Mussel beds are ‘as strong as steel’
Mussel beds are not a random clustering of mussels, but they contain patterns that resemble the arrangement of molecules and atoms in materials like bronze, steel or polymers. Post-doc Quan-Xing Liu of the University of Amsterdam and a team of ecologists and mathematicians from the Royal Netherlands Institute for Sea Research (NIOZ), Utrecht University and Leiden University, revealed their findings in the journal Proceedings of the National Academy of Science (PNAS) of July 1st.
'In our study we discovered that mussels form a pattern based on the mathematical principle behind phase separation, a process known in physics but unknown within ecology', highlights drs. Quan-Xing Liu, who carried out the study at the NIOZ and currently works as a post-doctoral researcher at the Institute for Biodiversity and Ecosystem Dynamics (IBED) of University of Amsterdam. Phase separation is the process where molecules and atoms of different types separate and form spatial patterns. It is an important physical process explaining the strength of alloys like bronze and steel, or polymers like rubber.
Until now, ecological models explain regular, self-organized spatial patterns, based on spatial differences in birth and mortality rates of organisms. The phase separation principle is solely based on movement and therefore deals with animal behaviour. 'This is a fundamentally different process of ecological pattern formation', Liu adds.
Liu and co-workers discovered the unique behaviour with a very simple experiment that can also be carried out in a school classroom. By spreading out the mussels in an aquarium and using a camera, they observed how the mussels move around and formed regularly spaced strings on the aquarium bottom. The strings then formed a net shape, very similar to the patterns found in the field. These net-shaped patterns help to protect the mussels against the pounding action of waves and predators such as sea gulls.
Liu then developed a mathematical description of the movement of mussels that he integrated into a computer model at IBED. To his surprise, this model was very similar to the classical model for phase separation developed by the well-known material scientists John Cahn and John Hilliard in 1958. Phase separation leads to the formation of honeycomb-shaped structures that make materials such as polymers and alloys such as bronze and steel very strong. The results highlight that the same process makes mussel beds very robust. Mussel beds are, so to say, ‘as strong as steel’.
More than mussel beds
Computational simulations of patterns in ecology are one of the strong points of the UvA’s Institute for Biodiversity and Ecosystem Dynamics (IBED). The results of Liu’s study also extend well beyond the ecology of mussels. Aggregation and pattern formation is common to many animals. By demonstrating in their analysis the potential of applying phase separation models to ecological systems, Liu and co-workers alert colleagues in their field to the possibility of using models and the fundamental of physics to explain ecological phenomena. Potential applications include aggregation in foraging birds, and mound building in social insects. Moreover, it highlights that even in the 21st century, scientists can discover unifying principles that explain phenomena in seemingly unrelated fields such as material science and ecology.
Quan-Xing Liu, Arjen Doelman, Vivi Rottschäfer, Monique de Jager, Peter M.J. Herman, Max Rietkerk, Johan van de Koppel. Phase separation explains a new class of self-organized spatial patterns in ecological systems. PNAS July 1 2013, doi: 10.1073/pnas.1222339110