Populations are kept in check by something the ecologists call “the resistance of the environment.” What does the author mean by that phrase?

The author means that popuations are kept in check naturally. These natural forms of population control include the food and water supply, weather conditions, and the presence of predators.

Populations are kept in check by something the ecologists call the resistance of the environment, and this has been so since the first life was created. The amount of food available, conditions of weather and climate, the presence of competing or predatory species, all are critically important.