Effects of soil abiotic factors and plant chemical defences on seed predation on sea fennel (Crithmum maritimum)

Soil abiotic factors influence plant defensive traits by shaping the costs of defence production and these bottom-up effects on plants can in turn affect insect herbivory. However, few studies have disentangled direct and indirect effects of soil abiotic factors on plant defences and insect herbivory. To address this gap, we tested the influence of soil abiotic factors on seed predation via changes in plant defences for sea fennel (Crithmum maritimum), a common coastal plant in southern Spain whose seeds are consumed by specialist caterpillars (Aethes species). To this end, we estimated seed predation on plants across several sea fennel populations, as well as measured different types of putative chemical seed defences (phenolics, terpenes) and soil abiotic factors (macro- and micro-elements, physicochemical variables). We found a positive association between seed chemical defences (terpenes such as α-thujene, α-pinene, β-pinene, β-myrcene, α-terpinene, y-terpinene, and thymol methylether) and seed predation. In addition, a few macro- and micro-elements such as Ca, S and Sr negatively correlated with seed defences; other macro- and micro-elements or physicochemical variables had no detectable association with defences. Despite observed effects of soil abiotic factors on defences and of the latter on seed predation, there was no detectable indirect effect of soil abiotic factors on seed attack. Our findings suggest that variation in a few key soil macro- and micro-elements in coastal environments can exert an important influence on seed chemical defences in sea fennel, with potential consequences for interactions between sea fennel and seed predators.