Guam has 40 times more spiders than nearby islands have. Biologists argue that this is a consequence of the accidenta...

Victoria on May 7, 2020

Hi @Isaac-Jeantete,

Happy to help!

The biologists conclude that Guam has more spiders than nearby islands as a result of the introduction of the brown tree snake and the resulting loss of bird species.

Why? Because the brown tree snake eliminated 10 of 12 native bird species and many birds prey on spiders.

So, we are looking for the answer choice which strengthens the conclusion that the loss of native bird species (caused by the introduction of the brown tree snake) caused the proliferation of spiders on Guam.

(B) is incorrect because the method used to survey spider populations is irrelevant to the biologists' conclusion.

(C) is incorrect because it has no impact on the biologists' conclusion. If there are more spiders on Guam than nearby islands, it would make sense that there would also be more spiderwebs. However, this does not work to support the conclusion that the loss of bird species is the cause of the increase in spider populations.

(D) is incorrect because it does not impact the biologists' conclusion. The proliferation of two species does not support the conclusion that the loss of birds is the cause of the concurrent increase in spender populations.

(E) is tempting as a failure to eradicate brown tree snakes would limit the possibility for the recovery of Guam's bird species; however, the passage notes that the snake eliminated 10 of 12 bird species, thereby precluding the possibility of recovery. Additionally, we do not need to support that the snake is still impacting Guam's birds, we are focused on the causal relationship between the elimination of bird species and the increase in spiders.

(A) is correct because it supports the conclusion that the decrease in birds caused the increase in spiders. If birds compete with spiders for insect prey, then the number of birds on the island would always be tied to the number of spiders as both would be impacted by the availability of insect prey. The loss of birds would result in both less predation on spiders (as articulated by the passage) and an increase in the availability of food for spiders. This supports the conclusion that the loss of bird species due to the brown tree snake was the cause of the increase in spider populations on Guam.

Hope this is helpful! Please let us know if you have any further questions.

Isaac-Jeantete on June 1, 2020

Thank you!

guibrasil1993@gmail.com on May 24, 2021

Can you please explain why C is wrong. From what I understood in the stimulus Guam has more spiders than nearby islands and the absence of birds due to snake is offered as an explanation. It also says that birds use the spider webs in their nests. If there are more spider webs would that not support the contention that snakes worked like the scientist hypothesized? That is the snakes removed the birds which were removing the spider webs leaving more webs than in other islands. Could you please explain why this is wrong. Thank you.

Emil-Kunkin on October 3, 2022

C does not strengthen the argument that the extra spiders is due to the snake-induced lack of birds. This reinforces the fact there are more spiders, but it does not explain why. The fact that there are more spider webs is a direct consequence of there being more spiders. regardless of the cause of there being more spiders, the additional webs is a pretty inevitable consequence.