Q8 - Over the last thousand years, plant species native

[ohthatpatrick]

Question Type:
Strengthen (an explanation)

Stimulus Breakdown:
CURIOUS FACT: for the last millenium, plants have been going extinct more often on islands than on mainlands.
EXPLANATION: Island plants don't have defenses against large land mammals, because mammals don't arrive on islands until they're colonized by humans.

Answer Anticipation:
What would bolster this explanation? They didn't spell it out, but I'm assuming that humans HAVE gotten to most of these islands?

We'd like any answer that reinforces the story that island plants have been killed by the large mammals that humans brought.

Most correct Strengthen answers on causal arguments are some form of Covariation answer (f.e. "the islands with the most humans and large mammals are the ones with the higest rate of extinction").

Correct Answer:
D

Answer Choice Analysis:
(A) This is about absolute numbers. The argument was comparing RATES of extinction between islands and mainlands.

(B) We don't care how well mainland plants perform if they move to island habitats. They're mammal-adapted, so we're not surprised they do well.

(C) This weakens by providing an ALTERNATE explanation for why island plants have gone extinct.

(D) YES, this increases the plausibility of the explanation. If the theory is that when humans arrive, large mammals arrive, and these heretofore oblivious plants get decimated, then this answer choice heavily corroborates that storyline.

(E) This weakens by undermining the plausibility that island plants have gone extinct from mainland land mammals eating them.

Takeaway/Pattern: This is a Covariation answer (when the CAUSE arrived, the EFFECT tended to arrive]. That's always our #1 suspect when we're Strengthening a causal explanation.

#officialexplanation

[RitikaK173]

I would think that human colonization would lead island plants building their defenses, so the rate of extinction of native plants would decrease after human colonization. I originally crossed out answer D for this reason.

[ohthatpatrick]

Well, just remember, your job here isn't to scrutinize whether you think an answer choice is likely to be true or false.

They are all to be treated as true, as the question stem instructs.

IF YOU KNOW THAT
native island plants tend to start going extinct way more rapidly right after humans arrive

DOES THAT SUPPORT
the idea that native island plants are going extinct because they're getting eaten by large land mammals?


Yes, because we were previously told that usually "when humans arrive is when large land mammals arrive".



As a secondary thought, your suspicion of how native plants would react to the arrival of humans and large land mammals is correct -- they would start to evolve defenses against the large land mammals. However, evolutionary adaptations can take thousands of years to manifest. Often, populations don't have enough time to "react to" a new environmental stressor. For example, if an alien dropped a sulfur bomb on the Earth and made the atmosphere way more sulphuric than it currently is, we would probably just go extinct.

If, perchance, there are already some humans with a genetic mutation that allows them to survive while breathing in a higher concentration of sulfur, then THOSE humans might live. And if they can find each other and breed and pass along that saving DNA mutation, then the human species could potentially survive in its adapted sulfur-tolerant form.

But if there aren't already any (or enough) humans with a sulfur-tolerant genetic mutation, the sulfur bomb could easily wipe out all of us before enough generations of breeding could roll the genetic dice enough times to see if we luck into some mutation that increases our tolerance of sulfur.

[WesleyC316]

So I picked (B) because I thought it was blocking out an alternative explanation that the dramatic climate on the islands are the cause of extinction of island plants, because the mainland plants are doing just fine there.

I crossed (D) out because I thought it was suggesting human activities are the actual cause, thereby weakening the argument.

What am I missing here?

[Yu440]

I crossed (D) out because I thought it was suggesting human activities are the actual cause, thereby weakening the argument.

What am I missing here?

Me too... Are we suppose to assume the populations of land mammals definitely established on the island after human colonization?

[ohthatpatrick]

There's been some love for (B), thinking that it would rule out the idea that "inhospitable" climate is killing off plant species on islands.

Is that a very plausible alternative story?
The climate would have had to change, because these island plants already evolved on the island so, they'd be already adapted to the island's climate.

If the climate has changed a bunch for these islands, hasn't it also changed for the mainland? Why wouldn't we expect species loss in both areas?

We'd be creating a pretty specific alternate explanation story for (B) to rule it out.
We'd be investing the possibility that over 1000 years the climate of islands (all over the world) have changed to where native plants can no longer live there, whereas over those same 1000 years the climate of mainland hasn't changed enough to mess with native plants there?

Also, I'm not sure that saying we transplanted a given species to an island would even show that the climate of the island is still hospitable to native plants. After all, say the climate of an island got much cooler than before and so many of its heat-loving native plants died off. Meanwhile, we've been able to establish a cool-weather fern on this island, since the new climate is perfect for that outsider.

------------ on to (D) -------

Strengthen / Weaken has nothing to do with proving / refuting. It's just about nudging plausibility in one direction or another.

Based on (D), it still might be humans or the large mammals they bring with them that are causing the native plants to die.

But if someone has a theory that "native island plants are being ruined by their exposure to large mammals + they're not exposed to large mammals until humans arrive", then that theory would assume that you wouldn't see major losses to these native island plants until AFTER the arrival of humans.

(D) confirms that assumption, so it strengthens the plausibility of the "large mammals are the culprit" theory.

Hope this helps.