|Animal-sediment relationships revisited: cause versus effect|
Snelgrove, P.V.R.; Butman, C.A. (1994). Animal-sediment relationships revisited: cause versus effect. Oceanogr. Mar. Biol. Ann. Rev. 32: 111-177
In: Oceanography and Marine Biology: An Annual Review. Aberdeen University Press/Allen & Unwin: London. ISSN 0078-3218, more
|Authors|| || Top |
- Snelgrove, P.V.R.
- Butman, C.A.
Over the last few decades, many studies have correlated infaunal invertebrate distributions with sediment grain size, leading to the generalization of distinct associations between animals and specific sediment types. When these data are compiled and reviewed critically, however, animal-sediment relationships are much more variable than traditionally purported. There is, in fact, little evidence that sedimentary grain size alone is the primary determinant of infaunal species distributions. In addition to observed variability in animal-sediment relationships, a clear mechanism by which grain size per se limits distributions has not been demonstrated. Furthermore, sediment grain size has usually been determined on completely disaggregated samples which may have little relevance to what an organism actually encounters in nature. Likewise, patterns have been documented using primarily sediment and biological samples that were not integrated over the same vertical scales within the bed, or on samples that were integrated over much larger vertical scales than those relevant to most organisms. Thus, the grain-size distributions described for a given habitat may be very different from those within the ambit of the organism. In addition to grain size, other proposed causative factors include organic content, microbial content, food supply and trophic interactions, but no single mechanism has been able to explain patterns observed across many different environments. One common generalization is that deposit-feeders are more abundant in muddy habitats and suspension-feeders dominate sandy habitats. A predominant hypothesis to explain this pattern is that suspension-feeders are excluded from muddy habitats by amensalistic interactions with deposit-feeders. In most studies that tested or evaluated trophic-group amensalism, however, the hypothesis generally became qualified to such a degree that it was no longer meaningful. Critical re-examination of data on animal-sediment relationships suggests, in fact, that many species are not always associated with a single sediment type, and that suspension- and deposit-feeders often co-occur in large numbers. Furthermore, a number of species alter their trophic mode in response to flow and food flux conditions; therefore, the simple dichotomy between suspension- and deposit-feeding is no longer valid.The complexity of soft-sediment communities may defy any simple paradigm relating to any single factor, and we propose a shift in focus towards understanding relationships between organism distributions and the dynamic sedimentary and hydrodynamic environment. Grain size covaries with sedimentary organic matter content, pore-water chemistry, and microbial abundance and composition, all of which are influenced by the near-bed flow regime. These variables could directly or indirectly influence infaunal distributions via several compelling mechanisms. Moreover, because the sedimentary environment in a given area is, to a large extent, the direct result of near-bed flow conditions, factors such as larval supply and particulate flux that are similarly determined by the boundary-layer flow may be particularly important determinants of species distributions. It is unlikely that anyone of these factors alone can explain patterns of distribution across all sedimentary habitats; however, meaningful and predictive relationships are more likely to emerge once the influence of these dynamic variables is examined systematically in controlled experiments.Much of the early research on animal-sediment relationships was conducted at a time when it was technologically and conceptually infeasible to evaluate and manipulate many relevant aspects of the hydrodynamic and sediment-transport regime, which correlate directly with, and are responsible for, sediment distributions, as well as factors such as food supply and larval supply, which may correspond with sediment distributions. Most studies of animal-sediment associations were conducted when the complex inter-relationships among these variables was poorly understood. Recent major advances in measuring or simulating (i.e. in laboratory flumes) critical aspects of the dynamic sedimentary environment and in understanding relationships among hydrodynamics, sediment transport and benthic biology have created an entirely new framework for studying animal-sediment interactions. Sorely needed are laboratory and field measurements and experiments that utilize this new technology, capitalize on the emerging interdisciplinary focus of research in the oceans, and test specific, innovative hypotheses concerning animal distributions and their environment.