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Permanent decrease in population would occur due to-

A) Migration B) Natality C) Emigration D) Mortality E) Territoriality .

It's obvious that C) & D) options are possible . But I'm confused about which one is more correct and what is the cause ?

The answer is given : C

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closed as off-topic by rg255, James, MattDMo, WYSIWYG Jun 10 '16 at 20:10

This question appears to be off-topic. The users who voted to close gave this specific reason:

  • "Homework questions are off-topic on Biology unless you have shown your attempt at an answer. For more information see our homework policy." – James, MattDMo, WYSIWYG
If this question can be reworded to fit the rules in the help center, please edit the question.

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    $\begingroup$ please see this post and this post $\endgroup$ – rg255 Jun 10 '16 at 8:49
  • $\begingroup$ Emigration is a permanent phenomenon. Mortality, on the other hand, is dependant on various factors such as epidemics, health, life expectancy, etc. A change in these factors leads to a consequent change in the mortality rate. For instance, if an epidemic plaguing a country is eradicated, then the rate of decrease in population would slow down. Hence, it can lead to an increase in population as compared to the population during the epidemic. $\endgroup$ – Irena Jun 10 '16 at 9:07
  • $\begingroup$ Aside the fac that the post is off-topic, It is definitely a poorly phrased question. Is it really the way the exact way the question was phrased? There was not even the term "size" after "population"? Both C and D sounds equally correct. $\endgroup$ – Remi.b Jun 11 '16 at 9:09
  • $\begingroup$ @ Remi.b : The question was on my MCQ book & i didn't commit any changes as i had doubt with the answer . If you have problem , Do an edit or don't give advices $\endgroup$ – Rupam Saha Jun 12 '16 at 3:35
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I don't know if this is just me, but I'd consider this a pretty badly worded question. For models of population dynamics, the rate at which the size of a population changes over time is normally accepted to be the net balance of four fundamental processes: Birth, Immigration, Death, and Emigration ('BIDE'; see for example here). Birth and immigration increase a population, death and emigration decrease it. Unless you exterminate a species then it's going to be pretty difficult to assert that a population decrease from either source is 'permanent' - new individuals are likely to be born into the local population and/or immigrate eventually.

An example that specifically refutes the assertion that 'emigration results in a permanent decline in population size' would be a source-sink metapopulation, in which emigration repeatedly/constantly occurs from the 'source' population but it doesn't permanently decrease in size.

As a lesser quibble, although 'migration' can be taken to refer to temporary movement/resettling, it is pretty common to consider immigration and emigration to each be components of migration.

So I would certainly argue that for the question as asked, 'D' is at least as valid an answer as 'C', but in both cases it is impossible to say a permanent decrease would occur. At the same time, if it was changed to 'could occur', you could make a case that 'A' could be a valid answer (since 'migration' includes emigration).

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I feel answers A & C are more obvious. option D is countered by Birthrate. also, migration is a temporary affair, check its meaning (An instance of moving to live in another place for a while). whereas emigration is to permanently move to a new location and populate that area.

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  • $\begingroup$ Through Migration , i tried to mean Immigration $\endgroup$ – Rupam Saha Jun 11 '16 at 8:59

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