Patterns of species diversity in the deep sea as a function of sediment particle size diversity

Abstract

UNDERSTANDING the processes that generate and maintain patterns of species diversity is a major focus of contemporary ecological and evolutionary research. In the deep sea, species diversity varies geographically and bathymetrically^1–3, and may attain levels that rival tropical communities⁴. Many hypotheses have been proposed concerning the forces that shape patterns of species diversity in the deep sea⁵, but so far it has not been possible to relate these patterns to potential causes in a direct quantitative way. The nature of sediments should be important in structuring deep-sea communities because deposit feeders rely on the sediments for nutrition and comprise most of the organisms in the deep sea⁶. The composition of soft sediment communities is influenced by sediment particle size^7,8. Shallow-water deposit feeders selectively ingest particular size fractions of the sediments^9,10 and there are interspecific differences in particle size preference^11–13. Partitioning of sediments with respect to size may be more likely in the deep sea if there is strong selection for macrophagy as a result of reduced food supply and digestive constraints imposed by feeding on deposits¹⁴; macrophagy would permit species to ingest selectively the more labile components of the sediments. If deposit feeders in the deep sea partition the sediments with respect to size, species diversity may in part be a function of sediment particle size diversity. Also, sediment particle size diversity may reflect habitat complexity because the organisms live on or within the sediments^15–21. Here we show that species diversity is a significant positive function of sediment particle size diversity. The relationship seems to be scale-invariant, accounting for a similar proportion of the variance at inter-regional, regional and local scales. Bathymetric patterns of species diversity also appear to be largely attributable to changes in sediment characteristics with depth. These results suggest that sediment diversity has an important role in determining the number of species within a community and identify a direct environmental factor that potentially influences species diversity in the deep sea.