Examining ecological interactions of urban tree and vertebrate species in New Delhi, India

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Vallari Sheel and Madhusudan Katti, Forestry and Environmental Resources, North Carolina State University, Raleigh, NC

Background/Question/Methods
The treescape of most Indian cities consists of a mix of native and non-native trees. Many vertebrates use these trees but we know little of their interactions with different tree species, as pollinators and seed dispersers, and for food, nesting and roosting habitat. Trees such as Ficus spp., Syzigium cumini and Bombax ceiba are well known to support diverse wildlife in forests, but we lack systematic data from urban spaces. We examine the ecological interactions of native and non-native tree species with diurnal vertebrates (birds, arboreal mammals) in New Delhi, India.
We aim to examine: a) which urban vertebrate species depend on which tree species for food (fruit, nectar); b) whether non-native tree species support more/fewer vertebrates than native trees; and c) if any tree species plays a “keystone” role in providing food resources for urban vertebrates. In March 2020, we started with the goal of monitoring 20 focal tree species (N = 10 individuals/species) systematically (1-3 hours/tree, within 4 hours of sunrise in 6 public parks) in their flowering/fruiting season for vertebrate activity, noting abundance and activity (feeding, nesting, perching, roosting) of all vertebrates on each tree. The COVID-19 pandemic and its recent resurgence in India forced us to pivot to additional data from secondary sources.
Results/Conclusions
We have monitored 123 individuals of 16 tree species till now with fieldwork constrained by the pandemic. We found higher levels of vertebrate association for Bombax ceiba (24 spp), Ficus virens (19), F. religiosa (17), Ehretia laevis (17), Morus alba (16) and Erythrina variegata (13) than for Butea monosperma (3), Bauhinia variegata (3), Cassia fistula (2) and Plumeria obtusa (0). While current data suggests a significant difference in food provision by some native vs. non-native trees, statistical testing is constrained by limited data due to the pandemic. With ongoing fieldwork, we aim to use food web network analysis to robustly identify significant food resource tree species.
Bombax ceiba, Ficus virens and F. religiosa trees are significant food sources for many urban vertebrates, cementing a similar role in cities as in forest ecosystems. Fruiting species including Morus alba and Ehretia laevis also provide food for frugivores, but ornamental species, such as Bauhinia variegata and Bauhinia X blakaena support few vertebrates. Our findings at the end of this study will have significant management implications by informing tree plantation programs in Delhi (and other Indian cities) to better manage, conserve and enhance urban biodiversity.