Urbanization results in highly fragmented, heterogeneous landscapes that disrupt biodiversity, trophic interactions, and ecosystem services. Urban gardens in these patchy landscapes can act as refuges for biodiversity while increasing access to food and enhancing the well-being of gardeners. Ecological networks map interactions among insect and plants inhabiting gardens along a gradient of surrounding land-use type composition. Examining patterns that emerge from these networks may also provide insights into insect community characteristics that contribute to the maintenance of ecosystem services such as biological pest control and food access. Understanding how urban landscapes influence ecological diversity and structure in gardens will subsequently improve our understanding of the role of urban agriculture in sustaining biodiversity and ecosystem services in urban settings. We surveyed 25 gardens in the California Central coast to characterize networks of insect natural enemies and herbivores on Brassica oleracea plants. We ask: 1) how does human management at local and landscape scales influence insect biodiversity and ecological networks? and 2) when are ecosystem services and other benefits enhanced? We sampled gardens monthly and identified insects using dichotomous keys and online resources. Generalized linear models tested for local and landscape effects. Network analysis using ‘bipartite’ in R is ongoing.
Results/Conclusions
Over five sampling periods we recorded 69,006 insects including aphids, other hemipteran herbivores, lepidopterans, parasitoid wasps, predatory and herbivorous beetles, and spiders. Preliminary generalized linear model results show that gardens with higher herbaceous plant species richness have a greater abundance of both non-aphid herbivores and predatory natural enemies. Agriculture within 2 km of gardens lowered abundance of parasitoid wasps and all natural enemies, perhaps evidence that agricultural fields exert a greater attractive pull relative to urban gardens. Gardens with higher vegetative complexity attracted less natural enemies overall, indicating the possibility that natural enemies as a whole gravitate to less complex landscapes. Older gardens have more non-aphid herbivores while newer gardens attracted a greater abundance of parasitoid wasps and a correspondingly higher incidence of parasitized aphids. Similarly, gardens with more leaf litter as ground cover, common in established gardens, had a lower abundance of parasitoid wasps and fewer parasitized aphids. Preliminary ecological networks suggest that more complex landscapes may simplify insect networks. Thus, garden management decisions have tradeoffs, attracting and benefiting certain groups of insects with neutral or opposite effects for others. Further, the importance of urban garden habitat for insect communities appears to be relative to surrounding landscape characteristics.