Saltmarsh ecology

TREE vol. 6, no. 6, June

Perhaps we should expect nothing more from such a young field. There is, for example, a rather obvious cultural gap between practising paleontologists who use protozoa as stratigraphic markers, and ecologists who are more concerned with the activities of living cells and their role in food webs. And while much effort has been directed at understanding and quantifying the basic elements of protozoan energy budgets in the laboratory, this information has only rarely been related to the results of in situ studies. Perhaps the most fundamental question to be tackled concerns the

true complexity of microbial food webs. The book provides a good synopsis of the range of types of protozoa found in the marine environment, butthe important question of the number of trophic transfers that organic carbon makes on its journey from algae to fish (with a reduction in yield of about 50% at each stage) is not embraced. Initial indications are that these webs are extremely complex and that the number of transfers is variable and sometimes very large. When we have a better impression of where organic matter is channeled, and of the number of transfers in microbial food

1991

webs, we will be able to apply basic energetic information with more confidence. And then we will have a clearer impression of whether protozoa are ‘sinks’ or ‘links’, assuming anyone still wants to ask the question. B.J. Finlay Institute of Freshwater Ecology, The Ferry House, Ambleside, Cumbria LA22 OLP, UK

Reference 1 Sherr, E.B. and Sherr, B.F. (1991) Trends Ecol. Evol. 6, 50-54

Coastal Saltmarshes Saltmarsh Ecology by P. Adam, Cambridge University Press (Cambridge Studies in Ecology), 7990. f55.00/$95.00 hbk (xii+ 461 pages) ISBN 0 52124508 7 Saltmarshes have long been a major focus of interest for ecologists. Demarcation of community structure in coastal saltmarshes is typically sharp, although the ecosystem is highly dynamic, and indeed sometimes subject to catastrophic change. Characteristic and ever-changing habitat conditions are reflected in distinctive structural and physiological features of the biota, and in recent years much research has been conducted on the ecophysiology of halophytes. Paul Adam has authoritatively appraised a wealth of studies addressing these and many other points of interest of saltmarsh ecology. A particularly welcome feature is that saltmarshes are treated on a world scale, thus giving a global overview (absent from most of the literature on saltmarshes) of the geographical variation of the vegetation of saltmarshes. Although the book is entitled simply Saltmarsh Ecology, it is, as admitted by the author, heavily biased towards plants. Nevertheless, there is a fair scatter of references to animals and the reader is directed to the review concerning saltmarsh fauna by Daiber’. Treatment of the plant ecology of coastal saltmarshes is extensive, but with only passing reference to seagrass beds and mangrove swamps, and with very little on inland saline areas. The clear-cut zonation of vegetation often seen in saltmarshes has in the past frequently been considered to be the result of the progressive trapping of sediment,

leading to orderly successional changes. Although such changes have been confidently described in the literature, few have been supported by long-term observations or by stratigraphic evidence. Adam justifiably concludes that ‘most successional diagrams must be regarded as hypothetical’, and he points out the need for more information in this context. Physiological traits of saltmarsh plants enabling them to tolerate the high and changing salinities have attracted much research. Indeed, other important related considerations, such as anaerobiosis and the mechanical effects of tidal action, have been neglected. In recent decades, emphasis has been placed on the nature and role of balancing osmotica and the mechanisms of salt tolerance. While there is now a framework for interpretation of the halophyte-glycophyte continuum in physiological terms, Adam rightly notes that we are still some way from explaining the basis of the distinctive distribution of many species in terms However, the of microhabitats. existence of physiotypes (although ‘neither taxonomic nor strictly ecological groupings’) has been recognized, as discussed here together with many other considerations in a major chapter entitled ‘Coping with the environment’. Throughout the book, the author has aimed at indicating areas of ignorance. One of these is the population ecology of individual species. While the germination of halophytes, especially under laboratory conditions, has been extensively studied, many features of the behaviour of species in the field are poorly known. Nevertheless, there is substantial documentation, given a balanced evaluation here, of clinal behaviour in

Armeria maritima and of aspects of the life history of Plantago maritima and species of Salicornia (the latter being the only saltmarsh annuals for which quantitative data are available on their life history). Research on the functioning of saltmarsh ecosystems is heavily centred on Spartina marshes; these studies are discussed and the need for the generation of testable hypotheses on the control of ecosystem processes is stated. The final chapter, ‘Modification, management conservation’, and discusses such issues as grazing, manipulation of species abundance, reclamation and pollution. With likely sea-level rise as a result of the’greenhouse effect’, with marshes disappearing, a plea is made for the involvement of ecologists and engineers in policy decisions on coastal management. Avaluablefeatureofthe bookisthe list of well over a thousand references, about one-third of which were published in the last decade. There is also a full index, including topics and organisms. Nomenclature is mostly up-to-date, but Agropyron junciforme (sic, but misprints are few) is now Elymus farctus. All interested in the plant ecology of saltmarshes will find the book a fund of information (tables, graphs and other illustrations are included) presented in a clear and thought-provoking manner.

A.J. Willis

Dept of Plant and Animal Sciences, Sheffield University, Sheffield SlO 2TN, UK.

Reference 1 Daiber, F.C. (1982) Animals of the Tidal Marsh, van Nostrand Reinhold