When Cities Have Trees That Don’t Belong, the Birds Notice

“Urban ecology begins with the trees we plant and ends with the species we forget”

Why in the News?

A recent study by researchers from the Indian Institute of Human Settlements (IIHS), Bengaluru, published in Ecological Applications, has revealed that urban heat and tree cover type are crucial determinants of bird biodiversity in Indian cities. The study, focused on Bengaluru, found that native vegetation supports more biodiversity, whereas non-native or exotic trees, which dominate over 77% of the city’s green cover, may adversely affect urban ecosystems.

Background: Urban Heat and Biodiversity Decline

India’s cities are experiencing unprecedented warming, urban sprawl, and fragmentation of natural habitats, all of which have direct and indirect effects on biodiversity. 

• The Urban Heat Island (UHI) effect, where cities are significantly warmer than surrounding rural areas, worsens heat stress, especially for flora and fauna.
• Birds, being sensitive bio-indicators, offer early signs of ecological stress. 
• The IIHS study used satellite thermal imaging and crowd-sourced bird data from eBird to assess the correlation between heat zones and biodiversity in Bengaluru. 
• Findings showed that cooler zones had more bird diversity, and areas with exotic trees or no vegetation exhibited significant species decline.

Features of the Study

Scope and Methodology

• Authors: Ravi Jambhekar, Dilip Naidu, Jagdish Krishnaswamy (IIHS).
• Data: Bird distribution from eBird, thermal patterns from satellite data.
• Focus: How green cover quality (not just quantity) affects bird distribution.

Key Findings

• Bird diversity is highest in areas with lower surface temperatures and native vegetation.
• Heat islands, typically asphalted or concretised regions, show the lowest avian richness.
• Exotic tree plantations like Jacaranda and Tabebuia, often used for aesthetic purposes, do not support local biodiversity and may even hinder it.

Challenges Identified

Exotic vs. Native Vegetation

• Exotic species, though fast-growing and visually appealing, do not support local insects, birds, or microbes.
• 77% of Bengaluru’s trees are non-native, offering little food or nesting grounds to native birds.
• Example: Trees like Jacaranda bloom vividly but have no ecological relationship with Indian birds or insect life.

Habitat Simplification

• Tree-planting drives often ignore habitat complexity, focusing only on canopy trees.
• Native ecosystems include a layered structure: herbs, shrubs, grasses, vines, and trees, each supporting specific fauna.
• The conversion of wetlands and grasslands into tree parks may eliminate open-habitat species.

Urban Planning Disconnect

• City planners often ignore historical land use.
• Bengaluru’s original land mosaic included wetlands, irrigation tanks, and open scrublands, which have disappeared under unscientific afforestation.

Heat Vulnerability and Inequality

• Ecological degradation correlates with social inequity:
  • Poorer neighbourhoods face higher temperatures, have less green space, and greater health risks.
  • Access to parks is often restricted or unequally distributed across economic classes.

Ecological Insights

Birds and Heat Stress

• High temperatures reduce birds’ reproductive success (e.g., thinner eggshells).
• Heat affects the availability of food insects, nectar, and fruits more rapidly than birds themselves.
• Some open-habitat birds like those of grasslands may not prefer dense tree cover, further underscoring the need for habitat diversity, not just more trees.

Impact on Ecosystems

• Tree monocultures and invasive exotics disrupt:
  • Pollination networks
  • Soil microbiomes
  • Native insect diversity
• Loss of wetlands and open grasslands affects migratory and endemic species.

Comparative Perspectives

Dehradun vs. Bengaluru

• In Dehradun, green cover correlates positively with species richness.
• In Bengaluru, the type of green cover matters more than its amount. Native species attract native birds; exotics do not.

Pune’s Mosaic Approach

• Pune’s landscape includes savannah patches and secondary forests.
• Conservationist Gurudas Nulkar emphasises restoring photosynthetic capacity by including grasslands and wetlands, not just afforestation.

Way Forward

Native Flora

• Urban forestry must focus on:
  • Local trees (e.g., Peepal, Banyan, Neem, Arjuna)
  • Shrubs and grasses are adapted to local conditions.
• Avoid visual landscaping preferences in favour of ecological relevance.

Urban Biodiversity Planning

• Develop city-level biodiversity goals.
• Integrate land-use zones (e.g., wetlands, grasslands, scrublands) into master plans.

Historical Restoration

• Reconstruct habitat mosaics based on historical land cover data.
• Urban green spaces must be designed in line with the natural character of the terrain.

Inclusive Green Access

• Ensure equitable access to green spaces for marginalised communities.
• Avoid gated parks and create public green corridors that link neighbourhoods.

Decentralised Green Infrastructure

• Encourage:
  • Home gardens
  • Vertical greens
  • Pocket parks
  • Community wetlands
• Strategic greening around heat hotspots can offer maximum climate relief.

Research and Citizen Science

• Build more open-access biodiversity datasets like eBird.
• Encourage citizen surveys to monitor ecological changes and intervene early.

Conclusion

The IIHS study delivers a powerful reminder: urban greening without ecological context is misguided. Merely increasing tree numbers without considering species type and habitat suitability can degrade rather than enhance biodiversity. Indian cities like Bengaluru must shift from tree-counting to ecosystem restoration, and from cosmetic planting to meaningful ecological integration.

MAINS PRACTICE QUESTION

Question: Urban greening strategies often ignore ecological relevance and historical land use, thereby threatening biodiversity rather than preserving it. In light of recent ecological studies on Bengaluru, discuss the limitations of afforestation as a biodiversity strategy and suggest urban planning measures to promote ecologically viable green spaces.

PRELIMS PRACTICE QUESTION