This document provides an overview of ecosystems and ecological concepts. It defines key terms like population, ecology, ecosystem, producers, consumers, decomposers, biotic and abiotic factors. It describes different types of natural ecosystems like forests, grasslands, deserts, tundra, taiga and aquatic ecosystems. It also discusses the relationships between living and non-living things within an ecosystem and the levels of organization. The document introduces concepts like the law of tolerance, zones of tolerance and intolerance, and limiting factors.