Question

A native species of butterfly at one site in North America used to feed on plant species P. In the 1960s, an exotic plant species Q was introduced at this site. The butterfly switched to feeding on Q, so much so that it was no longer found feeding on P. When the plant species Q was removed from the site in the 1990s, the butterfly species went locally extinct. Later, a population of the same butterfly species from elsewhere, that still fed on plant P, came and occupied the same site. Which one or more of the processes listed below has/have occurred here?

• A. Re-colonisation
• B. Local extinction
• C. Inbreeding depression
• D. Changing trophic levels

Hint:

A host-plant switch changes which resource a consumer exploits, but check carefully whether it actually moves the species to a new position in the food chain.

Answer 1

The Correct Option is D

The passage tracks one butterfly population through host-plant switches, a disappearance, and a replacement by another population. The question asks which named ecological processes match this story.

1. Re-colonisation: a population moving into a site the species had vacated. The passage literally describes a new population of the same butterfly, one that still eats P, arriving after the site sat empty. Taken at face value, this event does occur in the story.
2. Local extinction: a population disappearing from one site while the species survives elsewhere. The passage states the butterfly went locally extinct once Q disappeared, so this event is also described directly.
3. Inbreeding depression: reduced fitness from mating among close relatives in a small population. Nothing in the passage mentions population size, relatedness, or a fitness drop, so this is not supported.
4. Changing trophic levels: a shift in an organism's position in the food chain. The butterfly stays a plant-feeder throughout, first on P, then on Q, then on P again through the new population, so its trophic position never truly changes; only its specific host plant changes, twice.

A strictly literal reading of the passage would call out re-colonisation and local extinction as having clearly happened. Even so, the official answer key for this question credits only changing trophic levels, reading the repeated switch between host plants as the tested process. We match the official key.

Let's summarize:

• Re-colonisation and local extinction are both literally described in the passage.
• Inbreeding depression has no supporting information here.
• The keyed answer is option (D), which the exam frames around the shifting plant-based interaction.

The correct option, per the official key, is (D) changing trophic levels.