The Urbane Ecologist

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

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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].

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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.

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

  1. I mentioned this study in passing in a post in February: http://jeffollerton.wordpress.com/2014/02/16/there-were-hummingbirds-over-the-white-cliffs-of-dover/

    As you can see, it surprised me that anyone would suggest global homogenisation of urban biotas as a hypothesis. Would be interested to know what you think

    • Hi Jeff,
      Thank you for your response! Apologies for replying so late–I get quite intimidated by comments, seeing as I get so few of them, and the fact that you work in the field in which you are asking the question made me even more nervous. I was taking a few days to brush up on papers on biodiversity to make sure I had a coherent response.

      But my response is the same as I had when I first read your comment. I don’t think the authors were referencing a serious, rigorous hypothesis about the global homogenization of urban biotas. I don’t think there is much research to propose something like that, much less test it. I think they were referencing the general fear ecologists have of declining biodiversity. In addition, given that the paper sounded like they were trying to make it accessible to more than just people in their field makes me think that they may also have been tapping into growing fears of cultural homogenization that comes with globalization. That being said, I don’t think it is unreasonable to predict, in very general terms and within certain obvious limits, that cities are likely to be biotically homogenized especially compared to non-urban areas, given their very similar environments (e.g. presence of concrete, etc) and development histories and the rapid exchange of species. For example, I think tropical cities in Asia are likely to have more homogenized biota compared to non-urban tropical parts of Asia. I think that they may, in the longer term, become more similar to tropical cities in East Africa (due to, for example, higher introduction rates due to trade routes and similar abiotic conditions like temperature) than European cities. I also think that climate change will have an interesting part to play in determining whether or not cities become more homogenized, given the changes in ranges, etc. But do I think that cities across the globe will homogenize almost completely eventually? No.

      I hope that my answer was satisfactory and that it isn’t too rambly! Thank you for taking the time to write! I’m unreasonably excited by comments!

  2. Hi there [sorry, I don’t know your name, couldn’t see it on your site!],

    Please, don’t apologise, not at all necessary. I also get excited by comments, it’s always good to know that someone, somewhere is bothering to read what we write! 🙂

    Your reply is well argued and perfectly reasonable, and I would agree that there’s a degree of homogenisation within climate zones. There’s also some homogenisation between zones: I don’t know if you saw any of my posts from my trip to Brazil in November, but the first bird I saw was a feral pigeon and the first bee was a honey bee!

    But the quote from the lead author was: “Owing to the fact that cities around the world share similar structural characteristics – buildings, roads etc – it is thought that cities share a similar biota no matter where they are in the world”

    The last 8 words are the key and give the wrong impression to a general audience, I think. I know from my own experience that, when talking to journalists, it’s important to be precise about what we mean. Otherwise it gives a false impression of research findings, or of the rationale for doing research.

    Keep up the blogging!

    Best wishes,

    Jeff

    • Hi Jeff –
      You are right–their wording did give the wrong impression! Agree with you there.

      Thank you for the kind words and for pointing out that my name was missing! Believe it or not, I had not realized that I’d never published my name. I’m ready to share it, though, despite missing the partial anonymity I had… It’s Savina Venkova, and it’s now on my About page!

      Best wishes, and looking forward to reading more of your posts!

  3. Awesome website, thanks for sharing !!

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