This document discusses population ecology and how populations change over time. It uses the example of sea otters, kelp forests, and sea urchins to demonstrate how their populations interact. When sea otters were hunted, the sea urchin population increased and kelp decreased, but reintroducing sea otters caused the reverse effect. Population characteristics like density, growth rate, and geographic distribution are influenced by many factors and impact ecosystems. Population growth can be exponential at first but levels off into a logistic curve as resources become limited.