In Toronto, surveys showed that besides the typical local vegetables , farmers grew an additional 16 vegetable crops to supply the local community with foods unavailable in local grocery stores. These crops included Asian vegetables, such as bok choy, long bean, hairy gourd, and edible chrysanthemums and substantially increased the vegetative diversity of the urban garden system .Plant diversity is a principle predictor of insect diversity at small spatial scales , and plant diversity and small-scale structural complexity is important for tree-dwelling arthropods , ground-dwelling arthropods , web spiders , grasshoppers, bees, and ground-dwelling beetles . However, arthropod species richness has been shown to decrease with increasing impervious surface and intensive management in urban green areas, and intensive UA would presumably have a negative impact of species richness . In a study of urban backyard gardens in Toronto, invertebrate abundance and diversity was enhanced as the number of woody plant structures and plant species diversity increased, and backyard gardens had higher abundances of winged flying invertebrates when compared with urban grasslands and forests . Likewise, within domestic gardens in the UK, invertebrate species richness was positively affected by vegetation complexity, especially the abundance of trees . In Pennsylvania, butterfly diversity increased with native plantings within suburban gardens , and parasitoid diversity increased with floral diversity within urban sites . Because allotment gardens often exhibit a rich abundance of flowering plants and thus a prolonged season for nectar supply, allotment gardens can support urban pollinators for long periods of time . In a survey of 16 allotment gardens in Stockholm, the number of bee species observed per allotment garden ranged between 5 and 11, including a large number of bumble bees, which were observed on a total of 168 plant species,mobile vertical farm especially those in the Lamiaceae, Asteraceae, Fabaceae, Boraginaceae and Malvaceae . In a survey of gardens in Vancouver, a mean richness of 23 bee species were found across the different garden types sampled .
Similarly, community gardens in NYC were found to provide a range of ornamental plants and food cropsthat supported 54 bee species, including species that nest in cavities, hives, pith, and wood . In another study in NYC community gardens, the authors found that butterflies and bees responded to sunlight and floral area, but bee species richness also responded positively to garden canopy cover and the presence of wild/unmanaged areas in the garden . In Ohio, bee abundance in private, backyard gardens increased with native plantings, increases in floral abundance, and taller herbaceous vegetation . Overall, these studies support the idea that UA management with high vegetation diversity can have positive effects on invertebrate biodiversity in urban systems.Wildlife friendly features implemented in gardens can increase vertebrate diversity . Practices such as planting fruit/seed-bearing plants, limiting the use of pesticides and herbicides, and constructing compost heaps and bird tables increase bird and vertebrate abundance and diversity . Numerous avian studies have also shown that gardens with sufficient native vegetation can support large populations of both native and exotic bird species at the local level , and at the landscape level, garden heterogeneity can increase the overall diversity of insectivorous birds . Heterogeneity that includes native plant species may be particularly important, as studies of suburban gardens in Australia show that nectarivorous birds prefer native genera over exotic genera as foraging sites . For non-avian vertebrates, garden size and management style is critical for persistence in urban areas. Baker and Harris reported 22 mammalian species/species groups recorded in garden visitation surveys within the UK; however, mammal garden use declined as housing became more urbanized and garden size and structure decreased. Key findings from a range of garden studies show that in addition to high cultivated floral diversity, the three dimensional structure of garden vegetation is an important predictor of vertebrate abundance and diversity .
Increases in the vegetation structure and genetic diversity of domestic garden habitats have been shown to improve the connectivity of native populations currently limited to remnants and aid in the conservation of threatened species . For example, one study in Latin America documented that garden area and tree height were positively related to the presence and abundance of iguanas within urban areas, and increased patio extent allowed for greater iguana movement across the urban landscape . In addition to habitat quality, habitat connectivity may also affect the ability of ground dwelling animals to persist in the urban landscape ; thus, UA systems will need to be connected to other vegetated areas to allow for landscape movement. These studies show that garden structures or management practices that provide food and nesting resources or movement corridors can be important strategies for maintaining vertebrate diversity in cities.Ecosystem services are often a function of biodiversity levels , thus the composition, diversity, and structure of plant and animal communities within and around UA are important to consider for urban ecosystem service conservation. specifically, biodiversity provides opportunities for ecosystem services that city planners value–including energy efficiency, storm water runoff, air pollution removal, carbon storage and sequestration, and water quality provision . Within agricultural systems, ecosystem services like water storage, pollination, and pest control increase US crop production resilience and protect production values by over $57 billion per year . However, there remains a large knowledge gap around the provisioning of services in UA systems. This is especially concerning given increasing global food demands, climate-related crop failure, and consistent limitations in fresh food access within urban centers . We posit that UA systems provide a suite of ecosystem services, and that the extent and quality of the services are largely dependent on the biodiversity and vegetative structure of the UA system. Thus the form and management of urban gardens can radically influence service provision. Small garden patches are able to supply structural habitat diversity and carbon storage , while allotment gardens can potentially support ecosystem services such as pollination, seed dispersal, and pest regulation to the broader urban landscape .
In contrast, reductions in biodiversity can cause a reduction in the resilience of urban ecosystems overall . specifically, we review some of the most studied and important ecosystem services to the urban agricultural system: pollination, pest control, and climate control.As mentioned previously, urban agriculture can support a diverse assemblage of bees and butterflies , and the number of native flowering species can positively impact bee abundance and diversity . This may have large implications for fruit set and crop production given that crops experience higher or more stabilized fruit set in habitats with greater native bee diversity . Additionally, floral cover can positively impact conspecific pollen deposition by attracting a greater number of pollinators into an urban garden . Some studies suggest that pollinator foraging and dispersal needs are best supported by a network of small,hydroponic growing natural habitat fragments across urban areas . In general, bee foraging distance correlates with body size , and some larger bodied bees can regularly fly >1 km from their nest to forage on floral patches . Thus proximity to natural habitat can increase bee abundance, diversity, and pollination success for a wide range of crop species and may similarly impact bee diversity within urban landscapes. Research in rural and exurban habitats suggests that bumble bee nesting densities are positively impacted by the proportion of suburban gardens and wooded habitat and that bees are willing to forage further for high diversity flowering patches . Furthermore, both nesting density and dispersal are negatively impacted by the amount of impervious cover in a landscape , revealing the potential for urban landscapes to obstruct pollinator foraging and dispersal.Likewise, heavy development that leads to shaded and closed-off garden areas tend to limit local pollinator diversity . Overall, urban landscapes that maintain diverse natural habitat fragments and minimize impervious cover can promote bee nesting and dispersal. Insights from these studies and others suggest that pollination services may be higher in urban gardens if natural habitat patches and diverse flowering resources are available.Biological control is a method of controlling pest populations through the utilization of other organisms . Bio-control has been used for centuries within agricultural systems and could potentially enable sustainable crop production in cities without the reliance of toxic chemical pesticides. This is especially useful in high density urban areas where human exposure to toxins is more risky . The natural enemy complex responsible for bio-control includes predators, parasitoids, and pathogens that regulate pest populations . Different natural enemy taxa often have specific habitat preferences; therefore management for bio-control in urban areas requires knowledge of those factors that influence the abundance and richness of natural enemies.
One of the most effective groups of natural enemiesis parasitic Hymenoptera, which reduce herbivorous insect damage to urban trees, ornamental landscape plantings, and residential fruit and vegetable gardens . Bennett and Gratton showed that local and landscape scale variables associated with urbanization influence parasitic Hymenoptera abundance and diversity in residential and commercial properties along a rural to urban landscape gradient in Wisconsin. They found that parasitoid abundance was a positive function of flower diversity, and parasitoid diversity decreased as impervious surface increased in the surrounding landscape. This suggests that parasitoids benefit from increased floral resource availability and decreased impervious cover, similar to patterns described for pollinators.Below-ground natural enemies can prey on soil-dwelling stages of insect pests in urban lawns, often reducing the frequency and intensity of pest outbreaks . Yadav et al. tested if changes in urban habitat structure of gardens and vacant lots influenced below-ground bio-control services rendered by invertebrate and microbial communities. They showed that ants and microbial communities contributed a majority of the bio-control service, with ants exhibiting significantly higher bio-control activity than microbes, particularly in vacant lots. The high levels of below ground bio-control activity in vacant lots and urban gardens could serve as a foundation for building sustainable pest management practices for urban agriculture in cities. A number of other natural enemies provide bio-control services in UA landscapes, such as birds, bats, spiders and beetles , but there is still very little research done regarding their role in urban agriculture. The use of organic composts to support pest control by encouraging predatory species has shown some success . More work will be required to understand how urban systems, and especially urban agriculture, affect foraging behaviour in higher trophic level natural enemies .As climate models continue to indicate an increased likelihood of heat waves in urban areas, there has been great interest into the relationship between green infrastructure and mitigation of the urban heat island effect . Two main approaches have been proposed as solutions to reduce the urban heat island effect, maintaining more urban green space and reducing impervious surfaces. Increasing the proportion of green space within the urban matrix can reduce both surface and air temperatures . However, the variety of vegetative infrastructure, management, and plant species within UA systems will vary in their cooling potential. Akbari et al. predicted that up to a quarter of the cooling effect by urban trees in US cities are a result of garden/street trees contributing direct cooling of adjacent buildings, and this effect is dependent on tree size,species, maturity, and architecture. At the garden level, vegetation can influence the energy loads on individual buildings, but how this impacts air temperatures across the wider urban environment is still unclear . However, considering the potential impact that increased vegetation has toward regulating temperatures, there could be big implications on energy use and comfort levels for urban communities. Additionally, gardens located in areas unsuitable for buildings or established as buffer zones along rail corridors and highways, may be helpful in balancing the urban microclimate. Gardens also provide storm attenuation services to the urban matrix. Vegetation, trees especially, intercept intense precipitation and hold water temporarily within their canopy, thus reducing peak flow and easing demand on storm drains . In German cities, allotment gardens used on green belts to facilitate drainage have been shown to reduce heat and demand for air conditioning . In contrast, hard paving increases impervious surface, and in Leeds, UK, increased hard paving in residential front gardens has been linked to more frequent and severe local flooding .