Island Biogeography theory/ Mc Arthur and Wilson theory

 Island Biogeography Theory

Island is any isolated habitat that is totally separated from other of its kind like oceanic island, habitat

island etc. Islands as ecological systems have such salient features as simple biotas and variability.

in isolation, shape, and size.

Biogeography is the study of geographical distribution of species and the reasons for the pattern of

distribution.

Island Biogeography is the study of pattern in the distribution of species on islands which are influenced

by evolutionary and ecological processes related to characteristic island like isolation and area.

Mac Arthur and Wilson first published the theory of island biogeography. Island Biogeography theory

also known as Mac Arthur and Wilson theory was developed to explain the species richness. Also called

as equilibrium theory of island biogeography.

Background: It is well established that the number of species on islands decreases as island area decreases.

Probably the most obvious reason why larger areas should contain more species is that larger areas

typically encompass more different types of habitats. However, MacArthur and Wilson (1967) believed

this explanation to be too simple and they published their theory describing the factors determining

number of species on an island.

Basis of Island Biogeography theory: In their equilibrium theory of island biogeography, they argued:

(i)that island size and isolation themselves played important roles – that the number of species on an

island is determined by a balance between immigration and extinction.

(ii) that this balance is dynamic, with species continually going extinct and being replaced (through

immigration) by the same or by different species.

(iii) that immigration and extinction rates may vary with island size and isolation.

So island biogeography theory holds that the number, of species on an island is determined by the

equilibrium between the immigration of new species and the extinction of those species already present. As

rates of immigration and extinction depend on the size of islands and their distance from the mainland, a

general equilibrium can be diagrammed, as presented in Figure 9-8.

Four equilibrium points are there representing :

S1-A small, distant island predicted to have few species-S1

S2-A small, nearby or a larger, distant island, predicted to be intermediate in terms of species richness.

S3-A large, nearby island that should support many species.

S4-A Large and far island at distant

This model demonstrates the interplay of isolation, natural selection, dispersal, extinction, and speciation

that has attracted the attention of population ecologists and evolutionary biologists to island

biogeography for more than a century.

Explanation

These patches, which vary in size—large and small—and distance—near and far—fit the theory of island

biogeography as proposed by MacArthur and Wilson (1963). For example, a patch of forest may be

located in a “sea” of agricultural cropland, isolated from other patches in the landscape. The effect of

patch size and isolation appears to have a pronounced influence on the nature and diversity of species 

within these landscape patches. Preston (1962) formalized the relationship between the area of the island and the number of species present as follows:

                                                     S= c^z

where S is the number of species, A is the area of the island or patch, c is a constant measuring the

number of species per unit area, and z is a constant measuring the slope of the line relating log S and log

A (in other words, z is a measure of the change in species richness per unit area). Thus, the theory of island biogeography states that the number of species of a given taxon (insects, birds, or mammals) present on an island or within a patch represents a dynamic equilibrium between the rate of immigration of new colonizing species of that taxon and the rate of extinction of previously established species.

Case 1: Taking Immigration first

Starting with point contain no species at all. The rate of immigration of species will be high, because any colonizing individual represents a species new to that island. However, as the number of resident species rises, the rate of immigration of new, unrepresented species diminishes. The immigration rate reaches zero when all species from the source pool. The immigration graph is drawn as a curve, because immigration rate is likely to be particularly high when there are low numbers of residents and many of the species with the greatest powers of dispersal are yet to arrive. This condition gives graph 1.1a.

Case 2: Taking Extinction

The rate of species extinction on an island (Figure 21.11b) is bound to be zero when there are no species there, and it will generally be low when there are few species. However, as the number of resident species rises, the extinction rate is assumed by the theory to increase, probably at a more than proportionate rate. This is thought to occur because with more species, competitive exclusion becomes more likely, and the population size of each species is on average smaller, making it more vulnerable to chance extinction. Similar reasoning suggests that extinction rates should be higher on small than on large islands as population sizes will typically be smaller on small islands (Figure 21.11b)

Case 3: In order to see the net effect of immigration and extinction, their two curves can be superimposed.

Major predictions based on the equilibrium theory include the following:

(1) there is an equilibrium for a given island biota that is achieved when the extinction and immigration

rates are equal.

(2) The immigration rate is primarily affected by the distance between the island and its continental

colonizing source, and the extinction rate varies primarily with island area.

(3) for a given island, the immigration rate decreases and the extinction rate increases with increasing

number of species already on the island.

(4) the number of species at equilibrium increases with island area and this increase should be faster on

more remote islands.

(5) the number of species at equilibrium decreases with island-continent distance and this decrease should be faster on smaller islands.

(6) the species turnover rate at equilibrium is greater on less distant and smaller islands.