Urbanization often degrades biodiversity. As cities grow, vital habitats are destroyed or fragmented into patches not large enough to support complex ecological communities. In the city, the species may become endangered, or even locally extinct as the urban jungle swallows up the previously natural areas. Ironically, urban growth is often responsible for the introduction of non-native species, either by accident (such as the brown rat, Rattus norvegicus), or deliberately, for food, pets, or aesthetic reasons.
The growth of cities may reduce biodiversity by fragmenting or destroying large areas of natural habitats on which many species depend. A growing population leads to an expansion of urban areas and an increase in the demand for natural resources such as timber and fossil fuels. This inevitably leads to habitat destruction, which has been called "the largest factor contributing to the current global extinction event. The rate of urbanization is alarming,"
The extent of severe fragmentation impacts on biological diversity depends to a large extent on the size of the patch of habitat remaining. Many species require large contiguous habitat patches in order to maintain stable populations. So smaller spots usually contain fewer types than larger spots.
Biohomogeneity refers to the replacement of native native (and often endemic) species with non-indigenous, invasive, and global species. Often the same non-indigenous invasive species found in many cities around the world The result is an overlap of high species between regions of previously distinguished living organisms, homogeneity of biological communities and a decrease in regional and global biodiversity.
Because a decrease in biodiversity and increased urbanization are closely related, research has also shown that in some cases biodiversity actually peaks in suburban areas for certain groups such as insects and plants. The reason is that the arid land contains large numbers of plant species that have not been introduced. Local while the jungle was not.