Synurbization
From Wikipedia, the free encyclopedia
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]
Population density increase
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 taxa due to the similar conditions and selective pressures that urban environments across the world present.[2][11][14][15][16]
Urbanization transforms ecological conditions where animals live due to habitat fragmentation, human proximity, and attraction to anthropogenic foods that change selective forces. For instance, with small mammals, striped field mice urban-inhabiting are bolder, more exploratory, and phenotypically plastic than their rural counterparts. They adapt to anthropogenic shelters, foods, and novel ecosystems from their tolerance of human-modified environments, the colonization of bolder mice, and urban-specific trait selection.[17] White-footed mice adapted through behavioral and genetic changes also exist in fragmented, urban parks, and while they require more plasticity in response to urban predation patterns, adaptability promotes success within these fragmented patches.[18] Birds like the noisy miner have been observed conducting innovative behaviors (i.e., opening sugar packets) that mirror those of small mammals and other birds in the same urban setting, yet taxonomic differences exist in the response to the challenges of novel habitats.[19] Urbanized ecosystems act as a filter for which species can be innovative, adaptable, and risk-prone; urbanized small mammals and other species can flourish more than other species due to social learning and temporal-spatial adaptability, suggesting more of an ability to navigate the changes humans implement in their ecosystems.[17][18][19]
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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.[20] The increase in population density has been highly correlated with the reduction in species wariness, as well as their intra-species 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.[2][16]
Decreased wariness and increased boldness
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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] When presented with the option of feeding in a farmland or a forested area, bolder individuals across a range of wildlife species consistently demonstrated a preference for the farmland despite the risk of encountering humans, whereas shy individuals selected the safer, forested area for foraging.[21] Across various species, increasing levels of synurbization have been positively correlated with an increase in boldness.[14][22][23] Species that colonized urban environment earlier consistently show increased boldness when compared to more recently synurbanized species, reflecting cumulative long-term processes rather than short-term plasticity alone.[24]
Increased intra-specific aggression
Intra-specific aggression refers to antagonistic interactions toward members of the same species.[25] The increase in intra-specific aggression may be the result of the increase in population density, as having more members of a species in a smaller unit area, all whilst still competing for the same resources, like food, shelter, and nesting sites, has been linked to competition and higher aggression levels.[2][11][16][26] While these patterns favor the survival and reproductive success of bold or competitive individuals, they also introduce the risk for injuries and stress, and they negatively affect the fitness and survival of subordinate individuals.[26][27]
Reduced migration
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Urban microclimates and predictable access to anthropogenic food sources have reduced the need for migratory species to leave during the colder seasons.[2][28][29] As a result, many urban populations of migratory species become sedentary and remain city residents throughout the whole year, reducing energetic expenditure and decreasing exposure to hazards such as storms, predation, and navigational errors.[2] In turn, this reduced migratory behavior, coupled with the preferential climate, access to man-made structures and shelters, and lower predation, extends the breeding season of urban inhabitants compared to rural populations.[2][28]
Change in the circadian rhythm
Circadian rhythms, largely governed by light exposure, structure both the behavior and physiology of organisms to correspond with natural temporal cycles and external environmental cues.[30][31] As the degree of urbanization increases in an area, so does the amount of light pollution from sources like street lamps, cars, homes, and large billboards, leading to an increase in the amount of artificial light exposure and an altered circadian rhythm that is out of sync with the daylight cycle.[2][31]
In response to this increase in light exposure during nighttime hours, urban populations of small mammals have adapted their behavior to be active throughout the whole day, showing no preference for either nocturnal or diurnal activity.[32] Synurbic bird populations, such as members of the European blackbird species, have been shown to have a shortened duration of their circadian period and have modified their behavior to begin daily activities earlier in the mornings.[33] Beyond the influence of artificial light, the continuous anthropogenic noise and movement in urban environments eliminates the predictable onset of quiet and low activity associated with night in the remote wilderness, further contributing to circadian disruption.[34][35] Multiple mammal species that typically exhibit foraging activity during the day in their natural habitats have become primarily nocturnal when adjusting to live in urban settings as a way to avoid human contact.[35][36][34] Others demonstrate a reduction in total daily activity, resulting in greater time spent resting at the expense of foraging and other beneficial behaviors that increase survival and reproduction.[34][37]
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]
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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.[38] 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.[39]
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]
Behavioural homogenization
Urban environments often favour a recurring suite of traits, sometimes referred to as an 'urban wildlife syndrome', including reduced fear of humans, greater use of anthropogenic food, shifts in daily activity, and increased use of human-made structures.[40] Such repeated shifts may reduce behavioural diversity across individuals, populations, and species - a process termed 'behavioral homogenization' - potentially affecting ecosystem functioning, animal cultures, and human–wildlife interactions.[41]
Changes in Physiology
Urban animals undergo a range of physiological changes that track closely with the behavioral shifts required to survive in human-modified landscapes.[42] The expansion into an urban environment and subsequent alterations to dietary choices and foraging behaviors, new movement patterns, and constant exposure to urban stressors all contribute to shaping the distinct physiology observed in urbanized species.[43][44][45]
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.[43] 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.[46] Beyond microbial impacts, urbanization also modulates the immune defenses of wildlife by shifting the composition and distribution of pathogens relative to natural habitats.[47] 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.[47]
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][48][44] In Silver Gulls in particular, this difference was exclusive to male members of the species and was correlated with an improvement in body condition.[45] 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]
Increased lifespan
Animals living in urbanized populations are, on average, living substantially longer than their rural population counterparts.[2] This is due to the interplay of various factors such as reduced migratory behaviors, extended reproductive seasons, and predictable access to food and shelter.[15][16][28][29]
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