0198566123.pdf

18 ISLAND ENVIRONMENTS

continental fragment islands like Madagascar, New

Zealand, and New Caledonia, are characterized by

high endemism and some interesting ancient

lineages, which biogeographers attribute to their

ancient origins. Yet, the biotas of these ancient

fragments of continent may not necessarily be as

ancient as their rocks suggest. For instance, several

Madagascan mammal taxa such as the Lemurs,

the Tenrecidae, and the Viverridae, although

relatively primitive, are nonetheless estimated to

have diverged from their ancestral lineages during

the Tertiary period, roughly between 45 and

26 Ma: much later than the separation from

Africa. McCall (1997) points to geological evidence

as suggesting that areas of the Mozambique

channel were dry land during this period of the

Tertiary, and that subsequent subsidence has

opened up what is currently a wide clear water

channel. His interpretation has been disputed by

e.g. Rogers et al. (2000), who contend that at best

there were merely a few isolated dots of land in

the channel during this period.

The idea of vanished intercontinental land-

bridges and island arcs across ocean basins to

explain disjunct distributions has in general been

discarded in favour of plate-tectonic mechanisms,

afforced by the processes of long-distance

dispersal known to populate remote oceanic

island groups. However, the evidence of subaerial

exposure on a few seamounts within the

Mozambique channel suggests the possibility that

Madagascar’s mammals may well have used, if

not a solid land bridge, at least a set of now-

vanished stepping stones, in colonizing the island

Rafting Land-bridges

Jump-dispersal Separation by plate tectonic processes

Four hypothetical means of species reaching islands: rafting, land-bridges, jump-dispersal, and the separation of a fragment of continent by

plate tectonic processes (sketch by John Holden from Tarbuck, E. J. and Lutgens, K. (1999) Earth: introduction to physical geology, 6th edn,

© 1999, p. 472, Fig. 19.4. Reprinted by permission of Pearson Education, Inc., Upper Saddle River, NJ)