The ecology of herbivore-induced silicon defences in grasses

Silicon as a defence against herbivory in grasses has gained increasing recognition and has now been studied in a wide range of species, at scales from individual plants in pots to plant communities in the field. The impacts of these defences have been assessed on herbivores ranging from insects to rodents to ungulates. Here, we review current knowledge of silicon mediation of plant-herbivore interactions in an ecological context. The production of silicon defences by grasses is affected by both abiotic and biotic factors and by their interactions. Climate, soil type and water availability all influence levels of silicon uptake, as does plant phenology and previous herbivory. The type of defoliation matters and artificial clipping does not appear to have the same impact on silicon defence induction as herbivory which includes the presence of saliva. Induction of silicon defences has been demonstrated to require a threshold level of damage, both in the laboratory and in the field. In recent studies of vole-plant interactions, the patterns of induction were found to be quantitatively similar in glasshouse compared with field experiments, in terms of both the threshold required for induction and timing of the induction response. The impacts of silicon defences differ between different classes of herbivore, possibly reflecting differences in body size, feeding behaviour and digestive physiology. General patterns are hard to discern however, and a greater number of studies on wild mammalian herbivores arerequired to elucidate these, particularly with an inclusion of major groups for which there are currently no data, one such example being marsupials. We highlight new research areas to address what still remains unclear about the role of silicon as a plant defence, particularly in relation to plant-herbivore interactions in the field, where the effects of grazing on defence induction are harder to measure. We discuss the obstacles inherent in scaling up laboratory work to landscape-scale studies, the most ecologically relevant but most difficult to carry out, which is the next challenge in silicon ecology.