Wildlife living in cities exemplify ‘urban trait syndrome,’ study says

What can city dwellers do to help create and nurture a healthier environment for urban wildlife?


Animal species that live in cities must be adaptable to survive, including not being too picky about what they eat. Now, a global consortium of scientists has conducted a study and termed the collection of characteristics that go along with learning to have this flexibility “urban trait syndrome.”

The study used data from six continents and 379 cities, primarily gathered by the eBird program at the Cornell Lab of Ornithology, for which environmental observations are compiled by volunteers, reported the Cornell Chronicle.

“The idea of an ‘urban trait syndrome’ is that there is a collection of characteristics which are commonly observed in urban species, and therefore give [us] reason to believe species who possess those traits (characteristics) are likely to do well in the city, and those species with different (or the opposite) traits won’t be very common in urban areas,” Johan Kotze, co-author of the study and a professor in urban ecosystem studies at the University of Helsinki, told EcoWatch in an email. “Our study found that there are likely to be multiple ‘urban trait syndromes’ and therefore multiple ways that species can do well and survive in the city. We propose these syndromes and the taxa (with their generalized characteristics) that likely fall within them.”

The study, “Urbanisation generates multiple trait syndromes for terrestrial animal taxa worldwide,” was published in the journal Nature Communications.

Not every species group has the same urban trait syndrome characteristics, however.

“The most pronounced changes among city-dwelling organisms are in reproduction and foraging,” said Frank La Sorte, a senior research associate at the Cornell Lab and co-author of the study, as the Cornell Chronicle reported. “For example, city birds tend to be smaller, eat a wider variety of foods and produce smaller clutches than their rural counterparts. Smaller clutch sizes in urban birds have been associated with higher survival rates and increased growth.”

In addition to birds, reptiles and beetles have a tendency to be smaller in cities than they are in the country.

“[T]here are several reasons why this might be the case, but future research is needed to say for sure. It could be related to the lack of food resources, and/or to climate. Cities are usually warmer than the surrounding countryside, which at least for ectotherms (like insects) infer a metabolic cost, i.e., a higher metabolic rate… which means that more food is needed to maintain basic metabolic functions. If these needs are not met, it may result in smaller individuals,” Amy K. Hahs, lead author of the study and geographic information systems ecologist at the Australian Research Centre for Urban Ecology at the Royal Botanic Gardens in Melbourne, Australia, and Kotze told EcoWatch in an email.

Marco Moretti, senior scientist with the Swiss Federal Research Institute and co-author of the study, told EcoWatch that the amount of suitable habitat can also affect the size of urban dwelling animals.

“I would say that ‘in general,’ cities host species that are small, because cities lack large green spaces where larger species can easily move and find the resources needed. Cities are rather a mosaic of small patches and this favours small mobile species. But of course, there are exceptions, such as foxes and badgers,” Moretti said. “For insects, such as bumblebees, this could be a strategy to cope with heat stress, but also it is the result of less resources (flowers and insect biomass) and maybe also lower nutritional contents of the food resources.”

Ground beetles’ mobility was found to be higher in cities, while the mobility of birds and reptiles was lower.

Mobility plays a key role in how an animal searches for food. Four kinds of foraging behaviors were identified by the researchers in the urban species they studied, including central place forager; mobile generalist; mobile specialist; and site specialist.

“Generalists can make use of lots of different types of resources, so it is more likely they will be able to find the things they need as they are less reliant on a specific resource which might be scarce or patchy in the urban landscape. Specialists, on the other hand, may find it harder to find specific resources in patchy and degraded urban landscapes, which could spell their demise in the city,” Hahs and Kotze told EcoWatch.

Bees as well as birds are considered central place foragers, which means they establish a “base of operations” and leaf out to forage for food, reported the Cornell Chronicle.

“The most common dietary strategy for birds in urban areas is to be a generalist – they’ll eat a variety of different foods instead of specializing,” said La Sorte, as the Cornell Chronicle reported. “You see this clearly among such common city birds as the rock pigeon, European starling and house sparrow. The specialists gradually disappear.”

Hahs told EcoWatch that one way to know if specific birds will be present in an urban environment is if their usual food sources are available.

“Birds are a very diverse group, so a common way to think about them is by the types of food they eat. For example, there are insectivores who eat small insects, granivores who eat seeds, birds of prey that eat reptiles and other animals, and omnivores that eat lots of different things,” Hahs said. “Our study showed that all of these different types of birds are present in cities, as long as their food resource is available. In fact, the availability and type of resources appear to be a crucial aspect of the success (or not) of species present in urban environments.”

Healthy ecosystems require biodiversity, but in urban areas it is reduced as dietary specialists vanish and the variety of species becomes more and more homogenized.

“Maximising opportunities to support taxa with different urban trait syndromes should be pivotal in conservation and management programmes within and among cities. This will reduce the likelihood of biotic homogenisation and helps ensure that urban environments have the capacity to respond to future challenges. These actions are critical to reframe the role of cities in global biodiversity loss,” the authors of the study wrote.

Urban ecology suffers from a dearth of comprehensive data. Information is lacking for most species other than birds, as it has not been compiled systematically to allow for accurate cross-city comparisons.

“Urban biodiversity studies are also heavily biased taxonomically towards plants and birds. Other speciose and functionally-important groups, such as insects, amphibians, bats, and reptiles are severely impacted by urbanisation but poorly studied,” the researchers wrote in the study.

EBird and the expansion of other citizen-science programs could help fill this knowledge gap.

“Although considerable progress has been made toward understanding the impacts of urbanisation on global biodiversity, certain key research gaps remain. The scientific literature is geographically biased towards larger metropolitan areas of the Northern Hemisphere and Australia. Meanwhile, most biodiversity hotspots are in the tropics and the Southern Hemisphere and have received less attention,” the study’s authors wrote. “Despite the increasing importance of functional traits in the ecological literature and recent efforts to integrate functional aspects of biodiversity into urban ecological research, most urban biodiversity investigations remain focused on taxonomic diversity. This hampers our ability to develop a mechanistic understanding of the impact of urbanisation on biodiversity; creates additional challenges when making cross-taxon or cross-region comparisons; and hinders our ability to effectively conserve species with different life histories and habitat requirements.”

Conscious efforts need to be made to reimagine and modify cities into more natural, livable environments for animals and humans.

“Preserving habitat is critical,” said La Sorte, as reported by the Cornell Chronicle. “Ecosystems in cities are heavily transformed and managed, and intact native vegetation tends to be scarce. The more components of an ecosystem that are preserved and supported, the healthier the overall urban environment will be. That support could be in the form of expanded parks and green spaces, or by supplying artificial nesting resources as ways to partially compensate for habitat lost to city expansion. It’s a more nuanced approach to urban conservation, aimed at keeping cities healthy for nature and people by accounting for the needs of many different types of species.”

What can city dwellers do to help create and nurture a healthier environment for urban wildlife?

“Lots of easy and not so easy things. For instance, we should not be afraid to keep our back gardens a bit wilder than the traditional heavy management with highly manicured lawns. Keep dead and decaying wood in urban woodlands. Plant native species (including trees), pets (especially cats) should not spend a lot of time outdoors, reduce artificial light at night where possible, reduce the use of insecticides/pesticides, etc,” Hahs and Kotze told EcoWatch. “At a large scale, promote planning that [gives] wildlife the opportunity to move across the cityscape by connecting greenspaces.”

Moretti told EcoWatch that an urban garden can go a long way in providing an oasis for urban creatures.

“Provide a stable environment (e.g. home garden) with native food resources (not necessarily feeding them artificially), but by planting flowers or providing different structures, shelfs, different habitat… that allow the most diverse biodiversity to establish that can sustain themselves,” Moretti advised. “Avoid ecological traps… [Create a] group of discussion and exchange at the neighbourhood level that can discuss and decide about how to favour urban biodiversity in the quarter. Each garden is not an island. Speak out and make pressure to the municipalities and decision makers for more green of quality (not necessarily more in term of quantity – often not possible in dense cities), but of quality (lower management intensity, more resources, more heterogeneity and less manicured green).”


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