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Chapter 10
Human-Caused Extinction Of Birds
David W. Steadman
Curator of Birds, Florida Museum of Natural History, University of Florida, Gainesville
Because they are so conspicuous and appealing to the human senses of sight and sound, birds always have attracted more than their fair share of our zoological attention. Almost by necessity, therefore, birds have played a prominent role in our understanding of the processes by which species become rare, endangered, and finally extinct. The resulting literature on avian conservation biology has proliferated for decades and now is part of the information explosion, with all of its benefits and frustrations.
Each year we read of additional species of birds whose existence no longer can be demonstrated. One of the latest is the Colombian grebe, Podiceps andinus, whose demise in highland Colombia is attributed to the loss of wetlands, the introduction of exotic fish, and hunting (Fjeldsa, 1993). Declaring a species extinct can be a tricky business (Diamond, 1987); the discovery of even one living individual, regardless of the long-term viability of the species, refutes the claim. In other words, negative evidence (such as finding no grebes) can be refuted by even one bit of positive evidence (finding a grebe). To discover a few living Colombian grebes, however, is unlikely to prevent extinction of the species, given the vulnerability of very small populations to demographic stochasticity (Caughley, 1994; Gabriel and Burger, 1992), catastrophes (Lande, 1993), genetic viability (Lynch et al., 1995), and disease (Wilson et al., 1994).
Most species declared extinct by ornithologists never have been rediscovered. One exception is the Cebu flowerpecker, Dicaeum quadricolor, endemic to the Philippine island of Cebu (5,088 km2), where 8 of the 12 endemic subspecies of birds already are gone (Dutson et al., 1993). Considered extinct since 1906, up to four individuals of the Cebu flowerpecker were observed in 1992 and 1993
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in the last remaining patch (<2 km2) of closed canopy forest on the entire island. The prospect of long-term survival for the Cebu flowerpecker is remote, given the scarcity of its habitat and its small population. Only 2 years after its rediscovery, the Cebu flowerpecker may be gone for good.
This chapter discusses extinction, the final stage of endangerment. Extinction really is forever, in spite of what we are led to believe in dinosaur movies. Extinction is occurring today at unprecedented rates across a broad range of terrestrial and aquatic habitats (McNeely, 1992). Because species richness of birds is so high in tropical forests, the single most prolific cause of endangerment and extinction in birds (and many other groups of organisms) is the destruction of tropical forests (Phillips et al., 1994; Whitmore and Sayer, 1992; Wilson, 1992; Remsen, 1995). Deforestation and other types of habitat loss also deplete avian communities in temperate (Willson et al., 1994; see Figure 10-1) and high latitude areas, which typically have fewer species of birds to lose than tropical areas.
We have no evidence that any of the species of birds now endangered or that have gone extinct in recent millennia would be in their predicament if not for human activity. While extinction does occur naturally, human impact has increased rates of extinction by orders of magnitudes over background rates (Steadman et al., 1991; Wilson, 1992), and therefore is the only significant cause of our current "biodiversity crisis."
FIGURE 10-1
An adult and three downy chicks of the Least Bittern (Ixobrychu sexilis) at Presque Isle State Park, Pennsylvania. Like so many species of birds, the Least Bittern is strictly dependent upon wetlands, and therefore becomes rarer and more localized as wetlands are lost to human activities.
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Background
A comprehensive attempt to set global conservation priorities for birds (i.e., to avoid further extinction) has been compiled by Bibby et al. (1992), who identify 2,609 species of birds (27% of all living species) with breeding ranges of less than 50,000 km2, designating these as ''restricted-range species" (RRS). Sets of these species tend to occur together on islands or in well-defined areas of a particular continental habitat, especially tropical or montane forests. An "endemic bird area" (EBA) is any place where two or more RRS are sympatric. The 221 EBAs each contain 2-67 RRS. Many of the EBAs correspond roughly or rather exactly with centers of endemism in other organisms, such as vascular plants, butterflies, amphibians, reptiles, or mammals. The total numbers of both EBAs and RRS are about evenly divided between islands and continents. The tropics have 76% of all EBAs and more than 90% of all RRS.
The data on EBAs and RRS provide a rough but informative idea of the potential for extinction of birds in upcoming decades. For example, the country with the most RRS is Indonesia (Table 10-1), where most islands are unprotected. The EBA with the most RRS is the Solomon Islands, where none of the land is officially protected (Table 10-2). Of the 10 EBAs with the most RRS, only one has more than 15% of its land under protection. We are rapidly approaching the point of no return for hundreds of species.
Humans cause the extinction of birds in four major ways: (1) direct predation; (2) the introduction of nonnative species; (3) the spread of disease; and (4) habitat degradation or loss. Direct predation includes hunting (killing living birds), gathering eggs, or removing nestlings for captive rearing. The introduc-
TABLE 10-1 Ten Countries With the Most Restricted-Range Species (RRS) of Birds
Country
Occurring RRS
Confined RRS
Threatened RRS
Number of EBAsa
Indonesia
411
339
95
24
Peru
216
106
51
18
Brazil
201
122
67
19
Colombia
189
61
51
14
Papua New Guinea
172
82
18
12
Ecuador
159
32
38
11
Venezuela
120
40
17
8
Philippines
111
106
36
9
Mexico
102
59
23
14
Solomon Islands
96
43
16
4
a Endemic bird areas.
SOURCE: Modified from Bibby et al. (1992:Appendix 2).
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TABLE 10-2 Ten Endemic Bird Areas (EBA) With the Most Restricted-Range Species (RRS) of Birds
EBAa
Occurring RRS
Confined RRS
Threatened RRS
Land Area (103 km2)
% Land Area Protected
Pacific Islands, Indonesia
Solomon Islands
67
42
10
32
0
New Britain and New Ireland
57
34
4
46
0-5
Tanimbar and associated islands
46
23
10
5.6
0-5
Northern Moluccas
44
27
6
29
0
Central New Guinean midmountains
41
27
2
98
5-10
Central America
Costa Rican and Panamanian highlands
54
51
2
27
10-15
South America
Ecuadorian dry forests
51
44
14
57
5-10
Southeast Brazil
48
41
23
50
10-15
Tepuis
42
36
0
35
60-70
Western Andes of Colombia and Ecuado
41
35
12
27
10-15
a Under natural conditions, each of these EBAs is primarily tropical forest of some sort.
SOURCE: Modified from Bibby et al. (1992:Appendix 1).
tion of nonnative animals exposes indigenous birds to new predators, competitors, parasites, or pathogens, and thus is related to categories (1) and (3). Habitat degradation or loss can be direct (deforestation, draining wetlands, plowing prairies, toxic pollution, etc.) or due to encroachment by nonnative plants (category 2).
Much of our biodiversity crisis is due to human impact of recent centuries, especially the past few decades. Cooperative research by archaeologists, geologists, and biologists has shown, however, that most plant and animal communities were not pristine in preindustrial times. Our skills with tools and fire have set us apart from other animals for tens of millennia. All human societies, even nonagricultural hunter-gatherers, have had various effects on their environment (e.g., Betancourt and Van Devender, 1981; Burney, 1993; Diamond, 1992; Klein, 1992; Martin, 1990; Martin and Klein, 1984; Steadman, 1991). Hunting, fishing, and gathering often focus on certain species or groups of species (Redford, 1992). Agriculture affects natural ecosystems in more diverse ways, including modifications of landscape, soils, and water supply through deforestation, erosion, channeling, flooding, draining, etc., as well as the elimination or propagation of selected species of plants and animals.
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An understanding of past plant and animal communities is important for long-term estimates of community stability and change (Betancourt et al., 1990; Webb et al., 1993), such as how thoroughly and rapidly ecosystems might recover from disturbances. The responses of plants and animals to future climatic changes will be compromised by human-caused habitat fragmentation (Peters, 1988). Some species may be unable to disperse across tracts of disturbed habitat. When feasible, past distributions can aid in planning translocation programs, with an overall goal of preserving species assemblages that at least approach those of a less disturbed state.
Continents
The distinction between continents and islands is useful, although there is a continuum in land area and isolation from small, remote oceanic islands, such
Barranca del Cobre, Chihuahua, Mexico. On level ground, corn is cultivated here by the Tarahumara. The largest pine trees have been removed, resulting in the extinction of the Imperial Woodpecker (Campephilus imperialis), a close relative of the Ivory-billed Woodpecker (C. principalis), which is extinct in the United States and nearly so in its only other locale, Cuba.
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as Ascension or St. Helena, to large "continental" islands, such as Madagascar, Borneo, or New Guinea, to a relatively small and isolated continent such as Australia. Regardless, we now may be at a threshold where human-caused extinctions of birds—heavily biased thus far toward species on islands (compare Tables 10-3 and 10-4)—will occur at comparable rates on continents. For example, the number of threatened restricted-range species on continents compared to islands is 247 versus 165 (Table 10-1) and 51 versus 32 (Table 10-2). The most influential factor in this new trend is habitat loss, fueled by advances in human technology and population pressure.
Another important factor in endangerment and extinction, much more species-specific than habitat loss, is direct exploitation. This may take the form of hunting (often for food; "gamebirds" such as waterfowl, currasows, guans, chachalacas, pheasants, quail, grouse, pigeons, and doves are most affected) or taking young as pets (raptors, parrots, and songbirds are the most exploited, although many other types of birds are also taken; see chapters in Beissinger and Snyder, 1992). The combination of habitat loss and direct exploitation has had verifiable impacts on one or more species from virtually every family of birds.
Unlike on some islands (see next section), we know little about the prehistoric impact of people on birds on most continents. A partial exception is North America, where people first arrived about 11,000 years ago (Haynes, 1992). Between 20 and 40 species of birds became extinct at this time, probably because of ecological dependencies on the more than 40 species of ground sloths, mammoths, mastodons, horses, tapirs, camels, and other large mammals that died out across the continent (Steadman and Martin, 1984). If humans were involved in the mammal extinctions (Martin, 1990), then they indirectly caused the extinctions of the birds as well. Before the large mammal communities collapsed, the carrion-feeding California condor lived as far away from California as Texas, Florida, and New York (Emslie, 1987; Steadman and Miller, 1987). Most
TABLE 10-3 Minimum Estimates of Human-Caused Extinction of Continental Birds since A.D. 1600
Continent
Number of Species
North America
5-7
Central America
3
South America
3
Africa
1
Europe and temperate Asia
3-4
Tropical Asia
1
Australia
1
Total
17-20
SOURCES: Mountfort (1988) and herein.
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TABLE 10-4 Minimum Estimates of Extinction of Island Birds
Region
Number of Species Prehistorica
A.D. 1600-1899
A.D. 1900-1994
Pacific Ocean
90
28
23
Indonesiab
0
0
2
Indian Ocean
11
30
1
Philippinesb
0
0
1
Caribbean Sea
34
2
1
New Guinea and Melanesia
10
2
3
Atlantic Ocean
3
3
1
Mediterranean Sea
10
0
0
Total
158
65
32
a The prehistoric category consists of species from prehistoric cultural contexts, and includes only species already described. Many other prehistoric extinct species have been found but remain undescribed. For each region, an even greater number of extinct species are undiscovered because of incomplete archeological sampling.
b There is no interpretable prehistoric record of birds from Indonesia or the Philippines.
SOURCES: Modified from Johnson and Stattersfield (1990) and Milberg and Tyrberg (1993).
other scavenging birds perished completely. In the next 10,000 years before the arrival of Europeans, however, only two North American birds are known to have become extinct: a flightless duck, Chendytes lawi, of the Pacific coast (Morejohn, 1976; Guthrie, 1992) and a small turkey, Meleagris crassipes, from the Southwest (Rea, 1980).
Bones from late prehistoric archaeological sites in North America document birds such as the trumpeter swan, Mississippi kite, swallow-tailed kite, whooping crane, sandhill crane, long-billed curlew, great auk, Carolina parakeet, ivory-billed woodpecker, common raven, and fish crow in localities well outside of their modern ranges. While intertribal exchange of birds might account for some of these range extensions, most seem to reflect former indigenous populations. Prehistoric hunting, trapping, and habitat modification, as well as natural climatic change, may have caused these range contractions. It probably is no coincidence that three of these same species (the great auk, Carolina parakeet, ivory-billed woodpecker) are now extinct and the whooping crane is endangered. Some birds, such as the California condor, bald eagle, and golden eagle, were hunted for feathers, bones, or ceremonial purposes more than for food (Bates et al., 1993; Emslie, 1981; Rea, 1983; Simons, 1983).
In the past 200 years, at least five species of North American birds have been lost (great auk, Labrador duck, passenger pigeon, Carolina parakeet, ivory-billed woodpecker). Each of these, except the woodpecker, represented a monotypic genus. Two other species, the Eskimo curlew and Bachman's warbler, are either extinct or virtually so. The California condor, whooping crane, red-
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cockaded woodpecker, black-headed vireo, golden-cheeked warbler, and Kirtland's warbler persist today only in dangerously small, localized populations. Without management, three of these species already would be extinct. Even if no more habitat is lost, some of these last eight species are likely to become extinct in the next 200 years, as may others that are known to be endangered, in decline, or localized today (such as greater and lesser prairie chickens, sage grouse, piping plover, bristle-thighed curlew, common nighthawk, redheaded woodpecker, sedge wren, loggerhead shrike, California gnatcatcher, cerulean warbler, and Henslow's sparrow; reviewed in Ehrlich et al., 1992). Losing 20-25 species per millennium may seem slow to politicians and economists, but is a devastatingly high rate of extinction from an evolutionary standpoint. We have no evidence that speciation will offset any of these losses.
As serious as this situation seems, North American birds may be facing much less extinction in the next century or two than avifaunas from many other parts of the world, especially tropical regions. Each country or EBA listed in Tables 10-1 and 10-2 is tropical. With its many social, economic, political, legal, and ethical complexities (Rudel, 1993; Rush, 1991), tropical deforestation continues at rates far beyond sustained yield. Many tropical regions, especially those that are mountainous or covered with swamp forests, remain poorly surveyed for birds. In fact, it is often the case that localities are not surveyed until after human disturbances (clearing for roads, airstrips, settlements, etc.) allow access.
Long-term (decadal or more) ecological data are lacking for most tropical localities. Where such data have been gathered, decreases in species richness have been detected. At the San Antonio cloud forest in the western Andes of Colombia, for example, the approximate number of species of forest birds declined from 128 in 1911, to 104 in 1959, to 92 in 1989-1991 (Kattan et al., 1994). Caused primarily by forest fragmentation, these "local extinctions" are steps leading toward full extinction of species. Although all feeding guilds are involved, large canopy frugivores and understory insectivores have lost the most species at San Antonio.
Throughout the tropics, but perhaps especially in South America and Melanesia, the extent of local, regional, and full extinction of birds is undocumented for many species. This clearly justifies the call for more surveys (Kattan et al., 1994). Many such surveys will be fascinating biologically but ultimately futile for preventing extinction as long as so much habitat continues to be lost. How personally frustrating it must be for scientists such as Gretton et al. (1993), who determined that only 20-30 pairs of Gurney's pitta (Pitta gurneyi) still exist, yet territories of this colorful but secretive songbird are being lost each year to deforestation in Thailand. The gap between scientific knowledge and conservation policy often is large (Remsen, 1995). It may be safe to say that no single country on any continent has preserved enough habitat to secure the long-term survival of its current avifauna.
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Islands
The relatively small land areas of islands result in small populations of organisms that tend to be more vulnerable to extinction than those on continents (Diamond, 1985). Of 108 species of birds known to have become extinct since A.D. 1600, 97 (90%) lived on islands (Johnson and Stattersfield, 1990). By their calculations (97 extinct versus 1,750 extant species on islands, 11 extinct versus 7,500 extant species on continents), the probability of extinction during the past 4 centuries has been about 40 times greater on islands than continents. Although both 97 and 11 are underestimates (see Tables 10-3 and 10-4), this ratio probably is more or less valid.
The plight of island birds did not begin, however, in A.D. 1600. As summarized in Table 10-4, even more human-caused extinctions already had occurred on islands in prehistoric times (Milberg and Tyrberg, 1993; Steadman, 1995). Because of how incompletely we have sampled the zoo archaeological record of island birds thus far, the known prehistoric extinctions are a small fraction of those that actually occurred. These losses seem to have been due to the same processes that still exterminate species on islands today: predation by humans and introduced species, nonnative pathogens, and habitat destruction (Collar and Andrew, 1988; Kirch, 1983; Olson, 1977; Olson and James, 1982; Savidge, 1987; Steadman et al., 1984, 1990).
The losses of birds on oceanic islands consist of: (1) extinction (loss of all populations of a species); (2) extirpation (loss of a species on an entire island, with other population[s] surviving elsewhere); and (3) reduced population (loss of individuals from a surviving population). The last two categories are steps leading toward genuine extinction, which represents irreversible losses rather than the short-term fluctuations in populations near continental source areas that biogeographers often call "extinctions" when studying faunal turnover. Research in the Galapagos Islands (Steadman et al., 1991), Hawaiian Islands (James, 1987), and Tonga (Steadman, 1993) has shown that natural (prehuman) rates of genuine extinction in island birds may be at least 2 orders of magnitude less than posthuman rates.
Nearly all islands in Oceania (Melanesia, Micronesia, and Polynesia; Figure 10-2) were inhabited prehistorically by humans (Irwin, 1992). Birds provided fat, protein, bones, and feathers for the human colonists, who also cleared forests, cultivated crops, and raised domesticated animals (Steadman, 1989). The prehistoric extinction of Pacific island birds is known from studying bones from archaeological sites. Micronesia is not as well studied as Polynesia, but seems to differ only in details of taxonomy and chronology (Steadman and Intoh, 1994). Although Melanesian islands also have lost a variety of seabirds and landbirds (Balouet and Olson, 1989), a larger percentage of the indigenous avifauna survives on large Melanesian islands than on Polynesian or Micronesian islands, or on small Melanesian islands. This may be due to the buffering effects that steep
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FIGURE 10-2
The Pacific Ocean, showing major island groups.
terrain, cold and wet montane climates, and human diseases have had on human impact. As predicted from biogeographic theory (MacArthur and Wilson, 1967), birds tend to be easier to extinguish on low, flat islands (which often are small) than on high, steep islands (which often are large).
While Pacific islands have a well-earned reputation as the source of much modern extinction of birds, most species of landbirds already were extinct when Captain Cook opened the region to European influence 220 years ago. Lost were as many as 2,000 species of birds, dominated by flightless rails, but also including moas, petrels, prions, pelicans, ibises, herons, swans, geese, ducks, hawks, eagles, megapodes, kagus, aptornithids, sandpipers, gulls, pigeons, doves, parrots, barn-owls, strigid owls, owlet-nightjars, and many types of passerines (Milberg and Tyrberg, 1993; Steadman, 1995). Assuming that about 9,600-9,700 species of birds exist today (Monroe and Sibley, 1993; Sibley and Monroe, 1990), the world avifauna would be about 20% richer in species had islands of the Pacific remained unoccupied by humans.
In the Hawaiian Islands alone, at least 62 endemic species of birds have become extinct since the arrival of Polynesians nearly 2,000 years ago (James and Olson, 1991; Olson and James, 1991). As is true throughout Oceania, the number of extinct species known from the Hawaiian Islands increases with each new season of archaeological or paleontological field work. Most of the extinct Hawaiian species were gone before the arrival of Europeans, including most species within the spectacular endemic radiations of cardueline finches (the Hawaiian "honeycreepers") and flightless geese, ducks, ibises, and rails (James et al., 1987; Olson and James, 1982, 1984). In New Zealand, at least 44 endemic species of landbirds have become extinct in the past millennium, featuring many endemic species of moas, waterfowl, hawks, rails, and passerines (Anderson, 1989; Holdaway, 1989; Worthy and Holdaway, 1993).
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Outside of the Hawaiian Islands and New Zealand, prehistoric Polynesian birds are known from the Marquesas Islands, Society Islands, Cook Islands, Henderson Island, Easter Island, Samoa, Tonga, and Polynesian outliers in Melanesia (Steadman, 1995). In these island groups, rails, pigeons, doves, and parrots have undergone the most extinction. Rails have lost more species than any other family. Most island rails, in the Pacific and elsewhere, were flightless forest species rather than volant wetland or grassland species. Any Pacific island with a relatively thorough prehistoric record of birds has yielded bones of one or more endemic species of flightless or nearly flightless rails. The three islands with the best fossil records (Ua Huka, Mangaia, and 'Eua) each have produced two to four endemic species of flightless rails; the records from even these islands are incomplete. Two species of rails occur among only seven bones of landbirds known from remote Easter Island (see below). If not for human activities, at least 800 islands in Oceania probably would be inhabited today by flightless rails. Assuming one to four endemic species per island, rails alone may account for as many as 2,000 species of birds that would be alive today had people not colonized Oceania. The only surviving flightless rails in the tropical Pacific (east of New Guinea) are three species of Gallirallus (from Okinawa, Guam, and Solomon Islands; Diamond, 1991), Nesoclopeus woodfordi of the Solomon Islands (Hadden, 1981), and Porzana atra of Henderson Island (Graves, 1992). All except the last species is endangered. Just think of the possibilities for studying comparative systematics, evolution, biogeography, ecology, and behavior if hundreds or thousands of species of flightless rails still were alive. Sadly, we have wiped out nearly everything, leaving only some bones from one of nature's most dramatic examples of adaptive radiation.
Seabird losses in Polynesia have been greatest for petrels and shearwaters, although the distribution and population size of various albatrosses, storm-petrels, tropicbirds, frigatebirds, boobies, gulls, and terns have been reduced as well. Species nesting on or within the ground have been lost to predation from nonnative mammals and erosion of topsoil associated with deforestation. Abbott's booby, now confined to Christmas Island in the eastern Indian Ocean, once was widespread in the South Pacific (Figure 10-3). One island away from extinction, Abbott's booby is one of many examples of seabirds whose modern breeding range is but a tiny fraction of what it once was.
The loss of native birds is more complete on remote Easter Island (160 km2, elevation 507 m) than on any other island of its size in Oceania. Although Easter Island was forested at first human contact about 1500 years ago, deforestation was virtually complete by about 550 years ago (Flenley et al., 1991). Small samples of bird bones, associated with Polynesian artifacts 600-900 years old (Steadman et al., 1994), show that Easter Island once sustained at least 22 species of seabirds, including 12 tubenoses (albatrosses, fulmars, petrels, prions, shearwaters, storm-petrels). Only 1 of the 22 species of seabirds still nests on Easter Island itself, while 7 of them occur on one or two of its offshore islets. When
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Complete Table on previous page.
44 are rails, sandpipers, pigeons, doves, and parrots whose bones dominate zones 1-4 and decline sharply by zone 5 (Table 10-6). Most species of indigenous landbirds are not recorded above zones 5-7, which are 700 to 500 years old. The most common flightless rail, Porzana rua, is last recorded in zone 8. Two species of doves are the only extinct/extirpated landbirds recorded above zone 8. Each of the four surviving landbirds from MAN-44 (a duck, rail, kingfisher, and warbler) tolerates forest clearance. The bristle-thighed curlew (Numenius tahitiensis), a rare shorebird that nests in Alaska, has been killed and eaten for centuries on its Pacific island wintering areas (Table 10-6), where about 50% of the adults become flightless during wing molt (Marks, 1993).
Because of its depauperate modern avifauna, the Kingdom of Tonga did not qualify as an EBA in Bibby et al. (1992). Bones from caves on 'Eua (85 km2, elevation 300 m) indicate, however, that at least 27 species of landbirds lived on this Tongan island before the arrival of people about 3,000 years ago (Steadman,
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TABLE 10-6 Bones of Landbirds from Tangatatau Rockshelter (site MAN-44), Mangaia, Cook Islands, Ranging from Zone 1A (Oldest) to 17 (Youngest)
Zone
Native Birds
1A
IB
2-3
4
5-7
8
9-14
15
17
Total
Migratory Shorebirds
Plovers
Pluvialis dominica
—
—
2
—
—
—
—
—
—
2
Curlews
Numenius tahitiensis
5
—
2
2
—
—
—
1
—
10
Native Landbirds
Ducks
Anas superciliosa
—
—
3
1
2
2
8
11
3
30
Rails
**Gallirallus ripleyi
10
5
12
16
1
—
—
—
—
44
Porzana tabuensis
—
—
2
1
—
1
—
1
1
6
**Porzana rua
11
44
41
21
2
1
—
—
—
120
**Porzana new sp.
—
2
1
1
—
—
—
—
—
4
**Porphyrio? new sp.
—
1
—
—
—
—
—
—
—
1
Sandpipers
**Prosobonia new sp.
1
1
1
1
—
—
—
—
—
4
Pigeons, Doves
*Gallicolumba erythroptera
3
5
3
5
—
—
—
—
—
16
**Gallicolumba new sp.
—
—
—
—
—
—
7
1
3
11
**Gallicolumba nui
3
3
—
3
—
—
—
—
—
9
*Ptilinopus rarotongensis
1
4
4
2
—
—
—
1
—
12
*Ducula aurorae
1
2
1
1
—
—
—
—
—
5
*Ducula galeata
3
1
5
2
—
1
—
—
—
12
Parrots
*Vini kuhlii
14
41
15
23
1
—
—
—
—
94
**Vini vidivici
42
14
3
14
2
—
—
—
—
75
[*/** Vini kuhlii/vidivici]
2
2
—
5
—
—
—
—
—
9
Kingfishers
Halcyon mangaia
1
2
2
2
—
—
—
—
1
8
Warblers
Acrocephalus kerearako
6
6
1
2
—
—
—
—
—
15
Total NISP
All Species
103
133
98
102
8
5
15
15
8
487
Migratory Shorebirds
5
0
4
2
0
0
0
1
0
12
Native Landbirds
98
133
94
100
8
5
15
14
8
475
/** Native Landbirds
91
125
86
94
6
2
7
2
3
416
% NISP */** Native Landbirds of All Nonfish Bones
72
57
5
21
4
0.4
2
0.5
2
11
Total Species
All
13
14
16
16
5
4
2
5
4
19
Migratory Shorebirds
1
0
2
1
0
0
0
1
0
2
Native Landbirds
12
14
14
15
5
4
2
4
4
17
*/** Native Landbirds
10
12
10
11
4
2
1
2
1
13
*=extant species, extirpated on Mangaia.
**=extinct species.
NOTES: Dashes indicate no record. Taxa in brackets are not different from others listed more specifically. NISP represents number of identified specimens.
SOURCE: Updated from Steadman and Kirch (1990).
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1993, 1995; Table 10-7). Only 6 of these same species still survive on 'Eua. If they were still alive, most species of 'Eua's extinct forest birds probably would qualify as RRS under the criteria of Bibby et al. (1992), thereby changing considerably the international conservation priority of Tongan birds.
The 'Eua avifauna has been depleted irreversibly. As sampled thus far, the pheromone landbirds consisted of 27 forest and no nonforest species, compared to 9 forest and 4 nonforest species today (Table 10-7). Forest frugivores/ granivores have declined from 12 to 4, nectarivores from 4 to 1, omnivores from 3 to 0, insectivores from 6 to 3, and predators from 2 to 1. These losses are more complete for ground-dwelling than midlevel/understory or canopy species, which might be expected with predation from humans, rats, dogs, and pigs. The means of pollination and/or seed dispersal for various Polynesian forest trees undoubtedly has been restricted or eliminated by the loss of so many nectarivorous, frugivorous, and granivorous birds (Franklin and Steadman, 1991).
Discussion
Humans cause the extinction of birds by overhunting, by introducing nonnative species, by spreading disease, and through habitat destruction. For many, perhaps most, species of birds in danger of extinction today, habitat destruction is the most serious threat. Loss of habitat is why tropical regions now have more potential for extinction than temperate or polar regions. Even in the United States, however, with our many and often admirable environmental laws, much natural habitat has been lost or seriously degraded. We are fortunate that more species are not already gone. Given current trends in human population growth and habitat loss, this good fortune is unlikely to persist in the United States or anywhere else. That 20% of all living species and 70% of all threatened species of birds are confined to only 2% of the Earth's land surface (Bibby et al., 1992) is both a blessing and a curse. On the one hand, it means that we might be able to save many species by protecting relatively small areas. On the other hand, we can lose many species by allowing just these same places to be degraded.
Across the world, extinction rates of birds increase whenever humans enter a previously uninhabited region. Human colonization of the Pacific islands alone led to the extinction of as many as 2,000 species of birds, dominated by flightless rails, but also including species in many other families. A crude but conservative estimate of overall losses of birds in Oceania is that an average of at least 10 species (including 2-3 that are endemic to the island and 2-3 that are endemic to the island group) has been lost already on each of the approximately 800 major islands, yielding a minimum total loss of perhaps 2,000 species. Each of the 16 Polynesian islands that has yielded 300 or more prehistoric bird bones approaches or exceeds 20 extirpated species, and none of these records is complete.
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TABLE 10-7 Chronology and Community Ecology of Indigenous Resident Landbirds from 'Eua, Tonga
Resident Landbirds
Pre-human Recorda
Archaeological Recordb
Extant in 1988
Guildc
Herons
Egretta sacra
—
X
X
NF
**Nycticorax new sp.
X
—
—
GP
Ducks
Anas superciliosa
—
—
X
NF
Hawks
*Accipiter cf. rufitorques
X
—
—
DR
Megapodes
**Megapodius alimentum
X
X
—
GF
*Megapodius pritchardi
X
—
—
GF
**Megapodius new sp.
X
X
—
GF
Rails
**Gallirallus new sp
X
—
—
GO
Gallirallus philippensis
—
X
X
NF
*Porzana tabuensis
—
X
—
NF
**Gallinula new sp.
X
—
—
GO
Porphyrio porphyrio
—
X
X
NF
Pigeons, Doves
*Gallicolumba stairi
X
X
—
GF
**Didunculus new sp.
X
X
—
MF
Ptilinopus porphyraceus
X
X
X
CF
Ptilinopus perousii
X
X
X
CF
**Ducula david
X
X
—
CF
**Ducula new sp.
X
X
—
CF
Ducula pacifica
—
X
X
CF
Parrots
*Vini solitarius
X
—
—
CN
*Vini australis
X
X
—
CN
**Eclectus new sp.
X
X
—
CF
Barn-Owls
Tyto alba
—
X
X
NR
Swifts
Collocalia spodiopygi
—
X
X
AI
Kingfishers
Halcyon chloris
X
X
X
MI
Trillers
Lalage maculosa
X
X
X
MI
*/**cf.Lalage sp.
—
X
—
MI
Whistlers, Robins
*/**Eopsaltria sp.
X
—
—
MI
Monarchs
*Clytorhyncus vitiensis
X
X
—
MI
*Myiagra sp.
X
X
—
MI
Warblers
/**Cettia sp.
X
—
—
MI
Thrushes
*Turdus poliocephalus
X
X
—
MF
Starlings
Aplonis tabuensis
X
X
X
MF
Honeyeaters
*Myzomela cardinalis
X
X
—
CN
Foulehaio carunculata
X
X
X
CN
White-Eyes
**Zosteropidae new sp.
X
—
—
CO
Total Species
27
26
13
Total */** Species
21
14
3
# of Sites/# of Bird Bones
1/401
14/888
—
Guild Totalsc
Al
0
1
1
CF
5
6
3
CN
4
3
1
CO
1
0
0
DR
1
0
0
GF
4
3
0
GO
2
0
0
GP
1
0
0
MF
3
3
1
MI
6
5
2
NF
0
4
4
NR
0
1
1
Total Forest Species (All Categories of Guilds except NF)
27
22
9
Food Categories
Frugivores
12
12
4
Nectarivores
4
3
1
Omnivores
3
0
0
Insectivores
6
6
3
Predators
2
1
1
Height Categories
Aerial
0
1
1
Canopy
10
9
4
Midlevel/understoryd
10
9
4
Ground
7
3
0
*=extirpated species.
**=extinct species.
a>3000 years before the present (BP)(1 site).
b3000-200years BP (15 different sites).
cAl=aerial insectivore; CF=canopy frugivore/Granville; CN=canopy nectarivore; CO=canopy omnivore; DR=diurnal raptor; GF=ground frugivore/granivore; GO=ground omnivore; GP=ground predator; MF=midlevel/understory frugivore/granivore; Ml=midlevel/understory insectivore; NF=nonforest species; NR=nocturnal raptor. Certain distinctions between C and M are arbitrary.
d Includes the hawk and owl.
NOTE: Dashes indicate no record.
SOURCE: From Steadman (1993, 1995).
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Complete Table on previous page.
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The rapidity of extinction on oceanic islands has been influenced by the ruggedness of terrain and the size and permanence of human populations. After the arrival of humans, extinction of birds is what we have come to expect; survival is the exception. While it is too late to maintain or restore the entire natural avifaunas of most oceanic islands, rigorous habitat protection, predator control, and translocation can save much of what is left (Franklin and Steadman, 1991). Nevertheless, because most island biotas are so degraded, programs for endangered species on islands face difficult challenges to ensure the long-term survival of the species that remain. We know what needs to happen to save most species, but without improved conservation funding and coordinated changes in human activities, many of the challenges will not be met.
On tropical islands in all oceans, the prehistoric and ongoing extinction of birds has consequences far beyond losing the birds themselves. For example, the loss of hundreds of populations and a few entire species of Pacific seabirds probably has influenced marine food webs, in which seabirds are top consumers. Extinct Pacific island landbirds undoubtedly were involved in the pollination and seed dispersal of indigenous plants, many of which may lack natural means of intraor interisland dispersal today.
Conclusions
Rather than despair at what already is lost, I would argue that the extinct species of birds should inspire us to save those that remain. I also would hope to elevate the scientific status of recently extinct species so that their study is regarded as an important component of modern biology. Hundreds of species that should be living today exist now only as skins or bones in museums. Who will study these ''relics of a lost world" (Graves, 1993) to learn more about their phylogeny, biogeography, and ecology? Very few young ornithologists are being trained in systematics, especially in areas other than molecular systematics. This is a serious situation, given that even such a widespread and locally common species as the yellow warbler (Dendroica petechia), when studied more comprehensively than ever before (2,500 skins examined by Browning, 1994), has been found to include two previously unnamed subspecies in Alaska and northern Canada. As pointed out so clearly by Trombulak (1994:590), training in conservation biology should include much more than "applied ecology and field population genetics."
Whether their last gasp was thousands, hundreds, or only tens of years ago, virtually all birds lost since the last ice age would still be alive if not for humans; they would be feeding, preening, singing, nesting, molting, and doing anything else that living birds do. You could use them to test behavioral hypotheses. You could see them during field work, vacations, or maybe on the way to work. They would be illustrated in field guides and be part of your overall biodiversity consciousness. (A field guide to South Pacific birds would
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depict as many species as a field guide to South American birds!) Pacific island rails would compete with Darwin's finches and Hawaiian finches as "textbook" examples of adaptive radiation.
We need to squeeze out as much knowledge as we can about recently extinct species of birds. This chapter began by noting that, relative to other animals, birds are conspicuous and therefore more studied. This is supported by the low rate at which new living species are being discovered. The number of "good" new species of birds described per year has varied from 6.0 in 1938-1941 to 2.6 in 1941-1955, 3.5 in 1956-1965, 3.1 in 1966-1975, 2.4 in 1976-1980, and 2.4 in 1981-1990 (Vuilleumier et al., 1992). While estimates of valid descriptions of species, just like estimates of extinction, have margins of error, sometime within the past decade we probably reached the point where more living species of birds are going extinct per year than are being newly described. This is a debt that cannot be repaid.
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Representative terms from entire chapter:
extinct species
