omidsa wrote:
While the most abundant and dominant species within a particular ecosystem is often crucial in perpetuating the ecosystem, a “keystone” species, here defined as one whose effects are much larger than would be predicted from its abundance, can also play a vital role. But because complex species interactions may be involved, identifying a keystone species by removing the species and observing changes in the ecosystem is problematic. It might seem that certain traits would clearly define a species as a keystone species; for example, Pisaster ochraceus is often a keystone predator because it consumes and suppresses mussel populations, which in the absence of this starfish can be a dominant species. But such predation on a dominant or potentially dominant species occurs in systems that do as well as in systems that do not have species that play keystone roles. Moreover, whereas P. ochraceus occupies an unambiguous keystone role on wave-exposed rocky headlands, in more wave-sheltered habitats the impact of P. ochraceus predation is weak or nonexistent, and at certain sites sand burial is responsible for eliminating mussels. Keystone status appears to depend on context, whether of particular geography or of such factors as community diversity (for example, a reduction in species diversity may thrust more of the remaining species into keystone roles) and length of species interaction (since newly arrived species in particular may dramatically affect ecosystems).
1. The passage mentions which of the following as a factor that affects the role of
P. ochraceus as a keystone species within different habitats?
(A) The degree to which the habitat is sheltered from waves
(B) The degree to which other animals within a habitat prey on mussels
(C) The fact that mussel populations are often not dominant within some habitats occupied by
P. ochraceus(D) The size of the
P. ochraceus population within the habitat
(E) The fact that there is great species diversity within some habitats occupied by
P. ochraceus Passage: Keystone Species
Question: Detail Factor
The Simple StoryFirst, the passage defines a particular term: a keystone species is one whose effects are much larger than would be predicted from its abundance. In other words, this type of species has a larger impact on the ecosystem than would normally be expected.
How do you know for sure that a species is a keystone species? According to the passage, you can’t just remove that keystone species from the environment to see what happens. “It might seem” that a certain characteristic would define a keystone species, but the language it might seem signals that the author is going to disagree. Sure enough, the author provides a certain example about a possible keystone predator and then shows how that predator may or may not be a keystone species.
The author finally concedes that this question depends on two things: context (with an example given) and length of species interaction (with further elaboration given).
Sample Passage MapThis passage contains just one paragraph, so think about how to delineate different parts of it on your Map. Here is one way to map this passage. (Note: abbreviate as desired!)
KS: >> effect than expctd
Mid: How to ID KS? Hard.
2 things: context + interaction length
The “Mid” designation means “the whole middle of the passage is about this.” KS is an abbreviation for keystone species and ID is an abbreviation for identify.
Step 1: Identify the QuestionThe language mentions which of the following in the question stem indicates that this is a Detail question. Your job is to find this detail in the passage: what factor affects the role of the P thing as a KS in different habitats?
Step 2: Find the SupportScan for the P thing; it’s introduced in line 12.
“It might seem that certain traits would clearly define a species as a keystone species; for example, Pisaster ochraceus is often a keystone predator because it consumes and suppresses mussel populations, which in the absence of this starfish can be a dominant species. But such predation on a dominant or potentially dominant species occurs in systems that do as well as in systems that do not have species that play keystone roles. Moreover, whereas P. ochraceus occupies an unambiguous keystone role on wave‐exposed rocky headlands, in more wave‐sheltered habitats the impact of P. ochraceus predation is weak or nonexistent, and at certain sites sand burial is responsible for eliminating mussels.”
Step 3: Predict an AnswerThe first two sentences talk about P as a KS, but the example doesn’t discuss different habitats until the third sentence, beginning moreover. In one location, P has an unambiguous keystone role, but in another area, P’s predation is weak or nonexistent, so it can’t be playing a keystone role there.
What aspect of the habitats influence this difference? In the first location, there are a lot of waves. In the second, the area is sheltered from the waves.
Step 4: Eliminate and Find a Match(A) CORRECT. This choice reflects the circumstances described in the passage. When the area is exposed to the waves, P can be a keystone species. When the area is sheltered from the waves, P is much less central to the system.
(B) The example does not address what other animals are preying on the mussels or how that would affect P’s status as a KS.
(C) The passage does state that P consumes and suppresses mussel populations, which could be dominant in the absence of this starfish, suggesting that mussels are not dominant when P is present. However, the next sentence indicates that such predation on a potentially dominant species in not a guarantee of status as a KS in a given habitat.
(D) This choice is a trap. While it makes logical sense that a large population would have a significant impact on an ecosystem, the first sentence specifically defines a KS as one whose effects are much larger than would be predicted from its abundance. In other words, the species is having a much larger impact than you would expect based on its population size—but the effects don’t necessarily vary based on population size itself.
(E) The passage mentions species diversity (line 27) only as a general example of how a reduction in diversity could impact keystone roles in an ecosystem. It does not address species diversity specifically with respect to the P examples.
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