Spatial aggregation of herbivores and predators enhances tri-trophic cascades in paddy fields: rice monoculture versus rice-fish co-culture.
Rice-fish co-culture system has a history of more than 2,000 years in Asia and has been recognized as one of FAO's Globally Important Agricultural Heritage Systems (GIAHS). To date, few studies have explored rice-fish co-culture effects on the relationship between predator-prey aggregations and associated tri-trophic cascades of predatory spiders, insect herbivores and rice. Indeed, predators have been shown to increase prey spatial aggregation but it is still not clear how fish (tertiary consumers) affect aggregations and interactions of their prey on multiple trophic levels (i.e. predatory spiders and rice insect herbivores). Here, we conducted a 2-year experiment to measure the abundance and distribution of herbivores (leafrollers, stemborers and planthoppers) and their predators (spiders) along with rice yield in rice-fish co-culture versus monoculture-rice (fish free) systems in eastern China. We analysed spatial aggregations of herbivores and predators using geostatistics and used correlations to examine tri-trophic interactions. Results showed that co-culture enhanced predatory spider and herbivore spatial aggregations, which in turn increased biocontrol of herbivores by predators, decreased herbivore damage and increased rice productivity. Synthesis and applications. Fish presence increased spatial aggregation of two different trophic levels, which enhanced a tri-trophic cascade of predatory spiders, insect herbivores and rice in co-culture systems. Our approach linking spatial aggregations of herbivores and predators with trophic cascades can be applied to study trophic interactions in other ecosystems. Incorporating fish into monoculture paddy fields can increase both natural pest control by predatory spiders and crop productivity via aggregation, and thus increase multiple ecosystem services.