Island Facts and Theories

Authors


Encyclopedia of Islands . Gillespie, R. G., and D. A.Clague , editors. 2009. University of California Press, Berkeley, CA. 1111 pp. $95 (hardcover). ISBN 978-0-520-25649-1.
The Theory of Island Biogeography Revisited. Losos, J. B., and R. E.Ricklefs, editors. 2009. Princeton University Press, Princeton, NJ. 494 pp. $49.50 (paperback). ISBN 978-0-691-13653-0.

Islands are occupied by a disproportionately large number of terrestrial species per unit area relative to continents. Islands are also occupied by a very high percentage of endemic species, ranging from ancient relicts to the most recent evolutionary departures from continental forms. Islands are highly prone to anthropogenic disturbance, however, and species that inhabit islands have high probabilities of extinction. For these reasons I believe that conservation of island ecosystems and biotas has disproportionate urgency. It is surprising to me that the topic of conservation is not developed in either Encyclopedia of Islands or The Theory of Island Biogeography Revisited. In fact, conservation is not even an entry in Encyclopedia.

Encyclopedia is an eclectic ensemble of accounts of particular islands, island-like entities (such as dead whales), subjects studied on islands, and topics relevant to islands. In their opening, “Guide to the Encyclopedia,” the editors claim the book is “… a comprehensive, complete… reference dealing with all of the physical and biological aspects of islands and island habitats.” That is overstated. There are more than 7000 islands in the world, and it would be necessary to average over seven islands per page just to provide names, locations, and the most elementary features of each. In that same sentence the editors state that the book is “authoritative;” it is very much that. There are 236 entries by 300 authors. I have read over 100 of them, and not one was poorly done. In addition to accounts of individual islands and archipelagos, a few of the topics covered are archaeology, beaches, cold seeps, dwarfism, flightlessness, climate change, inbreeding, kipuka, missionaries, plate tectonics, research stations, surf, tides, and whales. One of the most fascinating pieces is “Whale Falls” by A. Baco: dead whales sink to the sea floor and become dense islands of food to successional stages of consumers-–perhaps the weirdest of all island analogs.

I found very few errors. That Encyclopedia contains relatively little information about the West Indies (Antilles) seems peculiar in view of those islands’ faunal, geological, and historical importance. Most of my favorite islands are not even mentioned (e.g., Anegada, Aves, Dry Tortugas, Guana, Jost Van Dyke, Kaho’olawe, and Sombrero). One of my most favorite, and my candidate for world's most beautiful island, Dominica, is mentioned, but only for its remarkable lizard ecomorphs.

I think the most amazing account is called “Kick ‘em Jenny,” by J. Lindsay. I visited Kick ‘em Jenny on June 15, 1964. It is a small, steep, rugged basalt pinnacle, fully exposed to wind and sea, in the southern Grenadines on the Grenada Bank of the Windward Islands, Lesser Antilles. I found thousands of nesting sea birds, some low shrubs, and three species of lizards. Lindsay states, however, “… it is the most frequently active volcano in the Lesser Antilles and the only known submarine volcano in the region…” and “… the highest point on the crater rim is about 180 m below sea level.” Lindsay further notes: “A sustained eruption from Kick ‘em Jenny might cause the volcano to break the surface and develop a small island.” This is not my Kick ‘em Jenny! The small island that I know by that name is already there and has been for a long time.

This encyclopedia is a rich trove of information and deserves a place in every natural history library. The book is tied to The Theory of Island Biogeography Revisited in numerous ways. The first editor of the encyclopedia, R. Gillespie, is the author of the excellent lead account in Theory Revisited, “Adaptive Radiation,” and a contributing author of the chapter “Island Biogeography of Remote Archipelagos.” Authors B. Baldwin, T. Schoener, D. Simberloff, K. Triantis, and R. Whittaker are featured in both books. Two specific accounts in the encyclopedia provide an excellent introduction to Theory Revisited: “Island Biogeography, Theory Of” by J. Fernandez-Palacios, some of whose publications on this subject are cited in the second book, and “Species–Area Relationship” by D. Spiller and T. Schoener.

The Theory of Island Biogeography Revisited is, like the encyclopedia, an eclectic ensemble, albeit of narrower scope. Following an informative foreward by R. May and preface by editors J. Losos and R. Ricklefs is a delightful chapter by E. O. Wilson: “Island Biogeography in the 1960s.” This essay reaches back much farther in time than the 1960s, recalling the formative works of Matthew and Darlington, taking the reader through the collaboration between MacArthur and Wilson, and on to Simberloff's experimental work. There follows a more detailed and expansive historical overview, “Island Biogeography Theory,” by M. Lomolino, J. Brown, and D. Sax. This chapter is a rich review of the theory.

For me, the term revisit conjures notions of critical reevaluation and reconsideration. There is little of either in this book. The theory of island biogeography distills down to the notion that any island, given time, will contain a stable number of species, a number that is balanced by additions of species due to immigration and speciation and losses of species due to extinction and that is determined by the island's planar area. Quantitatively, S=C x Az, where S is the number of species on an island, C is a constant that varies among taxonomic groups (e.g., birds, reptiles, beetles, bats), A is the island's area, and z is a constant that for islands equals about one-quarter (0.2–0.3). This equation is based largely on Darlington's (1957) classical assessment of West Indian herpetofaunas. This relation, however, is not consistent at the lower end of the curve (Lazell 1964): very small islands (like Kick ‘em Jenny) are apt to have more species than predicted by a factor of three. In my opinion this greatly enhances the importance of conserving small islands.

The late Karl Koopman (1983) and I (Lazell 1983, 2005) found that the elevation of an island explains greater variation in species richness than area. Unfortunately, the elevational dimension of island size largely has been left out of this volume. Nevertheless, T. Schoener in “The MacArthur–Wilson Equilibrium Model” notes the relation between elevation and area—larger islands often have higher elevations than smaller islands—but Schoener references planar area, which is less than surface area when elevation is heterogenous.

In developing “A General Dynamic Theory of Oceanic Island Biogeography ” R. Whittaker, K. Triantis, and R. Ladle note that species–area models are often “inadequate,” that the theory of island biogeography is “not a complete theory” and “not a precise predictive model.” They explicitly include island characteristics such as elevation, age, and land-cover diversity in addition to area in their description of factors determining species numbers. In considering “Toward a Trophic Island Biogeography,” R. Holt discusses different values for the exponent z. I. Hanski, discussing metapopulation dynamics, notes that the theory of island biogeography is “largely history and [has been] replaced by many more specific models.” Hanski also notes that the theory's “conservation applications were simplistic. … But this is all wisdom based on hindsight.” W. Laurance, in “Beyond Island Biogeography Theory,” reevaluates the theory as “a caricature of reality.” He notes, however, how important the theory has been in efforts to conserve natural areas, even if its predictive benefits have been “woefully limited.” D. Simberloff and M. Collins provide a chronicle of exceptions to predictions derived from the theory in a discussion of birds of the Solomon Islands, but many of these deviations may result from anthropogenic disturbance. In discussing remote archipelagos R. Gillespie and B. Baldwin note that “… we must continue to recognize the evolutionary and ecological differences among lineages…, rather than searching for general patterns across taxa.” Exactly so.

In “The Speciation–Area Relationship,” J. Losos and C. Parent concentrate on large islands that are fragmented into veritable archipelagos by cyclic interglacial sea-level rise. They consider species richness that evolved within such islands in contrast to diversity resulting from immigration and conclude that insular “age-dependent effects on species richness outweigh area effects.” In one of the book's few considerations of botanic diversity, J. Terborgh, in “The Trophic Cascade on Islands,” bemoans “anthropogenic shifts”: “Documenting the ecology of these last remaining intact islands before alien species arrive and transform them should be a research goal of the highest priority.” This highlights my point that conservation is critically important if we are to maintain natural biological diversity. Generally, however, the possible relevance of species–area relations to conservation is scantly noted in this book.

Three chapters in Theory Revisited deal with avian diversity on islands, but they are only tangentially related to the theory of island biogeography. S. Clegg examines evolutionary changes following colonization and calculates that in generating new species, selection is far more important than genetic drift, but that drift is more important than founder effects. For Lesser Antillean birds, R. Ricklefs finds that “… rates of colonization and/or extinction have changed dramatically in the past…” and that “… it is the old populations that are at risk of extinction.” Ricklefs notes that “… taxon cycles are intrinsic to biological systems.” In a fascinating account of speciation in Galapagos finches, P. Grant and B. Grant detail histories of speciation. Their descriptions are often based on intimate knowledge of the birds involved, right down to individuals, and reveal differentiation in sympatry in some cases.

A final chapter by M. Vellend and J. Orrock discusses the history of the theory. It appears that the theory of island biogeography largely mirrors genetic models developed earlier. For example, Sewall Wright's work in 1940 on island-mainland population genetics uncannily presages the theory if one simply substitutes the word species for allele. In fact, similar analogies of genes and species go back to the 1927 work of J. B. S. Haldane. Vellend and Orrock note “… the processes underlying changes in allele frequencies and species abundances are much the same.” They conclude, “… hopefully the process of cross fertilization between models and concepts in community ecology and population genetics can… provide the inspiration for future advances….” Perhaps so.

In summation, The Theory of Island Biogeography Revisited falls far short of a really thorough reevaluation and reappraisal of the theory of island biogeography as codified in 1967 and applied since. At the very least, one must question the value of codifying a theory in a precise mathematical formulation if that formula provides inaccurate projections almost every time. Yes, as a general rule, big buckets hold more than small buckets, but the circumstances and histories of each bucket impinge critically on just how much each bucket holds at any given time. Should theoretical approaches concentrate more on the disproportionate species richness of small islands in terms of species per unit area? And what of the conservation significance of older, insular lineages that are more prone to extinction?

Ancillary