The agricultural landscape matters: spider diversity and abundance in pomegranate orchards as a case study

Spiders are effective biological control agents in some agroecosystems. Their ability to control pest insects depends on their species diversity and abundance, which can be affected by environmental variables at different spatial scales. We investigated the effects of climatic gradient, landscape properties and local variables on spider diversity and abundance in pomegranate orchards. Spiders were sampled twice during the pomegranate growing season in 2015 in 12 orchards along the rainfall gradient in Israel. We examined whether spider diversity and abundance are explained by rainfall gradient, insect abundance and habitat structure within the orchard, or the landscape composition surrounding each orchard. Spider diversity and abundance were unrelated to rainfall or habitat structure, but were positively associated with agricultural landscape evenness and its interaction with insect abundance. We propose a novel hypothesis, the agricultural landscape evenness hypothesis, predicting that diversity will increase with greater evenness in the percentage of area occupied by different habitats surrounding the crop. These results point to the potential importance of a diversity of habitat types surrounding a crop field in promoting natural enemy abundance and diversity.