Better Forests, Better Cities

Sarah Jane Wilson Edith Juno John-Rob Pool Sabin Ray Mack Phillips Scott Francisco Sophie McCallum
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Chapter 5


Biodiverse forests are more resilient and often provide more reliable water and climate services than depleted or monoculture forests. Biodiverse forests can help to improve mental health, support key ecosystem services, and provide the blueprints for medicine. Forests in cities can be highly biodiverse, but many species will only thrive in forests outside of cities. City action on both fronts is critical.

Andres Felipe Perez/Shutterstock


Cities gain tremendous benefits from biodiversity (Box 3) inside and outside their boundaries (Figure 17). Biodiversity is a key feature of resilient ecosystems (Yadav and Mishra 2013), and cities rely on resilient systems for a wide range of services. Within cities, biodiversity can have positive impacts on mental health (Fuller et al. 2007; Wood et al. 2018). Around cities, biodiversity supports key regulating and provisioning ecosystem services, such as pollination (Cardinale et al. 2012; IPBES 2016; Alvarez-Garreton et al. 2019; Yu et al. 2019). Outside city boundaries, global biodiversity directly supports human health, food security, and climate regulation and provides the blueprints for new medicines (Chivian and Bernstein 2010; Hisano et al. 2018). The value of services from natural ecosystems, which rely on biodiversity to function, has recently been estimated at $44 trillion, or over half the global economy (World Economic Forum 2020).

Biodiversity is becoming a major topic in cities around the world. Activist groups, such as Extinction Rebellion, are demanding action to protect Earth’s species. Often, young and deeply engaged leaders are at the forefront of the charge, creating action plans and mobilizing political support and resources (Bagley 2019; Nwanevu 2019). Despite their distance from many of the world’s natural ecosystems, it makes sense that grassroots action is originating among city residents: cities have both a lot at stake and the strongest foothold to effect change in the cultures, regulatory frameworks, technologies, and market systems that are currently responsible for global biodiversity loss. The future of cities relies on a collective ability to keep natural ecosystems functioning. Cities contain highly concentrated populations that depend almost entirely on resources and ecosystems far outside their boundaries. Partly because of this, biodiversity is increasingly appearing on city agendas, although mainly at the inner forest level (Brende and Duque 2021).

But global biodiversity is rapidly declining. Because of the importance to human well-being everywhere, including within cities, multiple international agreements exist to support biodiversity conservation via sustainable development, including the Convention on Biological Diversity and the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services (IPBES n.d.). Yet despite these efforts, the world is already in the midst of the sixth “global extinction” (Kolbert 2014). Around 1 million species are at risk of going extinct, many within the next few decades (IPBES 2019), mainly from habitat loss and climate change. Up to 150 species are estimated to go extinct every day (CBD Secretariat 2007), and current extinction rates are 100–1,000 times greater than the “background” rate of extinction (Pimm et al. 2014; Ceballos et al. 2015). Unlike past mass extinctions, the current extinction event is being driven by humans, through habitat loss, climate change, overexploitation, and invasive species (Tilman et al. 2017; Mazor et al. 2018).

Reduced biodiversity could have widespread impacts on the functioning of ecological support systems on which cities rely (Ceballos et al. 2017; Tilman et al. 2017). When native biodiversity is lost, ecosystems often become more vulnerable to disturbance and become less reliable—and less effective—at providing vital services. Habitat fragmentation in highly biodiverse areas can even lead to spillover of pathogens from wildlife reservoirs to human hosts (Borremans et al. 2019; Evans et al. 2020). It is uncertain how much biodiversity loss can be sustained before ecosystems see major loss of function (Dirzo et al. 2014; Moore 2018).

Forests are bastions of biodiversity, and biodiverse forests also provide more reliable, plentiful benefits across health, water, and climate. Forests are estimated to hold well over 60 percent—to perhaps 90 percent—of Earth’s terrestrial biodiversity28 (Erwin 1982; Wilson 1988; Reid and Miller 1989), and forest conservation is essential to maintaining the diversity of life on Earth (Wilson 1988; Brooks et al. 2006; Mittermeier et al. 2011). But forests are being rapidly cleared, and deforestation is a leading cause of global extinction (Tilman et al. 2017)—a widely reported estimate states that 135 species a day are lost through deforestation alone, but this is difficult to pinpoint as the total number of forest species and their ranges are unknown (Wilson 1988). In 2020, 12.2 million ha of tropical forest were lost, of which over a third—4.2 million ha—were in humid tropical primary forests, which are extremely important sources of biodiversity (Weisse and Goldman 2021). People in cities are responsible for an astounding 75 percent of tropical deforestation per year through their consumption. Their direct choices and indirect influence are key to conserving forest biodiversity globally and, with it, important benefits upon which they rely.

Maintaining urban forest biodiversity is increasingly appearing on urban agendas, but municipal policies and practices to protect forest biodiversity in forests outside their boundaries are rare.29 In this section, we highlight nine key ways—based on the authors’ synthesis of the literature review—that forest biodiversity near and far supports cities and city residents—and, in turn, how cities can work to support and conserve global biodiversity.

Figure 17 | The Biodiversity Benefits of Forests at Three Levels

Source: Authors. Adapted from Cities4Forests n.d.a

Box 3 | What Is Biodiversity?

Biodiversity is the variation among genes, species, and communities of living organisms on Earth.a Species-level diversity is commonly used to assess biodiversity in forests and other ecosystems, and it is the main metric used in this section. Biodiversity can be measured at many scales, from local to global.b At the patch scale, promoting biodiversity could mean ensuring many native species are present; at the city scale, it could mean protecting native habitats and increasing connectivity. On the global scale, biodiversity conservation requires conserving a wide range of species and habitats, especially those that are rare, threatened, and/or species that have restricted ranges.

Endemic species—those with very limited ranges—are especially vulnerable to extinction, and the number of endemic species in an area often shapes global efforts to conserve biodiversity.c Many of the world’s tropical cities are in areas with high levels of endemic species, especially tropical islands and mountains. An area with many non-native species can also be considered “biodiverse,” but because many non-native species used in human environments are widespread, these areas contribute less to overall global biodiversity.

Sources: a. Chivian and Bernstein 2010; b. NRC 1999; c. Brooks et al. 2006; Mittermeier et al. 2011.

Nine Things Cities Need to Know About Biodiversity

Cities benefit from biodiversity and can also support it. The following nine points outline how biodiverse forests are vitally important to cities and what benefits they deliver to cities (Figure 18).

Figure 18 | Nine Things Cities Need to Know About Biodiversity

Source: Authors.

1. Biodiverse forests provide more goods and services to cities

Biodiverse forests are more resilient and have higher ecosystem function—key for climate change mitigation, water purification and regulation, and maintaining other forest benefits over time.

Biodiverse forests produce and sustain more ecosystem services on which city residents rely (Fischer et al. 2006; Flynn et al. 2011; Cardinale et al. 2012; Oliver et al. 2015). People in cities rely on forests for a large suite of different services, including mental and physical health, clean water, and climate regulation on many scales. To provide benefits, forests must be able to persist and recover from changes in the environment, including storms, droughts, and a changing climate. Maintaining higher levels of biodiversity can help them do that. “Higher biodiversity can be thought of as biological insurance”: when an ecosystem has many species fulfilling similar roles, it can continue to function even if some of those organisms are lost because the roles overlap (Yachi and Loreau 1999; Brandon 2014, 3). Biodiverse forests have high function and high “resilience,” or the ability to resist and rebound from shock, especially in comparison to monoculture plantations (Holling 1973; Folke et al. 2004; Thompson et al. 2009; Isbell et al. 2011; Brandon 2014; Liu et al. 2018).

Resilience is especially important for delivering forest benefits to city residents in the long term. Resilient forests persist for longer time periods. In cities, planting and maintaining a diversity of trees can prevent widespread tree loss when a new pest or disease is introduced (Santamour 2004; Raupp et al. 2006; Guyot et al. 2016). For example, in the United States, the introduction of Dutch elm disease and the emerald ash borer decimated millions of elm and ash trees, with extensive losses in urban areas (Santamour 2004; Raupp et al. 2006). Larger urban trees provide greater benefits (e.g., shade and cooling), so structurally diverse forests (with large, old trees and small, young trees) can provide additional resilience against environmental threats. Biodiverse forests in and out of cities can better weather storms, species invasions, weather fluctuations, and often human disturbance (Fischer et al. 2006; Flynn et al. 2011; Cardinale et al. 2012; Oliver et al. 2015).

2. Biodiverse forests store more carbon, more reliably

Conserving forests with high native biodiversity is a climate-biodiversity win-win for cities looking to take action on either front.

Having cities invest in forests is vital to climate change mitigation efforts. Intact and native forests sequester more carbon, store carbon longer, and provide much greater biodiversity conservation benefits than degraded forest or monoculture plantations (Holl and Brancalion 2020; Watson et al. 2020). A greater diversity of species and functional traits leads to more thorough carbon capture and storage throughout the ecosystem (e.g., aboveground versus belowground carbon; Díaz et al. 2009; Seddon et al. 2020). The higher resilience of biodiverse forests makes them a more reliable carbon sink—especially in the face of climate change, which increases extreme weather events and the need for forests to adapt to changing conditions (Turner et al. 2009; Brandon 2014; Seddon et al. 2019). Cities looking for cost-effective ways to action on climate change should consider integrating intact, biodiverse forest conservation efforts outside of cities into their climate change agendas—especially when considering procurement and sourcing options.

3. Biodiverse, intact forests protect watersheds

Native, biodiverse forests in watersheds are more effective than planted monocultures at supplying key water resources to downstream cities (Box 4; Case Study 5; Alvarez-Garreton et al. 2019; Yu et al. 2019).

The structure, impact on soils, and greater resilience of native forests create better conditions for storing and filtering water and are more likely to persist than many alternative land uses and monoculture tree plantations. For example, in south-central Chile, converting native forest to pine plantations had a negative impact on water availability, resulting in reductions in runoff (Little et al. 2009) and streamflow in the summer dry season (Box 4; Lara et al. 2009). Conversely, increases in native forest cover were linked to increases in streamflow in the dry season, when water was most needed.

Box 4 | How Non-native Trees and Plantations Decreased Water Yield in the Andes Mountains

In the Andes, millions of city residents rely on functioning paramos (tropical high-altitude ecosystems), alpine grasslands, and cloud forests for their water supplies.a In the last fifty years, the Andes have undergone both extensive deforestation and planting of non-native trees species.b

An extensive review showed that plantations of non-native trees had negative effects on water yields in nearly every situation, except when they were planted on highly degraded soils.c Non-native tree plantations reduce downstream water yields compared to native grasslands and native forest.d Cloud forests—which capture mist from passing clouds and convert it into precipitation—are especially important for supplying water year-round. Andean cloud forests are also the most biodiverse forests in the world.e

Sources: a–d. Bonnesoeur et al. 2019; e. Bruijnzeel and Proctor 1995; Myers et al. 2000; Bruijnzeel et al. 2011.

4. Biodiversity in the world’s forests provides the blueprints for new medicines and pharmaceuticals

Even though only a small percentage of Earth’s plants have been tested, medicines in wide use come from plants—many of which come from highly biodiverse tropical forests.

Medicinal compounds from forests are commonly used by city residents around the world. More than half of all commercial medicines are based on compounds derived from species in the wild (Figure 19; Chivian and Bernstein 2010). More than 28,000 plant species have been recorded as having a medicinal use (Allkin 2017), and an estimated 25 percent of modern medicines are derived from plants (Farnsworth and Morris 1976; Robinson and Zhang 2011).

Medicines derived from forest products include antibiotics, anticancer agents, anti-inflammatory compounds, and analgesics (Sen and Samanta 2015). Examples include morphine, aspirin, vinblastine (a lifesaving treatment for Hodgkin’s disease), and vincristine (which treats acute childhood leukemia; Chivian and Bernstein 2010). In the developing world, 70–95 percent of the population—including many people living in cities—rely on traditional medicines, such as herbal medicines derived from natural environments such as forests, for primary care. These medicines make up an $83 billion per year industry (Robinson and Zhang 2011).

Figure 19 | New Approved Drugs between 1981 and 2010

Notes: Twenty-two percent derived from natural product, 4 percent natural products, 15 percent biologics (derived from mammals), 6 percent vaccines, 29 percent synthetic, 24 percent from synthetic sources but modeled or mimicking a natural product (Newman and Cragg 2012). This means that 26 percent of drugs are either natural products or are directly derived from them. If including direct and indirect sources (and not including biologics and vaccines), 50 percent of drugs come from plants.

Source: Authors. Adapted from Newman and Cragg (2012).

And this is just the tip of the iceberg—fewer than 1 percent of plant species on Earth have been examined for medicinal potential (IUCN 2011). As forests—and tropical forests, in particular—house a disproportionate share of the world’s biodiversity, conserving forest is critical for future medical discoveries.

As the world’s biodiversity declines, opportunities to discover new medicinal compounds decrease. “As species vanish, so too does the health security of every human. Earth’s species are a vast genetic storehouse that may harbor a cure for cancer, malaria, or the next new pathogen—cures waiting to be discovered” (Mittermeier et al. 2011). A telling example comes from Australia. In 1980, two frog species were discovered in the rain forest with a unique reproductive strategy: they would ingest their own eggs to protect them and later regurgitate their young. A unique compound around the eggs prevented them from being digested—exactly the type of compound that would be useful in treating peptic ulcer sores on the stomach lining that cause burning pain. But before studies could be completed, both species went extinct due to habitat loss and other human activities (Chivian and Bernstein 2010). Peptic ulcers affect over 4 million people in the United States alone—and 1 in 10 people suffer from them (Harvard Health Publishing 2014). Extinctions like this happen more frequently than we would like to imagine (Wilson 1992). As forests are cleared, “hidden extinctions”—where undiscovered species disappear through extinction—reduce the biological bank that medicinal researchers have to draw from, and the possibilities for future medical advances.

5. Biodiverse forests support the world’s pollinators—and urban food supplies

Pollinators are considered essential for an estimated 35 percent of global food production (Klein et al. 2007). Many of these pollinators require native ecosystems—and biodiverse forests—to thrive.

Cities import 99 percent of their food—and are almost completely reliant on pollination services outside their boundaries. Within cities, pollination is important for urban trees, gardens, and so forth. But currently only an estimated 1 percent of global food production comes from urban agriculture30 (Clinton et al. 2018), and urban croplands account for only 5.9 percent of total croplands globally31 (Thebo et al. 2014). Most cities rely almost entirely on agricultural lands outside of cities and import the majority of their food.

Global food production relies on both wild and managed pollinators. Seventy-five percent of the 115 leading global food crops depend, to some extent, on animal pollination, accounting for 35 percent of food produced globally (Klein et al. 2007). The Food and Agriculture Organization of the United Nations estimated that pollination services in the countries responsible for 60 percent of the world’s agricultural production (Australia, Austria, Belgium, Canada, Denmark, Finland, France, Germany, Greece, Ireland, Italy, Luxembourg, Netherlands, New Zealand, Portugal, Spain, Sweden, the United Kingdom, and the United States) were valued at between $20 and $35 billion—per year. Some estimates at the global scale put this value higher: one study32 estimated that all pollination services had a value of $153 billion for global agriculture in 2005 (Gallai et al. 2009). Diversity of both wild and managed pollinators is important for providing more stable and effective crop pollination (Brittain et al. 2013; IPBES 2016).

But the world’s pollinators are in trouble. Land-use change (including deforestation), pesticide use, climate change, and diseases and parasites are affecting pollinators around the world (Mburu et al. 2006; Vanbergen 2013; Potts et al. 2016; Sritongchuay et al. 2019; Frick et al. 2020). Wild pollinators, which include insects and bats, and are important for both crops and wild plants and are in decline (Biesmeijer et al. 2006; Potts et al. 2010; Vanbergen 2013; Martin 2015). Wild bees and butterflies are especially threatened (IPBES 2016). Beekeepers in the United States and Europe have also reported annual hive losses of 30–50 percent for the past 10 years (10 percent is considered normal; Grossman 2013; Martin 2015).

Forests, along with other natural ecosystems, directly enhance pollinator populations in both temperate and tropical regions (Öckinger and Smith 2007; Nicholls and Altieri 2013; Bailey et al. 2014). For example, the Brazilian Atlantic Forest supports pollination processes for valuable crops, including coffee (Hipólito et al. 2019). Native pollinators tend to increase with proximity to natural habitat (Kremen et al. 2004). In many contexts, agricultural landscapes with natural areas or remnants near fields have more pollinators and higher crop yields (Morandin and Winston 2006; Monasterolo et al. 2015; Cusser et al. 2016). For example, in Japan native forests were shown to support native bee pollinators, whereas plantations did not (Taki et al. 2011), and in China, majority monoculture stands had 49–91 percent fewer bee species than native forests (Hua et al. 2016). Conserving, restoring, and maintaining the biodiversity of native forests outside of cities is crucial for maintaining pollinator populations and therefore food security within cities (Krishnan et al. 2020).

Forests, too, require native pollinators to function and provide ecosystem services. Over 85 percent of wild flowering plants are animal pollinated (tropical species, 94 percent; temperate species, 78 percent; Ollerton et al. 2011; FAO and UNEP 2020b). In tropical forest environments, for example, where wind pollination is rare, flowering plants are almost exclusively pollinated by a wide variety of animal pollinators (Bawa 1990). Nonagricultural forest benefits are rarely included in global valuation of pollinator services but are essential for people everywhere, including city residents (Mburu et al. 2006). Pollinators also make forests more resilient by maintaining genetic diversity of plant species (FAO and UNEP 2020b; Krishnan et al. 2020). Plant and pollinator diversity are inextricably linked and are essential for the provision of a wide range of ecosystem services (Kearns et al. 1998; Ollerton 2017). Losing pollinators via the loss of forest biodiversity is a negative downward spiral with serious implications for forests and agriculture.

6. Protecting biodiverse forests can reduce risks of zoonotic and vector-borne diseases and pandemics

Tropical deforestation can increase the transmission of infectious diseases—including novel ones—to humans.

In 2020, the COVID-19 pandemic spread around the world, with profound social and economic consequences and devastating loss of life. In many nations, cities were the hardest hit. Like many emerging infectious diseases, researchers suspected the virus responsible (SARS-CoV-2) was animal borne (zoonotic), and that it may have jumped from bats, civets, or pangolins (Mallapaty 2020).

The novel coronavirus is not an outlier: 61 percent of infectious diseases and 75 percent of new and emerging pathogens originate in animals (Taylor et al. 2001; Karesh et al. 2012). Although many of these pathogens persist among animal hosts for ages, deforestation, environmental degradation, agricultural intensification, and increased human presence in biodiversity hotspots—especially in the tropics—can increase human-wildlife interactions and allow contagious viruses and bacteria to leap from animals to humans (Wolfe et al. 2007; Karesh et al. 2012; Jones et al. 2013; Borremans et al. 2019).

Deforestation may play a role in the spread of emerging infectious diseases as well as outbreaks of established vector-borne disease (Evans et al. 2020; Guégan et al. 2020):

  • In West and Central Africa, clearing intact rain forest has been associated with Ebola virus outbreaks due to increased animal-to-human contacts, which promote the possibility for disease transmission (Olivero et al. 2017).
  • Yellow fever has been linked to deforestation (Wilcox and Ellis 2006) because it increases ground-level activity of the virus’s vectors (Monath and Vasconcelos 2015).
  • Deforestation has been tied to malaria incidence via mosquito abundance in Latin America (Guerra et al. 2006; Karjalainen et al. 2010), but not in Southeast Asia (Guerra et al. 2006) or Africa (Bauhoff and Busch 2020).
  • Deforestation and habitat fragmentation are also implicated in the spread of Lyme disease: as biodiversity declines, animal hosts of ticks such as mice become overabundant (Allan et al. 2003; LoGiudice et al. 2003).

Evidence suggests that maintaining high levels of biodiversity can decrease transmission of some infectious diseases (Civitello et al. 2015). Preventing deforestation and minimizing environmental degradation—especially in biodiversity hotspots such as the world’s tropical rain forests—could play a role in preventing the next pandemic (Evans et al. 2020; UNEP 2020; Alimi et al. 2021).

7. Access to biodiverse nature in urban areas provides measurable benefits to urban residents

Biodiverse forests can provide more reliable and richer health benefits to urban residents.

Access to natural environments is linked to an impressive list of mental and restorative health benefits (Hartig et al. 2014). Nature generally implies some degree of native biodiversity and function. The few studies on nature and health that incorporate biodiversity often indicate positive relationships (or, in some cases, no effect; Fuller et al. 2007; Lai et al. 2018; Marselle et al. 2019; Ngheim et al. 2021; Wood et al. 2018). The quality and type of green space also matters; for example, a study in Australia showed that wooded areas provide more mental health benefits, and that access to nature—not just green space—is important (Astell-Burt and Feng 2019).

Urban forests can be the only source of nature for many city dwellers. For example, a survey of New York City park users in 2013–14 found that 50 percent of people who use city parks only experience nature in urban parks (Pregitzer et al. 2019). Maintaining high-quality natural areas within cities is key for many residents to access the health and other local benefits biodiverse forests provide.

Urban forests can also provide early childhood nature experiences—and may be the only source of them for many children in cities. Exposure to nature in childhood can have lasting impacts on an individual’s feelings towards the environment (Sampaio et al. 2018). Spending time in nature early in life is associated with pro-environmental behavior later in life in both developing and developed nations (e.g., Evans et al. 2018; Rosa et al. 2018). Time in nature can also be important for children’s development—but children are getting less and less of it, in part because of the loss or inadequate amount of natural, biodiverse areas in cities (Louv 2005).

Biodiversity in the urban forest contributes to the distinctive character of cities around the world and can help connect people to place and encourage ecotourism (Hausmann et al. 2016). For example, Rio de Janeiro is internationally known for its astounding biodiversity and nature in the city, which is a point of pride to city residents and a major attraction for visitors worldwide. Working together to restore ecosystems can also connect people to place as well as to each other (Higgs 2003).

8. Urban forests can house high biodiversity

Urban forests can be highly biodiverse, but they also tend to have more invasive species, “generalist” species, and fewer endemic species than rural forests in the same habitat type (Concepción et al. 2015; Ducatez et al. 2018; Borges et al. 2021).

Urban areas occur in many of the world’s most biodiverse landscapes. For example, 422 cities33 are found in the world’s “hotspot” regions—areas of critical importance to biodiversity conservation with both high endemic biodiversity and extensive habitat loss (Seto et al. 2011; Weller et al. 2017). Cities are highly transformed environments, but many still support surprisingly high levels of biodiversity (Box 5). Their potential as migration corridors is also key for migratory species. In some cases, moderate levels of urbanization (e.g., lower-density settlements in suburbs, less homogeneity, larger and more evenly distributed green spaces) can even increase the biodiversity of certain groups, such as plants and certain bird species (Marzluff et al. 2001; Chace and Walsh 2006; McKinney 2008).

Box 5 | Cities Can Support High Biodiversity

The Secretariat of the Convention on Biological Diversity highlights a number of key examples of high biodiversity in cities:

  • More than 50 percent of the plant species found in all of Belgium can be found in Brussels.
  • Mexico City supports approximately 2 percent of all the known species in the world, including 3,000 species of plants, 350 species of mammals, 316 species of birds, and many more.
  • Nairobi National Park has over 100 mammal species and 400 bird species.
  • In São Paulo, Brazil, 1,909 plant species and 435 animal species have been recorded, 73 of which are endemic to the Brazilian Atlantic Forest.
  • Singapore has recorded many native species, including 2,145 vascular plants; 52 mammals; 364 birds; 301 butterflies; 127 dragonflies; 103 reptiles; 400 spiders; 66 freshwater fish; and 255 hard corals.

Source: CBD Secretariat 2014.

Urban forests often support lower biodiversity and fewer endemic species than forests outside of cities (Boxes 6 and 7). Despite the ability of some urban areas to support relatively high levels of biodiversity, these areas are often different from ecosystems found outside of cities. Urbanized areas often report lower species richness (total number of species) or species density (species per unit area) than nonurbanized areas (Marzluff et al. 2001; Chace and Walsh 2006; McKinney 2008; Aronson et al. 2014).

Cities often support different types of species, including more exotic and invasive species and fewer wildlife species, than nonurban areas (specifically animals with large ranges, highly specific habitat requirements, and large mammals; Adams 1994; Marzluff et al. 2001; Chace and Walsh 2006; McKinney 2008; Aronson et al. 2014). Non-native species are often intentionally planted, sometimes because they are well adapted to city conditions (Potgieter et al. 2017) but can spread into and threaten biodiversity in nearby forests. They also limit the opportunity to conserve locally endemic or other native species. Non-native, invasive species can be detrimental to ecosystem services, spread disease, produce allergens, or lead to “biotic homogenization,” where forests in different places become increasingly similar because common invasive species dominate at the expense of other species (Gaertner et al. 2017).

Both the size and connectivity of natural areas in cities are crucial for supporting urban forest biodiversity. A global meta-analysis showed that these two factors were more important than other biodiversity-friendly management techniques or the surrounding landscape, and that areas of more than 50 ha are often needed to conserve more vulnerable species (Beninde et al. 2015).

Box 6 | Biodiversity Comparisons between the Urban Tijuca National Park, the Rural Serra dos Órgãos National Park, and the Atlantic Forest, Brazil

Table B6.1 | Species Richness in Tijuca National Park, Serra dos Órgãos National Park, and the Atlantic Forest



Serra dos Órgãos National Parkb

Atlantic Forestc

Area (ha)




























Sources: a. Freitas et al. 2006; Maps of World 2017; Parque Nacional da Tijuca n.d.; b. Cronemberger and Viveiros de Castro 2009; Parque Nacional da Serra dos Órgãos n.d.; c. Mittermeier et al. 2011; Ribeiro et al. 2011.

Tijuca National Park is one of the largest, most well-known, and biodiverse urban parks in the world, and it is also a major tourism attraction.a Encompassing 3,953 hectares (ha) of primary and secondary tropical rain forest in the city of Rio de Janeiro, Brazil, it spans an impressive elevation, from 400 to 1,021 meters above sea level, especially for its size.b

Yet despite this, it still supports only a fraction of the species housed in forests outside Rio de Janeiro, the extremely biodiverse Atlantic Forest. Considered a hotspot, the Atlantic Forest is estimated to hold 20,000 plant species, 8,000 of which are considered endemic.c For example, Serra dos Órgãos National Park, 90 kilometers outside of Rio, encompasses more than 20,000 ha (the size was increased to 20,030 ha in 2008—note that species surveys represent only 10,000 ha) of remnant Atlantic Forest, with much higher numbers of animals across all taxa (Table B6.1). Bird species diversity is especially notable, as 142 are endemic to the Atlantic Forest.d

Sources: a. Freitas et al. 2006; Pougy et al. 2014; b. Drummond 1996; Pougy et al. 2014; Parque Nacional da Tijuca n.d.; c. Mittermeier et al. 2011; d. Mallet-Rodrigues et al. 2007; Cronemberger and Viveiros de Castro 2009.

Box 7 | Forest Biodiversity in New York City

New York City (NYC) is the most densely populated city in North America, with more than 8 million people in 78,000 hectares (ha).a Despite this, the city has 21 percent forest cover and expansive green spaces, including the iconic Central Park. These areas collectively support many species, including a relatively high proportion of native plant species (Table B7.1).b A study of municipal parklands found that 82 percent of natural area forest canopies are native, but that only 53 percent of the understory is native, indicating that many flowers and groundcovers are introduced exotics.c

NYC still only supports a small percentage of the species found in the state. This is to be expected—the state is much larger than the city—but also important because NYC both impacts and relies on nearby forests for watershed protection and recreation (Table B7.1). New York State has the greatest area of old-growth forests in the northeastern United States.dIntact forests support species that the city cannot—many mammals require large, intact areas of forest, and more than 20 species of birds in the state require interior forest habitat for nesting.e

Table B7.1 | Species Richness in NYC and New York State


New York City

New York Statea

Area (ha)





















Sources: a. Johnson and Smith 2006; b. NYC Department of Parks & Recreation n.d.; Decandido et al. 2004; c. NYC Audubon n.d.; d, e. DEC n.d.; f. Encyclopaedia Britannica 2020.

Sources: a. U.S. Census Bureau 2019; b. McPhearson et al. 2013; NYC Department of Parks & Recreation n.d.; c. Pregitzer et al. 2019; d, e. Johnson and Smith 2006.

The case studies in Boxes 6 and 7 highlight the difference between biodiversity in some of the world’s best examples of biodiverse urban natural areas and the forests found outside of cities. Collectively, these studies illustrate that well-managed and cared-for urban forests can have high biodiversity and can protect threatened and/or endemic species. Nonetheless, they are unable to support all the species in a given area. Conservation efforts outside of cities are therefore essential for conserving global biodiversity.

9. Tropical forests hold the majority of Earth’s terrestrial biodiversity and are therefore essential to urban well-being and sustainability

Where should cities invest in global biodiversity? Global biodiversity conservation priorities based on different metrics consistently identify tropical forests as priorities. Because cities around the world directly impact tropical forests via their consumption, they are in a strong position to improve their own biodiversity impacts through local policies that reduce negative impacts or invest in conservation.

Tropical forests hold up to 90 percent of the world’s land-based species. The tropics cover about 40 percent of Earth’s surface but are home to over 90 percent of bird species and 75 percent of other groups, including mammals, freshwater fish, ants, and flowering plants. The species richness (total number of species) of mammals, birds, and plants increases consistently from the poles to the tropics (Barlow et al. 2018). Within the tropics, forests hold an enormous amount of this biodiversity—most (65–90 percent) of the world’s land-based biodiversity is found in tropical forests, which are far more biodiverse than temperate and boreal forests34 (Wilson 1988; Reid and Miller 1989; WRI et al. 1992).

Cities looking to conserve global biodiversity need to look both inside and outside their boundaries. Efforts to conserve biodiversity that focus only on urban areas are limited by area, degree of habitat intactness, and—critically—their geographical location. The Amazon rain forest is estimated to contain 16,000 tree species. Canada and the United States combined are estimated to hold fewer than 650 but are approximately 3.6 times the size of the Amazon. Even more impressive is that this number—650 species—can be found in one hectare of tropical rain forest, indicating both the Amazon’s high level of species richness (Coley and Kursar 2014) and the potential return on investment for cities promoting biodiversity conservation in the tropics. This pattern holds true across multiple types of species as well as multiple tropical forests. Madidi National Park in Bolivia may be one of the most biodiverse areas in the world (WCS 2012; Brandon 2014). In contrast, the temperate Yellowstone National Park supports relatively high biodiversity for a temperate area but has only a fraction of the species (Figure 20; NPS 2019).

Figure 20 | A Comparison of the Species Found in Tropical Madidi National Park and Temperate Yellowstone National Park

Source:Authors. Adapted from data from WCS (2012) and NPS (2019).

The Amazon, Brazilian Atlantic Forest, Congo Basin, and other parts of Central Africa contain about 50 percent of the species on Earth in only 7.5 percent of its land area and have the highest species richness per unit area in the world (Jenkins et al. 2013). That is why most of the many methods to prioritize where to invest to conserve global biodiversity point to the tropics (Myers et al. 2000; Brooks et al. 2006).

Tropical forests are home to many endemic species (Figure 21). Endemic species are critical for global biodiversity conservation because they are more vulnerable to habitat destruction in a specific place. There are six times more endemic species in tropical areas than temperate ones (Barlow et al. 2018). Tropical mountains (notably the Andes) and islands in particular are home to species found nowhere else (Myers et al. 2000; Kier et al. 2009).

Most global biodiversity hotspots are in the tropics. Thirty-five global biodiversity hotspots have been identified using species “irreplaceability” (endemism) and “vulnerability” (the chance that they stand of being wiped out due to habitat loss35; Mittermeier et al. 2011). These hotspots cover 2.3 percent of the world’s land area, down from 15.9 percent originally, due to extensive habitat destruction in the last century. Yet they still house an astounding proportion of the world’s species (Figure 22): the tropical Andes alone has 30,000 plant species, of which 15,000 are endemic (Mittermeier et al. 2011). Nearly all 35 hotspots are in the tropics and subtropics, and all are either completely or partially forest biomes.

Figure 21 | Relative Forest Biodiversity Significance, 2018

Notes: Forest biodiversity significance is based on the “distribution of forest mammal, bird, amphibian, and conifer tree species” (Hill et al. 2019). Darker areas show higher significance values; white areas are not classified as forest.

Source: Hill et al. (2019).

Figure 22 | Percentage of the World’s Terrestrial Vertebrates and Plants Endemic to the 35 Global Biodiversity Hotspots

Source: Authors. Adapted from Mittermeier et al. (2011).

Caveats and Considerations

Measuring biodiversity is difficult and inexact. This applies to estimates of global or regional biodiversity and global extinction rates. These numbers give a range for the vastness of biodiversity and changes over the decades but are imprecise by nature. However, they still have value and are instrumental in communicating the general trends; for example, that extinction rates are increasing or that there is higher biodiversity in the tropics, which indicate when and where action is needed.

Biodiverse forests are important for their untapped medicinal potential, but this can lead to instances of traditional medicinal knowledge being stolen or co-opted. Bioprospecting—“the exploration of biodiversity for new biological resources of social and economic value”—can be especially extractive in the context of pharmaceuticals (Beattie et al. 2011). Although the importance of biodiversity for new discoveries should not go unacknowledged, neither should the contributions of local and Indigenous peoples to building this knowledge and their rights to these resources. When exploring these topics, cities should ensure that there is equitable benefit sharing and proper recognition of intellectual property.

Debate remains around the concept that higher biodiversity decreases the risk of infectious diseases, a hypothesis known as the dilution effect (Salkeld et al. 2013; Civitello et al. 2015). When applied generally, the link between biodiversity and infectious disease transmission is less conclusive, oftentimes mitigated by the specific disease’s system. However, the argument that deforestation and forest degradation contribute to the spread of infectious diseases is largely separate from the dilution effect and more strongly supported, though these mechanisms could reinforce each other. The conclusion that protecting biodiverse forests to decrease the spread of infectious diseases therefore still stands.

Some argue that biodiversity has inherent and cultural value. While not quantifiable in the same manner as the topics focused on in this section, there are arguments that biodiversity is important intrinsically and should therefore be protected. Sometimes this is described as “existence value,” which does not require any direct use for there to be value but instead simply the knowledge of its existence is enough.

Concluding Thoughts

Conserving biodiversity is critical to provide essential services that city dwellers rely on every day. Biodiverse forests are more resilient, and thus a more reliable source of forest benefits. Inside the city, biodiverse forests support improved mental health, pollination, and, in some cases, tourism. Forest biodiversity outside of cities supports services that we rely on every day, including food security (via pollination and global rainfall patterns), medicinal resources and research, climate change mitigation, and even buffering against future pandemics.

Conserving tropical biodiversity can be a double (or triple) win for cities because the world’s most biodiverse forests are also critical for providing other benefits (carbon sequestration; global hydrology cycles). Cities could consider investing in biodiverse forests to which they are connected by flows, such as migratory bird routes, rainfall patterns, or in areas of high carbon density.

Although cities are far removed from the world’s most biodiverse forests, they have a big impact on these environments and can play a critical role in conserving them. When it comes to biodiversity conservation, tropical forests are key. Conserving native, intact forests outside of cities in the tropics is the most important measure that cities can take because these forests contain far higher levels of native biodiversity than most secondary forests, temperate forests, and urban tropical forests. They are also especially important for conserving endemic species (Holl and Brancalion 2020). People in cities consume the majority of the world’s goods and commodities, many of which currently harm diverse forests—but this impact could be greatly reduced through better sourcing and changing consumption habits. Cities can also conserve biodiversity with their boundaries, including by maintaining and expanding diverse natural areas throughout cities.