The Urbane Ecologist


First global urban biodiversity study: anthropogenic drivers are major culprits of decreasing bird and plant diversity

In February, the Proceedings of the Royal Society published “A Global Analysis of the Impacts of Urbanization on Bird and Plant Diversity Reveals Key Anthropogenic Drivers“. This is the first study to have been done that compares the biodiversity of cities worldwide and is so far the largest global compilation of urban biodiversity data.

Despite the fact that over half of the world’s people live in cities [1], they only cover about 3% of the world’s surface [2]. In addition to the fact that the needs of urbanites go beyond the boundaries of a city and affect people and ecosystems around the globe, cities are located in places that were not only beneficial to the people who settled there, but allowed diverse and rich ecosystems and networks to flourish. In other words, the characteristics of a place that attracted people to a particular place are usually what make those same places “biodiversity hotspots”–i.e. a species-rich region [3-5]. In these regions, however, species are threatened by many anthropogenic changes, such as habitat loss and species introductions [6].

For a long time, people have recognized that biodiversity is important to cities and that more research on urban ecology needs to be done [7-9]. However, a global synthesis such as this paper had been lacking. Previously studies surveyed the biodiversity of several species worldwide, or used species diversity studies focused on individual cities to extrapolate the effects of urbanization on biodiversity worldwide. What we do know is that cities are novel ecosystems [10] with very fragmented or otherwise changed natural environments, high densities of artificial structures, and impervious surfaces that retain a lot of heat [11].

Many scientists hypothesized that the world’s cities would have very similar species compositions, due to very similar patterns in development and spatial structure, in addition to human-mediated movement of species from one place to another [12]. The frequent movement of species from their native environment to a non-native one increases the likelihood of exotic species becoming invasive and interacting with habitat alteration to destroy the finely tuned balance of local ecosystems [13]. This study, however, showed that cities may be better at retaining regional diversity than expected.

This study compiled a list of urban bird species for 54 cities and a list of vascular plant species for 110 cities. These cities fell on 36 countries, 6 continents, and 6 biogeographic realms. The cities covered a range of population size, geographical areas, and establishment dates (4000 BC to AD 1971). They examined the bird and plant diversity in these cities, how homogenized the biota were, species density in urban vs non-urban environments, and the how species density correlated with anthropogenic and natural variables.

What were the authors’ key findings?

  • Bird and plant species were significantly different among cities. Of the over 10,000 recognized bird species worldwide, 20% occur in cities, representing nearly three-quarters of all bird families. Of the almost 280,000 recognized  vascular plant species worldwide, 5% occur in cities, representing two-thirds of all plant families.
  • Within biogeographic realms, cities retained similar compositional patterns. This is good! It means that urban biotas have not become as globally homogenized as we feared, and that they continue to reflect regional species pools.
  • The number of exotic species, which are an increasingly grave threat to global biodiversity [14], varies broadly among cities. On average, cities have more native bird and plant species than exotic, and in general the proportion of exotic bird and plant species to native is similar. However, Australasia has a significantly higher proportion of exotic species–this is true in New Zealand cities in particular, which have had many exotic species deliberately introduced [15] and also has unfilled ecological niches.
  • “The relative proportion of exotic plant species is much greater than that of exotic bird species.” This likely indicates that the processes underlying how urban bird and plant communities are assembled are different. Factors that are involved in determining how communities are assembled include introduction rates [16], establishment rates, and varying needs for survival or success.

What are the most common birds and plants found in the world’s cities?

  • These four birds occur in over 80% of cities: the rock pigeon, the house sparrow, the European starling, and the barn swallow
  • These plants occur in all biogeographic realms: annual meadow grass, shepherd’s-purse, chickweed, ribwort plantain, and the common reed. Most of these plants were introduced to Europe before 1500 AD, meaning that they developed urban populations in European cities before they became successfully established in cities around the world.

What were the findings relating to species threatened with extinction?

  • Bird species that are threatened by extinction were found in 30% of cities.  Singapore had the largest number.
  • Threatened plants were found in 8% of cities. Singapore and Hong Kong had the largest number.
  • At a larger scale, the greatest number of threatened bird and plant species were found in Indo-Malaya. The Nearctic has the fewest threatened bird species. The Palearctic has the fewest threatened plant species.
  • These proportions are probably conservative, especially for the plant species, because national lists often include species that are not assessed by the IUCN.

What were the general patterns of biodiversity that were observed?

  • The highest densities of bird species were found in Palearctic species, mainly in European cities, and the lowest densities were in Nearctic and Australasian cities.
  • The lowest densities of plant species were in cities in Indo-Malaya and Australasia.
  • Compared to non-urban areas, species densities were low in cities. Indo-Malaya and Australasia experienced the greatest loss in plant species density compared with non-urban levels. This is particularly troubling, as these regions are important biodiversity hotspots [17] and are also regions were urban land area is projected to increase [18].
  • Bird and plant densities were best explained by the anthropogenic features of the city.
    Bird density was negatively associated with urban landcover, perhaps indicating that managing vegetation structure is an important part of bird conservation in cities.
    Plant density was positively associated with the cover of intact vegetation and city age. In other words, plant species are best preserved in older cities where there were more tracts of vegetation left unfragmented.

This study showed that although cities can and do support regional biodiversity and native species, urbanization substantially decreases biodiversity, at least compared to non-urban environments. The authors suggest that focusing on conserving and restoring the native vegetation within cities could help bolster bird and plant diversity, thus hopefully counteracting likely declines in biodiversity that accompany urbanization [18].


Literature Cited

[1] UN. 2011 World Population Prospects: 2010 Revision. New York, NY: UN.

[2] Schneider A, Friedl MA, Potere D. 2010. Mapping Global Urban Areas Using MODIS 500-m Data: New Methods and Datasets. Remote Sens. Environ. 114, 1733-1746.

[3] Luck GW. 2007. A Review of the Relationships between Human Population Density and Biodiversity. Biol. Rev. B82, 607-645.

[4] Kuhn I, Brandl R, Klotz S. 2004. The Flora of German Cities is Naturally Species Rich. Evol. Ecol. Res. 6, 749-764

[5] Cincotta RP, Wisnewski J, Engelman R. 2000. Human Population in the Biodiversity Hotspots. Nature 404, 990-992.

[6] Williams NSG et al. 2009. A Conceptual Framework for Predicting the Effects of Urban Environments o Floras. J. Ecol. 97, 4-9.

[7] United Nations Environment Programme. 2007. Report of the Cities and Biodiversity: Achieving the 2010 Diversity Target. Montreal, Canada: UNEP/CBO.

[8] Sukopp H. 2002. On the Early History of Urban Ecology in Europe. Preslia 74, 373-393.

[9] Pickett STA et al. 2011. Urban Ecological Systems: Scientific Foundations and a Decade of Progress. J. Environ. Manage. 92, 331-362.

[10] Hobbs RJ et al. 2006. Novel Ecosystems: Theoretical and Management Aspects of the New Ecological World Order. Glob. Ecol. Biogeogr. 15, 1-7.

[11] Rebele F. 1994. Urban Ecology and Special Features of Urban Ecosystems. Glob. Ecol. Biogeogr. Lett. 4, 173-187.

[12] La Sorte FA, McKinney ML, Pysek P. 2007. Compositional Similarity Among Urban Floras Within and Across Continents: Biogeographical Consequences of Human-Mediated Biotic Interchange. Glob. Change Biol. 13, 913-921.

[13] Winter M et al. 2011. Plant Extinctions and Introductions Lead to Phylogenetic and Taxonomic Homogenization of the European Flora. Proc. Natl. Acad. Scie USA. 106, 21721-21725.

[14] Butchart SHM et al. 2010. Global Biodiversity: Indicators of Recent Declines. Science. 328, 1164-1168.

[15] Atkinson IAE, Cameron EK. 1993. Human Influence on the Terrestrial Biota and Biotic Communities of New Zealand. Trends Ecol. Evol. 8, 447-451.

[16] Hulme PE et al. 2008. Grasping at the Routes of Biological Invasions: A Framework for Integrating Pathways into Policy. J. Appl. Ecol. 45, 403-414.

[17] Myers N, Mittemeier RA, Mittemeier CG, da Fonseca GAB, Kent J. 2000. Biodiversity Hotspots for Conservation Priorities. Nature 403, 853-858.

[18] Seto KC, Guneralp B, Hutyra LR. 2012. Global Forecasts of Urban Expansion to 2030 and Direct Impacts on Biodiversity and Carbon Pools. Proc. Natl. Acad. Sci. USA 109, 16083-16088.

Leave a comment

“Sand Wars”

Denis Delestrac had come home to Barcelona, having wrapped up a documentary. On a Sunday afternoon, he decided to enjoy the weather and recharge his batteries by taking a long walk on the beach close to his home, where he had strolled many times before.

The beach had shrunk since he last saw it. It had gone from being a wide, expansive beach to being about 10 feet wide. Shocked by the sudden difference, he realized that he had no way to answer very basic questions: How did the sand get to the beach? What is sand? How does a coastline disappear?

So he went home and Googled “sand”. As often happens with Googlings, one thing led to another and he came across articles about “sand wars” and “sand mafias”. He was stunned by how little he knew about sand, and he became fascinated by it. He began learning more by reading Sand: The Never-Ending Story by Michael Welland and many other books and articles.

One day he got a call from a producer who had seen the documentary he had recently finished wrapping up. The producer said, “I saw your last film and I really want to work with you on whatever you decide to do next.” And Delestrac said, “I want to work on sand.”


“Sand?! What is there to say about sand?”

It was a chorus that Delestrac was to hear many times over the course of making “Sand Wars“, but its story was so compelling that funding for a documentary was assured and many people became interested in seeing it come to fruition. I saw the documentary at DC’s Environmental Film Festival, where Delastrac and Welland were present for a Q and A afterwards. My eyes opened to a whole new environmental issue and I have been ranting about sand to everyone who will listen. Here, I’m summarizing what I learned.*

Despite how little thought we give to sand, it is the “foundation of modern development” and as crucial to our civilization as air and water. It finds its way into almost every product we can think of and comprises almost every single building and paved road on the planet. Sand, however, is also a worldwide problem that affects all countries and all continents. The demand for sand is so large that coastlines are disappearing. Its story is also dramatic and tantalizing—understanding the current state of sand almost immediately brings us into contact with “sand mafias”, multinational organizations that send ship fleets worth millions of dollars to prowl the world’s oceans, inequality, and refugees. Mind blown yet?

Sand is in everything from microchips, wine, glass, paper, plastics, and paint. A major source of the consumption, however, is construction, where sand is a dominant building material. According to “Sand Wars”, this is the amount of sand needed for the following building projects:

  • House: 200 tons
  • Hospital: 3,000 tons
  • 1 km Highway: 30,000 tons
  • Nuclear Plant: 12,000,000 tons

Annual consumption of sand adds up to 15,000,000,000 (15 billion) tons. After air and water, it is our most used commodity. And in our overwhelming demand for sand lies our problem. As is the case with many of our other natural resources, we have exhausted easily accessible supplies and search for it in expensive, investment-heavy ways that have deeply felt consequences that are difficult to mitigate. Most sand we now use comes from beneath the ocean and is obtained with dredgers. A single dredger can displace 4,000—400,000 square meters of sand. Given the very high cost of these dredgers, companies can only begin to make money when they have a fleet—and this in turn means that the sand that is taken from the world’s oceans is in the hands of a few dredging companies.

Dubai is a fascinating case study in the role of sand in development, and illustrates the broader problem—and incredible irony—inherent in many Middle Eastern cities’ rapid urbanization and consequent dependence on imported sand. Dubai’s rapid development into a heavily constructed metropolis with the world’s tallest building makes in an excellent example of a global trend. A key reason for this sand consumption has been their construction of artificial islands, such as the famous “Palm”, which used 150,000,000 (150 million) tons of sand. A newer project, “The World”—300 artificial islands intended to shape into a map of the world—have used three times as much sand as the Palm. However, due to the financial crisis, the project has been abandoned and there are many islands sitting empty near Dubai.

Middle East = sand and camels in many people’s minds. You would think that cities in the Middle East have no shortage of sand with which to build all their roads and buildings and artificial islands. Unlike the sand that comes from the ocean, which is angular and rough and packs together well, desert sand has been rounded out from being blown around for thousands of years. Being round and smooth means it doesn’t pack together well. Thus development in these countries identified with sand has caused ocean sand to be dredged and imported into the Middle East. About 3,500 companies import sand from Australia to the Gulf. The import of sand is worth $70 billion in international trade a year.

Islands in Indonesia have borne many of the consequences of sand dredging. When dredging occurs near coastlines, it damages them irreparably. The missing sand from below the ocean floor collapses, leading to land above the water sinking. This is particularly terrifying for island communities. “Islands exist by a natural conspiracy,” says Welland. Waves, wind, currents, gravity, and other factors have to align in a particular set of conditions to allow the formation of an island. Thus, once the sand is dredged up below an island and its coastline begins to collapse, there is no way to fix the problem. And once a coastline collapses, boundaries change—making sand a geopolitical issue. Twenty-five islands have disappeared from Indonesia. A lot of the sand that has been dredged below them and has contributed to their demise goes to Singapore. Singapore’s land mass has increased 40% in the past 20 years. 130 square kilometers along Singapore’s shores has already been converted from water to land, and by 2030, another 100 square kilometers will also have done so. Much of the sand that is used in this development is dredged illegally, and contributes to worldwide sand trafficking operations. The Singaporean government is the largest customer of illegal sand traders.

In the Maldives, years of divers heading underwater to fill bags of sand that then get sold have resulted in the severe destruction of coastlines that has prevented islands from being able to resist sea level rise. As a consequence, several islands have had to be evacuated due to their having become uninhabitable. These refugees abandon the islands for neighboring islands and create a need for housing. You can probably guess what this means at this point—these people, who are fleeing destruction caused by the need for sand, contribute to nearby construction booms, and consequently generate an increasing demand for sand. And this sand comes from other islands.

Mumbai also has sand crime to contend with—sand smugglers are the most powerful criminal organization in India. And in Mumbai, construction must keep up with rural to urban migration and consequent increases in sand consumption. In India, construction firms have tacit agreements with the sand mafia, who are also connected to the government, administrative bodies, and the police. There are 8,000 dredging sites on the Indian subcontinent.

Tangiers, Morocco has experienced an influx of people looking for a home on the beach. This has led to a quick rise in construction. The structures made from sand in Tangiers are made from 40-45% stolen sand pillaged from Tangiers’ beaches. Ironically, the very people who flocked to Tangiers to see and live in the picturesque beach vistas and look at and play with the sand are being sheltered in homes that contribute to the transformation of beaches into “lunar landscapes”. What is scary, too, is the fact that much of these buildings are mixed with seawater. Sand and saltwater make a corrosive, unstable mixture that does not make for long-lasting buildings.Tangiers is far from alone—all beaches on the planet are shrinking at an accelerating rate, due largely in part to sand dredging.

In Florida, a state heavily dependent on tourists who come to admire and enjoy its beaches, nine out of ten beaches are in the process of disappearing. In some areas, up to half the GDP depends on beaches. Some cities invest astronomical sums in “beach replacement therapy” which is, in essence, dredging up sand from the bottom of the ocean and then spitting it out on a beach. This is hardly a solution—like the sand it replaces, replacement sand is washed away. One $17.5 million dollar beach replacement project was washed away in a year. Coastal engineering firms profit from this sand problem, too—frequently, they project longer durabilities for these projects than they actually last. Sand barriers which are built against extreme storms are ramped-up versions of these beach replacement therapies and also fail and cost a lot of money. Further, altering the coastline in one area by filling it with sand inevitably results in the disappearing coastline of another.

This huge problem with sand is not purely due to the incredible demand that exists for this material. The construction industry profits from making buildings regardless of whether they are filled or not. Globally, many buildings are being built that are unaffordable to many people but are being held vacant, for investment. In Mumbai, 50% of the houses that are build stay vacant for investment. Consider, too, the huge Chinese cities that have been built, only to remain ghost towns—although their construction industry flourishes. In Spain, where a housing crisis has been so severe it has made global headlines, 30% of the homes built since 1996 have remained empty. Whole airports have been built that have never had a plane fly through them. And although there are so many vacant houses in the world, their prices prevent many from being able to afford them. A third of the global population live in slums. Governments profit from this situation. In fact, the leading consumer of sand is the state. However, there is no global policy, and very few national policies, that even come close to addressing the many problems inherent in our poor management of sand. Frighteningly, it will likely take some time before sand is addressed. Humans have historically waited for problems to get far worse before they get debated or get better. As an example, water issues have become so bad that policy exists to address them; there is some debate on soil issues, which will hopefully resolve itself into good policies; but on sand, there isn’t even a debate yet.

There are some piecemeal efforts to address the many problems derived from our consumption of sand, but they are piecemeal and frequently hampered by lobbying from the industry. Some of the solutions include making houses from other materials, such as corn bales (or hemp, which was not discussed in this documentary); and glass or building recycling to replace sand. One encouraging example has been Denmark, which placed a tax on natural sand and then used the funds to subsidize sand recycling. They now use 90% recycled sand (i.e. glass).

You can find more information on “Sand Wars” here (available in a variety of languages), or you can watch Delastrac’s TED talk on the same topic.  It will be aired on PBS on April 2–don’t miss it!

*The information presented here was written based on my notes from the documentary “Sand Wars” and the Q&A with Welland and Delastrac. I have tried to ensure that my notes were correct and that I did not present any erroneous facts or figures–but I felt it was so important and such an interesting issue that the potential of getting one or two facts wrong was worth any change in perspective I offered my readers. So please forgive any errors–but make sure to leave a comment and I’ll correct them–and read up on sand and watch the documentary for yourself!

Leave a comment

DC’s been busy…

Almost 400 protesters were arrested yesterday at the White House for protesting Keystone XL! Conveniently, the police were well prepared and waiting to arrest them via buses. Protesters had been trained in nonviolent action on March 1. You can see videos from the live stream here. Great photos here. Thank you to every single one of you who was out there yesterday, and I’m grateful to those of you who got arrested!

In happier news, DC’s Environmental Film Festival schedule is up! Theme: Our Cities, Our Planet. 

Leave a comment

This week’s adaptation-related news

Check out the Rockefeller Foundation’s 100 Resilient Cities blog.

This month’s Global Roundtable on the Nature of Cities  was “What are the social justice implications of urban ecology, and how can we make sure that “green cities” are not synonymous with “gentrified” or “exclusive” cities?” Nine contributors from the United States, China, India, and Brazil, offer their perspectives.

I came across this article that gives advice to teachers on teaching the seasons–and how to introduce the changes in seasonal trends to their students.

This article gives a short review of the lack of transparency in the global market for rare earth elements, which are crucial to modern tech.

This article describes the Gazelle Valley Park, Jerusalem’s first urban nature park.

This article reviews the colossal problems of Mexico City’s sewage system.

A study predicts that temperature-related mortality will increase 257% by 2050 in the UK as a consequence of climate change. Here is an article summarizing it.

Johannesburg’s Mayor Parks Tau said at the C40 Mayors Summit that major cities in developing countries find it difficult to attract funds for infrastructure projects from international financing institutions like the World Bank. This article discussed the personalities of several key players in the C40 summit–such as Mayor Tau, Michael Bloomberg, Annise Parker–and touches on how the public role of a mayor may change based on how involved they become in international groups. This article takes it even further and gives them classroom nicknames–Rio de Janeiro’s Paes is Mr. Popularity, and Bloomberg is Head of the Class.

This article reviews the World Bank’s scheme to balance water scarcity and poor hydro management with energy production.

Sao Paulo, already experiencing drought, is running out of water. California’s droughts have become longer and more severe. A report by Ceres found that 47% of oil and gas wells recently fracked in the United States are experiencing drought.

This Guardian article introduces us to “bucket brigades”, community groups which measure air pollution and use their findings to campaign for cleaner air.

Richard Branson hosted a summit to develop plans to “green” the Caribbean, with a focus on renewable energy.

Vivienne Westwood announced that climate change, not fashion, is now her priority, and has joined George Clooney, Chris Martin, Paloma Faith, and other celebrities in supporting Greenpeace’s Save the Arctic Campaign. As a way of promoting sustainable consumption, she has decided to stop expanding. Here is an album of the celebrities modeling the tshirts!

Disappointingly, communities secretary Eric Pickles has said that it “does not matter if climate change or fluctuation in weather patterns” were the ultimate cause of Britain’s floods. A live news feed about the floods and related issues can be found here. The chairman of Britain’s Environment Agency stated that there is not enough money to defend both “town and country” (urban and rural areas) from floods and that the island faced a choice in prioritizing one over the other.

Nature paper came out predicting changes in species distributions as a consequence of climate change. (Here’s a readcube pdf.)

China is planning a major reorganization of its Ministry of Environmental Protection and is likely to give that ministry more power. The Ministry of Land and Resources may be dissolved.

I’m a little late to the party on this one, but a Nature paper found the flooding as a consequence of climate change had little impact on migration in rural Pakistan, whereas heat stress has had a large impact. Relief efforts, however, have focused on the former.


Get every new post delivered to your Inbox.