UC Berkeley

Abstract

Biodiversity and Ecosystem Services in Urban Agriculture: Evaluating Local andLandscape Effects on Parasitic Hymenoptera and Biological Control Services

By Joshua Earl Arnold

Doctor of Philosophy in Environmental Science Policy and Management

University of California, Berkeley

Professor Miguel A. Altieri, Chair

Urbanized areas are the fastest-growing habitat worldwide. In the United States, over 80% of the population now lives in cities. It is expected that urban populations will continue to increase significantly in the coming decades. Matching urban population growth is an increase in urban food production; urban agriculture (UA) has grown 30% in the United States in the last three decades. Growing food in the city has become an increasingly common pathway to affordable, nutrient-rich, and culturally appropriate foods for people who live in high-cost cities.

A myriad of issues complicates urban agricultural production. Once urban farmers gain access to land, they must address many abiotic factors unique to cities that disrupt the ecosystem services many agroecological practices rely on. Increased impervious surface and decreased canopy cover affect hydrological and biogeochemical cycles and increase urban temperatures. Past land uses can affect soil quality and composition. These abiotic factors often exacerbate crop damage from herbivorous insects. Herbivorous pests in urban agriculture can become more persistent and increase in abundance in response to favorable environmental conditions in cities and cause damage to crops.

In urban agriculture, management of pests is almost universally accomplished through cultural practices as pesticides are rejected for environmental and health reasons. Many urban agriculturalists turn to agroecological pest management (APM) practices to increase on-farm beneficial insects and regulate pest populations. Agroecological practices proven on rural farms, such as crop diversification and floral resource provisioning, have been implemented to varying degree in the built environment, often with conflicting results. This research focuses on understanding urbanization impacts on agroecological pest management in urban agriculture. Specifically, how on-farm diversification schemes affect biological control services from parasitic Hymenoptera (PH). Recognizing how these biological control services function in fragmented urban landscapes is vital to urban farmers.

Understanding agroecological pest management practices and factors that may affect ecosystem function on urban farms necessitates understanding urban farm biophysical composition. Over three years, biophysical data were collected on twenty-nine urban farms in the San Francisco Bay Area. The physical composition of urban farms were measured, including overall size, areas of production, and percentage of land not in agricultural production. Indicators of specific management practices, such as type and percentage of mulch and ground cover, floral diversity, and crop and non-crop biodiversity, were recorded, and overall production was assessed. We found that practices associated with APM are widely adopted and are often practiced concurrently. Our research shows that urban farms are highly productive, and most crops grown feed local community members. Land use and spatial composition of urban farms varied, but the production area as a percentage of the total area is often low, and areas set aside for pollination gardens or beneficial habitat are common.

As agroecological pest management in urban agriculture is an understudied topic, a systematic review of research specific to UA and biological control services was conducted. Previous findings recorded significant impacts on both natural enemy and herbivorous pest populations in response to landscape and local effects, but findings remain inconsistent. Local management factors related to agroecological practices, including increased floral abundance, mulch and leaf litter, high plant species richness, and structural diversity, had significant beneficial effects on natural enemy abundance, richness, and biological control services.

We conducted a two-year experiment testing the effects of local management practices and landscape effects on parasitic Hymenoptera, aphids, and crop damage on common Brassica crops. Two fundamental hypotheses in conservation biological control: the enemies and the floral nectar provisioning hypotheses, were tested in novel urban agroecosystems. Local and landscape factors were measured and assessed for their influence on PH populations on eleven San Francisco Bay Area urban farms. Farms were selected to represent a variety of sizes and surrounding imperviousness. Our research indicated that Local factors, including increased mulch coverage, crop richness, and percent of non-crop areas, were predictors of increased PH abundance and aphid parasitism rates.

To test the effects of floral provisioning on PH we sampled thirteen common floral species across community partner sites to link common floral species in urban farms to PH families and subfamilies known to utilize aphids as hosts. We found that PH had no feeding preference, and floral species had little impact on PH abundance. To assess the second criterion of the nectar provision hypothesis, a demonstrable reduction in pests or crop damage, we looked at aphid abundance, rates of parasitism, and overall crop damage on brassicas. Our results show that farms with increased floral richness have lower aphid counts per plant. Our findings indicate that on-farm habitat manipulations can increase ecosystem function, supporting the enemies hypothesis in fragmented urban agriculture sites.