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 , they only cover about 3% of the world’s surface . 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 .
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  with very fragmented or otherwise changed natural environments, high densities of artificial structures, and impervious surfaces that retain a lot of heat .
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 . 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 . 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 , 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  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 , 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  and are also regions were urban land area is projected to increase .
- 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 .
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