This document provides an overview of unit 3 on communities and populations in ecology. It defines key terms like community, predation, competition, and symbiosis. Specifically, it explains that predation regulates populations and can lead to evolution, while interspecific competition can result in extinction or specialization. The three types of symbiosis - mutualism, commensalism, and parasitism - are also defined. Food webs and chains within a community are described. Succession and climax communities are briefly outlined. Students are expected to learn and apply these concepts by creating a food web for their local community.