The study of relationships-between organisms or between an organism and their environment

ecology is the study of the relationship between living organisms and their environment. an ecologist is someone who studies those relationships. an ecosystem is a place, such as a rotting log, a forest, or even a schoolyard, where interactions between living and non-living things occur.

explanation:

the answer is true when observing or determining population size, a direct observation is use, in which each organism is counted in a population or in an area. one benefit of this measure is that you get the exact population count but it will take a long time to  determine  the  size  of a  population.