Final answer:
The 'target effect' refers to the expectation that larger islands will have more immigration due to their size presenting a larger target, thereby attracting more colonizing species than smaller islands.
Step-by-step explanation:
The statement that describes the "target effect" is: D. larger islands are expected to have more immigration. This concept is part of island biogeography theory, which tells us that larger islands present a bigger "target" for species dispersing from the mainland or other islands, hence they are more likely to receive more immigrants. In contrast, smaller islands are less likely to be colonized due to fewer species finding and settling on them, less geographic diversity, and a higher chance of extinctions.
Island biogeography also relates to the concept that smaller populations are more vulnerable to genetic drift. Genetic drift occurs more quickly in small populations because random changes in the frequency of alleles have a greater impact when there are fewer individuals. This is illustrated in the statement that the more individuals in a population, the larger and more stable is its gene pool.