The two natural resources that will be most affected as people try to meet the growing demand for food are - land and water.
As the food demand grows according to the population, more land is needed to plant the food and vegetation. Hence, more water is needed to plant these food items. So overall, land and water are required to fulfill these requirements.
You take any of the "signs of change in populations of organisms and their environments". For imagine, Australopithecus (early man) turning to Homo sapiens. In that way they could tell that you took this journey.
