A food web is a network of linked consumer-prey interactions. The apex predators in food webs are typically large animals that feed on other species but have no predators themselves. In his first lecture, Dr. James Estes goes over concepts that can be used to understand the effects of apex predators on their environment. To investigate these interactions, scientists look for “perturbations” that result in a change in the number of apex predators. Perturbations could be due to natural events such as disease, or human activities such as hunting. By comparing ecosystems with or without apex predators, ecologists can determine the direct and indirect impact of these animals on their environments.