The Urban EcosystemT

he agglomerations of roads and buildings in which hundreds of thousands to millions of personscoexist produce major changes to the landscape. Urban areas can even modify the local climate.The presence of urban areas does not mean, however, that fauna and flora disappear. On the

contrary, even though some native animals and plants may completely dieoff, other species manage to adapt to urban areas. Often, these speciesare the ones that find it impossible to live away from people.

86 HUMANS AND THE BIOSPHERE ECOLOGY 87

1.2 billion acres(480 million hectares)The total area occupied by allthe great urban centers of the world. This is equivalentto 4% of the planet’s surface area.

280The number of offspringthat a single female ratproduces in its lifetime.The female can becomefertile just 48 hoursafter giving birth.

Urban MicroclimateThe microclimate of cities has characteristic propertiesthat result from the massive concentration of concrete.

Urban areas often lack green spaces, and their microclimatesare driven by the gaseous emissions from factories, automobiles,and other by-products of human industry, such as pollution.

World of Concrete Concrete and polluted city airprovide ideal conditions for

some species; however, others areonly relatively successful in thisunnatural environment. Some species(such as rats and cockroaches) adaptso well to city life that they becomea nuisance to people.

Survival Strategies The city presents advantages for numerousspecies. Its climate is more stable, and

there are no large predators. It is a complexenvironment, however, and it requires a number of strategies to be able to survive.

FLEXIBLE DIET

Many species adopt new eating habits. In many cases, their diet is based on the garbage discarded by people.

HUMAN CONTACT

Many species lose their fearof people as they come incontact with them. The minimum safe distance between humans and animalsshrinks, and some animals (such as pigeons) even establish physical contact.

LEARNING NEW BEHAVIORS

Animals learn how to tear openbags of garbage and stealfood. Cases have been reported in which birds (theblue tit, Parus caeruleus)have opened bottles of milkin Great Britain.

LOWER DENSITY, BETTERSURVIVAL

Although it would appear that the density of a given population of animals is less in the urban environment than in the wild, eachindividual animal is able to live longer in general.

Champions at Adaptation Some species have adjusted so magnificently to urban life thatthey form a part of the urban landscape in almost all cities

around the world.

HEAT ISLAND

The temperature in large cities is, on average, 2.7° F (1.5° C) warmerthan in the surrounding area. When the temperature falls at night, theconcrete from roads and buildings radiates some of the heat it absorbed during the day. This temperature difference between citiesand surrounding areas can be as great as 9° F (5° C).

HUMIDITY

It tends to be lower, although rainfall is heavier and more plentiful.

SOLAR RADIATION

It is between 15 and20% less, because thesky is usually dimmedby particulate matter,carbon dioxide, andwater vapor.

WINDS

Structures create concrete barriersthat reduce wind strength by 20%.

AIR QUALITY

The air in cities contains largeamounts of pollution and particulate matter.

Parks and plazas

In cities, these areas contain the greatest biodiversity. Native species coexist with introduced species. Migratorybirds even use these places for stopovers.

Below Ground This region is inhabited by significantcolonies of insects and rats, which comeout at night to find food. In addition,underground areas contain a variety offungi, bacteria, and worms.

High Above Birds and bats make nests onroofs and on window ledges.Other animals (such as insectsand rodents) use buildings asliving spaces, reaching altitudesthat they would never be able toreach in nature.

Sterile concrete? Concrete does not stop themarch of life. Several kindsof plants can grow fromcracks or holes in the pavement. Plants also usethe water that collects inthese spaces. The plants can,in turn, provide a home fornumerous microorganisms.

PIGEONS AND SPARROWS

These birds are present in all majorurban centers. They nest in the city’strees or buildings and incorporategarbage into their diets.

RATS

They can live underground and eat cockroaches and garbage.

SPIDERS

A large number of species have adapted to urbanlife. Some even build their nests inside houses.They feed on insects.

COCKROACHES, ANTS, AND MOTHS

They have adjusted extremely well to livingin cities with people. They are practicallyimpossible to eradicate.

MOSQUITOES

Not only do they live alongside humanbeings, but the females evenfeed on human blood inorder to reproduce.

BATS

These mammals nest in high places.Although feared for their appearanceand nocturnal habits, they help controlbothersome populations of mosquitoes.

RuralResidentialsuburbs

Commercial

Tem

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DowntownUrban residence

ParksSuburbanresidence

Rural