The community structure of bird assemblages on urban Strangler figs

Figs have been regarded as keystone plant resources that support diverse tropical vertebrate frugivore communities. Planting or conserving large fig trees, such as stranglers, has therefore been proposed for enhancing urban biodiversity. We compared the diversity and community structure of bird assemblages on strangler figs with non-fig urban trees as well as between the fruiting and non-fruiting fig trees in an urban setting in Singapore. The total bird abundance across all the fig trees when in fruit was 4.5-fold higher than on non-fig trees and 3.5-fold higher than when the same fig trees were not fruiting, but only attracted two more species. On individual trees, after accounting for the presence of mistletoes, tree height, the area covered by buildings and road lane density, and distance to natural vegetation, mean diversity was not different between non-fig trees and fig trees when they were not in fruit. On the other hand, when fruiting, each fig tree on average had 1.4 more species, 3 more counts of non-native birds, and 1.6 more counts of insectivorous birds than when not fruiting. There was significant compositional turnover between non-fig trees and non-fruiting fig trees, while community dispersion was significantly lower among fig trees in fruit. Our results demonstrate that fig trees provide fruit and non-fruit resources for birds in an urban landscape but do not necessarily support a more diverse total bird assemblages than non-fig trees. Instead, bird communities on fruiting urban figs would be highly homogeneous and dominated by a few species.