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Journal of Experimental Marine Biology and Ecology 245 (2000) 149–151
www.elsevier.nl / locate / jembe
Book review
Reef Evolution
by Rachel Wood; Oxford University Press, Oxford; 1999; paperback 414 pp.; GBP 22.50; ISBN (paperback) 0-19-857784, (hardback) 0-19-854999.
Community ecology is concerned with a wide temporal spectrum, since it must encompass the interacting effects of behaviour, population dynamics, evolution and history. Among these, history has received least emphasis, doubtless because historical data are harder to acquire but perhaps also because of a belief that factors such as competition, predation, niche partitioning and coevolution are of greater importance. Within this scenario, coral reefs and tropical rain forests have become paradigms for the biodiverse, highly integrated community, which being the product of prolonged coevolution, is fragile and irreplaceable. It is clear from the geological record, however, that reef communities have been eradicated and replaced numerous times. Ecological scrutiny of palaeontological data, therefore, should provide the valuable insight of history.
Rachel Wood portrays this insight in her exposition on the evolution of reefs. The eight chapters are grouped into three parts. Part I introduces ancient and modern reefs, defining a reef as any carbonate structure that develops in situ and provides significant topographical relief. Today’s coral reefs are the most resplendent in history, but also significant are lesser structures including those formed by stromatolites, sponges, calcareous algae, non-photosymbiotic corals, sabellariid and serpulid polychaetes, vermetid gastropods and bryozoans. The vexed question ‘‘what is a community?’’ is aired, heeding the message from the fossil record that reef communities have constantly changed through local extinction and recruitment of component species, that new communities have developed from whatever was suitable in the geographically available biota and that former co-occupants of reefs have subsequently occurred in completely different associations. The major chapter in this section presents a history of reef communities, inferred from the fossil record by combining taphonomic and ecological principles. We see how Proterozoic and Paleozoic reefs derived most of their carbonate from inorganic processes or microbial activity and flourished in more sheltered and probably more turbid conditions, as do modern non-scleractin-ian reefs.
Because they predominate in shallow water, reefs are strongly affected by changes in sea level, climate and water chemistry. Part II analyses the role of
150 Book review / J. Exp. Mar. Biol. Ecol. 245 (2000) 149 –151
chemical change on reef growth and then reviews the collapse and recovery of reef communities in association with mass extinctions. Supersaturation of sea water with calcium carbonate is a critical determinant of carbonate accumulation. It is also correlated with other factors such as sea-surface temperature, light intensity, turbidity and salinity, with the result that interpretation of reef-building processes has some-times fallen foul of the correlation-causation trap. Contrary to received wisdom, reef environments are characterized by considerable physical variation. The old concept of coral reefs as stable ecosystems maintained in a state of long-term equilibrium, is untenable. Nevertheless, most mass extinctions coincided with major fluctuations in climate and sea level. Although reef communities disappeared, the component species persisted in non-reef environments, tending to reform once conditions again favoured carbonate accumulation.
Even during periods of geological stability, species forming reef communities also occurred, more thinly spread, in different habitats. Perhaps the high population density of modern frame builders, resulting in reef formation, is simply a conse-quence of escape from competition with faster growing space occupiers. Part III develops this theme. Most Palaeozoic reef species occupied sedimentary habitats, tending to have relatively small cup-shaped, stalked and branching morphologies. Their dominance would have been threatened by the Mesozoic radiation of infaunal species and consequent bioturbation. Modular construction allows great architectural flexibility, in particular the development of a peripheral meristem that enables colonies to encrust solid substrata, safe from bioturbation but in the face of competi-tion from other encrusting organisms. A partial competitive refuge occurs in turbul-ent, nutrient-poor water, where the carbonate skeleton offers mechanical support and whose growth itself is promoted by the physicochemical conditions in such places. Competition for space still exists, but is checked by biological and physical distur-bances that have been the subject of classical experimental research over the past half-century. The final chapter explodes two myths about photosymbiosis. First, host-symbiont relationships are evolutionarily labile and this, rather than tight coevolution, imparts long-term stability since once old partnerships have vanished new ones readily form. Secondly, although deducing the presence of photosymbiosis from fossilized material is fraught with difficulty, it is clear that photosymbiosis is not a pre-requisite for reef formation and has not significantly increased carbonate accumu-lation on geological time scales.
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Fossil reefs provide a geological record of community development that is unique in ecological detail. The lessons from history are important, making this book relevant to any student of community ecology, marine or not.
R.N. Hughes
