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Sprinkled across the tropical Pacific, the innumerable islands of Oceania are home to some of the most unique bird communities on the planet, and they sustain species found nowhere else on earth. Many of the birds that live in this region are endangered, however; many more have become extinct as a result of human activity, in both recent and prehistoric times.
Reconstructing the avian world in the same way archeologists re-create ancient human societies, David Steadman—a leading authority on tropical Pacific avian paleontology—has spent the past two decades in the field, digging through layers of soil in search of the bones that serve as clues to the ancient past of island bird communities. His years of indefatigable research and analysis are the foundation for Extinction and Biogeography of Tropical Pacific Birds, a monumental study of the landbirds of tropical Pacific islands—especially those from Fiji eastward to Easter Island—and an intricate history of the patterns and processes of island biology over time.
Using information gleaned from prehistoric specimens, Steadman reconstructs the birdlife of tropical Pacific islands as it existed before the arrival of humans and in so doing corrects the assumption that small, remote islands were unable to support rich assemblages of plants and animals. Easter Island, for example, though devoid of wildlife today, was the world’s richest seabird habitat before Polynesians arrived more than a millennium ago. The forests of less isolated islands in the Pacific likewise teemed with megapodes, rails, pigeons, parrots, kingfishers, and songbirds at first human contact.
By synthesizing data from the distant past, Steadman hopes to inform present conservation programs. Grounded in geology, paleontology, and archeology, but biological at its core, Extinction and Biogeography of Tropical Pacific Birds is an exceptional work of unparalleled scholarship that will stimulate creative discussions of terrestrial life on oceanic islands for years to come.
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Extinction & Biogeography of Tropical Pacific Birds
By David W. Steadman The University of Chicago Press
Copyright © 2006 The University of Chicago
All right reserved.
Geography and Geology
Stretching about 15,500 km north to south and 19,500 km east to west, the Pacific Ocean is frighteningly large. No place on earth is so geographically perplexing as the 25,000 islands sprinkled across the Pacific that make up Oceania. Ranging from mere rocks to New Britain at 35,742 [km.sup.2], these islands lie in clusters over 13,000+ km from Palau to Easter Island (Figure 1-1). The clusters (= island groups or archipelagos) vary geometrically from chains to seemingly random scatters.
In the eastern Pacific, the islands close (<1500 km) to Central or South America sustain primarily or exclusively Neotropical biotas and thus are not part of Oceania. These include Juan Fernandez, Galápagos, Cocos, Clipperton, Revillagigedos, and many less isolated islands, none of which harbors a biota of Old World origin or has evidence of prehistoric habitation by Oceanic peoples. I do not consider the massive island of New Guinea to be part of Oceania in spite of its many biotic ties with the Bismarcks and Solomons. From my nesophilic perspective, New Guinea, which was joined to the Australian continent as recently as 12,000 years ago, is more of a continent.
Modern politicalboundaries in Oceania may not agree with those based on geology, biogeography, or ethnicity. Further confusion ensues because of European-induced name changes that are now obsolete, such as for Tonga (the Friendly Islands), Samoa (Navigator's Group), or the Cook Islands (the Hervey Group). Palau, often called "Pelew" by non-Palauans until the mid-1900s, is officially the Republic of Belau. For the island of Emirau in the Bismarcks, the names Emira, Squally, Storm, Sturminsel, Keruë, and Hunter can be found in 20th-century writing (Steadman & Kirch 1998).
Inconsistency occurs as well in definitions of archipelagos, such as Mangareva being regarded as the largest of the Gambier Islands, although it also has been classified as the only eroded volcanic island in the Tuamotu Group. Both statements are correct if you consider (as I do) the Gambier Islands as part of the atoll-dominated Tuamotu Group. Tuamotu, which is East Polynesian for "many (tua) islands (motu)," was called the Puamotu Group or Low Archipelago in the 19th and early 20th centuries.
Dividing Oceania into Polynesia ("many islands"), Micronesia ("small islands"), and Melanesia ("black islands") is useful but imperfect for culture (Clark 2003) or biogeography. Fiji, for example, can be pigeonholed culturally into neither Melanesia nor Polynesia (Frost 1979). The birdlife of Fiji also has multiple affinities, although tends to resemble that of Tonga or Samoa (West Polynesia) more than that of Melanesian islands to the west (Steadman 1993a). Cultural diversity in Melanesia far exceeds that in Polynesia or Micronesia (Spriggs 1997, Kirch 2000). Within both Micronesia and Melanesia are remote, usually small islands that were colonized prehistorically, and often remain inhabited today, by seafaring Polynesians (the "Polynesian outliers"; Kirch 1984b, 2000).
A useful distinction is that between Near Oceania and Remote Oceania. Near Oceania consists of islands near New Guinea that were colonized by people ca. 35,000 to 30,000 years ago, whereas Remote Oceania comprises all other Pacific island groups, in which people first arrived (also from the west) less than 3500 years ago (Green 1991). As detailed in Chapters 2, 5, and 9-14, these two culturally derived terms also are useful in biogeography because so many terrestrial taxa are unknown on Pacific islands north or east of Near Oceania, which consists of the Bismarck Archipelago and the Solomon Island Main Chain (Buka through Makira; Figure 1-2). Outlying islands that are politically part of the Solomon Islands today (Rennell, Bellona, Ontong Java, Sikaiana, and the Santa Cruz Group) are part of Remote Oceania.
Islands and island groups on Papua New Guinea's continental shelf, such as the D'Entrecasteaux Islands and Louisiade Archipelago, are not part of Oceania. Neither are other islands near New Guinea, such as Karkar and Bagabag (volcanos that are not part of the Bismarcks) and Japen and the Aru Islands, which were connected to New Guinea ("land-bridge" islands) during times of glacially lowered sea levels. My coverage of birdlife in Oceania excludes the immense continental island of New Guinea (currently divided politically into the independent Papua New Guinea in the east and West Papua or Irian Jaya in the west, the latter controlled by Indonesia), except for pertinent comparisons, especially in Chapter 17. The Bismarcks through New Caledonia can be called Island Melanesia to distinguish them from New Guinea (Spriggs 1997).
This book has many maps because familiarity with the shapes, sizes, and location of islands and archipelagos is essential for critical thinking in biogeography. The geographic sequence by which I will cover most topics is west to east in the southern hemisphere from Melanesia to West Polynesia to East Polynesia (Figures 1-2 through 1-4), then back to the far western equatorial and northern hemisphere part of Oceania in Micronesia (Figure 1-5). Except for occasional faunal comparisons, I will not cover the Hawaiian Islands or New Zealand, two isolated Pacific island groups that are faunistically largely independent from the rest of Oceania.
I will provide the land area and elevation for every island where I tabulate data on the past and present distributions of birds. Data for land areas and elevations can be inconsistent among different sources, with variation exceeding ±5% in some cases. I have tried to choose what seem to be the most reliable sources. In some cases I have measured island area directly from maps.
Important geographic references for Pacific islands include Kennedy (1966), Douglas (1969), Bunge & Cooke (1984), Carter (1984), Menard (1986), Motteler (1986), New Zealand Government (1986), Nunn (1994), Ridgell (1995), Lobban & Schefter (1997), Rapaport (1999), and Lal & Fortune (2000). I will draw regularly from these sources, citing more detailed studies as needed, especially for geology. My geographic and geological narratives are influenced as well by my own observations on 100+ Pacific islands.
To facilitate comprehension by nongeologists, I will minimize the use of geological jargon, with apologies to geologists. A partial geological time scale (Figure 1-6) also should be helpful. I must begin with geochronological abbreviations. A million years is "My," whereas million years ago or a million years old is "Ma." Similarly, a thousand years is "ky" and a thousand years ago or a thousand years old is "ka." Thus an island that is 10,000,000 years old (10 Ma) may have had an eruptive phase that lasted 2,000,000 years (2 My) from 10,000,000 to 8,000,000 years ago (10-8 Ma). A low sea-level stand from 22,000 to 19,000 years ago (22-19 ka) lasted 3000 years (3 ky).
The geochemical (isotopic) dating methods most pertinent to Pacific geology are potassium-argon and argon-argon (K-Ar, Ar-Ar) for volcanic rocks, uranium series (U-series, sometimes involving uranium-thorium or U-Th) for carbonate rocks, and radiocarbon ([sup.14]C) for carbonate rocks and organic materials. The potential effective age range is ca. 2-5 ka to 4000+ Ma (several thousand years to 4+ billion years) for K-Ar and Ar-Ar dating, ca. 5 to 350 ka for U-series dating, and ca. 0.2 to 45 ka for 14C dating. Terminology specific to [sup.14]C dates will be explained in Chapter 4.
Most islands in Oceania lie either on the Pacific Plate or Indo-Australia Plate (Drummond et al. 2000a, b; Figure 1-7 herein). Exceptions are Easter Island (Rapanui), which is on the Nazca Plate just east of the small Easter Plate or Easter Microplate (Handschumacher et al. 1981, Woollard&Kulm 1981), and the westernmost Micronesian islands (Palau, Marianas, and part of Yap) on the Philippine Sea Plate (Dickinson & Shutler 2000). Islands on the Pacific and Nazca Plates typically are made of dark, heavy volcanic rocks characteristic of thin oceanic crust, such as basalts (Calmant & Cazenave 1987, Dostal et al. 1998, Helffrich & Wood 2001). Islands on the Indo-Australia Plate (= Australia-India Plate, India Plate, Australia Plate, etc.) and the Philippine Sea Plate are more geologically varied; in spite of localized plutonic rocks (those formed in the earth's mantle) or oceanic-type volcanic rocks, they generally consist of rocks that are lighter in weight and color, with "andesitic" mineral suites more characteristic of continental crust (Polhemus 1996, Dickinson 2001). Reflecting this fundamental geological difference, the boundary between the Pacific Plate and the Indo-Australian and Philippine Sea Plates is called "the Andesite Line" (see Tarling 1965).
Islands on the Pacific Plate average younger than those on the Indo-Australia Plate. Nearly all currently emergent islands on the Pacific Plate are <5 Ma, with most dating to <3 Ma. This does not mean that the Pacific Plate was island-free >5 Ma; most older islands are now submerged. Land was available for terrestrial life in Remote Oceania long before the subaerial age of any individual island.
Except for New Caledonia and New Zealand, evidence is lacking that Pacific islands, even those in Near Oceania, ever were connected to Australia or any other continent (Dickinson & Shutler 2000, Dickinson 2001). New Caledonia and New Zealand are Gondwanan fragments originally associated with Australia and Antarctica, although their separation from continental landmasses occurred during dinosaur times in the Cretaceous (Kroenke 1996). Therefore, I believe that most of the vertebrate fauna (species still living and those that became extinct since human arrival) of New Caledonia and New Zealand can be explained by Cenozoic over-water dispersal rather than by late Mesozoic or early Cenozoic vicariance (rifting landmasses). Because the islands in the rest of Oceania always have been islands, overwater dispersal must have been involved at some point in their terrestrial biogeographic history.
Ocean depths average greater, and subsurface contours around islands average steeper, on the Pacific Plate than the Indo-Australia Plate (Summerhayes 1967). Deep trenches may occur at convergent plate boundaries where the Pacific Plate's thinner, heavier crust subducts beneath the lighter crust of the Indo-Australia or Philippine Sea Plates. Dramatic examples are the 10,882 m deep Tonga Trench (Scholl et al. 1985, Pelletier & Louat 1989) and the 11,022 m deep Marianas Trench (Fryer 1995). Island formation behind these trenches features uplifted limestone islands on the forearc belt close to the trench, and active volcanoes slightly farther from the trench (volcanic arc) that largely represent melting of subducted oceanic crust (Dickinson et al. 1999; Figure 1-8 herein). Divergent plate boundaries, also known as spreading centers, usually are not areas of island formation (Mayes et al. 1990).
Pacific islands can be classified into seven major types, each with substantial variation. The first type is the active volcano, such as Tofua or Kao in Tonga (Figures 1-9, 1-10). Active volcanoes can be found along island arcs (often trending north to south) associated with subduction zones at convergent plate boundaries (Taylor 1992, Clift et al. 1995) or along linear chains (often trending east-southeast to west-northwest) associated with midplate hotspots (Okal & Batiza 1987, Keating 1992, Dickinson 1998b, Hieronymus & Bercovici 1999). Active volcanic islands vary from large ones such as Savai'i in Samoa (1821 [km.sup.2], elev. 1858 m) to tiny, ephemeral ones such as Metis Shoal (Late Iki) in Tonga (Sutherland 2000) or Kavachi in the Solomon Islands (anonymous 2000, Holden 2000). These last two islands still come and go above the ocean's surface as their lava domes expand and contract. When Lister (1891) visited Metis Shoal in 1889/90, it was 46 m in elevation and had been above water for about a decade. When I flew low over Metis Shoal (July 1995) it was ca. 120 x 100 x 20 m high and had been above water for only weeks. Since then it has submerged and risen again. Most active volcanic islands date to <2 Ma, often <1 Ma. Their shorelines often are cliffy and bouldery with little or no beach development. Subsurface contours also are steep, with deep nearshore waters that generally lack coral reefs.
The next type is the eroded volcanic island. Usually 1-8 Ma and no longer volcanically active, the bedrock is basaltic to andesitic. Eroded volcanic islands can have steep, knife-edge topography (especially in young stages as in Tahiti, Mo'orea, and many Marquesan islands) or gentler slopes at later stages such as Babeldaob in Palau. A coastal plain of variable width and continuity usually occurs, with some beach development (Gillie 1997a). The beach sands may be mineral (bedrock-derived), calcareous (biogenic), or combinations thereof. Eroded volcanic islands can be large such as Tahiti in the Society Islands (1042 [km.sup.2], elev. 2237 m) or very small such as Morotiri in the Tubuai Islands (0.3 [km.sup.2], elev. 105 m). The three islands in the Manu'a Group of American Samoa are small, very steep, and high (Figure 1-11). The nearby, larger eroded volcanic island of Tutuila (145 [km.sup.2], elev. 652 m) also is steep but not so precipitous as to inhibit forest growth over most of its surface (Figure 1-12). Fringing reefs often develop around eroded volcanic islands. Gaps in the reef may be found near stream mouths when terrigenous sediment inhibits coral growth (Richmond 1997b).
Raised limestone islands, sometimes just called limestone islands, are based in Oceania on emergent coralline limestones, i.e., uplifted coral reefs dominated by calcium carbonate (CaC[O.sub.3]). The uplifted reefs are tens to hundreds of meters thick and cap a core of volcanic rock (Menard 1986, Spencer et al. 1987). These islands range in size from tiny rocks less than 10 m across to such major islands as Rennell (676 [km.sup.2], elev. 154 m), in the Solomons, or Niue (259 [km.sup.2], elev. 68 m), lying between Tonga and the Cook Islands. Although some raised limestone islands are found in most island groups, major concentrations of these islands include the Lau Group in Fiji, Tonga (Figures 1-13 to 1-15), the southern Cook Islands, Tubuai, Palau, and the southern half of the Mariana Islands. Raised limestone islands can have a wedding-cake profile (sets of terraces and cliffs) that represents multiple phases of reef formation and uplift. While the volcanic foundations of raised limestone islands are highly variable in age and can date from only several to occasionally as much as 20 to 30 Ma, these islands became totally submerged (typically through subsidence after hotspot or plate-boundary volcanism; Dickinson 1998b, Dickinson et al. 1999) before being uplifted. Emergence of many raised limestone islands has occurred only during the past 1-2 My or less.
The shorelines of raised limestone islands may be cliffy, gentle with beaches (the sand usually calcareous), or combinations thereof (Dickinson 2004b; Figures 1-13 to 1-15 herein). Limestone islands usually have fringing reefs or occur in clusters more or less surrounded by a barrier reef, as in Palau or the Vava'u Group of Tonga. Because limestone is so prone to develop caves and rockshelters (areas under projecting rock ledges; Farrand 1985) with high-pH, low-energy sediments, this type of island is an excellent source for prehistoric bird bones (Chapters 3, 4). Some raised limestone islands (and atolls; see below) also are covered with phosphate deposits [[Ca.sub.3]([P[O.sub.4]).sub.2]], often in the form of clayey phosphorite (typically [Ca.sub.10][(P[O.sub.4]C[O.sub.3]).sub.6][F.sub.2-3]; Allaby & Allaby 1999:410). Phosphorite deposits are believed to be derived from ancient seabird guano (Rodgers 1948, Stoddart & Scoffin 1983). These deposits can persist even on submerged atolls, known as guyots (Cullen & Burnett 1987).
Atolls typically are a set of low, sandy islets encircling a central lagoon and surrounded by a fringing reef (Bryan 1953, Stone 1953, Wiens 1962, Newhouse 1980, Devaney et al. 1987). No volcanic rock is exposed on atolls (Figures 1-16, 1-17), where calcareous sand overlies the subsurface limestone (which can be as much as 1300 m thick; Leopold 1969) that caps the volcanic rock. On some atolls, reefal limestone or indurated calcareous rubble are exposed intertidally or just above the tideline (Dickinson 1999a). Richmond (1997a) classified the 16 islands in the Gilbert Group of Kiribati into four types: (1) "reef island," for those with exposed, low-lying clastic (sands and gravels) carbonate buildup and little if any lagoon; (2) "atoll, deep lagoon," for those with a lagoon >10 m deep and the land separated by channels into islets; (3) "atoll, shallow lagoon," differing from the last only in depth (<10 m) of lagoon; and (4) "atoll, enclosed lagoon" for those with an essentially continuous landmass or reef surrounding the lagoon. His "reef island" more or less is what I call a "sand cay" below. A shallow, doubly convex layer of fresh water, known as the Ghyben-Herzberg lens, often develops on an atoll's larger islets (Arnow 1954, Wiens 1962, Niering 1963), greatly enhancing the potential for agriculture and therefore human habitation. As current global warming (Wigley & Raper 2001) melts ice at high latitudes and altitudes (Kindler & Hearty 2000, Bradley 2001) and therefore increases sea level (Cabanes et al. 2001, Church 2001, Arendt et al. 2002), many Pacific atolls are threatened with salinization of water supplies and terrestrial habitats, if not downright inundation, in upcoming decades or centuries.
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Table of Contents
Chapter 1. Geography and Geology
Chapter 2. Terrestrial Flora and Fauna
Chapter 3. Human History
Chapter 4. Birds Living and Dead, on Islands and in Museums
Chapter 5. Melanesia
Chapter 6. West Polynesia
Chapter 7. East Polynesia
Chapter 8. Micronesia and Remote Central Pacific Islands
Chapter 9. Megapodes
Chapter 10. Rails
Chapter 11. Pigeons and Doves
Chapter 12. Parrots
Chapter 13. Other Nonpasserine Landbirds
Chapter 14. Passerines
Chapter 15. Seabirds
Chapter 16. Extinction
Chapter 17. Dispersal, Colonization, and Faunal Attenuation
Chapter 18. Equilibrium and Turnover
Chapter 19. Species-Area Relationships
Chapter 20. Community Ecology
Chapter 21. Conservation Biology
Chapter 22. Conclusions, and Suggestions for Future Research