Many studies have described the effects of urbanization on species richness. These studies indicate that urbanization can increase or decrease species richness, depending on several variables. Some of these variables include: taxonomic group, spatial scale of analysis, and intensity of urbanization. Recent reviews of birds (the most-studied group) indicate that species richness decreases with increasing urbanization in most cases but produces no change or even increases richness in some studies. Here I expand beyond the bird studies by reviewing 105 studies on the effects of urbanization on the species richness of non-avian species: mammals, reptiles, amphibians, invertebrates and plants. For all groups, species richness tends to be reduced in areas with extreme urbanization (i.e., central urban core areas). However, the effects of moderate levels of urbanization (i.e., suburban areas) vary significantly among groups. Most of the plant studies (about 65%) indicate increasing species richness with moderate urbanization whereas only a minority of invertebrate studies (about 30%) and a very small minority of non-avian vertebrate studies (about 12%) show increasing species richness. Possible explanations for these results are discussed, including the importance of nonnative species importation, spatial heterogeneity, intermediate disturbance and scale as major factors influencing species richness.

Effects of urbanization on species richness: A review of plants and animals

Low-density rural home development is the fastest-growing form of land use in the United States since 1950. This ''exurban'' development ( ;6-25 homes/km 2 ) includes urban fringe development (UFD) on the periphery of cities and rural residential development (RRD) in rural areas attractive in natural amenities. This paper synthesizes current knowl- edge on the effects of UFD and RRD. We present two case studies and examine the patterns of biodiversity response and the ecological mechanisms that may underlie these responses. We found that many native species have reduced survival and reproduction near homes, and native species richness often drops with increased exurban densities. Exotic species, some human-adapted native species, and species from early successional stages often in- crease with exurban development. These relationships are sometimes nonlinear, with sharp thresholds in biodiversity response. These effects may be manifest for several decades following exurban development, so that biodiversity is likely still responding to the wave of exurban expansion that has occurred since 1950. The location of exurban development is often nonrandom relative to biodiversity because both are influenced by biophysical factors. Consequently, the effects on biodiversity may be disproportionately large relative to the area of exurban development. RRD is more likely than UFD to occur near public lands; hence it may have a larger influence on nature reserves and wilderness species. The ecological mechanisms that may underlie these responses involve alteration of habitat, ecological processes, biotic interactions, and increased human disturbance. Research on the patterns and mechanisms of biodiversity remains underdeveloped, and comparative and experimental studies are needed. Knowledge resulting from such studies will increase our ability to understand, manage, and mitigate negative impacts on biodiversity.

/pdf/effects-of-exurban-development-on-biodiversity-patterns-44wo9biwo3.pdf

Effects of exurban development on biodiversity: patterns, mechanisms, and research needs

The patterns of biodiversity changes in cities are now fairly well established, although diversity changes in temperate cities are much better studied than cities in other climate zones. Generally, plant species richness often increases in cities due to importation of exotic species, whereas animal species richness declines. Abundances of some groups, especially birds and arthropods, often increase in urban areas despite declines in species richness. Although several models have been proposed for biodiversity change, the processes underlying the patterns of biodiversity in cities are poorly understood. We argue that humans directly control plants but relatively few animals and microbes—the remaining biological community is determined by this plant “template” upon which natural ecological and evolutionary processes act. As a result, conserving or reconstructing natural habitats defined by vegetation within urban areas is no guarantee that other components of the biological community will follow suit. Understanding the human-controlled and natural processes that alter biodiversity is essential for conserving urban biodiversity. This urban biodiversity will comprise a growing fraction of the world's repository of biodiversity in the future.

/pdf/urban-biodiversity-patterns-and-mechanisms-44jidpph9z.pdf

Urban Biodiversity: Patterns and Mechanisms

Human activities dramatically change the abundance, diversity, and composition of species However, little is known about how the most intense human activity, urbanization, alters food webs and trophic structure in biological communities Studies of the Phoenix area, situated amid the Sonoran Desert, reveal some surprising alterations in the control of trophic dynamics Species composition is radically altered, and resource subsidies increase and stabilize productivity Changes in productivity dampen seasonal and yearly fluctuations in species diversity, elevate abundances, and alter feeding behaviors of some key urban species In urban systems—in contrast to the trophic systems in outlying deserts, which are dominated by limiting resources—predation by birds becomes the dominant force controlling arthropods on plants Reduced predation risk elevates the abundance of urban birds and alters their foraging behavior such that they exert increased top-down effects on arthropods Shifts in control of food web dynamics are probably common in urban ecosystems, and are influenced by complex human social processes and feedbacks

https://www.jstor.org/stable/10.1641/0006-3568(2005)055%5B0399:TDIUC%5D2.0.CO;2

Trophic Dynamics in Urban Communities

We describe the richness, abundance, and ecological characteristics of bees in community gardens located in heavily developed neighborhoods of the Bronx and East Harlem, NY. In total, 1,145 individual bees, representing 54 species (13% of the recorded New York State bee fauna) were collected over 4 yr. The nesting habits of these species include bees that nest in cavities (33% of species), hives (11% of species), pith (1.9% of species), wood (1.9% of species), or soft/rotting wood (7.4% of species) substrates. Soil-nesting individuals were relatively rare (25% of individuals), perhaps due to a lack of proper soils for nesting sites. Parasitic species were scarce (5.6% of species, 2.6% of individuals), most likely because of an absence or rarity of host species. Overall, exotic species were abundant and constituted 27% of the total individuals collected and 19% of the identified species. We compare these results to several bee faunal surveys in New Jersey and New York State, including newly reported species lists for Central Park and Prospect Park in New York City. Relative to other studies, bee richness of the urban gardens is reduced and composition is biased toward exotic and cavity-nesting species. Nevertheless, despite their small size and location within highly urbanized areas, urban community gardens harbor a diverse assemblage of bees that may provide pollination services and opportunities for ecological exposure and education.

http://files.meetup.com/235362/NYCurban_garden_bees2008.pdf

Bee Richness and Abundance in New York City Urban Gardens

Effects of urban land use on pollinator (Hymenoptera: Apoidea) communities in a desert metropolis

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Effects of urban land use on pollinator (Hymenoptera: Apoidea) communities in a desert metropolis