Population and Community Biology – serie
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Often the meanings of words are changed subtly for interesting reasons. The implication of the word 'community' has changed from including all the organisms in an area to only those species at a particular trophic level (and often a taxonomically restricted group), for example, 'bird-community'. If this observation is correct, its probable cause is the dramatic growth in our knowledge of the ecological patterns along trophic levels (I call these horizontal patterns) and the processes that generate them. This book deals with vertical patterns - those across trophic levels -and tries to compensate for their relative neglect. In cataloging a dozen vertical patterns I hope to convince the reader that species interactions across trophic levels are as patterned as those along trophic levels and demand explanations equally forcefully. But this is not the only objective. A limited number of processes shape the patterns of species interaction; to demonstrate their existence is an essential step in understanding why ecosystems are the way they are. To achieve these aims I must resort to both mathematical techniques to develop theories and statistical techniques to decide between rival hypotheses. The level of mathematics is likely to offend nearly everyone. Some will find any mathematics too much, while others will consider the material to be old, familiar ground and probably explained with a poor regard for rigour and generality.
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The last ten years have seen an enormous increase in the development and application of multivariate methods in ecology; indeed the perceived importance of these methods for elucidating the complex interactions observed in community studies is shown by the number of recent books devoted to introducing the more common multivariate techniques to ecologists (Williams, 1976; Orloci, 1978; Whittaker, 1978a, b; Gauch, 1982; Legendre and Legendre, 1983; Pielou, 1984) and by the chapters added to new editions of more general texts on quantitative ecology (e.g. Greig-Smith, 1983; Kershaw and Looney, 1985). Two reasons can be put forward to explain this development. The first is undoubtedly the increasing availability of cheap computing power which makes it feasible to analyse the large data matrices involved in community studies. The second, perhaps less widely appreciated, is the change in emphasis of theoretical work on multivariate analysis, away from the development of formal statistical models and associated distribution theory towards descrip tive techniques for exploring pattern in data sets and providing succinct summaries and displays. This new approach, termed 'pattern analysis' by Williams (1976), has led to a range of statistical techniques which have been enthusiastically taken up by ecologists to replace the collection of ad hoc procedures developed over the years for analysing community data.