Explain why it is often necessary for ecologists to estimate the size of a population..?

Question

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Answer 1

Because many times we are unable to identify the numer of individuals exactly.  In some cases we can do an actual count and know exactly how many organisms we have.  It is easy if the animals are large and there are a few of them.  It is hard if they are small or hard to see or the population is frequently changing (immigration and emmigration or births and deaths).  So often times an estimation is the most cost effective way to get an idea or how many organisms are in a population.

Answer 2

Your question has two meanings.

1. Why would an ecologist want to know the size of a population?

The size of a population determines the ecological impact that the population will have, through the consumption of resources and production of waste.

2. Why would an ecologist be forced to estimate rather than measure the size of a population?

All measurements of population are estimates.  It is impossible to account for all members of a population.