Conclusions

We found that gender, distance to garden from gardeners’ homes, and age played a significant role in influencing the number of rare plant species found in the garden plots. Specifically, older women who lived closer to their gardens tended to plant more rare species. When considering garden management and landscape characteristics, age of garden and percentage of bare soil significantly predicted number of rare plant species/cultivars, with younger, less bare gardens supporting more rare plant species. Larger gardens support a higher number of rare bird species and gardens surrounded by more urban land cover support more rare bee species. Finally, a higher number of rare bees is associated with a higher number of rare plants. Our results highlight the importance of both garden and gardener characteristics in mediating plant, bird, and bee rarity. Intentional human management of urban gardens can support rare species both directly and indirectly, impacting urban biodiversity and ecosystem function.