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58 SMITHSONIAN CONTRIBUTIONS TO PALEOBIOLOGY
COMPARISONS WITH OTHER ISLANDS
The 20 species of seabirds recorded as fossils from Amsterdam
Island do not include Phoebetria fusca or Sterna vittata,
which currently breed there, suggesting that up to 22 species
had breeding populations. Species that are very rare in the fossil
record, however, such as Pachyptila desolata, Puffinus carneipes,
and P. griseus (each with one individual), are likely to
have been nonbreeding vagrants or the result of skua kills, so a
more realistic estimate is that no more than 19 species of seabirds
bred on Amsterdam Island in the recent past.
Amsterdam Island is similar to the Crozet and Kerguelen islands
in that most breeding species are seabirds; however, with
at least 34 and 30 breeding species of seabirds, respectively,
these subantarctic island groups have far richer communities
than Amsterdam did (Jouventin et al., 1984). Although the fossil
faunas of these islands have not been studied, it is likely that
neither island group has suffered species extinctions as happened
on Amsterdam because some islands in each group have
remained free of predators (Jouventin et al., 1984).
The Mascarene islands Mauritius and Rodriguez are well
known for their extinct, endemic land birds (Newton, 1888;
Hachisuka, 1953; Gill, 1967), but the fossil seabirds have
largely been ignored (Bourne, 1968). These islands are much
farther north than Amsterdam, and various tems and boobies
dominate the fauna, with a few species of petrel present.
Bourne (1968) reported Wedge-tailed Shearwaters {Puffinus
pacificus), a larger shearwater (possibly P. carneipes), and the
Mascarene Petrel {Pterodroma aterrima) in addition to numerous
remains of the White-tailed Tropic Bird {Phaethon lepturus)
in a small collection of fossils from Rodriguez. The discovery
of bones of Barau's Petrel {Pterodroma baraui) on
Amsterdam extends the range of this species, previously
known only from Reunion, and provides a faunal link with the
Mascarene Islands. Pterodroma aterrima, also known only
from Reunion Island, probably bred on Rodriguez (Bourne,
1968), and it may also have reached Amsterdam occasionally.
Studies of the fossil avifauna of St. Helena Island in the Atlantic
Ocean (16°S) have revealed remains of 21 species,
among which the greatest loss of species and individuals was
five of the six resident petrels, three of which were endemic
(Olson, 1975). Farther north in the Atlantic, the fossil avifauna
of the eastern Canary Islands, which lie off the African coast
between about 28°N and 29°N, is dominated by remains of two
extinct species of Puffinus (Alcover and McMinn, 1995).
In the tropical Pacific Ocean the fossil faunas of numerous
islands have been studied, and all are rich in terrestrial species:
seabirds are dominated by procellariids, but toward the equator,
the species diversity of tems, boobies, tropicbirds, and frigatebirds
often equals or exceeds that of procellariids (Olson
and James, 1991; Steadman, 1995). On every island studied,
several species of land birds and populations of seabirds were
exterminated following the arrival of humans in the last few
thousand years (Steadman, 1989, 1995; Olson and James,
1991). Often losses, particularly of land birds, have exceeded
50% of the original species diversity, and, among seabirds,
some of the greatest losses have been petrels and shearwaters,
especially in eastern Polynesia (Steadman, 1989).
In the South Pacific the fossil faunas of the North and South
islands of New Zealand have been studied extensively: Millener
(1990) listed 34 species that became extinct in the late Holocene
following the arrival of humans, to which Scarlett's
Shearwater {Puffinus spelaeus), South Island Adzebill {Aptornis
defossor), Bush Wren {Xenicus longipes), and Long-billed
Wren {Dendroscansor decurvirostris) should be added, for a
total of 38 species. In the New Zealand region, seabirds have
not suffered the same degree of loss as elsewhere because, although
numerous populations of seabirds have been extirpated
from the main islands, colonies on offshore islands have ensured
their species survival, with the exception of Puffinus
spelaeus (Holdaway and Worthy, 1994). The fossil faunas of
the New Zealand subantarctic islands have not been studied.
Conclusions
Like most islands where numerous extinctions followed the
arrival of humans and commensal mammals, Amsterdam Island
had a naive avifauna (Milberg and Tyrberg, 1993) unable
to cope with predation by people and rats. Its location far from
other landmasses was no doubt responsible for the paucity of
land-bird species, and its relatively southern position resulted
in a seabird fauna composed mainly of petrels. Studies of fossil
faunas from islands throughout the world, reviewed above,
show that petrels are particularly susceptible to the effects of
humans and to predation by introduced commensal mammals.
Amsterdam Island conforms to this generality in that only 10 of
at least 19 species that formerly bred there survive; in other
words, 47% of the original species have been extirpated. Five
of these 10 surviving species, however, are critically endangered
on Amsterdam. Only three species are endemic to Amsterdam
and St. Paul, and of these the land bird Anas marecula
is extinct, whereas Diomedea amsterdamensis and Pachyptila
macgillivrayi survive.