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

Abstract

Cities are sometimes characterized as homogenous with species assemblages composed of abundant, generalist species having similar ecological functions. Under this assumption, rare species, or species observed infrequently, would have especially high conservation value in cities for their potential to increase functional diversity. Management to increase the number of rare species in cities could be an important conservation strategy in a rapidly urbanizing world. However, most studies of species rarity define rarity in relatively pristine environments where human management and disturbance is minimized. We know little about what species are rare, how many species are rare, and what management practices promote rare species in urban environments. Here, we identified which plants and species of birds and bees that control pests and pollinate crops are rare in urban gardens and assessed how social, biophysical factors, and cross‐taxonomic comparisons influence rare species richness. We found overwhelming numbers of rare species, with more than 50% of plants observed classified as rare. Our results highlight the importance of women, older individuals, and gardeners who live closer to garden sites in increasing the number of rare plants within urban areas. Fewer rare plants were found in older gardens and gardens with more bare soil. There were more rare bird species in larger gardens and more rare bee species for which canopy cover was higher. We also found that in some cases, rarity begets rarity, with positive correlations found between the number of rare plants and bee species and between bee and bird species. Overall, our results suggest that urban gardens include a high number of species existing at low frequency and that social and biophysical factors promoting rare, planned biodiversity can cascade down to promote rare, associated biodiversity.

In defence of urban community gardens

“…Short‐lived and temporary community gardens will likely have different habitat features, plant composition, and may have a different conservation value than older community gardens (Ong et al., 2022). Furthermore, a lack of long‐term perspective may reduce the motivation of the community garden to engage with and connect with the surrounding neighbourhood, reducing spillover neighbourhood social cohesion benefits (Clarke et al., 2023).…”

“…In both cities, community gardens in landscapes with higher surrounding imperviousness are generally smaller. Yet, in Berlin, larger gardens have more ‘rarer’ plant species of low observed frequency (sensu Ong et al., 2022) and vegetation cover. In Munich, larger gardens have higher plant community diversity and higher total wild and cultivated plant species numbers (Figure 1; Conitz et al., unpublished).…”

Unterwegs in die Stadt der Zukunft

“…Abgesehen von den Beiträgen zur Pflanzenvielfalt auf Artenebene weisen Gemeinschaftsgärten auch eine hohe Sortenvielfalt auf (vgl. Ong et al, 2022).…”