Understanding Urban Biodiversity: A Blend of Nature and City Life

Cities are not lifeless concrete jungles. Beneath the pavement, between the buildings, and across the parks, a surprising amount of life thrives. Urban biodiversity – the variety of living organisms found within cities – is an increasingly important field of study as more than half of the global population now resides in urban areas. Understanding how nature coexists with city infrastructure is no longer a niche academic interest; it is essential for designing sustainable, livable cities.

What is urban biodiversity? 🔗

Urban biodiversity refers to the variety and abundance of life in a city, most commonly measured in terms of ecosystem types and extents – such as lakes, grasslands, wetlands, and forests – and the types and abundances of plant and animal species within them. This includes everything from microorganisms in the soil to birds nesting on building ledges, insects pollinating rooftop gardens, and trees lining streets.

The concept goes beyond simply counting species. Biodiversity has generally been used in a comprehensive manner to describe the variability of life, encompassing composition, structure, and function across all levels of biological organisation. In an urban context, this means examining how genetic diversity, species variety, and ecosystem processes all operate within heavily human-modified landscapes.

Although cities only account for roughly 3% of the Earth’s surface, they are often situated at important ecosystem junctions or in areas that had high levels of biodiversity before development. This is one of the reasons why cities, despite their drastically altered landscapes, can still harbour notable biological richness – though often in a much-changed form.

The complex urban ecosystem 🔗

An urban ecosystem is far more than buildings and roads. Urban ecosystems are composed of biological components – plants, animals, and other life forms – and physical components such as soil, water, air, climate, and topography, all interacting within a specified area. What makes urban ecosystems distinct is that human populations, their social institutions, economic activities, and built structures are part of this equation.

Grey infrastructure: the built environment 🔗

The term “grey infrastructure” refers to the engineered, human-made components of a city – buildings, roads, bridges, parking lots, drainage systems, and other hard surfaces. The horizontal structure of a city is a fragmented mosaic of small patches of landscaped vegetation, roads, residential neighbourhoods, and economic centres, while its vertical structure consists of tall buildings made of glass, metal, and concrete rather than natural vegetation.

Grey infrastructure significantly alters the local environment. Urban ecosystems tend to be warmer than surrounding ecosystems, show less rainwater infiltration into local soil, and produce higher rates of surface runoff after storms. Heavy metals, dust, fertilisers, pesticides, and pharmaceutical contaminants also concentrate in these areas. These conditions create environmental pressures that filter out many species while favouring those that can tolerate such stresses.

Green and blue spaces: nature’s urban refuge 🔗

In contrast to grey infrastructure, green spaces – parks, urban forests, gardens, green roofs, street trees, and even abandoned lots – provide critical habitat for urban wildlife. Blue spaces such as rivers, ponds, lakes, and wetlands further add to habitat diversity. Urban green infrastructure has emerged as a multifunctional strategy to reconcile urbanisation with biodiversity conservation and ecosystem services provision.

For instance, green roofs and walls can help reduce the urban heat island effect, manage stormwater, and simultaneously provide habitats for birds, insects, and plants. Parks and urban forests serve as recreational spaces for people while also functioning as carbon sinks and biodiversity hotspots. These green patches are not isolated luxuries; they are ecologically functional spaces that support real biological communities.

Informal green spaces such as utility easements and brownfield sites have also been found to be particularly important for supporting species richness. Research from Melbourne, Australia, showed that these overlooked spaces contribute to urban species richness at levels comparable to formal parks, underscoring the need to consider them in green space planning.

The urban matrix and biodiversity hotspots 🔗

Cities are not uniform environments. When ecologists study urban areas, they see them as a mosaic – a patchwork of distinct habitat types arranged across the landscape. Cities are unique mosaic landscapes with heterogeneity that reflects both the local, natural ecosystem and an urban landscape common to cities worldwide. This concept of the “urban matrix” is central to understanding how biodiversity is distributed across a city.

Urban patches as habitat islands 🔗

One useful way to think about urban biodiversity is through the lens of island biogeography. Green spaces like parks, green rooftops, and backyard gardens function as green “islands” within the cityscape, separated not by water but by a matrix of impervious concrete where many organisms cannot survive. Just as on oceanic islands, larger habitat patches in cities tend to support more species, and more connected patches allow greater movement of organisms between them.

Urban habitats are extremely diverse: parks, cemeteries, vacant lots, streams, gardens, campus areas, golf courses, bridges, and landfills all qualify. These habitats are dynamic, shaped by ecological, social, and economic forces. In highly built-up areas, habitat patches in the urban core can be isolated from each other by the built environment, making dispersal risky and difficult for poorly mobile organisms.

Surprising biodiversity in cities 🔗

Despite the challenges, cities can be remarkably biodiverse. Well-managed urban areas can support high levels of biodiversity, and many cities are already situated within globally recognised biodiversity hotspots. Studies have found that even domestic gardens near high-density city centres can support over a thousand macro-organism species, many of which are native.

Research in temperate regions shows that, on a small scale, urbanisation often increases the diversity of non-native species while reducing that of natives, typically resulting in lower overall species richness but higher total biomass. However, tropical and subtropical cities can retain very high diversity if small habitat patches are preserved throughout the urban area.

The key factor is habitat heterogeneity. A city with a greater variety of habitat types – from manicured parks to overgrown brownfields to tree-lined streets – will generally support a wider range of species than one dominated by uniform commercial development. Ecological research in urban areas has highlighted that this complex mosaic of land uses is what makes cities ecologically interesting and, potentially, quite productive.

Interactions of urban and natural ecosystems 🔗

The relationship between urban development and natural habitats is not one-directional. Cities alter existing ecosystems, but they also create entirely new ones – and many species are adapting to take advantage of these novel environments.

How cities alter natural habitats 🔗

The expansion of large urban areas results in the conversion of forests, wetlands, deserts, and other adjacent biomes into residential, industrial, and commercial zones. This conversion fragments remaining wild ecosystems into ever-smaller patches. The boundary zones between native ecosystems and modified areas – known as “edge habitats” – tend to favour generalist species while pushing out specialists that need very specific environmental conditions.

Over recent decades, urbanisation has been a major driver of habitat loss, and residential and commercial development ranks as the third most frequently cited threat to species on the IUCN Red List. Urbanisation creates physical and chemical barriers that limit species’ ability to find food, shelter, and breeding partners.

New habitats and novel ecosystems 🔗

At the same time, cities generate habitats that did not previously exist. Rooftop gardens, constructed wetlands, railway embankments, old walls, and even underground transit systems provide niches for various organisms. Urban ecosystems are highly dynamic and can provide useful insights into the management of biodiversity, characterised by a high level of heterogeneity often organised along gradients extending from the surrounding landscape to the town centre.

These novel ecosystems may not replicate natural ones, but they have ecological value. Brownfields, for instance, often develop spontaneous plant communities that attract pollinators and other wildlife. Green roofs can provide habitat for ground-nesting bees and other invertebrates. Even the cracks in pavements can support hardy plant species that, in turn, feed small herbivores.

Species adaptation to city life 🔗

Urban animal communities tend to be dominated by medium-size generalists – animals like raccoons, coyotes, foxes, and certain bird species that can survive across a wide range of environmental conditions. These species have traits that predispose them to success in disturbed, varied habitats.

But adaptation goes deeper than simply being a generalist. Many species in urban settings are evolving along different lines than their rural counterparts. For example, white clover in cities produces less hydrogen cyanide than rural populations – an evolutionary response to reduced herbivore pressure in urban areas. Urban birds commonly develop louder, higher-pitched songs to communicate over traffic noise. Brown rats in New York City show signs of evolving smaller teeth. Water fleas in urban ponds mature and reproduce faster than those in rural ponds.

Urban ecosystems provide unprecedentedly potent evolutionary stimuli due to their myriad novel pressures, contexts, and species assemblages. Cities essentially function as real-time laboratories for studying how organisms respond to rapid environmental change – both through behavioural flexibility within an individual’s lifetime and through genetic changes across generations.

The role of connectivity 🔗

For urban biodiversity to be sustained, isolated habitat patches need to be linked. Green spaces within urban green infrastructure should be interconnected via green corridors, wildlife corridors, and ecological networks to facilitate species movement, maintain genetic diversity, and strengthen ecosystem resilience. Without connectivity, small and isolated populations face higher risks of inbreeding and local extinction.

Habitat loss from forestry, agriculture, urbanisation, and industrial development remains the greatest threat to species in urbanised regions. However, targeted actions – such as planting native species along road verges, maintaining continuous tree canopy cover, and protecting waterways – can help stitch together fragmented habitats and support wildlife movement through the urban matrix.

Why urban biodiversity matters 🔗

Human survival is dependent on biodiversity because diverse organisms affect ecosystem processes and functions, and therefore ecosystem services – the benefits that humans derive from ecosystems. In cities, these services include air purification by trees, stormwater management by green spaces, pollination of urban gardens, and temperature regulation through shade and evapotranspiration.

There is also a deeply personal dimension. As the proportion of urban residents increases every year worldwide, the nature of urban ecosystems becomes increasingly important in shaping people’s views about natural ecosystems. Daily exposure to nature in the city – whether it is a bird singing outside a window or a butterfly visiting a balcony planter – may be a major factor in cultivating broader environmental awareness and concern.

Nature-based solutions for infrastructure could cost 50% less than grey infrastructure alternatives alone and deliver 28% in added value, including carbon sequestration, cleaner air, better health outcomes, and economic opportunities. Investing in urban biodiversity is therefore not just an ecological choice but a sound economic and public health strategy.

Looking ahead: building biodiversity-friendly cities 🔗

The evidence is clear that cities do not have to be ecological dead zones. With thoughtful planning, urban areas can support meaningful levels of biodiversity while simultaneously improving quality of life for residents. Key strategies include preserving and expanding green spaces, maintaining connectivity between habitat patches, prioritising native plant species in landscaping, incorporating green roofs and walls into building design, and protecting informal green areas like brownfields and utility corridors.

Cities and their surroundings can be part of the solution – they can host biodiversity hotspots, and urban green infrastructure can deliver important benefits as part of regional ecological networks. However, this requires that urban planning actively accounts for ecological values, not just aesthetic or recreational ones.

Urban biodiversity is not a contradiction in terms. It is a reality – one that grows richer or poorer depending on the choices cities make about how they grow, what they preserve, and how they integrate nature into their fabric.

What do you think? Does your city do enough to support biodiversity through its parks, green corridors, and building designs? How might recognising cities as living ecosystems change the way we approach urban planning and development?

References

1. https://www.frontiersin.org/journals/ecology-and-evolution/articles/10.3389/fevo.2024.1440477/full
2. https://link.springer.com/article/10.1007/s11252-024-01623-0
3. https://www.nature.com/articles/s44358-026-00138-0
4. https://www.britannica.com/science/urban-ecosystem
5. https://iucn.org/story/202305/embracing-biodiversity-paving-way-nature-inclusive-cities
6. https://knowablemagazine.org/content/article/living-world/2022/urban-evolution-species-adapt-survive-cities
7. https://wwf.ca/habitat/urban-areas/