Urban wildlife

Urban wildlife is wildlife that can inhabit urban/suburban environments or densely populated human settlements, such as towns and urban parks.

Some urban wildlife, such as house mice, are synanthropic, or ecologically associated with and even evolved to become entirely dependent on human habitats. For instance, the range of many synanthropic species is expanded to latitudes at which they could not survive the winter outside of the shelterings provided by human settlements.

Other species simply tolerate cohabiting around humans and use the remaining urban forests, parklands, green spaces and garden/street vegetations as niche habitats, in some cases gradually becoming sufficiently accustomed around humans to also become synanthropic over time. These species represent a minority of the natural creatures that would normally inhabit an area, and contain a large proportions of feral and introduced species as opposed to truly native species. For example, a 2014 compilation of studies (which were biased towards work in Europe, with very few studies from south and southeast Asia) found that only 8% of native bird and 25% of native plant species were present in urban areas compared with estimates of non-urban density of species.

Urban wildlife can be found at any latitude that supports human dwellings. The list of animals that will venture into urbanized human settlements to forage on horticultures or to scavenge from trash runs from monkeys in the tropics to polar bears in the Arctic.

Different types of urban areas support different kinds of wildlife. One general feature of bird species that adapt well to urban environments is they tend to be the species with bigger brains, perhaps allowing them to be more behaviorally adaptable to the more volatile urban environment. Arthropods (insects, spiders and millipedes), gastropods (land snails and slugs), various worms and some reptiles (e.g. house geckos) can also thrive well in the niches of human settlements.

Urban environments can exert novel selective pressures on organisms, sometimes leading to new adaptations. For example, the weed Crepis sancta, found in France, has two types of seed, heavy and fluffy. The heavy ones land near the parent plant, whereas the fluffy seeds float further away on the wind. In urban environments, seeds that float far often land on infertile concrete surfaces. Within about 5-12 generations the weed has been found to evolve to produce significantly more heavy seeds than its rural relatives. Among vertebrates, a case is urban great tits, which have been found to sing at a higher pitch than their rural relatives so that their songs stand out above the city noise, although this is probably a learned rather than evolved response. Urban silvereyes (an Australian bird) make contact calls that are higher frequency and slower than those of rural silvereyes. As it appears that contact calls are instinctual and not learnt, this has been suggested as evidence that urban silvereyes have undergone recent evolutionary adaptation so as to better communicate in noisy urban environments.

As a consequence of synurbization, animals that inhabit urban environments have differences in morphology, physiology and behavior when compared to animals that inhabit less urbanized areas. Hormone-mediated maternal effects are capable mechanisms of offspring phenotypic developmental modification. For instance, when female birds deposit androgens into their eggs, this affects many diverse aspects of offspring development and phenotype. Environmental factors that can influence the concentration of androgens in avian eggs include nest predation risk, breeding density, food abundance and parasite prevalence, all factors of which differ between urban and natural habitats. In a study that compared antibody and maternal hormone concentrations in eggs between an urban population and a forest population of European blackbirds, there were found to be clear differences in yolk androgen concentrations between the two populations. Although these differences cannot be attributed definitively (more studies have to be performed), they might result from different environments causing females to plastically adjust yolk androgens. Different yolk androgen levels are likely to program offspring phenotype.

Plant genetic variation has an influence on herbivore population dynamics and other dependent communities. Conversely, different arthropod genotypes have varying abilities to live on different host plant species. Differential reproduction of herbivores could lead to adaptation to particular host plant genotypes. For instance, in two experiments that examined local adaptation and evolution of a free-feeding aphid (Chaitophorus populicola) in response to genetic variants of its host plant (Populus angustifolia), it was found that, 21 days (about two aphid generations) after aphid colony transplantation onto trees from foreign sites, aphid genotype composition had changed. In the experiments, tree cuttings and aphid colonies were collected from three different sites and used to conduct a reciprocal transplant experiment. Aphids that were transplanted onto trees from the same site produced 1.7-3.4 times as many offspring as aphids that were transplanted onto trees from different sites. These two results indicate that activities of human perturbation that cause plant evolution may also result in evolutionary responses in interacting species that could escalate to affect entire communities.