Ecological and latitudinal aspects
The latitudinal cline of the diversity (i.e. the highest numbers of species in the tropics and gradual decrease poleward) is a well recognized and long established pattern in terrestrial ecology. It was documented e.g. for plants, bats, mammalian quadrupeds, reptiles or termites . A similar pattern has been postulated for marine biota. The first suggestion of the latitudinal diversity cline was formulated in late 1950ies for hard bottom epifauna. Sanders in his influential paper, postulated that the macrobenthos is more diverse in the tropical shallow seas than that in boreal locations. He explained that difference by his ‘time-stability theory’ – he perceived tropics as ‘biologically accommodated’ systems that experienced stable conditions for a long time, while boreal communities as ‘physically controlled’ by environmental stressors.
Clear gradients of increased diversity from the North Pole to the tropics were recorded for prosobranch mollusks collected along the Pacific and Atlantic coasts of North America. Similar pattern was described for bivalves of North Pacific continental shelf. A latitudinal cline in regional species richness (data pooled into bins of 10 degrees of latitude) was described for North Atlantic Bryozoa. Deep-sea benthic foraminifera sample species richness decreased with increasing latitude in both North and South Atlantic. Rex et al. described a clear latitudinal cline for deep-sea isopods, gastropods and bivalves in North Atlantic.
Several hypothesis have been proposed to explain the origins of the latitudinal cline of diversity in the sea. It was postulated that the decrease of the diversity towards higher latitudes can be explained by:
- time-stability hypothesis (sea Introduction)
- Rapoport’s rule (latitudinal differences in species range sizes) - smaller geographic ranges in the tropics permit more species to co-occur in a given area, broader geographic ranges of species in higher latitudes result in lower diversities
- species-area hypothesis – based on the species-area relationship
- differences in geological history
- gradients in solar energy input/levels of productivity
- ‘mid-domain effect’ - which assumes denser species packing around the geographic midpoint of distribution
- differences in rates of evolutionary processes (speciation and extinction)
How general is the pattern in the sea?
Some authors questioned the generality of the latitudinal pattern of diversity in the sea:
- Thorson claimed that latitudinal diversity gradient occurs in marine epifauna but not in infauna and attributed that difference to contrasting levels of heterogeneity in infaunal vs epifaunal habitats,
- Clarke noted that the pattern was documented mostly for taxa with a calcareous skeleton (Foraminifera, Bivalvia, Gastropoda) – he suggested that the low species richness in cold polar waters might be a pattern specific to calcifying taxa (resulting from higher energetic costs of calcification in low temperatures)
- no latitudinal effect was noted for protobrach bivalves in north-eastern Pacific as opposed to clear patterns for other bivalve groups - Roy et al. related these contrasts to differences in reproductive and developmental strategies employed by the studied taxa (planktotrophic vs non-planktotrophic mode of dispersal)
- several authors documented asymmetric patterns in studies encompassing both hemispheres: a sharp cline in the northern hemisphere contrasted with no clear gradient in the southern seas was observed e.g. for deep-sea gastropods, isopods and bivalves – these findings suggested that the observed latitudinal patterns could be produced by the impoverishment of north Atlantic and Arctic high latitude seas (that are considered to remain in a state of colonization after the last glaciations) rather than some globally operating processes
- no latitudinal effect was recorded in infaunal materials encompassing: 1) three selected localities in tropical (Java), temperate (North Sea) and polar (Svalbard) waters, 2) Norwegian continental shelf, 3) European continental shelf waters spanning from 36 to 81°N – comprising Marbeff database.
- no even latitudinal cline of diversity was observed for sessile macrofauna colonizing intertidal boulder fields in west North Atlantic, the diversity was only strongly depressed in High Arctic sites that was related to severe ice disturbance in littoral zonein in the Arctic
Other large scale patterns of marine diversity
A comprehensive studies of diversity of reef-building corals, bivalves and other taxa showed that the diversity of those groups is highest in the southern China-Indonesia-NE Australia region and decreases as one gets farther away (both longitudinally and attitudinally) from that area (Fig. 1). Crame showed that for bivalves the steepest gradients (both longitudinal and latitudinal) were recorded for the youngest clades. That finding supports the hypothesis that the south-east Asian tropical high-diversity area served as major center of speciation and the observed gradients represented an evolutionary radiation towards other tropical regions and higher latitudes.
Marine environment differs from the terrestrial systems in that it has an important third dimension – depth. Sanders suggested that deep ocean provides more stable environmental conditions and so hosts a more diverse ‘biologically accommodated’ benthic biota. Grassle & Maciolek reported as much as 698 species in their study of 90 000 benthic invertebrates collected off east US coast at 1500-2100 m. The parabolic pattern (with highest diversities at intermediate depths of 2000-3000 m and lower values on the upper slope and in the abyss) was documented for polychaetes, gastropods, protobranchs, cumaceans, invertebrate megafauna and fish megafauna in northwestern Atlantic and seemed to be a predominating depth related pattern in the seas. That pattern was related to bathymetric changes in average population growth rates and competitive replacement (regulated by predation, disturbance level and productivity). The increase of macrobenthic diversity towards the intermediate (2000-3000 m) depths was not observed e.g. in the Greenland Sea where the ecological patterns are masked by the historical and geographical constraints.
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