Patterns of diversity in benthic assemblages

In assessing aquatic biodiversity, we have long taken the view that there appear to be a few accepted main principles: that diversity decreases with increasing latitude and with increasing depth, that it increases with increasing habitat complexity (and thus with a greater number of niches), that it increases with the size of a habitat and with the productivity of that habitat (as long as that productivity is not artificially increased through eutrophi-cation), and decreases in what may be regarded as stressed areas such as estuaries. Hence, putting these together, complex shallow tropical areas such as coral reefs will (and do) have a high diversity, and temperate estuaries will (and do) have a low diversity. Furthermore, it is well known that on land there is a gradient of species richness with the tropics having more species than the boreal regions and the boreal more species than the polar regions. It was therefore natural to assume that this latitudinal gradient would also be found in the marine environment. Initial studies in the 1960s indeed found evidence that this was the case; there were found to be fewer species of bivalve molluscs in temperate areas than in the tropics, and fewer still in the Arctic. Yet remarkably few studies had been done in the southern hemisphere, so whether there was the same gradient there remained unknown. On land there is also a well-known gradient with altitude; the species richness of trees, for example, decreases the higher up one samples. What about the species richness of marine soft sediments in the deep sea where depths down to over 11 000 m occur?

One of Britain's most illustrious nineteenth-century naturalists, Edward Forbes, predicted in 1843 that no animal life would be found below a depth of 550 m since there was no light and the pressure was too great. Forbes had overlooked the work of Sir John Ross and his nephew Sir James Clark Ross, who as early as 1817 had obtained many living animals from depths of 1800 m in Baffin Bay, Canada. In 1869, the Sars (1872) (father Michael and son, Georg Ossian), trawled up a variety of marine life from depths greater than 550 m off the Norwegian Lofoten Islands. Forbes's views, however, were influential, and there was still no general acceptance that life existed at great depths. Thus, when the Royal Society set the aims for the Challenger expedition, one of the problems to be tackled was the distribution of organic life at all depths of the ocean and on the seabed. This expedition, which lasted from 1872 to 1876, was the first truly scientific oceanographic expedition; it obtained in all 133 dredge samples from the deep sea, which showed conclusively that life does exist at great depths. The number of animals obtained was relatively small, but most were new to science. For almost the next 100 years the pattern was repeated, with expeditions finding many new species of deep-sea animals, but each species being represented by relatively few individuals. Initially this pattern was thought to be an artefact resulting from the fact that the trawls and dredges were not closed, for it seemed likely that many animals were winnowed out on the way up from the great depths. In the 1950s and 1960s better gear became available and it became clear that the earlier expeditions had certainly lost material and that the density of animals was higher than they had found; however, more importantly, far more species were found than were anticipated. The pioneering work of Sanders (1968), Hessler (1974), and Hessler and Sanders (1967) showed clearly that the deep sea held large numbers of species. The question that arises, therefore, is whether there are differences between species richness in tropical, temperate, and polar regions and between the coasts and the deep sea.

4.4.1 The coast-deep sea continuum

Sanders (1968), in his paper describing the rarefaction method for comparing diversity, stimulated a debate that caught the imagination of many workers and resulted in a whole new direction in marine ecological research. Working at the Woods Hole Oceanographic Institution in the USA, Sanders had been studying the benthos of the deep sea for many years. He had amassed data from a wide variety of depths and geographical regions and was struck by two things. First, the deep sea had a high number of species, although the number of individuals per square metre was low. Since the total number of individuals is low and the number of species high, diversity is high however one measures it. Secondly, just as on land, the tropics had a higher diversity than boreal regions. A possible explanation is that natural selection has had a longer time to act in the tropics than it has in the ice-age-prone polar and boreal regions. The processes leading to high tropical diversity are, however, still under dispute. Another theory is that competition there is intense, which means that niches are smaller and so there are more species per unit area. A third theory is that there are more predators in the tropics, which keeps the abundance of prey species low, prevents competition, and thereby allows more species to coexist. The competition and predation theories appear to be mutually exclusive and, as will be shown later, this is at the root of the deep-sea diversity debate.

Sanders' rarefaction curves (Fig. 4.6) indicate that the diversity of the deep sea is higher than that of shallower areas, that tropical areas have a higher richness than boreal areas, and that the Pacific coast of the USA has a higher richness than the Atlantic coast. He explained this high-low diversity pattern by the stability-time hypothesis. He postulated that at one environmental extreme, the high intertidal, the fauna is subjected to environmental factors that fluctuate in an unpredictable manner, and as many species are not able to tolerate these unpredictable fluctuations the species complement is low. At one time species A may be dominant, but competitive exclusion does not occur because before this can happen the environment changes, giving a competitive advantage to species B. Sanders suggests that this results in species in intertidal areas having broad, overlapping niches. However, competition and predation effects also operate, and these lead to large fluctuations in population sizes and low diversity. This part of Sanders's argument has been misunderstood and wrongly quoted. Sanders regards species as adapting to the environment and not to each other, hence he calls this the physically controlled habitat. However, he does not say, as he has sometimes been quoted, that there are no biological interactions here. In fact, biological interactions of competition and predation may be very severe. The important point is that niche specializations do not occur since the environment is constantly fluctuating.

This point has also been made recently at the other end of the depth spectrum. Estuaries are also highly variable and so the environment is constantly fluctuating, particularly with respect to salinity, temperature, and dissolved oxygen. Its benthos is adapted to those conditions but, by default, other species with narrower environmental tolerances cannot exist there (Elliott and Quintino 2007). There are still biological interactions but within a low diverse system and so these are more likely to be intraspecific rather than interspecific. Finally, estuaries are geologically ephemeral (in many cases existing only since the last ice age), again resulting in little inherent speciation but more dependent on colonization, occasionally from fresh water but usually from the sea.

By contrast, the deep sea is an extremely constant environment, with no light and almost no changes in temperature, salinity, or oxygen from month to month and year to year. Furthermore, it has remained constant for a very long time (probably hundreds of thousands of years) compared to the glaciated boreal and polar regions. This constant environment over evolutionary timescales has enabled species to adapt to each other rather than

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