UA systems may contribute to the reduction of impervious surfaces in urban landscapes, thus increasing the drainage and infiltration potential of precipitation.As mentioned in the Urban agricultural systems and biodiversity provision section, it is important to understand that not all biodiversity is necessarily “good” biodiversity, and there may be a number of disservices that come from UA that can negatively impact the ecosystem functioning and health of cities. In some cases, there is the possibility of negative types of spill over from managed to natural systems or vice versa of weed, pathogen or pest populations, potentially harming native ecosystems and damaging ecosystem service delivery from natural systems . The juxtaposition of natural systems to UA systems also potentially leads to an increased opportunity for biological invasions and detrimental competition to native species . Genetic introgression within natural ecosystems by urban garden plants can negatively alter the genetic composition of native vegetative patches and affect the long term viability of these systems . UA areas may also lead to increased human health issues and disease transmission to urban populations. For example,UA systems provide increased mosquito breeding sites due to the presence of standing water from irrigation, and this may potentially increase the rate of mosquito-borne diseases in certain areas of the city . Additionally, in non-organic UA systems, there is the potential for spillover of chemicals into natural and human habitats,hydroponic grow kit leading to environmental pollution and air- or water-borne health risks .
There may also be potential competition for limited resources between UA and natural systems, such as competition for water in arid environments. All such potential disservices from within and outside of UA systems must be considered in optimizing the overall ecosystem services gained in an urban landscape.There are several research areas that deserve more attention so that urban land use trade offs can be better elucidated and biodiversity and ecosystem services can be promoted through the increased integration of UA in cities. We posit that the three key challenges to improving management of gardens for B&ES delivery are availability of urban space, environmental constraints, and lack of knowledge. Below, we provide three major strategies for addressing the key challenges of enhancing B&ES within UA systems.Increased urbanization will lead to greater competition for space in cities, making it difficult to maintain biodiversity supporting habitats. More research is needed to best take advantage of the limited space available for urban gardens and maximize biodiversity within these areas. Private yards. Private yards make up a significant proportion of green space in a city and do not require the acquisition of new space. Even small-scale, private gardens that present complex vegetation structure can provide appropriate habitats for organisms that have difficulty existing in the urban matrix .In the US, the National Audubon Society’s “Audubon at Home” project offers several management principles to increase bird biodiversity in backyards , and the National Wildlife Federation provides certification for ‘wildlife-friendly’ gardens. More research is needed to understand the effectiveness of these incentives to support native biodiversity and food production as many of the techniques are focused on the augmentation of ornamental or floral plants rather than food crops. Available public spaces.
Because greater housing density has been linked to smaller garden sizes, there is an acute need to better understand how UA can be supported within public green spaces, such as community gardens and easements to enhance ecosystem services . Even within small habitat strips, the conservation of plants known to attract pollinators or pest natural enemies can provide substantial B&ES while utilizing limited space , but more information regarding urban plant–animal interactions needs to be known in order to best augment such spaces effectively. Vacant lots. Vacant lots provide opportunities to create functional green spaces where industrial redevelopment is not likely to happen . UA in these areas can improve B&ES and provide physical and psychological health for people in cities . However, a better understanding of how to successfully rehabilitate vacant lots is needed in order to promote this option. Additionally, creating gardens in abandoned lots has implications on urban land tenure for garden management, and it would be helpful to investigate whether temporary gardens make positive contributions to B&ES and food production in the same ways that more permanent gardens do.A number of environmental changes come with urbanization and affect the agronomic conditions necessary for food production, such as water availability, nutrient supply, soil degradation, and pest pressure . Resilience to climatic change. We need investigations that examine how the choice of garden trees, shrubs, and other plants influence air and surface temperatures in the gardens, and the potential role of garden vegetation to lower energy use and costs in urban environments. Likewise, we need to develop a better understanding of the specific garden plantings that most enhance carbon sequestration in UA. On the flflip side, there is basically nothing known about how different UA respond to climate change or climate extremes, and how the urban environment in which UA is embedded may exacerbate climate effects. Thus, more research is needed to understand how plants in UA will respond to increasing temperatures, drought, changes in rainfall amount, nutrient deposition, and weather extremes. Water use.
Research on environmental constraints related to water use is also needed in UA, as irrigation is often required to provide water necessary for urban farming, especially in arid environments. Rainwater or grey water can be used for garden irrigation, and it is cheaper and at times more available than potable water-based irrigation, but UA gardeners must be aware of the potential pathogens and heavy metal contaminants that can cause human and environmental health problems , especially with water run-off from these sites. Soil ecology. Urban soils are usually compacted, have low levels of organic matter, altered soil moisture characteristics, and sometimes have lead or other heavy metal contamination due to urban environmental processes . A number of methods, such as cover cropping, mulching, producing in raised beds, and changing subsurface drainage through piping, can improve soil conditions to support food production . However, more research must be done to understand how to sustainably rehabilitate urban soils. The use of both organic and inorganic fertilizers in combination with nutrient-rich wastewater can lead to surplus nutrients in these systems . Alternative methods, such as “organoponics”, where organic compost is used as a growing medium instead of existing soils, need to be further explored to develop farming methods that are successful in the urban environment .Two oft-cited barriers to wildlife gardening are the lack of information to alter gardening methodologies for improved biodiversity and the ineffective transfer of knowledge to improve the sustainability of urban gardens . Methodologies for improved B&ES. There is an ever expanding data set on patterns of biodiversity loss in urban areas and the factors that positively correlate with the diversity of plants, arthropods,vertical farming racks and vertebrates in urban landscapes. The time is ripe for a quantitative review or meta-analysis of those specific habitat and landscape features of urban habitats that correlate with increases in species richness and abundance of biodiversity in general, and beneficial organisms in particular. For example, past studies document that urban gardens can increase predator or parasitoid diversity , but we still lack research that documents the garden features that enhance the specific act of predation by ants, spiders, birds, or other predators. Very little research has focused on how management intensity in UA systems will affect biodiversity and ecosystem services provision from these systems. Additionally, landscape level UA connectivity is important for the creation of networked biodiversity refuges and for the improvement of matrix permeability for organisms. There is a lack of knowledge in understanding how the movement of species between landscape elements can allow organisms to carry out functions at different points in space and time and maintain services that would otherwise be isolated and how UA fits into the larger general pattern. More research to understand the effects of garden management on landscape-wide biodiversity and movement will be necessary to determine the most critical management practices for promoting effective landscape connectivity. Knowledge transfer. Increasing our understanding of UA management practices and knowledge spread may be the most important area of research if we are to promote gardens that support B&ES. We need to determine which types of UA contribute disproportionately to food production under different geographic, weather, and socio-economic conditions so that urban gardeners can cultivate specific plant species best suited for their location.
Local ecological knowledge is generally low among urban residents; however, discussion between community members may encourage biodiversity-friendly gardening, either through neighborhood or community exchanges of information . More research is essential to understand how to identify the information most useful to urban gardeners and how to most effectively communicate this knowledge.The ecology of Phytophthora, a genus of fungal-like oomycetes historically erected and known for plant pathogenic species primarily associated with destructive diseases in agriculture, has undergone substantial reconsideration in recent years. The recent emergence of a number of Phytophthora-caused plant epidemics in forests and other non-agricultural ecosystems has clearly shown that many members of the genus have potential as invasive species that can threaten natural ecosystems. As a consequence of research in non-agricultural environments, a surprising diversity and abundance of Phytophthora species have been discovered, many previously undescribed. Incidental to this research has been the discovery that many species of Phytophthora are abundant in natural surface waters, especially in streams. Many such species are so widespread and regularly encountered that they are now considered resident, if not endemic, and characteristic of such environments. Nevertheless, isolates of well-known plant pathogenic species or species complexes are also regularly recovered, often without discernible symptoms or signs of disease on the vegetation. Though the prevalence of Phytophthora in surface waters is now well established, the ecology underpinning this phenomenon is largely speculative. Because these organisms are known primarilyas causes of often devastating plant diseases, the nature of their presence in these environments and its implications for the persistence and spread of pathogenic species are important considerations for disease prevention and management. There is also a growing interest to understand the role of Phytophthora, among other Peronosporales, in decomposition of vegetative matter in aquatic environments. The biology of Phytophthora, a genus of well adapted plant pathogens with a necrotrophic phase, suggests that their ecological role in leaf decomposition should be early colonization and breakdown of relatively fresh, live vegetative tissue. As they colonize leaves newly exposed in streams, they can open the integral tissues for colonization by saprotrophic organisms less able to penetrate the leaf cuticle, in a process analogous to ‘conditioning’ of leaf litter for palatability to shredder organisms. The co-occurrence of both known plant pathogens and primarily stream-associated Phytophthora in aquatic environments also raises the question of whether these taxa have similar or divergent modes of life and whether they compete for resources in these environments. In streams, vegetative litter is the primary source of nutrients for microorganisms, but the quality of vegetative tissues available varies with respect to senescence and degree of decomposition. Coastal forests of northern California largely consist of evergreen trees and shrubs and so green leaves are a regular component of leaf litter introduced into streams, especially in winter and spring when, based on the region’s climate, most rainstorms occur. Nevertheless, much vegetative litter is in the form of senesced leaves. California bay Nutt is a common, broad leaf evergreen component of northern California’s coastal forests and a frequently occurring tree species in riparian zones. It is also a primary source of P. ramorum inoculum in California forests affected by sudden oak death, epidemic mortality of certain species in the beech family resulting from P. ramorum infection of the vascular cambium of the main trunk. California bay leaves are highly conducive to sporulation by P. ramorum which, despite causing localized necrotic lesions and spots on leaves, nevertheless causes little damage to the tree species itself. Additionally, bay leaves are sclerophyllous, as is typical for broad leaf evergreen plants in this Mediterranean climate, and so they decompose slowly. Bay leaves are therefore both very common as leaf litter in northern California forest streams and a highly suitable substrate for P. ramorum.