Final answer:
The equilibrium theory of island biogeography states that the number of species on an island is determined by a balance between immigration and extinction rates. Islands farther from a mainland have lower migration rates, while smaller islands have lower immigration rates and less geographic diversity. Both factors contribute to lower species numbers and a higher probability of extinction on these islands.
Step-by-step explanation:
According to the equilibrium theory of island biogeography, the number of species on an island or in a fragment of habitat is dependent upon a balance between the rates of immigration and extinction.
Islands farther from a mainland have lower migration rates leading to lower species numbers. Additionally, smaller islands have lower immigration rates and less geographic diversity, limiting the potential for speciation and supporting smaller populations increasing the probability of extinction.