Herbivory could be an important pathway of biomass and nutrient loss from plants. This key ecological process regulates the flow of energy and matter to upper trophic levels and the composition, abundance and distribution of plant communities. Changes in plant architecture and abundance caused by herbivory have important effects in herbivore community structure. Particularly, when plants are habitat forming species such as seagrasses whose grazing-mediated changes in structure and abundance could cascade through the food web having profound ecological impacts. Grazing rates and the ecological effects of herbivory have been largely underestimated in seagrasses promoting the interest for understanding the factors regulating herbivory in these systems. Plants have evolve diverse strategies against herbivory and the resource allocation to these strategies is determined by a tradeoff between benefits (i.e. better defense against herbivores) and costs (i.e. less resources available for growth and/or reproduction). Thus, studies have found that under high resource conditions plants can better tolerate and/ or compensate herbivory.
In this study we investigate how increased resource availability affects the defense mechanisms of the Mediterranean seagrass Posidonia oceanica and how induced defense differs under high resource conditions. To this end, we simulated three fish-herbivory levels and two resource availability conditions in a factorial experiment with P. oceanica. We also explore which are the species-specific effects of grazing and nutrient-driven changes in leaf palatability by a series of feeding experiments with the main herbivores of P. oceanica meadows; the sea urchin Paracentrotus lividus and the fish Sarpa salpa.
Fertilization increased nutrient content in leaf and rhizome but decreased carbohydrate reserves (sucrose), fibers and leaf biomass. Simulated herbivory decreased nutrient content in leaves and rhizomes, induced the production of phenolic compounds, fiber and resulted in higher leaf toughness. Interestingly, only the intermediate herbivory level induced the production of leaves. The decrease in nutritive quality of repeatedly eaten plants could have negative feedbacks in plant-herbivory interactions since low nutritional value of food could be counterbalanced by increasing feeding rates. Despite compensatory feeding, low food quality could be a threat to the performance of herbivore populations if there is no other high quality food source available. Both herbivores responded similarly to the nutrient and herbivory-induced changes which affected plant palatability. The results of the feeding behavior experiments suggest that feeding preference of the main herbivores of P. oceanica meadows is driven by nutrient quality, chemical deterrents and structural traits.