In this unit, students explore how biotic and abiotic factors affect population growth. They explore the effects of competition on growth of different species of molds. Students have actively growing cultures with which to make an inoculation mix of mold spores—reproductive gametes—and mycedium—vegetative cells. They inoculate sterile plates containing nutrient agar with a single mold species or with a combination of two molds, and then measure the density and types of molds growing in each dish. Based on initial results and outside reading, students formulate one or more hypotheses about the factors that favor growth of one mold species over another and design and set up their own experiment to test one of the hypotheses.