Anti-herbivory protection by mutualism in marine ecosystems: The case of kelps and hydroids

Plant–herbivore interactions are critical for the functioning of ecosystems, particularly when habitat-forming species are involved. Kelp forests are among the most diverse and productive ecosystems in the world and, along the Chilean coast, they have a high economic importance. Lessonia trabeculata constitutes the main component of the subtidal shallow habitats in Chile. We quantified the consumption of kelp tissues by the snail Tegula tridentata (one of the most important grazers in Chilean kelp habitats), and the modification in this consumption by a guild of epibiotic hydroids growing on kelps. We performed two different sets of experiments with and without the possibility for the snails to select between kelps with hydroids and kelps without them. The consumption by T. tridentata on kelps without hydroids was between 3 and 4 times higher than on kelps with hydroids. Kelp is protected from herbivores by hydroids and can gain nitrogen during low concentration periods of this nutrient in water. Hydroids gain a substratum, and an elevated position above the seafloor where particle capture is facilitated by the effect of kelp in water currents. Predictions using densities of T. tridentate observed in different forests of L. trabeculata along Chilean coasts, show that herbivory pressure can drastically change depending on this density; in forests with high densities, the presence/absence of hydroids could be crucial. Hydroids have shown to be ubiquitous components of the fauna inhabiting the kelp holdfast and forming a rich community in comparison with other epibionts. Despite its low overall biomass, hydroids can be key elements in kelp productivity (affecting, therefore, the community associated to kelps). The kelp-hydroid mutualistic relationship can change the interactions kelp-herbivore.