Structural Modification of Aquatic Environments by Shells: Implications for Human-Mollusc Interactions
Physical ecosystem engineering is the structural modification of environments by organisms. Many mollusc species are noticeable physical ecosystem engineers, not least because of their ability to produce shells. Here I postulate three general structural roles of mollusc shells in aquatic ecosystems, namely: (1) substrata for organismal attachment, (2) refuges from predation, physical or physiological stress, and (3) controllers of near-bed flows and material transport in the benthic environment. Then, I illustrate (a) that changes in resource availability or abiotic conditions caused by shell production have important consequences for other organisms, (b) that colonization of shelled habitat depends on individual shell traits and spatial arrangement of shells, which determine access of organisms to resources and the degree to which biotic or abiotic forces are modulated, (c) that shell production will increase species richness at the landscape level if shells create resources that are not otherwise available and species are present that use these resources, and (d) that changes in the availability of resources caused by shells and the resulting effects on other organisms have both positive and negative feedbacks to the engineer in question. Last, I exemplify how ongoing changes in shell production and distribution resulting from human activities (e.g., exploitation, introduction and invasion of shelled molluscs; use of mollusc shells in habitat restoration) can affect the structure of benthic environments as well as the composition and functioning of aquatic ecosystems.
Keywords: Physical ecosystem engineering, Shell and mollusc beds, Substrata, Refugia, Fluid transport, Feedbacks to engineers.
Figure: Mussel (Brachidontes rodriguezii) bed in a Patagonian rocky intertidal platform (El Espigón, Río Negro Province, Argentina; 41° 07’ S, 63° 00’ W). Mussel shells provide surfaces for algae and sessile invertebrate attachment and increase the three-dimensional structure of the rock bed, thus retaining sediments and creating interstitial space protected from the impacts of wave splash, sand abrasion, desiccation, and predators (© María Bagur).