Ornithology, Evolution, and Philosophy 123

172 4 Ornithologist and Zoogeographer
major portion of the bird fauna of Melanesia and Polynesia. David Lack (1976)
postulated that rather than decrease in immigration rate with distance, “ecological
impoverishment” explains the decrease in species numbers on remote islands. This
is very doubtful because birds are able to utilize broad resource types.
Colonizing organisms may affect the members of the indigenous fauna either
as competitors/enemies (e.g., rats) or as vectors of disease organisms. Island birds
are particularly vulnerable: 80 percent of all birds that became extinct in historical
times were island birds, even though the number of island species is less than 10
percent of all bird species. A comparatively rapid faunal turnover is taking place on
islands independent of the arrival of man. Mayr (1940i, 1965n, 1967f) established
this important evolutionary phenomenon and formulated the simple rule that
the percentage of endemic island species increases with the size of the island at
a double logarithmic rate (Mayr 1965p). The smaller the island the more rapid the
turnover which is due to the extinction of the native element and its continuous
replacement by newimmigrants. Mayr (1942e: 225) stated: “Thereis littledoubt that
[…] well-isolated islands are evolutionary traps, which in due time kill one species
after another that settles on them.” Apparently the genetic composition of island
populations becomes so uniform that any change of environmental conditions
including the introduction of a disease may be fatal.
In a report as chairman of a zoogeography committee of the Seventh Pacific
Science Congress, Mayr (1954j) refuted once more previous attempts to build
landbridges in the Pacific which is an old ocean basin. Volcanic eruptions of the
Tertiary never formed extensive land masses, but merely stepping stones passable
only for organisms capable of transoceanic dispersal.
All Pacific archipelagoes are “effectively accessible” to dispersing birds. In most
of these islands empty niches for birds were filled more rapidly by colonists from
outside than by evolution of endemic taxa in situ (Diamond 1977). However, the
Marquesas, Society and Fiji Islands are sufficiently remote for birds to have permitted
speciation within the archipelago at least in some groups (the pigeons Ptilinopus
mercierii and P. dupetithouarsii on the Marquesas, the kingfishers Halcyon
tuta and H. venerata on Tahiti (Society Islands), and the “flycatchers” Mayrornis
versicolor and M. lessoni on Fiji). Only the Galapagos and Hawaii archipelagoes
are so remote and immigration has been so low that intra-archipelagal speciation
became dominant (“adaptive radiation” of the Geospizidae and the Drepaniidae,
respectively).
Northern Melanesia
During the 1970s, E. Mayr and J. Diamond investigated jointly the biogeography,
ecology and evolution of the island birds of northern Melanesia (Bismarck and
Solomon Islands) regarding the origin of the montane avifauna (Mayr and Diamond
1976f), the species-area relation (Diamond and Mayr 1976a), and the
species-distance relation (Diamond, Gilpin and Mayr 1976d). In these studies
more than half of the work and results are J. Diamond’s. Most montane species