What Is an Ecosystem?
How living things and their environment form connected communities — from a puddle to an entire ocean.
More Than Just Animals and Plants
An ecosystem is a community of living things interacting with each other and with their nonliving environment. It includes every organism — from the tallest tree to the tiniest bacterium — plus the air, water, soil, sunlight, and temperature that surround them. An ecosystem can be as large as the Amazon Rainforest or as small as a rotting log in your backyard.
The key idea is that everything in an ecosystem is connected. Change one part, and the ripple effects spread through the entire system.
Biotic vs. Abiotic Factors
Scientists divide ecosystem components into two categories. Biotic factors are the living parts: plants, animals, fungi, bacteria, and all other organisms. Abiotic factors are the nonliving parts: sunlight, temperature, water, soil, air, and minerals. Both work together to shape what can live in a particular place and how those organisms behave.
For example, a desert ecosystem has intense sunlight and very little water (abiotic factors), so only organisms adapted to extreme heat and drought — like cacti, scorpions, and kangaroo rats — thrive there (biotic factors). Change the rainfall, and the entire community changes.
Energy Flow: Who Eats Whom
Energy enters most ecosystems through sunlight. Producers (plants and algae) capture that energy through photosynthesis and store it in their cells. Consumers get energy by eating — herbivores eat plants, carnivores eat other animals, and omnivores eat both. Decomposers break down dead organisms and recycle nutrients back into the soil for producers to use again.
This flow of energy creates a structure called an energy pyramid. Producers form the wide base, primary consumers sit above them, secondary consumers above that, and top predators at the narrow peak. Each level contains less energy than the one below it.
Types of Ecosystems
Earth hosts an enormous variety of ecosystems grouped into broad categories called biomes. Terrestrial biomes include tropical rainforests (warm, wet, incredibly biodiverse), deserts (dry, extreme temperatures), temperate forests (four distinct seasons), grasslands (wide open, dominated by grasses), and tundra (cold, frozen ground). Aquatic ecosystems include freshwater (lakes, rivers, wetlands) and marine (oceans, coral reefs, deep sea).
Each biome has its own characteristic climate, soil type, and community of organisms that have adapted to those specific conditions over thousands of years.
Balance and Disruption
Healthy ecosystems exist in a state of balance. Predators keep prey populations in check, decomposers recycle nutrients, and producers replenish the oxygen supply. But ecosystems are sensitive. Removing a single species — called a keystone species — can cause a cascade of changes. When wolves were removed from Yellowstone National Park, elk populations exploded, they overgrazed the riverbanks, trees disappeared, streams eroded, and dozens of other species declined.
When wolves were reintroduced in 1995, the entire ecosystem began recovering. This famous example shows how deeply interconnected every part of an ecosystem truly is.
Why Ecosystems Matter to Us
Ecosystems provide ecosystem services that humans depend on: clean air, clean water, pollination of crops, decomposition of waste, climate regulation, and natural resources like food and timber. Protecting ecosystems isn't just about saving animals — it's about maintaining the systems that support human life, too.
Coral reefs cover less than 1% of the ocean floor but support about 25% of all marine species. They're sometimes called the "rainforests of the sea" because of their incredible biodiversity — a single reef can be home to thousands of different species of fish, invertebrates, and algae.
Last reviewed: April 2026