Q: Why is population relevant to the environment?
Human economic activities impact the environment through land use, freshwater consumption, pollution, and so on. A larger population can increase these pressures, but not always in a linear manner. Environmental impacts depend not only on the size of the population, but also how wealthy those people are, the nature of their consumption, and how those products are produced.
For example, the average Northern European consumes more food and a larger variety of foods than the average West African. However, those two regions actually require a similar amount of cropland, per capita, for food production, because they use very different agricultural technologies.3 Ultimately, population size is an important, but not the only, factor determining human impacts on the environment.