- A population is a group of organisms of the same species that live in a specific area and interbreed. Populations have characteristics like size, density, and dispersion. - Population growth can be modeled. The simple model looks at birth and death rates, while the logistic model accounts for limited resources constraining growth. - Organisms have different life history strategies. R-strategists reproduce rapidly when conditions allow, while K-strategists have slower growth near an environment's carrying capacity. - An ecosystem includes all the organisms in an area along with their environment. Energy flows through ecosystems, with producers like plants capturing energy and consumers eating producers or other organisms.