The second day of the 12th International Seagrass Biology Workshop kicked off with a plenary lecture by Teresa Alcoverro, one of the worlds leading scientists in the field of seagrass-herbivore interactions.
Seagrasses are marine flowering plants, evolved from life on land about 100 million years ago. In coastal areas all over the world, seagrasses form extensive meadows, providing both food and shelter for all kinds of organisms. Seagrass and their associating fauna have a million year old co-evolutionary history. Some grazers like molluscs and isopods have specialized in eating the algae living on seagrass leaves; other organisms such as sea urchins, certain species of fish, turtles and dugongs eat the leaf tissue itself and therefore have a negative impact on the plants. The degree of impact will vary with the size, feeding preference and with the grouping behaviour and home range of the grazer. Schools of fish that exclusively eat all the young leaves for example will have a greater impact than small molluscs that forage on old leaves and additionally eat other benthic algae. The closer you look the more trophic layers you can discover in a seagrass ecosystem. Seagrass meadows are hotspots for herbivores and as a consequence these meadows attract predators. Predators show a preference for fragmented seagrass patches with a lot of edges. Alcoverro found that the fear generated by the presence of predators (e.g. sharks) in turn can reduce the amount of herbivory (e.g. turtle grazing), up to 50%. Cold spots of predators are richer in biodiversity of both prey species and other organisms, leading to the conclusion that herbivores, both direct and indirect shape these seagrass ecosystems.
Seagrasses in return have evolved to deal with grazing by using either avoidance or tolerance mechanisms. Seagrasses and their symbiotic epiphytes are known to contain several chemical components that could act as defenses, but their actual role in defense is probably low. Apart from self-defense, seagrasses were found to have certain escape mechanisms in order to avoid herbivore grazing: seagrasses can, for example, temporally increase their production when herbivore densities are low or have unpredictable or synchronized reproduction. Tolerance mechanisms focus on compensating for biomass loss due to herbivore grazing; for instance increased photosynthesis in existing leaf material or even increased growth rates. These mechanisms explain why grazing can actually have a positive effect on seagrass productivity – but only until a certain level. When turtle populations recover and their number rises, which is currently the case in certain marine protected areas, they can overgraze and deplete seagrass meadows, moving from one patch to another while the meadows they leave behind may not recover. Therefore, a fine balance between herbivores, seagrass standing biomass and highly resilient seagrass meadows are key to preserving both functional groups: we need to monitor the productivity, herbivory and predatory rates to truly understand the ecosystem and to be able to predict the future of seagrass meadows. Global change will for example lead to a rise in seawater temperature. How this temperature change will affect the aspects of seagrass-herbivore interactions is a one of the main questions for future seagrass research.