The Green World Theory: Diving Under Coastal Ecosystems

The world is green, apparently… This trivial assertion unleashed an ecological theory, which is now approaching half a century of age, and which sparked a great debate among ecologists.

The theory argues that producers, carnivores, and decomposers are all resource-limited, whereas herbivores are mostly predator-limited. Green biomass accumulates because predators and parasites keep their prey in check. Thus, herbivore densities are always maintained at a lower level than would be needed to deplete any plants they consume (Hairston et al., 1960). However, other authors took issue with the argument and suggested that while our world may be green, it may not be edible. They proposed an alternate possibility that the abundance of vegetation may instead be the result of plants fighting off their consumers through nutritional, chemical or structural defenses (Murdock, 1966). Nowadays, there is a general consensus that both ecological processes (a top-down process controlled by predation and a bottom-up process controlled by resource ability) are crucial to the functioning of ecosystems.

Within these biological interactions, herbivory is the process that focuses on the consumption of plants by herbivores, and it is considered as a key process in the functioning of the ecosystem since is one of the locomotives of natural selection (Schowalter 1981). In every piece of wood, bark, meadow, and forest, plants use mechanical and chemical tools to survive and reproduce in the company of herbivores that can consume them. Herbivory involves the transference of matter and energy from primary producers to higher trophic levels, affecting the physical structure of the community and the ecosystem and habitat productivity (Scott et al. 2018). Deepening these biotic interactions in coastal areas is entirely relevant because they inhabit some of the most productive ecosystems worldwide, such as seagrass and macroalgae (Duarte, 2002).

The existing literature indicates that the intensity of herbivory on coastal macrophytes can vary. In fact, there have been few comparisons of the variability in herbivory within and across types of coastal macrophytes, or the existing comparisons are not extensive. Moreover, former evidence suggested the existence of an oversimplification of the importance of grazing in both modern-day and historical seagrass food webs. This has impaired our ability to elucidate if there are any patterns in how herbivory varies within and across macrophytes and to identify the controls and consequences of that variability.

Previous studies on plant-herbivore interactions and food-preferences of herbivores in seagrass ecosystems have commonly reported that nutritional, structural, and chemical traits may determine leaf palatability and regulate plant quality as food (Zapata and McMillan 1979, Mariani and Alcoverro 1999, Cebrian et al. 2009, Prado and Heck 2011).

R. Jiménez-Ramos with her research team of the University of Cádiz, explored the herbivory on coastal macrophytes. In this work, an increase in the nutritional quality of seagrass leaves enhanced the intensity of herbivory on these tissues. This work strengthens the theory of the nutritional quality of macrophytes — those primary producers with the highest concentration of internal nitrogen are more vulnerable to being consumed (Cebrián et al. 2009). This pattern was similar in the acquired results from others work of the group, where they saw that under a scenario of global change, seagrasses become more vulnerable to being consumed due to their high nitrogen content in leaves (Jiménez-Ramos et al. 2017) and the presence of epiphytic algae (Jiménez-Ramos et al. 2018).

On the other hand, macrophytes can co-occur in coastal areas, creating mixed communities, and, therefore, herbivores have access to several food sources. Consequently, if herbivores have the capacity to make a choice among the different available food sources, it may generate different patterns of vegetation consumption within the ecosystem, which may increase or decrease the direct consumption of seagrass tissues. In our work, we have seen how the internal properties of different macrophytes influence the rates of herbivory as well as the diversity of species and their abundance. Under high abundance and diversity of macrophytes, herbivores selected the most palatable food against other sources of available food. In this case, the selected food was the macroalgae Ulva, a common ephemeral macrophyte under eutrophication events.

This may have great ecological implications since during eutrophication events plants are subjected to indirect (e.g. light reduction promoted by ephemeral algae growth, anoxic conditions, etc) and direct effects (e.g. ammonium toxicity) that negatively affect their growth and survival. Thus, the presence of ephemeral macroalgae may improve seagrass resilience under eutrophication events. However, given the potential influence of herbivores on macrophyte distribution and abundance, future studies should incorporate observations of feeding behaviour for a better understanding of macrophyte-herbivore interactions. Finally, the importance of considering biotic processes in conservation and management plans is emphasized in these works in order to enhance seagrass ecosystems resilience and to identify solutions to support the maintenance of key ecosystem services provided by seagrass meadows to our planet.

These findings are described in the article entitled, Food choice effects on herbivory: Intra-specific seagrass palatability and inter-specific macrophyte palatability in seagrass communities, recently published in the journal Estuarine, Coastal and Shelf Science. This work was conducted by Rocío Jiménez-Ramos, Fernando G. Brun, Luis G. Egea, and Juan J. Vergara from the University of Cadiz.