Animal-algal interactions are crucially important components of the processes causing patterns of distribution, abundance, patchiness and biodiversity in coastal habitats. The interactions involve obvious and direct processes of grazing (algae provide food) and provision of shelter. Other, more indirect intercations may be more important. For example, removal of grazers often leads to excessive growth of algae which prevent establishment or smother other organisms. In some cases, provision of shelter, for example, for predatory animals, can cause striking patterns in distribution of prey species. Without experiments, complex interactions cannot be understood. The complexity of interactions with or among plants and the subtlety of indirect interactions require clarity of logical thought and care in design and interpretation of the relevant experimental analyses.
