Untangling Ecological Complexity: The Macroscopic Perspective.—Brian A. Maurer. 1999. University of Chicago Press, Chicago, Illinois. ix + 251 pp. ISBN 0-226-51132-4. Cloth, $50.00. ISBN 0-226-51133-2. Paper, $18.00.—What patterns exist in communities? How do they arise? And, how can they be revealed? Those fundamental questions have motivated ecologists for a century, but only recently have the distribution and abundance of individual species across their entire geographic range been the units of analysis. Advocates of macroecology, as this approach has become known, emphasize that the small spatial scale and short duration of most community ecology research miss the processes that occur at larger scales, thereby losing the opportunity to derive generalizations or deduce mechanisms. Maurer has been involved in macroecology since it became a formalized discipline about a decade ago and, together with James H. Brown, has made some of the most important advances in the field. In this book, the author reviews some of those advances and addresses some new areas, but equally presents the philosophy, historical stimuli, and statistical basis for macroecology.
The reader eager to learn the fruits of the macroecologists' labors will need patience through about the first half of the book. The patterns revealed by macroecology are reviewed more thoroughly in Brown's 1995 book Macroecology. Maurer's approach is to first show why short-term, local-scale results can be misleading, and why macroecological processes should be expected. Simply put, species behave as statistical entities, making it necessary to back away from individual species or small assemblages to examine top-down processes. In the first five chapters, Maurer performs useful, if zealous, scholarship by using recent empirical data, ideas from the physical sciences, and early models of population and community dynamics to criticize reductionism and support his approach. The theoretician will likely find much stimulating material here, although those who have ever generalized from local data will likely find issues to challenge. But those with local data not agreeing with data from elsewhere (inevitably published by those of greater reputation) may feel vindicated by Maurer's later discussion of how niche variation across space can be expected to alter relative abundance and competitive interactions.
As an empiricist awaiting results, I found more of interest in the second half of the book. Even so, most of the conclusions described in Chapters 6–8 will be familiar to the reader who has followed the development of macroecology. This information is revisited in order to discuss mechanisms and demonstrate that the macroscopic perspective has yielded some important successes. These results will be especially significant to ornithologists because so many such conclusions are derived from analyses of North American Breeding Bird Survey (BBS) data. In fact, it is probably safe to say that the BBS data are the single most important resource in the development of macroecology. The main conclusions here are the following: species tend to reach their maximum abundance in the central part of their distribution; there is a positive relationship between mean local abundance and range size; there is a positive relationship between body size and range size; species with highest densities are those of intermediate body size; and the evolutionary trend within clades is toward larger mean body size. Population processes that could account for these patterns, such as the intuitively attractive idea that optimal environmental conditions lead to overproduction of offspring which will then disperse to peripheral, suboptimal areas of the range, are also considered. Note here that the conclusions come from analyses of multiple species, and are supported by theories of population processes. Interspecific interactions contribute only as part of density dependence. Similarly, Maurer argues earlier in the theoretical part of the book that niche partitioning and assembly rules are evidence only of local processes operating in ecological time.
So, what does determine the specific community in a given place? Based mostly on models, Maurer proposes in Chapter 8 that local communities owe their composition to phylogeny and local geography, meaning the location within the ranges of the species present. He shows that patterns, such as the species–area relationship and nestedness among communities of different sizes, can be explained best by models that incorporate geographic range. Perhaps, but the alternative models in that section seemed like rather frail straw men. To be fair, however, the intent was as much to demonstrate another approach to showing pattern as it was to attack these specific problems.
Chapter 9 is the first look at long temporal scales, in this case whether generalists or specialists are more likely to leave more descendant species. The conclusion, apparently supported by the only appropriate data set (for two clades of foraminiferans) is that specialists speciate more rapidly, but that they also go extinct more rapidly. As such, at any point in time, there are more species of generalists than there are specialists. We can only hope that the fossil record for birds will one day be complete enough to consider this question. Considering extant species, Maurer suggests that the abundance and range size of the specialist and species-poor Phaethornithinae (hermit) hummingbirds will be shown to be lower than for the generalist and species-rich Trochilinae. I doubt it, given that most of the hermits are lowland species with broad distributions, compared to the higher frequency of high-elevation, restricted distribution Trochilinae. But, like so many other questions raised in the book, this one awaits more data and analysis.
The book concludes with a brief chapter on macroecology and conservation. Maurer is optimistic about the contribution that the macroscopic approach can make to conservation, but the news so far is likely to make the job of managers more difficult. We have been shown that short term data are noise, that local demographic or community composition data can not be extended to other locations, and that rare species are likely to be doomed to small populations and narrow niches. On the other hand, Maurer neglects to mention that many local problems do not require a macroscopic look—tools such as the proper use of fire, control of alien species, or remediation of disturbance often provide acceptable outcomes. One objective recommendation Maurer makes, which has also been noted elsewhere, is that simple tallies of species richness do not necessarily indicate the most important areas for conservation, as the species total is likely inflated by widespread species or by species on the very edge of their geographic ranges.
Who should read this book? Anyone whose interest in community ecology extends beyond comparing species lists should recognize the potential for macroecological processes. It should be required reading for any reviewer who ever dismissed ecological results that did not match existing data from another location, or for any writer who claimed his or her system was representative of a broad geographical area. Ornithologists have been instrumental in providing the data essential for bringing the study of macroecology to where it is today; we should now prepare ourselves to see if the generalizations hold up. For example, will Amazonian birds, with highly specialized niches and low abundance, show the same patterns of distribution and abundance as revealed by the BBS data? Do generalizations from breeding birds hold up on the wintering grounds? Similarly, although Maurer does not mention it, there are useful contributions to be made from museum collections. For example, how do clinal variation and genetic structure across distributions accord with the niche conservatism that should arise from population processes across a geographic range? This book should provide the rationale and the impetus to take a macroscopic look at what we know about birds. Only time will tell how far macroecology will take us in answering the basic questions of ecology.
