Many of us have played in rock pools on the shore. The animal life there is as intriguing to children as Mario Brothers. I still play there if I can, when nobody is looking, sometimes with the addition of snorkel and mask (and fins.) In the tropics, as well as some kindly temperate shores, you can discover wonderful species such as nudibranch sea slugs. They feed on hydroids, soft coral and even sea anemones. On northern shores, the green crab,Carcinus maenas, or the rarer, edible alternative, with the green seaweed,Ascophyllum nodosum, or another fucoid, and the “snails” such as the periwinkle, Littorina littorea, and the carnivorous whelk, Nucella lapillus, represent the major players in our game, often covered with barnacles, Semibalanus balanoides, themselves preyed on by the whelks.
Under the pretence of scientific study for a published paper, several researchers have implied that this play can be called the study of Trait-Mediated Indirect Interactions (TMII.) Under the rules of this game, the whelks transmitted TMIIs between the crabs and the barnacles and crab-algae TMIIs were transmitted by winkles and whelks, adding up the scores from both sources. The winners were the crab-algae TMIIs if the whelk and the winkle were present. And the reason for that —-?
If the whelk (Nucella) was caused to consume fewer barnacles because of the risk from predation by crabs, more seaweed settled on and between the barnacles and remained in the little community!
This effect of predators indirectly on the spread of plants by eating the consumers is obvious to some of us. Reduced grazing is the driver of many “trophic cascades” in most ecosystems, with prime examples available in aquatic systems. This is why the experimenters studied the effect of removal of the predacious green crab on the food webs and the community of rocky shores in Maine. Geoffrey C. Trussell, Catherine M. Matassa and Patrick J. Ewanchuk of Northeastern University, Massachusetts and Providence College, Rhode Island, US, used laboratory mesocosms of the actual rock pools with the paper in RSPB entitled: “Moving beyond linear food chains:trait-mediated indirect interactions in a rocky intertidal food web”