Soil organic matter links organic farming to enhanced predator evenness

Abstract

Balanced communities of natural enemies (i.e., greater evenness) can strengthen biological control. Natural enemy evenness is generally higher on organic than conventional farms, but the reason(s) for this are unclear. One possibility is that applications of composted manure on organic farms provide habitat for predators as well as food for detritus-feeding arthropods that are important supplemental prey. We searched for potential links between soil organic matter and ground beetle biodiversity in 65 broccoli (B. oleracea) fields, spanning the U.S. west coast, on mixed-vegetable farms using organic or conventional management practices. We measured soil organic matter in each field while also collecting predatory ground beetles (Coleoptera: Carabidae) and detritus-feeding springtails (Collembola), which are potential alternative prey resources for ground beetles. We found that carabid activity density, species richness, and species evenness were higher on organic than conventional farms. Some organic farms integrated livestock alongside vegetable production, however this did not detectably impact ground beetles, springtails, or organic matter. In a path analysis built from our on-farm data, carabid community evenness correlated with soil organic matter, and subsequently, carabid evenness positively correlated with carabid activity density. However, springtail activity-density had no apparent relationship to soil organic matter, carabid activity, or carabid evenness. It is possible that increasing organic matter improved soil microclimate to the benefit of ground beetles, although we cannot exclude the possibility that carabids on organic farms were responding to other ecological or management practices that differed from those on conventional farms. More broadly, among its myriad soil-health benefits, enhancing organic matter could be one strategy for re-balancing natural enemy densities to increase evenness.