Bottom-up and top-down forces in a tropical intertidal ecosystem: The interplay between seagrasses, bivalves and birds

Seagrasses are called 'ecosystem engineers' and are essential for many coastal ecosystems. The intertidal area Banc d'Arguin is such coastal area. The seagrass Zostera noltii here bursting with life, where the red knot (Calidris canutus canutus) lives and feeds from the benthic life.
Seagrasses have an important bottom-up role as the foundation of the ecosystem. They create suitable living conditions for themselves and for many other species. Although seagrasses are crucial for biodiversity and coastal protection they are threatened by climate change in an unexpected way. Due to drought and heat waves seagrass suffers from desiccation stress and this alters the delivery of oxygen to their bivalve mutualistic partner (Loripes lucinalis) and their sulphide-oxidizing, gill-inhabiting bacteria (‘endosymbionts’). The breakdown of the mutualsim increases sulfide toxicity and speeds up the die-off of seagrasses.
Red knots have a unique remote sense in the tip of their bill which allows them to detect buried prey efficiently without direct contact. One would expect that knots would be able to deplete Loripes and break the mutualistic partnership. But in high seagrass densities the seagrass conceals their detoxifying shell. In combination with the toxicity of the shells this prevents a strong top-down impact on the system.
However, a computer model showed that the stabilized effect of the mutualism could be disturbed by knots. The model showed that in degrading seagrass knots may have been able to speed up the gradually degrading process. Here they may be able to eat the remaining Loripes which increases the toxic sulfide. Therefore, we can conclude that the top-down effect of knots is probably a cumulative effect that accelerates the disappearance of the seagrass in our study system.