Island dispi'rsal 



Origin of IS'orth Amrrican fauna 



Dispersal of animals from continents to islands 

 and from one island to another presents special prob- 

 lems. 



Many of the world's prominent islands occur on 

 the continental shelf and are separated from the main- 

 land only by shallow seas. At times of land emer- 

 gence, as when glaciers lock up quantities of water as 

 snow and ice enough to lower the level of the seas, 

 these islands become connected by land bridges to 

 the mainland, and dispersal of forms occurs. The 

 British Isles have been thus connected to Europe : 

 Japan, to Korea and Siberia : Sumatra, Java, and 

 Borneo to Malaya ; Xew Guinea and Tasmania, to 

 Australia ; and Newfoundland, to Labrador. On the 

 other hand, the Bermudas, Azores, Hawaiian and 

 other small Pacific islands, and possibly New Zealand, 

 the West Indies, and Madagascar, could not have had 

 mainland connections and thus have 'received their 

 present faunas by some means other than overland 

 dispersal (Chapter 20). 



Islands adjacent to continents, unless very small 

 or long separated, generally have faunas similar to 

 that on the nearby mainland. Oceanic islands are 

 more difficult to colonize, however, and often have 

 unique unbalanced faunas or chance assemblages of 

 species. Larger islands generally have a more varied 

 fauna than do small islands ; mammals, amphibians, 

 and fresh-water forms are often absent or scarce. Fly- 

 ing birds, bats, lizards, insects, snails, and small in- 

 vertebrate forms easily transported on rafts or blown 

 in by strong winds are better represented. 



Since island faunas are small in point of popula- 

 tion compared with the mainland, there is less com- 

 petition between species. A single genus or family 

 may adaptively radiate into new niches to form a vari- 

 ety of species or races, as did the group of finches in 

 the Galapagos Islands observed by Charles Darwin 

 and later studied by Lack (1945), and insects, honey- 

 creepers, and other forms in the Hawaiian Islands 

 (Zimmerman 1948). Because of lack of competitors 

 and predators, primitive animals isolated on islands 

 may survive long after their relatives on the mainland 

 have perished, as has, for instance, the reptilian Sphc- 

 nodon on New Zealand. Confinement of a species to 

 a limited range permits extensive inbreeding so that 

 the population becomes more homozygous in its vari- 

 ous genetic traits, hence much less adaptable to new 

 situations than are larger heterozygous populations. 

 Traits that would be eliminated by predation pressure 

 on mainlands sometimes become established in pop- 

 ulations on islands. All of these conditions render 

 oceanic island species liable to extinction by invasion 

 of mainland forms, and renders them impotent to re- 

 invade the mainland. 



A dry land bridge connected North America to 

 .\sia across the Bering Sea during most of the Ter- 

 tiary and during glacial jieriods in the Pleistocene 

 (Hopkins 1959). During early Tertiary, Alaska had 

 a temperate climate, but as the climate became pro- 

 gressively colder in late Tertiary, there was increased 

 filtering of animal groups having access to the bridge. 

 At the present time all land connection has, of course, 

 disappeared, although a few forms, particularly birds, 

 are able to iiop the narrow straits. Across this bridge 

 came a heavy traffic of Asiatic mammals (Simpson 

 1947, Savage 1958), birds (Mayr 1946), reptiles, 

 amphibians, and fish (Darlington 1957), modern in- 

 sects (Ross 1951, 1953), and other groups. There 

 was also some dispersal from North America into 

 Asia, but this reverse movement was much less strong 

 than the one from Asia into North America. 



During the early part of the Tertiary, all major 

 groups of mammals appear to have moved freely 

 across the land corridor so that there was considerable 

 similarity between the two continental faunas. How- 

 ever, from early Eocene to early Oligocene, new 

 orders, families, and subfamilies arose that were dis- 

 tinctive to each continent. Later in the Tertiary, there 

 were fewer groups dispersing across the bridge, and 

 these were largely confined to genera within the 

 iiigher taxa already established on each continent. 



There is no evidence for a land bridge between 

 North America and Europe via Greenland and Ice- 

 land since early Tertiary, although it is possible that 

 one existed earlier. A few animal forms may have 

 been able to hop from island to island across the 

 North Atlantic, but the fauna of North America and 

 Europe are not sufficiently similar to suggest any 

 recent close connection of the continents. Some pan- 

 tropical forms may have crossed the Pacific Ocean 

 from island to island to reach the Western Hemi- 

 sphere, but former land bridges across either the 

 Pacific or South Atlantic oceans are highly unlikely. 



A continuous land connection occurred between 

 North and South American in late Cretaceous- Paleo- 

 cene time. It was probably at that time that ancient 

 types of mammals, birds, reptiles, amphibians, insects, 

 and other groups got into South America from the 

 north and differentiated into distinct families and 

 other taxonomic categories (Dunn 1931). During 

 most of the Tertiary, no land bridge existed between 

 the two continents, although there were scattered is- 

 lands separated by relatively narrow water gaps which 

 some groups, particularly birds, may have been able 

 to use. The land connection now in existence was 

 apparently formed in the late Pliocene or Pleistocene. 



Previous to late Pliocene there were about 29 

 families of land mammals confined to South America 



Dispersal, migration, and ecesis 155 



