Patchy confidence

BOOK REVIEWS fundamental physical and biological constraints, such as the evolution of body sizes among lineages of species. Within these constraint boundaries, there might be considerable variation, which reflects the diversity of factors acting at the scale of local communities, as well as the uniqueness of their individual histories and stochastic effects. Maurer’s second argument is that, because of this variability in community structure, the meaningful patterns of communities are likely to be statistical, and these statistical patterns will emerge only when community properties are considered over sufficiently broad scales that they are no longer obscured by the idiosyncrasies of local situations. Maurer develops arguments based on the dynamics of ergodic chaotic systems to show how the nonlinear interactions among the many elements of community may cause such regularities in statistical properties at broad scales. The key to discerning the general properties of communities lies in recognizing the constraint boundaries and describing statistically the overall variation within the constraint envelope. Operationally, ‘the more expanded the spatial scale at which we view a population, the more likely we are to see some kind of significant pattern that has some repeatability or regularity among species’. And what, exactly, is a suitably broad scale? Here Maurer slips dangerously close to circularity, suggesting that the appropriate scales are those at which regularities are evident. In a strictly Popperian framework, this makes the propositions untestable, because a failure to detect regularities might indicate only an inappropriate scale rather than a falsification of the derived generalizations. Testability is also compromised because ‘in a system as complex as a community, there are so many events involved in the dynamics that it is essentially impossible to make precise predictions about outcomes’. Although some reductionists might well wince at the absence of the precise predictions, it is probably the nature of ecological systems that the only general predictions that can be made are more likely to be probabilistic than precise. And although much of the macroecology that Maurer advocates appears to be pattern-seeking rather than hypothesis-testing, he goes to considerable lengths to develop plausible biological mechanisms that might produce the statistical regularities he documents. There is more to ecology than hypothesis-testing. Maurer’s book is well written, tightly argued, and fascinating. He presents a powerful case for paying attention to a multiplicity of scales in ecological investigations, and for finding insights in broad patterns of variation and their limits (the constraints), rather than in central tendencies alone. The macroscopic approach Maurer advocates might not ‘untangle ecological complexity’ by itself, but neither will further reductionist,

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experimental work at a strictly local scale. Clearly, both are needed.

John A. Wiens Dept of Biology, Colorado State University, Fort Collins, CO 80523, USA ([email protected])

Reference 1 Asimov, I. (1951) Foundation, Gnome Press

Patchy confidence Metapopulation Ecology by I. Hanski Oxford University Press, Oxford Series in Ecology & Evolution, 1999. £45.00 hbk, £22.50 pbk (ix 1 313 pages) ISBN 0 19 854 066 3 / 0 19 854 065 5

etapopulation ecology has gained significance as we confront landscapes that are increasingly fragmented. This volume gives an excellent survey of the field with a careful account of both strengths and weaknesses of the approach. Metapopulation ecology concerns organisms that are dependent upon discrete patches of suitable habitat, many of which are so small that subpopulations confined to that patch can become extinct within a few years. The population as a whole might nevertheless persist because of successful colonization of suitable patches by surviving subpopulations. In modelling the phenomenon, the probability of extinction in a patch is taken to be a simple function of the patch area, and the probability of successful colonization is taken to be a simple function of the distances between patches, their areas and, possibly, the intervening terrain. Population size in a patch is neglected, and hence no detailed account of either extinction or colonization is possible within this framework. The prime example for the theory is the Glanville fritillary (Melitaea cinxia) on islands of the coast of Finland. Although that system is special, Hanski points out that ‘it is not productive to dismiss some ecological scenarios as uninteresting if some alternative scenarios are more frequent by somebody’s account. Any real-world situation is interesting, and a better understanding of a multitude of examples adds to our comprehension of ecological interactions in general.’ Hanski distinguishes metapopulation ecology from (1) landscape ecology, which is more heavily empirical and which generally regards habitat suitability as a continuous rather than a discrete variable, and (2) spatial dynamics in continuous space, which is more heavily theoretical and concerned with complex patterns that are generated by population processes alone. Hanski’s goal is ‘a close dialogue between models and ecological field studies’ and he has consistently

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0169-5347/99/$ – see front matter © 1999 Elsevier Science. All rights reserved.

favoured simple models over more complex ones. I heartily agree; this work is a convincing demonstration of the power of empirical investigation when combined with simple theory. Hanski’s preferred tool is a model based upon the ‘incidence function’, which is defined as the long-term (stationary) probability of a patch being occupied at any given time. This probability can be computed in terms of such quantitites as the patch area, its distance from other patches (and their areas) and the distribution of migration distances. One survey provides the patch information, and one or more surveys provides presence versus absence information that can be used to estimate parameters of the model. Migration information is more difficult to obtain: in the worst case one can attempt to estimate it together with the other parameters. The most serious defect with this approach is the assumption that the presence– absence data are from a population that has reached the quasi-equilibrium predicted by theory. It is impossible to check such an assumption without an extremely intensive long-term study; if such data were available, a more elaborate model would be appropriate. One can assess the consequences of failure of this assumption by performing simulations, but this idea has not been pursued very far. Another difficulty with the theory, as applied so far, is its neglect of dynamics of other trophic levels. For the Granville fritillary, Moilanen and Hanski1 included variates to describe habitat quality, but the results were not necessarily improved. The dynamics of specific larval parasitoids are very influential for the Granville fritillary, but the interaction has not been modelled. Hanski is scrupulous in pointing out difficulties with the statistical methods used and the possible unreliability of estimates, but he generally fails to provide confidence intervals or error estimates. His warnings would carry more weight if they were quantitatively based. Error estimates would show whether or not a single survey is sufficient to apply the incidence function model: I fear the worst. Are these ideas and methods suited to guide conservation efforts? Hanski puts little faith in model predictions, but he hopes that these methods might be suitable to rank alternative conservation plans. Only time will tell, but in the meantime there is a great deal to be gained by careful statistical treatment of data and more comprehensive simulation studies to test the efficacy of models and assumptions.

Donald Ludwig Depts of Mathematics and Zoology, University of British Columbia, Vancouver, BC, Canada V6T 1Z2 ([email protected])

Reference 1 Moilanen, A. and Hanski, I. (1998) Ecology 79, 2503–2515 TREE vol. 14, no. 8 August 1999