Rare species biodiversity, socio-demographics and local and landscape characteristics in Northern California community urban gardens

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 over 50% of plant cultivars
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 where 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.