Untangling Ecological Complexity: The Macroscopic Perspective.—Brian A. Maurer, 1999. University of Chicago Press, Chicago. vii + 251 pp., 37 text figures. ISBN 0-226-51132-4. $50.00 (cloth). ISBN 0-226-51133-2. $18.00 (paper).
Ecologists are starting to break away from their physics envy, and are realizing that they do not have to be embarrassed about studying large-scale patterns in nature. Brian Maurer, James Brown, and others have dubbed the study of such patterns, macroecology. After the hullabaloo that inevitably seems to follow the birth of new jargon in science dies down, one needs to consider what that jargon means and whether it contributes anything to our understanding of science. As I understand it, “macroecology” is a new term for a range of new statistical procedures applied to well-trodden issues such as biogeography and community ecology. The emphasis in macroecology is placed on the role played by regional ecological processes on local ecological interactions. In a way, macroecology is an attempt at integrating various well-known fields and stressing their importance to ecology. I stress that this does not denigrate attempts made by Maurer and colleagues. On the contrary, viewing old issues through new lenses has often produced some of the most important advances in science. After all, the new evolutionary synthesis of Dobzhansky, Huxley, Mayr, and Simpson assimilated advances made in genetics and evolutionary biology and in doing so, revolutionized both those fields and created many new and exciting avenues for research. Having said that, I do not believe that we are yet at the stage of proclaiming a major new breakthrough for macroecology.
Maurer draws quite heavily on ornithological data for his arguments on the importance of studying large-scale patterns in ecology. Much is made of data from the annual Breeding Bird Surveys (BBS) in North America, particularly with regard to body size distributions, density, and geographic range size. However, it would be hard to say that this book is an essential reference for the ornithologist. That was not Maurer's motivation when writing the book, nor should it have been. Maurer has intended this book as a route map for untangling complexity in large-scale ecological patterns, ornithological or otherwise. However, he limits his discussion to just a few of the most important issues. In particular, he discusses abundance:range size relationships, species:area curves (although in a rather limited manner), geographic range distributions, and body size distributions. The thread that runs through his arguments is that we need new tools to investigate those issues and their interactions with one another. He stresses the importance of geographic range structure of individual species in determining local patterns of species distributions. His point is well made that the assumption of a uniform distribution of abundance of a species across its range is usually wrong. He ably demonstrates, using bird species distribution patterns in North America, that this assumption can have many effects on our understanding of large-scale patterns in nature (e.g., species:area curves).
Maurer also addresses the important point that we unnecessarily restrict ourselves to use of linear statistical models (e.g., ANOVA, regression, etc.) in much of our statistical analyses of complex ecological data. This is surely a truism. However, the glib manner in which he dismisses use of such models does little to enlighten the reader. For example, he attempts to illustrate the problem with linear models by drawing on the example of Schluter's (1986) use of variance components to estimate the role of phylogeny and habitat similarity in determining convergence in community structure of two bird communities on different continents. Yet he provides so little information on that study (other than pointing out that phylogenetic effects are autocorrelative and not linearly additive) that the reader is not able to fairly draw conclusions on the limitations of such models. Furthermore, Maurer criticizes linear models because of pseudo-correlations, yet falls foul of the logical fallacy, post hoc ergo propter hoc [after the fact therefore because of the fact], himself (e.g., see his discussions of causation regarding the role of geographic range structure in species:area relationships). Consistency of a model with a particular dataset is not justification for the claim that the data show the pattern because of the model. A related issue is that several mechanisms may simultaneously contribute to an observed pattern. To be fair, Maurer is not alone in this type of claim; indeed Duhem's paradox (that several models may explain a pattern equally well) abounds in large-scale ecology.
A surprising aspect to this book is that the issue of macroecology and the particular contribution that Maurer and others have to offer only surfaces in Chapter 5 (page 91 of this 251-page book). However, the first four chapters contain a very thorough historical introduction to the issues central to the macroscopic perspective that Maurer is promoting. Indeed, Maurer's historical discussion of Lotka's singular contributions to ecology is particularly thorough and most interesting (although Lotka's models were considered groundbreaking at least in part because they were non-linear, and not linear as Maurer has suggested—L. Stone, pers. comm.). However, it is moot as to whether it is really necessary to go through all that to get to the substance that is macroecology. Had he left those first four chapters out and devoted the space to other large-scale issues, this book would likely have been a more complete attempt at a route map for macroecology.
I found Chapter 8, on the geographic assembly of local communities, to be the best chapter in the book. Maurer's comparisons and explanations of models that incorporate island biogeography theory, random dispersal, geographic range structure, and habitat heterogeneity were very enlightening. His demonstration of the importance of geographic range structure in species:area relationships was particularly convincing. This demonstration alone emphasizes Maurer's point that we need to consider the macro scale in ecology.
In Chapter 9 (“The evolution of species diversity at the macroscale”), Maurer considers Darwin's interesting hypothesis that species that have relatively large geographic ranges (and which tend to have high population densities) would be predisposed to be more successful in competition with other species and would have a higher likelihood of persisting over geological time. In addition, any species that arose from an ecologically successful species would also be successful. Over time, such species would replace those that were less successful ecologically. Those ecologically successful clades will be more diverse than related clades composed of species that have narrow ecological tolerances. Maurer uses two examples that support this hypothesis. He then dismisses Vrba's (1980) and Eldredge's (1989) demonstration of the reverse pattern in two clades of African antelopes, the Aepycerotini and the Alcelaphini. Vrba (1980) showed that the species in the most diverse clade, the Alcelaphini, have narrow ecological tolerances (i.e., are stenotopic), whereas the Aepycerotini consists of a single species with wide ecological tolerance, i.e., that example contradicts Darwin and Maurer's predicted pattern. Maurer dismisses that example as inappropriate by claiming that some supposedly stenotopic species in the Alcelaphini (wildebeest and relatives) actually constitute a single widespread polytypic species (i.e., contra Vrba and Eldredge's claim about this clade consisting of stenotopes with narrow distributions). However, Maurer's claim is unsubstantiated by data and demonstrates poor scholarship—the generalist Impala Aepyceros melampus (the single species in the Aepycerotini) is considered to be a generalist not just for its broad geographical distribution, but also because it eats a wide range of plants and is both a grazer and a browser, whereas the Alcelaphini are grazers only and in this regard are specialists. Because Vrba's data must be weighed against the two examples Maurer brings to bear in support of Darwin's hypothesis, we should hold back on claiming support for the hypothesis when the scorecard is still only 2 against 1. This example returns us to my point about the relatively narrow way in which Maurer approaches macroecology as an abundance:range size/shape issue.
I found this book surprisingly lacking in certain key aspects that must surely make up macroecology. In particular, I found his discussions of core-satellite species and nestedness issues rather glib. A couple of pages are devoted to the core-satellite issue and patterns of nestedness in species distributions are all but shoved under the table. Other macroscopic/macroecological issues that were not covered in this book include incidence:abundance phase planes and saturation curves.
In sum, Maurer has demonstrated that there are some interesting new ways to examine macro-scale patterns in ecology and that it is important to incorporate such patterns in local-scale analyses. However, I think that some of the major claims for this approach can be considered little more than advocacy as of yet. I would not rush out and buy this book, but I would like it to be available on my university library shelves for occasional consultation on specific issues.—