To start, discuss the fact that there are not infinite resources available on Earth. Do this by asking if the kids think this is true - if not, when and how could these resources run out?
Pick several kids to be "resources" and have them join hands to form a circle. (Try to select a number of students that appears to be too few for everyone else to fit inside their circle.) The rest of the kids are "consumers" and one by one the should fit under the arms of the resources and into "earth". As the carrying capacity is reached, the resource circle will break!