Pre-dispersal seed predation of gorse (Ulex europaeus) along gradients of light and plant density

Common gorse (Ulex europaeus) is one of the most invasive species worldwide. Biological control of gorse by two pre-dispersal seed predators (the weevil Exapion ulicis and the moth Cydia succedana) is used in New Zealand. Gorse shrubs are distributed along wide natural gradients, and this could influence seed predation. The aim of this study was to identify factors that influence seed predation along two natural gradients, of light availability and gorse density. Seed predation was studied in the native range of the species, in south-west France. A total of 140 shrubs in stands with different irradiance and population densities were monitored. The number of seeds damaged was determined at different scales: the pod, the shrub and the gorse stand. The multi-scale analysis revealed that weevil activity increased with the quantity of gorse seeds produced, mainly at the pod and plot scales. The moth appeared satiated by abundant seed production at the bush and plot scales. In addition, moth activity was maintained in shady plots where weevil activity decreased. On the whole predation intensity was high and varied little along the density gradient (about 60–80% of seeds destroyed). Conversely, predation intensity decreased significantly with shade (from about 80% in full-light plots to 25% of seeds destroyed in the shadiest plots). These results could help predict the impact of pre-dispersal seed predation on the dynamics of gorse populations along environmental gradients. The activity of the moth appeared to be complementary to that of the weevil because it was maintained where the weevil was rare (i.e. in shady environments). Thus, the joint presence of the two predators may be helpful in the context of biological control of gorse.