Implications of Island Biogeography

Insularization¹⁴ may have immediate impacts, as I mentioned earlier when we were talking about habitat fragmentation, some species require large tracts of undisturbed habitat for their continued existence. They may be lost almost immediately from small habitat islands. These are short-term insularization effects.

As a result of reduced habitat area, insularization is expected to lead to loss of local species diversity in the long term.

• Barro Colorado Island was formed in 1914 with the flooding of the Panama Canal. Of the 208 birds that bred on the island prior to flooding, 48 have gone extinct.

• At the last glacial maximum, all of the islands on the Sunda shelf--Borneo, Bali, Java, and Sumatra--were part of a much larger southeast Asian landmass. They were probably separated from the Malay Peninsula (and from one another) by 10,000 years ago. Based on the assumption that the species area curve for the present mainland also applied to the islands when they were part of the mainland, we can calculate how many species have been lost from each of the islands (Table 1).

Table 1: Faunal ``relaxation'' on islands of the Sunda shelf.

Island	Area km$^2$	Initial Number of Species	Present Number of Species	Number of Extinctions	Percent Extinctions
Borneo	751,709	153	123	30	20
Sumatra	425,485	139	117	22	16
Java	126,806	113	74	39	35
Bali	5,443	66	19	47	71

The loss of species as a result of insularization often referred to as ``faunal collapse,'' reflecting the animal bias of the people who have worked on the problem. Nonetheless, this aspect of island biogeography is broadly accepted, and is the basis for many estimates of extinction rates.¹⁵ More controversial, and much less certain, is the application of the equilibrium theory of island biogeography to the design of nature reserves.