This document discusses population density and carrying capacity. It defines population density as the number of individuals per unit area and provides examples of calculating population density. An activity is described where students measure the area of a garbage bag and determine how many people can fit on it at different levels of folding to understand the concept of carrying capacity. The document then discusses limiting factors on population growth and defines carrying capacity as the maximum population size an environment can sustain. It provides examples of limiting factors for human populations like water, food, living space, pollution and disease.