Community interactions among organisms can vary depending on the types of species present in the community. For intertidal primary producers, the presence of a competitive dominant, like Ulva spp., can force out a competitive inferior, like Fucus spp. But add a preferential grazer to the system, Littorina spp. in this example, and the dynamics quickly change, and the competitive inferior autotroph can eventually dominate the system.

The magnitude of the positive and negative effects of community dynamics can greatly alter our understanding of top-down and bottom-up processes in a given system. Matthew Bracken and former MSC colleagues, examined the facilitative (nutrient-cycling) and consumptive effects of snails (Littorina littorea) and two co-existing seaweed species (Ulva lactuca and Fucus vesiculosus).  The research, which appears in the Journal of Ecology, showed that in the presence Ulva, the snail’s consumptive and facilitative effects balanced each other out, causing no real detriment to the seaweed.