One of the most pressing challenges of the 21st century will be to feed a projected global population of 9 billion people whilst at the same time reducing humanity's agricultural and environmental footprint. With more than 50% of the human population now living in cities there is growing interest in the possibility of using urban agriculture (UA) to provide some of the food required by an expanding population. This thesis addresses two underexplored aspects of UA, namely its productivity and the role that biologically mediated ecosystem services play within it, using UA systems in Sydney, Australia, as case studies.
I monitored the inputs and outputs of urban food gardens in Sydney for one year to assess their productivity and resource use efficiency through economic and emergetic analyses and used spatial analysis to model the productivity of scenarios in which UA production in Sydney was substantially expanded. I then conducted surveys and experimental trials to identify pollinators and natural enemies in these gardens and measured predation and pollination service provision using sentinel plants and insects.
Urban agriculture, as performed by amateur gardeners, was found to be highly productive, yielding nearly twice as much produce per unit area as typical rural fruit and vegetable farms. However the inputs of labour and materials used in the process were substantial, calling into question the economic viability and sustainability of UA. Despite this, many of the inputs required could potentially be substituted for recycled materials common in urban areas, raising the possibility that alternative sourcing of inputs could significantly improve sustainability.
Spatial modelling concluded that Sydney could produce around 15% of its total food supply, or its entire vegetable supply, using street verges, unused spaces (e.g. vacant land, park margins) and 25% of domestic yard space. Sufficient water and organic matter is available within the waste stream of the city to provide the irrigation and soil amendments required. However human labour would be a major limiting factor, with the amount of labour available under a scenario where the entire population worked at the same rate as typical amateur gardeners falling an order of magnitude short of what is required to manage the available land. A small professional workforce may be able to adequately manage the available land, however this would create social challenges related to access to private property and ownership of the produce derived from it.
Pollinators within the UA systems studied were found to be diverse and highly active, resulting in sentinel plants not being pollen limited. The abundance and/or richness of floral resources within gardens was positively related to both visitation rate and richness of insect pollinators, however these same factors, surprisingly, exerted a negative influence on plant reproductive output. We did not find any relationship between pollinator communities and yield of food crops, but found these yields were highly influenced by the surrounding environment, likely mediated by abiotic factors.
Natural enemies, namely vertebrates and flying insects, were found to remove a substantial proportion of sentinel prey. Investigations of the trap nesting fauna showed it to be dominated by wasps that attacked spiders, implying that spiders were abundant in the studied sites and raising the possibility that these could also play a major role in sentinel prey removal had they not been excluded by our experimental methodology. Relationships with individual environmental variables were found to be relatively weak, likely due the heterogeneous nature of the gardens and their surrounds.
We conclude that UA can be highly productive and that urban areas contain many of the resources required for it to make a non-trivial contribution to overall food supply, including land, materials and ecosystem services. However social factors likely serve as a major barrier to the significant expansion of the practice.

A major challenge of the 21st century is to produce more food for a growing population without increasing humanity’s agricultural footprint. Urban food production may help to solve this challenge; however, little research has examined the productivity of urban farming systems. We investigated inputs and produce yields over a 1-y period in 13 small-scale organic farms and gardens in Sydney, Australia. We found mean yields to be 5.94 kg⋅m⁻², around twice the yield of typical Australian commercial vegetable farms. While these systems used land efficiently, economic and emergy (embodied energy) analyses showed they were relatively inefficient in their use of material and labor resources. Benefit-to-cost ratios demonstrated that, on average, the gardens ran at a financial loss and emergy transformity was one to three orders of magnitude greater than many conventional rural farms. Only 14.66% of all inputs were considered “renewable,” resulting in a moderate mean environmental loading ratio (ELR) of 5.82, a value within the range of many conventional farming systems. However, when all nonrenewable inputs capable of being substituted with local renewable inputs were replaced in a hypothetical scenario, the ELR improved markedly to 1.32. These results show that urban agriculture can be highly productive; however, this productivity comes with many trade-offs, and care must be taken to ensure its sustainability.

Urban agriculture (UA) can be highly productive in terms of yield per unit area, however productivity is limited by available land and high input requirements. We determined how much of the food supply of Sydney, Australia, could be produced through UA by synthesising yield data from 13 UA gardens with information on labour and key material inputs and using spatial analyses to assess available land area. We modelled three scenarios with varying proportions of available land used for food production; 25%, 50% or 75% of domestic yard space along with street verges and unused land (e.g. vacant lots). Around 15% of Sydney’s total food supply, or its entire vegetable supply, could be produced through UA under the low range scenario, increasing to 34% under the highest land use scenario. Under the low range scenario, all necessary irrigation water and organic soil amendments could be obtained from local waste streams, though these sources were insufficient to meet the needs of higher range scenarios. Available labour was a limiting factor in all scenarios, with the entire population being insufficient to meet labour needs required to maintain food production under efficiency and labour investment regimes typical of amateur urban gardeners. Establishing a professionalised UA workforce with greater labour efficiency would be required for managing the available land, however this scenario would likely require changes in public attitudes towards use of private land. These social issues, rather than physical limitations, may be the biggest factors preventing cities like Sydney from obtaining a non-trivial proportion of their food supply from UA.