Trends in Urban Biodiversity: Adapting to a Growing Urban World

Cities are expanding at an unprecedented pace. Research published in Nature Communications projects that future urban expansion could result in 11 to 33 million hectares of natural habitat loss by 2100. This growth is reshaping the natural world in profound ways – fragmenting habitats, filtering which species can survive, and creating entirely new ecological dynamics. Understanding the patterns and trends in urban biodiversity is essential for anyone who cares about how life on Earth will look in the decades ahead.

How urbanization reshapes biodiversity
Why mobility matters for survival
Biotic homogenization: when cities start to look the same
Floral species richness in urban landscapes
The rise of introduced species
What drives urban plant diversity?
Faunal species diversity and urban life
Mammals in the city
Birds: urban adapters and avoiders
Invertebrates: the overlooked majority
Reptiles and amphibians: the most vulnerable
The path forward: urban biodiversity conservation

How urbanization reshapes biodiversity

When cities grow, they don’t just take up space. They fundamentally alter how ecosystems function. The conversion of natural land into roads, buildings, and infrastructure breaks up continuous habitats into smaller, isolated patches. This process – known as habitat fragmentation – is one of the most significant drivers of biodiversity loss in urban areas.

A study examining 16 major cities over two centuries found that urbanization consistently decreases habitat area while simultaneously increasing fragmentation. The relationship between habitat loss and fragmentation during urban development tends to be monotonic, meaning both worsen together over time as cities expand. This creates a compounding effect: not only is there less habitat available, but what remains becomes increasingly isolated.

The consequences are far-reaching. Fragmented landscapes disrupt migration routes, limit gene flow between populations, and expose species to edge effects such as increased predation, invasive species incursion, and altered microclimates. According to a PNAS study on projected urban expansion, new urban land development is expected to directly threaten more than 800 species globally within the next few decades, with hotspot cities in sub-Saharan Africa, Southeast Asia, and South America facing the greatest risks.

Why mobility matters for survival

Not all species respond to urban fragmentation in the same way. A critical factor is mobility – how far and how easily an organism can move across a landscape. Highly mobile species such as birds, bats, and certain mammals can navigate between fragmented habitat patches, find food, locate mates, and maintain genetic diversity across urban barriers. Less mobile species, however, face a much tougher reality.

Amphibians are a prime example. These animals typically have poor dispersal abilities, with most species moving less than 300 metres during their lifetime. When roads, buildings, and concrete surfaces cut through their habitat, they struggle to reach breeding pools and terrestrial feeding grounds. Research from a global analysis of urbanization effects on amphibians confirms that species with limited migration and dispersal abilities respond most negatively to urban development. Globally, amphibian richness drops by approximately 36% along urbanization gradients.

Roads are particularly lethal for amphibians. During spring migrations to breeding sites, many frogs, toads, and newts must cross roadways, leading to mass road-kill events. Urban fragmentation reduces species diversity and abundance, and when combined with habitat degradation and loss of breeding sites, cities can become extremely challenging environments for amphibians. In fact, more than one-third of all amphibian species are currently threatened by urbanization-related pressures.

Biotic homogenization: when cities start to look the same

One of the more subtle but important trends in urban biodiversity is biotic homogenization. As cities filter out specialist species and favour generalists, urban wildlife communities around the world start to resemble each other. You’ll find pigeons, house sparrows, rats, and raccoons (or their ecological equivalents) in cities across different continents. This global convergence of urban species comes at the cost of local, unique biodiversity. Native specialists – species that depend on specific habitats or food sources – are increasingly replaced by a small set of cosmopolitan generalists.

Floral species richness in urban landscapes

Plant diversity in cities tells a complex story. On one hand, urbanization reduces native plant richness through land conversion and habitat degradation. On the other, human activity introduces a large number of non-native plant species – through gardening, landscaping, accidental transport, and deliberate planting – that often naturalize and persist in urban environments.

The rise of introduced species

A global meta-analysis published in Ecology and Evolution found that urbanization has positive effects on introduced plant species but negative effects on native species. Cities essentially act as ecological filters, selecting species that are more tolerant of disturbed conditions, heat, pollution, and drought. Introduced species, many of which evolved in similarly stressed environments or were specifically chosen for their hardiness, tend to thrive.

Research from Adelaide, Australia illustrates this trend well: plant species richness in the city increased by 46% between 1836 and 2002, driven almost entirely by the introduction of exotic species outpacing native extinctions. In Los Angeles, cultivated plant species originate from every continent except Antarctica, creating novel assemblages with no parallel in native ecosystems.

This doesn’t mean urban plant diversity is healthy or equivalent to natural diversity. The replacement of native flora with introduced species changes pollinator networks, soil ecology, and food webs. Native insects, for instance, may not be able to feed on or pollinate non-native plants, creating cascading effects up the food chain.

What drives urban plant diversity?

Several factors influence how rich or poor a city’s plant life will be. Habitat availability is a key driver – cities with more parks, gardens, green corridors, and vacant lots tend to support greater plant diversity. Patch size matters too. Research inspired by island biogeography has shown that the area of green patches, their distance from the city boundary, and edge effects are among the best predictors of both native and non-native spontaneous plant richness in urban settings.

Climate and latitude also play a role. Cities at lower latitudes tend to retain higher native plant richness even under urbanization, likely because warmer climates support a larger overall species pool. Meanwhile, a systematic review of global research identified land cover change and biotic invasion as the two most commonly cited urban drivers responsible for changes in plant communities. Urbanization reduces pollinator availability, promotes environmental homogenization, and causes shifts in the timing of flowering and other phenological events.

There is, however, a resilience story. Some urban plant species demonstrate remarkable phenotypic plasticity – the ability to adjust their traits in response to new environmental pressures without genetic change. Others undergo genuine evolutionary adaptation, developing traits suited to urban heat, pollution, and altered soil conditions over multiple generations.

Faunal species diversity and urban life

Cities are far from the biological deserts they were once assumed to be. A surprising range of animal species – from mammals and birds to reptiles and invertebrates – have found ways to persist and even flourish in urban environments. However, the strategies they use and their degrees of success vary enormously.

Mammals in the city

A study in Global Ecology and Biogeography identified 190 mammal species that regularly occur in urban settlements worldwide. The research found that all urban mammals tend to produce larger litters than their rural counterparts – a trait that likely helps offset higher mortality in city environments. Beyond litter size, successful urban mammals tend to be dietary generalists with broad trophic niches.

Raccoons, red foxes, coyotes, and various macaque species are among the most successful urban mammal exploiters. These species share key traits: they are omnivorous, behaviourally flexible, and capable of exploiting human food sources such as garbage, pet food, and bird feeders. Larger predators like mountain lions and wolves, in contrast, generally avoid urban cores due to spatial constraints and high conflict risk with humans.

The concept of synurbization – the process by which wildlife adapts to urban environments – creates a clear divide between “winners” and “losers.” Generalist species that can exploit novel resources thrive, while specialists with specific habitat requirements decline and may eventually face local extinction.

Birds: urban adapters and avoiders

Birds are the most studied group in urban ecology, and the trends are well established. Species richness typically declines with increasing urbanization, but certain species have become remarkably successful city dwellers. Peregrine falcons nest on skyscrapers, Cooper’s hawks exploit abundant prey in suburban neighbourhoods, and crows have become master urban scavengers.

Research suggests that cities can serve as important stopover sites for migratory birds and even as refuges for threatened species, provided that green spaces are maintained. In Delhi, for instance, ponds covering just 0.5% of the city’s land area support 37% of all bird species ever documented there. Urban birds have also shown behavioural adaptations, with some species singing at higher pitches to be heard above traffic noise.

Invertebrates: the overlooked majority

While mammals and birds get most of the attention, invertebrates – including insects, spiders, and other arthropods – are the most species-rich group in any ecosystem, including urban ones. Research from a study of arthropod biodiversity in Los Angeles found that cities can support high regional arthropod diversity, though community composition varies significantly depending on the taxonomic group and local habitat conditions.

Arthropods play essential ecological roles in cities: they pollinate urban gardens, decompose organic matter, cycle nutrients, and serve as food for birds and other wildlife. Interestingly, arthropod diversity in cities is influenced by the so-called luxury effect, where wealthier neighbourhoods with more vegetation cover and better-maintained gardens tend to support higher biodiversity than lower-income areas with less green space.

Urban pollinators, particularly bees, have become a focus of conservation concern. Cities can act as pollinator reservoirs, with green areas within them serving as ecological stepping stones connecting habitats across the landscape. However, habitat fragmentation, pesticide use, and the prevalence of non-native ornamental plants over pollen-rich native species continue to pose challenges.

Reptiles and amphibians: the most vulnerable

Among urban fauna, reptiles and amphibians tend to fare the worst. A study examining 17 cities in North America found that species richness for both groups was 10-20% lower inside cities compared to surrounding areas. For reptiles, impervious surface cover was the main predictor of decline, while for amphibians, loss of canopy cover was most strongly associated with reduced richness.

Amphibians face a particularly harsh urban reality. Their permeable skin makes them sensitive to pollution, their dependence on both aquatic and terrestrial habitats makes them vulnerable to fragmentation, and their limited dispersal ability means that once a local population disappears, recolonization is unlikely. Species that are habitat generalists or have lower dispersal requirements tend to persist better in urban settings, but many specialist species simply cannot cope.

The path forward: urban biodiversity conservation

The patterns are clear: urbanization transforms biodiversity, favouring mobile generalists over sedentary specialists, promoting introduced species over natives, and homogenizing ecological communities across the globe. But these trends are not inevitable. How we design and manage cities matters enormously.

Maintaining and creating green corridors that connect habitat patches can restore some degree of landscape connectivity, helping less mobile species move through urban areas. Preserving wetlands, even small urban ponds, can provide critical habitat for amphibians and waterbirds. Planting native species in parks and gardens supports local pollinators and maintains ecological food webs. And thoughtful urban planning that limits sprawl into biodiversity hotspots can prevent the worst impacts before they occur.

As researchers at Yale have emphasized, cities can be part of the solution to the biodiversity crisis, not just a cause of it. With 70% of the global population expected to live in urban areas by 2050, the decisions we make about urban design today will shape the future of biodiversity for generations.

What do you think? In your city or neighbourhood, have you noticed changes in the types of wildlife you see compared to a decade ago? And what small steps – like planting native species or preserving a local pond – could make a meaningful difference for urban biodiversity where you live?

References

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Table of Contents

How urbanization reshapes biodiversity 🔗

Why mobility matters for survival 🔗

Biotic homogenization: when cities start to look the same 🔗

Floral species richness in urban landscapes 🔗

The rise of introduced species 🔗

What drives urban plant diversity? 🔗

Faunal species diversity and urban life 🔗

Mammals in the city 🔗

Birds: urban adapters and avoiders 🔗

Invertebrates: the overlooked majority 🔗

Reptiles and amphibians: the most vulnerable 🔗

The path forward: urban biodiversity conservation 🔗