Induced herbivore resistance in seaweeds: a meta-analysis

1 Terrestrial plants can sense and respond to herbivory, which may lead to increased resistance towards further grazing if the responses have negative effects on the preference and/or performance of the herbivores. Marine plants (seaweeds) are exposed to a considerable grazing pressure by herbivores ranging from large, mobile fishes and sea urchins to small, sedentary crustaceans and molluscs. The number of investigations studying induced resistance in seaweeds has increased during the last decade, but empirical results are conflicting. 2 We performed a categorical meta-analysis to evaluate statistically the overall seaweed responses to damage or damage-related cues, and factors that may explain the observed variation in inducible seaweed resistance to herbivores. 3 We found a highly significant overall effect of damage on induced seaweed resistance to further herbivory. Division of the studies into different categories showed that brown and green, but not red, seaweeds induce significant resistance to further grazing in response to grazing by small crustaceans and gastropods, but not in response to large gastropods and sea urchins. The seaweeds showed stronger responses when exposed to damage for 11-20 days than in shorter or longer experiments. 4 Seaweeds are very important both as habitat and food for a wide range of marine animals. Our findings contribute importantly to the general ecological understanding of marine plant-herbivore interactions by showing that induced resistance in seaweeds is more common than previously assumed. Many recent marine investigations included in this study have not put emphasis on the ecological relevance and underlying mechanisms of the investigated plant-herbivore interactions. We suggest that the scientific value of future investigations concerning induced defences in marine algae would benefit from formulating more advanced and/or complex hypotheses including the genetic and biochemical mechanisms, cost and constraints of damage-induced civilian and defensive seaweed responses, as well as the effects of these responses on herbivores and other organisms/trophic levels, and on community structure and functioning.