Synurbization refers to the adaptation of wildlife to the unique conditions presented by urbanized environments, with a focus on how synurbic populations differ behaviorally, physiologically, and ecologically from populations of the same species living in their natural environments.[1][2][3] Urbanization, in terms of ecology, is the process by which natural landscapes are transformed into built environments through human development and intervention, leading to altered environmental conditions and ecological dynamics within the area.[1][3][4][5] Synurbization may occur as a response to the encroachment of cities into the existing habitats of wildlife, or through the colonization and expansion of wildlife into urban environments.[1][6][7][8] These adaptations are necessary for the survival of the population, as individuals possessing advantageous variations in traits or greater behavioral flexibility ultimately have higher fitness in these urbanized settings.[2][3][8][9][10] However, while some of the behavioral adaptations are favorable for the species and allow them to persist, some may have a negative impact on the species, and the broader consequences that stem from urbanization can negatively affect both wildlife and humans.[2][3][6][11][12][13]
When compared to members of their species that live in natural environments, urban-dwelling populations exhibit clear behavioral differences, collectively described as “urban wildlife syndrome” since the changes are not exclusive to one species, but rather are a pattern of behaviors that have been observed across multiple species due to the similar conditions and selective pressures that urban environments across the world present.[2][11][14][15][16]
Due to excessive "hand-rearing" in bird species, some individuals may become behaviorally crippled. This can remove innate survival skills that a species would otherwise possess. One of the most important behaviors that these young would lose is their fear of humans.[17]
Population density increase
Including the correlation between other variables, such as aggression and wariness, multiple studies show a population density increase.[2][11][15][16] Population density is the population number in one unit of area at a given time.[18] The increase in population density has been highly correlated with the reduction in species wariness, as well as their intra-specie aggression. As population density increases, wariness of humans decreases- this is known as a negative correlation. As population density increases, intra-specific aggression increases- this is known as a positive correlation.[15][16] Park spaces in urbanized spaces may contribute to this population density increase. These parks allow for species to mate, and access and be fed food by humans, with little to no predation.[16][2]
Decreased wariness and increased boldness
Wariness is the observable fear or caution that animals exhibit when encountering humans, typically shown through a startled response or rapid retreat.[2] Previous research hypothesized that population density is the main influence on this behavioral change.[11][16] Through coexistence, animals grow accustomed to human presence, not only becoming less startled, but also more willing to approach and physically interact with humans.[16] For urban squirrel populations in parks in particular, this behavioral change was in response to human willingness to feed them.[16] This is a demonstration of boldness, which is characterized by the propensity of the animal to take risks, explore novel environments, or approach potentially threatening situations.[8] Across various species, increasing levels of synurbization have been positively correlated with an increase in boldness.[14][19][20] Species that colonized urban environment earlier consistently show increased boldness than recently synurbanized species, reflecting cumulative long-term processes rather than short-term plasticity alone.[21]
A negative consequence of this behavioral shift is the killing of urban-dwelling animals that some humans perceive as a nuisance. Some humans have been observed to have an irrational fear of wildlife in their yards and resort to "intentional abuse", and "unethical harm."[17] Some urban species, especially those willing to come into close contact with humans, pose a legitimate health hazard through zoonotic disease transmission. [6][22] Another consequence of increased boldness, especially with urban deer populations, is an increase in vehicle collisions with wildlife. [22]
Increased intra-specific aggression
Intra-specific aggression is the aggression toward members of their own species.[23] Similar to reduced wariness, the increase in intra-specific aggression may be the result of the increase in population density. Having more members of a species in a smaller unit area, all whilst still competing for the same resources is likely to have the outcome of higher aggression levels.[2][11][16]
Increased lifespan
Squirrel having easy access to food due to humans having feeders for the wildlife.
Animals living in urbanized populations are on average living substantially longer than their rural species populations. This is due to many factors such as:[2]
Reduced migrations- The climate conditions and access to food seasonally has reduced the need for migratory species to leave during the colder seasons. Migratory species also face dangerous conditions and require high energy levels to endue a migration; without this need, the animals are being more safe and saving themselves time, and energy be remaining sedentary.[2]
Longer breeding season- The sedentary lifestyle from the reduction of migration, as well as the preferential climate, allows the animals to breed for much longer periods of time compared to the rural populations.[2]
Access to food- Since these animals live in urban areas, they encounter humans daily. Whether these are humans in a public park, a street, or even their own backyard; where there is a human, there is likely food. The reduction of wariness is favorable to animals in this sense because they can approach people in parks or the streets, and as a result gain food in return. Many people also have feeders in their backyard; another way for animals to easily access food.[15][16]
Favorable conditions- easier access to food, an abundance of sheltered areas, low predation, and favorable climates.[2][15][16]
Change in the circadian rhythm
Some hypothesize that changes in circadian rhythm are because of artificial light[2] from street lights, cars, homes, and large signs.
Changes in dwellings
Synurbic populations have been observed to alter their dwelling habits, incorporate materials commonly found in human-populated environments into their nesting material, and make use of man-made structures as shelters as they adapt to living in urban areas. [1][2][3][8]
An urban coot using part of a sunken boat as a nest in an Amsterdam canal.
Urban birds frequently modify their nesting behavior to account for the limited natural materials and suitable nesting sites in their new environment. While birds in rural areas use the abundant twigs, grass, moss, and other naturally occurring materials for their nests, urbanized birds have less access to these types of materials and adapt by using plastic, paper, and string to build their nests.[2][3] Many may also use the infrastructure of buildings, bridges, public parks, and utility poles for nesting and roosting.[2][3][8] In some species, such as the magpie, an increase in nest heights of urban populations has been documented in response to human activity close to the ground.[24] Other species, including Torresian crows and little ravens, adjust to urban living by forming large, communal roosts that improve individual fitness through collective predator defense and information sharing about food sources, behaviors that are absent from their rural counterparts.[25]
A number of small urban mammal populations have likewise been found to use buildings and other artificial structures as den sites or shelters, especially during the winter, improving their survival.[8] However, species that do not exhibit such behavioral flexibility have been unable to successfully colonize and persist in urban environments, which often have dense soil, spare vegetation, and fewer trees, increasing competition for the limited natural refuges remaining in urbanized areas and making the construction of burrows unfeasible. [1][8]
Changes in Physiology
Changes in gut microbiome and immunity
Rapid urbanization has more recently been linked to incidences of autoimmune diseases, such as irritable bowel syndrome, in humans due to the reduced diversity of the gut microbiota through Westernization of diet, pollution, and increased antibiotic use associated with urbanized populations.[26] Additionally, in urban birds, shifts in habitat use and diet have been shown to increase susceptibility to pathogens through alternation of microbial composition and diversity.[27] Beyond microbial impacts, urbanization also modulates the immune defenses of wildlife by shifting the composition and distribution of pathogens relative to natural habitats.[28] In feral pigeons, increasing urbanization was found to correlate with reduced natural antibody activity, suggesting possible immunosuppression or adaptive downregulation of innate defenses in heavily modified environments.[28]
Changes in body mass
Due to the altered ecological conditions of urbanized environments, urban animals often differ in body mass when compared to those in living in their native habitats, though the direction of change depends on the interplay of a variety of factors specific to the population in question. For some species that have demonstrated an ability to adapt to the stressors, selective pressures, and anthropogenic food sources presented by an urban environment, an increase in body mass has been observed.[12][29][30] In Silver Gulls in particular, this difference was exclusive to male members of the species and was correlated with an improvement in body condition.[31] However, other urban species that have been studied, such as rufous-collared sparrows and Eurasian red squirrels, exhibit lower body mass.[8][12] Potential explanations that have been proposed include the physiological toll of higher disease and parasite exposure, as well as the elevated stress associated with inhabiting non-native habitats for these populations.[8][12]
Consequences of Urban Development
The main consequence of urban development for wildlife is a decrease in its species and ecological diversity. The growing tendency towards synurbization observed in birds and mammals is a chance for enriching diversity of urban wildlife. Synurbization of some species could cause practical problems when their populations grow to high concentrations. An example of such problems is Canada goose in North American cities.[32]
1 2 Burton, Donald L., and Kelly A. Doblar. "Morbidity and mortality of urban wildlife in the midwestern United States." Proc 4th International Urban Wildlife Symposium. Vol. 171. 2004.
↑ Zuo, Tao; Kamm, Michael A.; Colombel, Jean-Frédéric; Ng, Siew C. (2018). "Urbanization and the gut microbiota in health and inflammatory bowel disease". Nature Reviews Gastroenterology & Hepatology. 15 (7): 440–452. doi:10.1038/s41575-018-0003-z. PMID29670252. S2CID4944320.
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