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1 July 2007 Extinction & Biogeography of Tropical Pacific Birds
Daniel Simberloff
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Abstract

The following critiques express the opinions of the individual evaluators regarding the strengths, weaknesses, and value of the books they review. As such, the appraisals are subjective assessments and do not necessarily reflect the opinions of the editors or any official policy of the American Ornithologists' Union.

David W. Steadman. 2006. University of Chicago Press, Chicago, Illinois. xiv + 594 pp., 243 text figures. ISBN 0-226- 77142-3. Paper, $45.00.—As reports of new data on anthropogenic avian extinctions on Pacific islands multiplied in the ornithological, paleontological, and conservation literatures for the past 20 years, the scope of the hecatomb was gradually recognized and the widespread view that pre-European human colonists trod lightly on the landscape was overthrown. These impressive discoveries have generally received, at most, passing mention in biogeographic papers, however. This is odd, given the prominence of Pacific island birds in the development of biogeographic theory, particularly the dynamic-equilibrium theory of island biogeography (MacArthur and Wilson 1963, 1967) and community-assembly rules (Diamond 1975). Steadman aims to summarize both his findings and those of other avian paleontologists and to reconsider biogeographic theories in light of this growing literature.

This will be an exciting book for many audiences: ornithologists, biogeographers, ecologists, evolutionists, paleontologists, conservationists, and anyone interested in island faunas and floras. Though aiming at an academic audience, Steadman superbly captures the adventure of his 25 years of exploring remote Pacific islands and relates the bird distributions to his own observations and to the literature on anthropology, ecology, sociology, geology, and botany. This is also a big book: 22 chapters in four parts. Part I comprises extensive descriptions of the geography and history of the islands, as well as details of how an avian paleontologist actually works. Part II describes the modern and prehistoric avifaunas of each island, and Part III reviews these avifaunas from a taxonomic perspective. The book is almost wholly about land and freshwater birds, but Part III includes a chapter on seabirds. In Part IV, Steadman assesses a variety of biogeographic and ecological theories and discusses conservation and research needs. It is possible that many ornithologists will turn quickly to Parts II and III, which contain fascinating details on such depleted taxa as megapodes and rails and accounts of extinct giant pigeons and other island oddities. Ecologists, by contrast, may restrict themselves to Part IV. This would be a mistake, as the cogency of Steadman’s arguments cannot be assessed without a good understanding of the underlying data.

Steadman does not pull his punches, and he telegraphs them, stating in the preface and reiterating frequently that, at least for the birds of these islands, he sees little value in the equilibrium theory of island biogeography or community-assembly rules. Subsequently, he also assails species-area relationships, taxon cycles, macroecology, and Monte Carlo simulations. He realizes that he will generate controversy, pleading for us not to write him off as a “weirdo.” And he does not help his case by occasional wholesale (and often gratuitous) scatterblasts at statistics, computers, and literature on other taxa, such as West Indian anoles, plus tirades that he himself terms “ranting.” However, in his primary quest, he usually restricts himself to just birds and just tropical Pacific islands, and he builds a strong case for questioning biogeographic and ecological dogma. His main contentions are two. First, the amount of prehistoric extinction on these islands is staggering in terms of fractions of the avifauna, and the fossil record for almost all of them is so poor that it is not even possible to reassess theories by simply replacing the erroneous figures with new, “complete” data. Second, virtually all of this extinction was caused not by any sort of equilibrium turnover, whether generated by interspecific competition or otherwise, but by humans through hunting, habitat destruction, and introduced species.

His data suggest that from 20% to at least 65% of land birds present on each island before human colonization are now known to be extinct, and there is good reason to believe that other species were present at human settlement that are now extinct and totally unknown because they are not yet represented by fossils. Worse, the paleontological research is so scattered that, even within single archipelagoes, it is usually impossible to construct species-area curves for pre-human-contact birds. Interestingly, there is little or no evidence of precultural bird extinction on these islands. These facts lead Steadman to propose, for birds of oceanic Pacific islands, an alternative model to both the equilibrium theory and the equilibrium-turnover interpretation of species-area relationships. In his scheme, birds begin to colonize oceanic islands about 100,000 to 500,000 years after they first rise from the sea, but at an ever-slower rate because the best dispersers get there first (not because of increasing competition). Extinction in the absence of humans is very rare, nearly nonexistent, until erosion and subsidence cause islands to sink. At some point, when they are very small (perhaps 1–10 km2 or even less in what Steadman refers to as “Remote Oceania,” and 50 km2 in “Near Oceania”—oceanic islands near New Guinea), such decreasing islands fall below the threshold sizes required for a minimum viable population (different species have different thresholds, but almost all are small). Except in the range of the very small sizes encompassed by the thresholds, there is no relationship between area and species richness. Further, except for extremely rare events, the only ongo- ing immigration is from nearby islands within the same archipelago; thus, on isolated islands such as Easter Island, there is no short-term immigration. Steadman allows the possibility that, on average, a given species on a large, high island may be able to forestall anthropogenic extinction for longer than on a small, flat island (though such extinction is not envisioned in the original equilibrium theory). But even this case is tentative, and he points to several examples of remarkably small islands that have maintained surprisingly large avifaunas.

Steadman presents a similar null model in contrast to Diamond’s (1975) assembly rules for birds of the Bismarck Archipelago. He sees most “checkerboard” distributions as artifacts of insufficient knowledge of human-caused extinctions. Fairly quickly, for each species, every island within an archipelago above that species’ area threshold and not isolated by more than 100 km would usually contain that species. Subsequently, human contact leads to extinctions, and interspecific competition has little to do with the resulting distribution, whether checkerboard or not.

Steadman frequently emphasizes that his models and his rejection of equilibrium and other theories are restricted to oceanic islands and are based on the accumulated empirical ornithological information of the past 25 years. Thus, he is at pains to suggest that, for certain taxa (not birds) on continental islands over very short time scales, MacArthur and Wilson’s (1963, 1967) dynamic-equilibrium theory might approximate reality. He exonerates MacArthur and Wilson on this count and indicts their successors for excessive application of dynamic equilibrium theory, neglect of conflicting data, and failure to consider alternative hypotheses. Similarly, he lauds Diamond (1975) for having developed the concept of assembly rules, given the “ecological climate of the time,” and for sustained Melanesian field work, though he is far less charitable to authors who subsequently applied the rules to tropical Pacific birds, and indeed to Mayr and Diamond (2001).

His long, intensive field experience with the Pacific birds has made Steadman acutely aware of the staggering losses they have already undergone and the threats they still face. He is a fine writer, and at many points, always relevant ones, he interjects poignant notes on conservation issues. Though these are often poetic, sometimes achingly so, they are never contrived or strained, but rather flow naturally from the topic at hand. Additionally, both his copious observations and his overarching biogeographic- ecological hypotheses suggest approaches to conservation, which are summarized in the penultimate chapter. His general pleas are for far better surveys, especially in Near Oceania but also in parts of Remote Oceania, and for much greater attention to the potential value of small (sometimes very small) islands (the latter in obvious contradistinction to the current fashion in conservation biology, beginning with refuge design rules associated with the equilibrium theory). Although his overall prognosis for Pacific island bird conservation is pessimistic, Steadman warns against crying wolf about particular island species, on two main grounds—inadequate surveys and taxonomic inflation. With regard to the former problem, he points to several birds listed variously as “rare” or “threatened” that are, in fact, common and sometimes widespread. He includes an interesting section with specific suggestions for translocating populations of threatened birds to new, usually very small, islands, and abundant mention of problems caused by introduced species, including “ornitho-Euro-trash.” This chapter, like the entire book, has many insights and observations about how local human populations interact with particular bird species (generally to the detriment of the latter) and some suggestions on how to ameliorate these interactions.

Steadman repeatedly fails to cite well-known ecological and biogeographic literature that would strengthen and support many of his arguments. To take just three examples, Connor and McCoy (1979) showed that species-area curves typically explain only about half the variation in species richness and that there is no empirical justification for representing the curve by a log-log rather than the semi-log plot Steadman prefers. Williams (1964) came to conclusions similar to Steadman’s on the tripartite nature of the curve, and Caughley (1994) stressed the impressive achievements of autecological case studies in conserving species and lamented the lack of relevance of general theory, including the equilibrium theory. Nevertheless, with its exhaustive description of these birds and their biogeography, plus its many insights and about 1,800 references, this is surely now the authoritative work on land birds of tropical Pacific islands, and I highly recommend it.

Literature Cited

1.

G. Caughley 1994. Directions in conservation biology. Journal of Animal Ecology 63:215–244. Google Scholar

2.

E. F. Connor and E. D. McCoy . 1979. The statistics and biology of the species-area relationship. American Naturalist 113:791–833. Google Scholar

3.

J. M. Diamond 1975. Assembly of species communities. Pages 342–444 in Ecology and Evolution of Communities (M. L. Cody and J. M. Diamond, Eds.). Belknap Press of Harvard University Press, Cambridge, Massachusetts.  Google Scholar

4.

R. H. MacArthur and E. O. Wilson . 1963. An equilibrium theory of insular zoogeography. Evolution 17:373–387. Google Scholar

5.

R. H. MacArthur and E. O. Wilson . 1967. The Theory of Island Biogeography. Princeton University Press, Princeton, New Jersey.  Google Scholar

6.

E. Mayr and J. Diamond . 2001. The Birds of Northern Melanesia: Speciation, Ecology, and Biogeography. Oxford University Press, New York.  Google Scholar

7.

C. B. Williams 1964. Patterns in the Balance of Nature. Academic Press, London.  Google Scholar

Appendices

Daniel Simberloff "Extinction & Biogeography of Tropical Pacific Birds," The Auk 124(3), 1101-1104, (1 July 2007). https://doi.org/10.1642/0004-8038(2007)124[1101:EBOTPB]2.0.CO;2
Published: 1 July 2007
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