Grand Challenges in Urban Agriculture: Ecological and Social Approaches to Transformative Sustainability

GeoSyntheSES Lab, Department of Geography and Programs in Rural Sociology and Ecology, Pennsylvania State University, State College, PA, United States, Department of Humanities, Pontifical Catholic University of Peru, Lima, Peru, 3 Faculty of Social and Behavioural Sciences, University of Zimbabwe, Harare, Zimbabwe, Department of Geography and Environmental Studies, University of New Mexico, Albuquerque, NM, United States, Department of Life Science Systems, School of Life Sciences, Technical University of Munich, Freising, Germany, Department of Geography, National Taiwan University, Taipei, Taiwan, Department of Urban Studies and Planning, University of California, San Diego, San Diego, CA, United States, Department of Environmental Science, Rhodes University, Grahamstown, South Africa, Department of Environmental Engineering, University of South Australia, Adelaide, SA, Australia, Department of Geography, Autonomous


INTRODUCTION
This synopsis of the Grand Challenges of Urban Agriculture (UA) is framed by the urgent need to understand and strengthen the expanding yet highly diverse roles of UA amid rapid global urbanization, failures of predominant food systems, and crises in systems of physical and mental health. More than half of humanity lives in cities today and by 2030 this is projected to grow to 60.4 percent, ∼5 billion people ( UN Habitat, 2020). More than 90 percent of urban demographic increase is anticipated to take place in the developing world.
Ecological and social dimensions of UA are situated in these expanding spaces of cities, towns, and villages (along with their urban fringe or peri-urban areas), and among their diverse populations. UA is further situated in the powerful, far-reaching influences of urbanization processes that occur within and beyond these spaces. UA is thus integral to the prospect of Urban Sustainability as SDG 11 ("Make cities and human settlements inclusive, safe, resilient and sustainable") of the U.N.'s 2030 Agenda for Sustainable Development.
Predominant agri-food systems are currently failing to provide healthy diets to the world while causing planetary externalities of environmental damage that both create and compound social injustices. As described below, UA has critical roles to play in strengthening food systems and the sustainability and justice of these functions in addition to benefits such as contributing vital new approaches to address crises of physical and mental health. Such contributions occur while recognizing the fuller scope of these societal problems. In response to such concerns, the Grand Challenges of UA serve as a clarion call for the integration of ecological and social research to advance this expansive frontier of sustainable food systems.
Addressing the Grand Challenges requires UA research to integrate across the biological, ecological, health, engineering, energy, and geophysical sciences as well as the disciplines of the social sciences and the fields of planning, design, and policy. UA research is well-suited to and indeed requires expanding applications of inter-and transdisciplinary frameworks such as social-ecological systems, social agroecology, sustainable livelihoods, environment-focused political ecology, cultural landscape and food-territory analysis, community studies, social justice and anti-racism movements, and sustainability science. To describe the social and ecological connections of UA through such frameworks, our synopsis of Grand Challenges refers to both the mechanisms of strong ties (linkages) and to the dynamics of more diffuse yet nonetheless powerful influences (entanglements).
Widespread engagement of diverse, non-academic stakeholders in UA reinforces the call to integrate ecological and social analyses through diverse knowledge systems and practices. The centrality of their engagement in UA highlights the urgency of strengthening solutions in projects, programs, and policies, thus further propelling the need for integrated ecological and social research.
Our article is intended to identify, highlight, and synopsize the Grand Challenges of ecological and social analyses in UA research, including future literature reviews and structured analyses. Thus, we call for research to address two principal themes of UA Grand Challenges: (i) UA ecological and social multi-functionality; and (ii) relationships of UA governance to social and ecological systems. We conclude with a call for research to leverage these Grand Challenges to strengthen sustainability and resilience through transformative UA.

UNDERSTAND AND SUPPORT CONNECTED UA ECOLOGICAL AND SOCIAL MULTI-FUNCTIONALITY
Several challenges of UA research are focused on determining the linkages and entanglements of ecological and social multifunctionality (Zasada, 2011). One principal focus has been the estimation of UA food yields, consumption levels, and efficiency (McDougall et al., 2019;Csortan et al., 2020). These estimations have occurred in UA case studies across a variety of geographic spaces and time frames (e.g., Crush et al., 2011;Karg et al., 2020). Though UA is unlikely to replace large-scale food systems, the potential benefits -and limitations -of UA's specific functions in a better food system remain elusive and constitute a vital challenge for UA research.
The range of research challenges associated with UA multi-functionality extends to both the direct social-ecological linkages of food, nutrition, and health as well as the crucial ecosystem services (Clinton et al., 2018), such as soilwater, pollinator, biodiversity, ecological restoration. and more diffuse yet influential entanglements with these factors and others. Prior to brief synopsis of each, we foreshadow the similar importance, which is examined in the next section, of UA's extensive governance-influenced ecological and social linkages and entanglements involving institutions, communities, and individuals.
Challenges of UA research on linked ecological and social multi-functionality are rooted in the diverse humanenvironment spaces of gardening, farming, foraging, livestock and food-animal tending, and other activities. UA research challenges traverse multiple social levels (e.g., individual, household, community) and spatial scales and settings (e.g., cities, towns and villages, core urban, urban fringe or periurban, individual plot, landscape). This rich diversity of spaceand place-based practices provides for the production and procurement of plants, animals, and biota of wide-ranging food, medicinal, and sociocultural goods as well as a panoply of potential related benefits such as outdoor enjoyment, social empowerment, food-environment experiences (see details below and Shackleton et al., 2017). The challenge of understanding these practices as integral parts of UA ecological and social multi-functionality is driven also by the expanding spectrum of urbanization impacts related to wide-ranging variations of the size, location, demographics, development, socioeconomic, and cultural characteristics of human settlements.
Active transformation of UA spaces extends from the shifting demographics of diverse neighborhoods located in and beyond city cores (Brown and Jameton, 2000) to UA in urban-fringe areas and urban-rural interfaces whose blurred boundaries reflect increased spatial inter-connectedness (Lerner and Eakin, 2011;Zimmerer et al., 2020a). Global urban expansion and differentiation are thus both strengthening and compounding the diverse processes and patterns affecting UA that present a new wave of research challenges. One example of recent urban expansion and differentiation propelling new research challenges is the estimated global-scale continuity of agricultural areas in urban fringe spaces (Thebo et al., 2014), a trend that co-exists with urbanization-driven farmland loss. Another important dimension is reflected in the emerging new global geographies of rapid urbanization. With ∼95% of urban growth in the next 30 years forecast to occur in developing countries, many research challenges will concentrate in these regions ( UN Habitat, 2020).
Understanding the social-ecological linkages of key urbanization processes-ranging from supply chains and demographics to technology development-contribute to new challenges in UA research amid these massive shifts (Saha and Eckelman, 2017;Carolan, 2020). These challenges extend in the reciprocal direction whereby it is vital to determine the influence of UA characteristics, as they unfold, on subsequent landscape, biodiversity, and resilience capacities of cities (McPhearson et al., 2014;Kremer et al., 2016a,b). Moreover, these changes challenge research to understand better how UA is being influenced through accelerated innovation, development, and implementation of new agricultural technologies such as indoor horticulture and vertical gardens amid numerous others (Thomaier et al., 2015;Hallett et al., 2016;Carolan, 2020).
Temporal dynamism, evidenced by historical UA case studies in the U.S. (Bassett, 1981), Europe, and wide-ranging international contexts (e.g., Lima, Peru, during the Spanish colonial period; Bell, 2018), reveal the short-and long-term social-ecological trajectories of UA in complex societies. These diverse trajectories include peri-urban areas and food chains, as well as attention to gardening per se (Doolittle, 2004), that evidence the fuller spatiotemporal scope of UA functions. These insights signal a challenge for new research to understand the range of UA temporal and spatial dynamics in relation to governance (see next section).
UA potentially offers additional key linkages to sustainable food systems through its benefits to physical and psychological health (Pollard et al., 2018). Estimated contributions of UAderived food to diet and nutrition, including factors such as protein intake, suggest its role is limited in overall scope but not trivial (Ward J. D. et al., 2014). Moreover, UA spaces such as gardens can play roles that are secondary in overall food supply but nonetheless crucial (and statistically significant) in regard to human nutrition (Jones et al., 2018). Expanding interest centers on UA-provisioned foods that deliver nutrition and health benefits due to quality and diversity, enhanced access, fit with sociocultural foodways, and potential contributions to food and nutrition security (e.g., Shackleton et al., 2009). These linkages are shown to include the benefits of outdoor exercise, personal satisfaction, and connection with nature (McLain et al., 2014).
Understanding and strengthening UA multi-functionality amid the global COVID-19 pandemic pose diverse research challenges. These include emphasis on UA capacities to address pandemic-worsened food and nutrition insecurity (e.g., Lal, 2020). Among those suffering the pandemic's most pronounced adverse effects on food and nutrition insecurity are large urban populations among the estimated 1.6 billion workers whose wages have been reduced and the estimated 71 million people being pushed back into extreme poverty ( UN Habitat, 2020).
At the same time, the challenges for UA research studying and strengthening multi-functionality extend well-beyond food and nutrition per se amid the global COVID-19 pandemic and potential post-pandemic phases. One priority is the analysis of potential shifts in UA ecological and social functions related to the COVID-19 pandemic. A particular area of research challenges is to examine social-ecological interactions of UA in time frames focused on both the COVID-19 pandemic as well as before and after. These interactions range from issues of sociocultural values and access, on the one hand, to ones of broad changes associated with potentially transformative UA (see final section below). They will need to focus, for example, on the impact of the COVID-19 pandemic on the potential expansion and possible transformation of UA functions, benefits, sustainability, governance, and justice dimensions and, similarly vital, how to strengthen these (Pulighe and Lupia, 2020).
Notwithstanding surging interest, specific UA linkages to generalizable health outcomes, both physical and psychological, are not well-understood and robust new analysis of this multifunctionality is needed (Siegner et al., 2018;Audate et al., 2019). Similarly, investigation is needed of proposed "multiplier effects" of entangled food, health, and nutrition benefits stemming from the kinds of knowledge and awareness that are gained in UA experience (Surls et al., 2015).
Substantial advances are taking place in the challenge to link site-based agroecological processes and pattern to broader contexts (Egerer and Cohen, 2020). The soil properties of community gardens, for instance, can be linked to neighborhood socio-demographic characteristics (Egerer et al., 2018). Other examples illustrate how UA typically hinges on multi-scale spatial flows of water, nutrient, and biotic resources (such as pollinators), as well as broadly defined social factors, knowledge, management, and policy Lin et al., 2015;Langellotto et al., 2018;Graefe et al., 2019). These landscape flows are being found to correspond to spaces or scales both within and across urban areas as well-traversing periurban and urban-rural interfaces (Soulard et al., 2018). Such findings furnish crucial inputs to UA design and planning, including elements of basic information on where and how UA can effectively support these agroecological functions.
A similar emergent challenge concerns research on climate change impacts and potential adaptations arising through UA ecological, social, and spatial processes (Egerer, 2020). Analysis of these interactions coalesce around specific topics such as UA relations to urban heat islands (Pearson et al., 2010) as well as runoff and flood-risk reduction, windstorm control benefits, and benefits to climate change mitigation such as carbon storage and sequestration and the reduction of energy use and emissions (Lwasa and Dubbeling, 2016;Egerer, 2020).
Related UA research challenges are centered on the complex ecological, social, and spatial factors that condition UA biodiversity. UA patches in cities and in city-fringe areas are being found to support agrobiodiverse plant and animal foods that represent domesticates, semi-domesticates, and wild foods as well as many underutilized species and integral agroecosystem components (e.g., soil biota, pollinators, beneficial insects; Lerner et al., 2013;Zimmerer and Carney, 2019; yet further systematic research is needed (Clucas et al., 2018). Distinct though related is the food agrobiodiversity of retail shops. Ranging from convenience stores to supermarkets, they intermix with UA spaces, thereby jointly influencing urban food environments (Duvall et al., 2010;Minaker et al., 2016).

DETERMINE GOVERNANCE RELATIONS TO UA SOCIAL AND ECOLOGICAL SYSTEMS
UA governance issues stem from organizational practices involving the wide-ranging realms of economics and policy as well as a diverse suite of community building, activist, and sociocultural practices. One challenge for UA research is to understand the social and ecological linkages of both short food chains connecting proximate production and consumption (Moustier and Renting, 2015;Yacamán Ochoa et al., 2020b) as well as long-distance economic specialization or even bulk commodity production that can connect UA farming to faraway producers and consumers (Hung, 2016;Opitz et al., 2016). Consumption practices, including such basic functions as cooking and access to food-preparing spaces, are integrally related to UA.
A similar research challenge corresponds to UA labor sourcing. It can entangle local networks of individuals, households, and communities, including many whose nonfarm income helps to support or even subsidize certain UA styles (Lerner and Eakin, 2011). By contrast, other UA systems incorporate farm worker and immigrant populations (Zimmerer et al., 2020a). Other approaches to UA activities are experience-based and center on the roles of race and antiracism (Reese, 2018). These can further incorporate demographic variation ranging from young people and the elderly in urban neighborhoods to the UA spaces of incarcerated persons.
Insights on the entanglements of UA governance reinforce the challenge of orienting integrated UA social and ecological research "beyond methodological city-ism" (Connolly, 2019). "Beyond city-ism" means framing UA research both within urban sites and extending the scale of analysis to the city fringe and, in some cases, to the connections with surrounding rural spaces and beyond. This challenge requires UA perspectives that can address the roles of infrastructural, planning, and design systems including transport, water, food, and green infrastructure as well as built-environment research (Pollalis, 2016;Lin et al., 2017;Pinna, 2017;Yacamán Ochoa et al., 2020a;Zimmerer et al., 2021).
A second challenge consists of calls to analyze UA in the multiple, inter-sectoral dimensions of FEWs (Food, Energy, and Water) and related resource-interaction frameworks. One approach well-suited to UA has positioned the role of people ("P") at the center of FEW analysis (FEWP; Caputo et al., 2020). This approach and others can be incorporated into UA research challenges in specific regional or national-level FEW frameworks (e.g., sub-Saharan Africa; Chirisa and Bandauko, 2015).
Policy issues represent many of the most visible, compelling research challenges on UA governance. Though previously minor, food is being transformed into a centerpiece of urban policy issues where research challenges extend well-beyond safety and narrow regulatory concerns (Pothukuchi and Kaufman, 1999;Morgan, 2009). Urban food planning and analysis are shedding new light on specific regulations of production, distribution, consumption, and social initiatives (Dawson and Morales, 2016), cultural practices (Hammelman and Hayes-Conroy, 2015), and case studies of specific cities (e.g., New York and Toronto; Cohen and Reynolds, 2014;Mulligan et al., 2018). Significant UA research challenges are centered on the need to create policies that can respond to the spatial, temporal, and human-environment complexities of UA social and ecological functions (see preceding section).
UA research challenges extend to analysis of overarching drivers such as neoliberal structural adjustment and globalization policies that propel the powerful food and urbanization trends setting the current stage for UA worldwide (Bosco and Joassart-Marcelli, 2017). These challenges encompass UA analysis needed to focus on influences of government agencies and non-state actors (e.g., NGOs; Olivier and Heinecken, 2017), individual program, projects, and solution-specific analysis (Pollard et al., 2017b;McClintock and Simpson, 2018), and effects related to broader networked urban green infrastructure (Rolf et al., 2020).
Research challenges related to expanding UA activism examine how specific governance issues entangle with ecological and social processes through social organizations, cultural practices, and spatial strategies. For instance, community-based governance initiatives such as food hubs can exert influence on UA production systems (Levkoe et al., 2018;Hammelman et al., 2020). Cultural imaginaries of urban political agroecologies might serve as a source of transformative rethinking and action. This challenge is designed as "non-celebratory, " meaning it subverts the triumphalism of naïve UA interpretation, by linking analysis to political economies and multi-layered realities of social power in cities involving racism, gendering, commodification, and other forces. Moreover, UA experience exerts an influence on the human relationship to symbolic meanings and ecological effects of urban nature, pushing research to think beyond the human-centered perspective. These UA research challenges are well-suited to further advancements via the conceptual orientations of political ecology, feminist political ecology, multi-species entanglements, and critical, more-thanhuman geography (Hovorka, 2006;Tornaghi, 2014;Classens, 2015;Nyantakyi-Frimpong et al., 2016).
This emphasis on bottom-up styles of UA governance reveals the power of a justice perspective for changing ecological and social issues (Gray et al., 2014;Reynolds and Cohen, 2016;Reynolds et al., 2020). It highlights the influence on UA of social justice movements allied with alternative food networks, "rights to the city" (Cabannes and Raposo, 2013;Shillington, 2013;Purcell and Tyman, 2015), food citizenship (Baker, 2004;Travaline and Hunold, 2010), food sovereignty (Block et al., 2012), food territories and landscapes, and citizen science (Pollard et al., 2017a).
The role of UA in citizen rights and practices, for example, can strengthen senses of belonging and place among diverse populations that include the housing insecure. These movements add crucial context to understanding UA's valuation of nature, as being documented in scientific and economic approaches (Keeler et al., 2019), while revealing the roles of the gentrification (induced through property valuation that can arise as a UA consequence; McClintock, 2018) and "sustainability fixes" (Pirro and Anguelovski, 2017).
Additional challenges in UA governance research are focused on systematic comparisons of multiple parameters, including social-ecological interactions, across case studies. UA case studies can be aggregated at such levels as cities, countries, and world regions (De Bon et al., 2010;Taylor and Lovell, 2014;Lohrberg et al., 2016;DuŽí et al., 2017;Gray et al., 2017;Soulard et al., 2017a;WinklerPrins, 2017;Diehl et al., 2020). Comparing and contrasting developing and developed countries, or the Global North and Global South, shows the convergence of certain UA trends along with continued distinctness (Bryld, 2003;WinklerPrins, 2017;Gray et al., 2020). Focus on the dynamics of social-ecological systems is emerging as an important challenge of comparative, site-based UA research (Lohrberg et al., 2016).

CONCLUSION: BROADENING THE UA FOCUS ON SUSTAINABILITY AND RESILIENCE, AND TRANSFORMATION
We conclude by arguing that integrated social and ecological analysis of UA can contribute solutions that expand the sustainability and resilience of food systems and cities (see also de Zeeuw and Drechsel, 2015;Soulard et al., 2017b;Egerer and Cohen, 2020). Our synopsis is centered on highlighting the Grand Challenges in UA research as well as other focal points of rapid social-ecological change that are spurring increased attention to transformation as a key research concept. Transformation refers to major, often abrupt social-ecological changes as analyzed in current or historical UA and is particularly well-suited to understanding the extensive scope of UA Grand Challenges (Tornaghi and Dehaene, 2019;Zimmerer et al., 2020a).
The concept of transformation frequently encompasses a prescriptive dimension that aims to bridge ecological sustainability with the justice concerns of communities and social movements (Warner et al., 2020;Zimmerer et al., 2020b), thus promises to align still further with new UA research challenges. Indeed, interest in UA among stakeholders, researchers, and others can be and is often fueled by its potential to generate social-ecological processes that strengthen community and social empowerment.
These UA social-ecological processes can propel the transformative sharing of resources, spaces, knowledge, and participation in community organizations and social movements. Such UA social-ecological interactions potentially incorporate and extend beyond concerns for sustainable ecosystems, production, and social empowerment per se. Here UA has the potential to generate the embodied experiences of collective values, worldviews, and cosmovisions that are similarly key to transformative sustainability (O'Brien and Sygna, 2013).