A short GRE question that puzzles me Exotic insect pests can produce both short-and long-term effects on forest ecosystems. Shortterm effects include the disturbances directly associated with the action of the pest, which may cause the defoliation, loss of vigor, or death of trees. Long-term effects are primarily mediated by changes in tree species composition and the consequent alterations of forest structure, productivity, and nutrient uptake. Exotic pests are more efficient than most abiotic disturbances (e.g., fire or wind) at producing long-term changes in species composition. Pests often target specific tree species and, if they become established, they usually remain as permanent components of the ecosystem. Shifts in forest species composition ramify through the ecosystem in many ways because tree species have different, often unique properties. The author of the passage mentions the “unique properties” of tree species primarily in order to help explain   A. why pests tend to target specific tree species B. why pests can have long-term effects on entire ecosystems C. how pests contribute to shifts in forest species composition D. how pests are able to become established in an ecosystem E. how some tree species are able to withstand the effects of pests I think it's B, the answer given online is C (usually correct, but not 100% and not from the official test organization either). My logic is, because the "shifts in forest species composition ramify through the ecosystem", the author wants to emphasize that pests have ultimate effect on the ecosystem. Thank you very much for help!