Describing assemblages of sediment-living organisms

One of the most fruitful aspects of ecological research is the search for common patterns in the bewildering variability of nature. Given current concerns about global warming, climate change, and habitat degradation, the determination and protection of biodiversity has become paramount. There are essentially three ways of describing an assemblage of organisms, and each of these gives more information on the patterns and interrelationships. First, we have the classical taxonomic method of identifying all species in the assemblage, to the highest taxonomic separation possible (usually to species) and then counting the abundance and weighing the biomass of each taxon. Secondly, we can determine the size and/or biomass spectra of all organisms in the assemblage irrespective of their identities, on the basis that organisms of different sizes or body weights play a different role in the ecosystem. Thirdly, we can determine the role that each organism can play in the system, again irrespective of its name, and define these as ecological groups or guilds—hence separating those feeding in different ways or those building tubes from their free-living associates (e.g. see Elliott et al. 2007 for a discussion of the guild concept). There are many methods of analysing assemblage data; for example Elliott (1994) identified over 25 groups of techniques for macrobenthic analysis (these are mentioned throughout this book and summarized in Chapter 11). Using these methods, when considering assemblages of marine organisms living in sediment, we can ask if there are any ‘rules’ that can be applied to patterns of abundance, size, and biomass distributions and how data on species distributions can be organized. Here, we first treat abundance, then size and biomass spectra, and finally how species assemblages can be assessed. Another way of describing assemblages is to examine the number of species and how abundance is distributed among species, although these are aspects of species diversity which will be addressed in the next chapter. In any sample of a biological community, whether marine, terrestrial, or freshwater, the immediately observable pattern is that most species are rare, represented by one or a few individuals, and only a few species are very common, represented by many individuals.