RC - Lyme Disease - GmatPrep

[alexjohnhumphries]

Can someone please explain why this is not A? No one has attacked that answer to my knowledge. It's just high-level reasoning why B is the answer.

[RonPurewal]

A asks whether the other species (which would compete with the mice as food sources for baby ticks) are found only in the areas also inhabited by white-footed mice.

that 'ONLY' is super duper hugely important here.

because of 'ONLY', the issue raised by A is...
...do those other species live ONLY where the mice also live?
...or, do they live in some places where the mice live, and in some others where the mice don't?
this distinction is irrelevant. we don't care whether the other species have 'a room of their own', so to speak.

--

[RonPurewal]

if you think A is relevant, i think i might know what you're thinking.
specifically, you might be ignoring the word 'ONLY'.

IF YOU IGNORE THE WORD 'ONLY', then A asks whether the competing species are found in areas also inhabited by white-footed mice.
now THIS... this would definitely matter. (if the species don't share ANY space, then the other species can't affect the mice.)

...but this is not what the words say.

if 'only' is present but you ignore/miss it, you will ALWAYS emerge with the wrong interpretation. it is impossible to 'get lucky' with such a mistake.
SLOW DOWN.

[Crisc419]

choice (b) presents the possibility that the deer tick population is currently limited by the availability of hosts. if that's the case, then increasing the number of hosts will NOT cause the population to dissipate or spread out -- instead, you'd just get exponential population growth, with the same density of deer ticks on disease-causing hosts (as well as more of them on your newly introduced hosts)

Hello, Ron, i don't understand why you used "with the same density", because in my mind "density" have the same meaning with "percentage".

if the deer tick population is currently limited by the availability of hosts, increasing other hosts without bacteria will increase the population of the deer tick but will not increase the population of the infected deer tick population.Therefore, i think, the percentage of the infected deer tick will decrease.

could you explain a little bit more?

thanks in advance.

Cris

[RonPurewal]

if the additional hosts are mice, they'll become infected with the bacterium.

[JacksonP938]

Here is where I get hung up with B. Hoping someone can help clear this up for me. I think B would be more relevant if the conclusion was that alternate food sources would decrease the proportion of infected ticks.

Let's just keep things really simple and assume before the alternate food sources are increased there are eight pods so-to speak of larvae. Let's assume there are four food sources. Three are white footed mice and one is a squirrel. Let's assume each pod needs exactly one food source to sustain itself and survive. In current state three pods would feed off mice and be infected, while one pod would feed on a squirrel and not be infected. The remaining four pods would die off.

Now let's say we introduce three squirrels. Now we would still have three pods feeding off of three mice and we would still have three infected pods. We would then have four pods feeding off squirrels that would not be infected. Now 7 pods in total would survive through the next iteration. One would die off.

In scenario one we have three infected pods. In scenario two we still have three infected pods. The only change is that the net population of ticks itself increased but the raw quantity of infected pods is the same.

The proportion of infected ticks decreased. 3/4 -> 3/7

Now let's consider the flip side, the population is not limited by food sources. Let's assume again that there are eight pods and eight food sources, 3 mice and five squirrels. Let's assume there is some other natural force driving the population of pods reproduced through each iteration to be the eight we are using as an example. If we introduce two more squirrels there will still be eight pods feeding off the same food sources.

In both scenarios the only factor that could influence the infected populations is the food preferences of the tick populations which would render the knowledge as to whether population was limited by food sources irrelevant.

[JacksonP938]

Here is where I get hung up with B. Hoping someone can help clear this up for me. I think B would be more relevant if the conclusion was that alternate food sources would decrease the proportion of infected ticks.

Let's just keep things really simple and assume before the alternate food sources are increased there are eight pods so-to speak of larvae. Let's assume there are four food sources. Three are white footed mice and one is a squirrel. Let's assume each pod needs exactly one food source to sustain itself and survive. In current state three pods would feed off mice and be infected, while one pod would feed on a squirrel and not be infected. The remaining four pods would die off.

Now let's say we introduce three squirrels. Now we would still have three pods feeding off of three mice and we would still have three infected pods. We would then have four pods feeding off squirrels that would not be infected. Now 7 pods in total would survive through the next iteration. One would die off.

In scenario one we have three infected pods. In scenario two we still have three infected pods. The only change is that the net population of ticks itself increased but the raw quantity of infected pods is the same.

The proportion of infected ticks decreased. 3/4 -> 3/7

Now let's consider the flip side, the population is not limited by food sources. Let's assume again that there are eight pods and eight food sources, 3 mice and five squirrels. Let's assume there is some other natural force driving the population of pods reproduced through each iteration to be the eight we are using as an example. If we introduce two more squirrels there will still be eight pods feeding off the same food sources.

In both scenarios the only factor that could influence the infected populations is the food preferences of the tick populations which would render the knowledge as to whether population was limited by food sources irrelevant.

I understand the following logic: To determine if infected ticks will decline we need to address whether or not the ticks would even feed off the added food sources. The preposition of limitations being driven by food sources does seem to answer that question. For these reasons I think B is the least flawed but I think this is a rare question in which GMAC did not think through the answer in enough detail.

[RonPurewal]

i'm not completely following your argument, but it seems to depend on the idea that a "pod" of larvae can just exist independently -- like, they can just hang out? where? -- until they "find" a host animal on which to feed. that's not how it works.

[AnirbanM72]

If the tick population is currently limited by the food source (if B is "YES"), then when you introduce more food (other host animals without the bacterium) the number of tick will increase because the ticks will feed on the extra food source.

so far, so good...

--

However, since the host animals do not carry the bacterium, the number of ticks that are infected will not increase.

still good...
note "will not increase" --> i.e., the number will remain exactly the same. it certainly won't decrease.

--

Thus, the number of infected ticks will be "diluted".

nope.
as you wrote above, the number of infected ticks will stay the same. it won't go up -- but it won't go down, either.

In contrast, if the tick population is currently not limited by the food source (if B is "No"), then when you introduce more food, the number of ticks will not increase because the ticks already have enough food.

yes, up to there.

--

They are not likely going to feed on the new food source. Thus, the number of infected ticks will remain the same.

no. you are assuming that the ticks will simply ignore the newly introduced host animals.
that's not a reasonable assumption; after all, a host animal is a host animal. the only reasonable assumption here is that the ticks will spread across the entire host population, which now includes the extra host animals.

you are spreading the same number of ticks over a greater population of host animals; therefore, the density of the tick population goes down.
lower density of ticks per host animal + same number of host animals with the bacterium = less bacterium.


Hey Ron,
I am not able to understand the blow-
lower density of ticks per host animal + same number of host animals with the bacterium = less bacterium.
What I thought is that even if more host are introduced, the deer tick will feed on all the available hosts, but the number of infected deer ticks wont go down. the infected deer ticks will remain the same as they were earlier before the introduction of new hosts. So, please help me to understand how can we have the desired effect as mention in the passage?

[Sage Pearce-Higgins]

The scenario Ron was referring to is, what would happen if the number of other, non-bacteria carrying animals were increased? Presumably the white-footed mice (the bacteria carrying animals) would be diluted. Now, if the number of ticks were kept the same, then they too would be spread out over a greater number of animals (all other things equal, of course). This is why it would be good to know whether the number of ticks would likely increase in this scenario. No increase in ticks - same number of ticks - more spread out over more animals - fewer ticks per animal - fewer infected ticks. Or, alternatively, Ticks increase - more animals just means more ticks - no change in ticks per animal - no change in number of infected ticks.