All the organisms and the environment they live in form an ____. The ____ is made up of populations of different ____. Each population lives in a ____ within the ecosystem. Measuring population size by counting all the organisms in an area is often ____. However, you can ____ by taking samples using a ____. ____ can be used to estimate populations and the changes in ____ (non-living) factors. ____ can estimate population size of animals in a habitat.

Sampling

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