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THE SCALE OF ISLAND LOSSES GLOBALLY 293

numbers of island versus continental species, island
forms have had about a 40 times greater probability
of extinction over the period since AD1600 than have
continental species (Johnson and Stattersfield 1990).
Recent palaeontological and archaeological stud-
ies have made it abundantly clear that island

endemics have been extinguished in significant

numbers from islands and archipelagos all over the

world, before the explorations of modern collectors

brought their biotas to the attention of Western

science (Table 11.3). In the case of island birds, at

least, the established prehistoric losses greatly

Box 11.1 The decline of Lake Victoria’s cichlid fishes

Lakes are ‘negative islands’, that is, they are

more or less isolated freshwater areas

surrounded by a hostile land matrix, usually

connected to other lakes through rivers. Lakes

thus behave as islands in many biogeographical

and ecological respects. Long-lasting lakes (e.g.

Lake Baikal in Siberia, the Great Lakes of the

African Rift Valley, and Lake Titicaca in South

America), have provided similar evolutionary

opportunities to oceanic islands, and in the case

of the African Rift Valley lakes (Tanganyika,

Victoria, and Malawi) outstanding illustrations of

radiations of fish species (Box 9.2).

Lake Victoria, shared by Kenya, Tanzania, and

Uganda, is the largest tropical lake and the

second largest freshwater lake in the world,

comprising some 3000 km^3 of water. It is the

unique habitat of c. 300 endemic cichlid fishes,

belonging to the genus Haplochromis, which are

thought to have diversified since the lake

recharged after 12 400BP(Box 9.2).

Unfortunately, much of this endemic richness has

been depleted following the introduction of the

Nile perch (Lates niloticus) in 1954 in order to

increase the size of the fishing catch (Mackay

2002). Today it is considered that at least half of

the lake’s native fish species have gone extinct or

are so severely depleted that too few individuals

exist for the species to be harvested or recorded

by scientists (Groombridge and Jenkins 2002).

Although predation by the Nile perch is

believed to be the major cause of decline,

important additional factors may include

increasing pollution and sediment load, excess

fishing pressure, and possible competition from

introduced tilapiine cichlids (Groombridge and

Jenkins 2002). The lake itself has now become

depleted of oxygen, as a consequence of the

massive alterations in trophic organization within

the lake. A shrimp tolerant to oxygen-poor waters

today provides the major source food for the Nile

perch, which itself forms the basis of a fishery

increasingly focused on the export trade rather

than providing a local protein source.

Table 11.2Summaries of known extinctions from islands, continents, and oceans since c.AD1600 (data from

Groombridge 1992; Steadman 1997a; Primack and Ros 2002). The values for several taxa differ from those given in

the first edition of this book. Figures given by different authorities vary depending on the criteria adopted, so the

values given here should be taken as of largely indicative value

Group Islands Continents Oceans Total % insular

Mammals 51 30 4 85 60

Birds 92 21 0 113 81

Reptiles 20 1 0 21 95

Molluscs 151 40 0 191 79

Insects 51 10 0 61 84

Plants 139 245 0 384 36

Total 504 347 4 855 59