Question

Island biogeography theory states that the number of species on an island is determined by immigration rates of new species to the island and extinction rates of species on the island. Immigration rates to an island decline as its distance from the mainland increases and extinction rates decrease with increasing island size. When the immigration and extinction rates on an island are equal, the number of species on the island reaches equilibrium.

Which of the following gives the correct combination of distance (near and far) and area (small and large) of islands where species ($S_1$-$S_4$) occur ?

• A. $S_1$- near island, small island
• B. $S_2$ - near island, large island
• C. $S_3$ - small island, far island
• D. $S_4$ - near island, large island

Answer 1

Answer: (D)

The island equilibrium model describes the number of species on an island based on the immigration and extinction rate of species on that island. It is observed that as the number of species increases, the rate of immigration begins to decrease. This is because as more and more species migrate, the chances for intraspecific competition increases. Also, as a new place is colonised, the rate of extinction increases because there is more competition. Further, first factor that affects immigration and extinction rates is how close the island is to the mainland. The mainland is the source of new immigrants to the island. Assuming all new species have to immigrate to the island from the mainland, closer islands will have more species on them than far islands, just because closer islands are easier to reach. Extinction is lower on islands close to the mainland because of the likelihood of immigration. Another factor is the size of the island. Larger islands have more space than smaller islands, so there are likely to be more resources available for species to use.