No scientist could challenge the assertion that Earth is the most biodiverse planet in the solar system. Few would challenge that lower-latitude ecosystems are more biodiverse than higher-latitude ones. But in this game of “biodiversity trumps” there are some closer calls. In the early 1990s, Fred Grassle and others challenged the ecological community with an assertion that the deep sea—cold, dark, and still fairly poorly sampled—could rival or exceed tropical rainforests and coral reefs for biodiversity. It was an astounding claim, and although the authors were careful to qualify it, they were widely criticized for the daring of their extrapolations. Nevertheless, high deep-sea diversity is now an established paradigm, and the excellent new book Deep-sea Biodiversity: Pattern and Scale, by Michael A. Rex and Ron J. Etter, provides a veritable barrage of data-missiles to hurl at any terrestrial-based doubters.
Those scientific missiles are not hurled by these authors, however. Indeed, this is a calm, analytical, and elegantly written book that leads the reader neatly through the most important studies in deep-sea ecology. It may feel like a book in the hand, but it reads like a polished review paper; the prose is wonderfully clear and concise. There are no flights of fancy, complex analogies, or firsthand accounts of derring-do on the high seas. There is just the data, the analyses, and the ideas of two highly respected researchers in the field.
The book, as befits its subtitle, is organized around the concepts of pattern and scale. It starts with a review of quantitative data on abundance and food supply, building then through chapters on local diversity, regional diversity, beta diversity, and finally the evolutionary origins of deep-sea biodiversity. Processes are invoked throughout, and the final chapter aims—and partially succeeds—at a challenging synthesis.
The chapters that discuss broader, macroecological trends in deep-sea biodiversity are the highlights; they build on several papers by the authors themselves and present a thorough review of how biodiversity patterns shift across two of the planet's greatest physical gradients— depth and latitude. Deep-sea Biodiversity presents the most convincing case yet for the unimodal pattern of species richness with depth, and provides an excellent discussion of the relative roles of actual ecological processes versus the bias of the mid-domain effect. With regard to the gradient in diversity with latitude, criticism has arisen of the authors' own papers on this subject. To their great credit, however, this section presents a comprehensive review of both sides of the argument, with the admission that low diversity in Norwegian Sea samples heavily (but not completely) influenced the patterns they published in their 1993 paper in Nature.
There has been a resurgence of interest in reproduction and dispersal as processes influencing deep-sea diversity patterns. Dispersal is probably a key process driving deep-sea biodiversity from both ecological and evolutionary points of view. One idea that relates dispersal to the bathymetric gradient in diversity is the source-sink hypothesis, first promoted by Rex and coauthors in a 2005 paper in American Naturalist. The premise is simple: The food-impoverished abyssal fauna are not reproductively viable, and these small populations are sourced from reproductive propagules that have come from larger populations at bathyal depths—hence the lower abyssal diversity. As with the authors' 1993 latitudinal-gradient paper, the source-sink idea has been criticized for its application only to mollusks from a limited range of samples. Deep-sea biologists (myself included), in discussions and in print, have since been keen to point out various samples containing healthy-looking abyssal species, their stomachs packed with food and their gonads packed with eggs. But, to be fair, Rex and colleagues have always been at pains to emphasize that their hypothesis was specifically to explain molluskan patterns, and they reiterate this here, making little play of source-sink dynamics as a “general theory” in deep-sea biology.
The concept of dispersal reappears in the final chapter on evolution. Here, the authors present what I believe to be one of the main paradoxes in deep-sea biology: If deep-sea species have such powers of dispersal, and if the habitat boundaries are so unconstrained, then how can so much speciation have taken place? There are perhaps two answers. The first is that deep-sea species are not all that good at dispersing across the vast distances of the abyssal plains. Here, Rex and Etter point out that many so-called cosmopolitan species may in fact be “constellations of cryptic species”—a wonderful turn of phrase that reflects the haplotype maps that geneticists use to illustrate them. Molecular genetics is now confirming this for some groups of species, but not all. The second idea, explored extensively in this final chapter, is that changes in depth are the most powerful bound-aries to dispersal. I would argue that the deep sea is not characterized by its deepness as much as by its huge range of different depths. In this concept, the bathyal regions are the engines of speciation, fueled by cyclic changes in paleo-oceanographic processes.
Howard Sanders and Robert Hessler published the first comprehensive, quantitative studies on deep-sea biodiversity in the late 1960s. Rex and Etter's book is dedicated to them and is a worthy tribute. Sanders and Hessler would (and will) no doubt be pleased that their ideas have withstood another 40 years of quantitative sampling, and that there is still enormous interest in the undoubted mystery of the deep sea.
