Earth’s ecosystems are best described as interconnected systems where living organisms and their physical environment interact to cycle energy and nutrients. They range from vast ocean basins covering 71% of the planet’s surface to a rotting log on a forest floor, and they exist at every scale in between. What unites them all is a simple principle: living things (plants, animals, fungi, bacteria) depend on nonliving elements (water, sunlight, soil, temperature) and on each other to survive.
What Makes Something an Ecosystem
Every ecosystem has two fundamental building blocks. The living components, called biotic factors, include everything from microscopic soil bacteria to apex predators. The nonliving components, called abiotic factors, include water, temperature, sunlight, minerals, and atmosphere. What turns these ingredients into an ecosystem is the interaction between them: plants pulling nutrients from soil, animals eating those plants, decomposers breaking down dead material and returning it to the soil. Remove or change one component and the entire system shifts.
Ecosystems also operate at wildly different scales. A puddle in a tree hollow is an ecosystem. So is the Amazon rainforest. So is the entire biosphere, the thin shell of life wrapped around the planet. Ecologists study patterns that recur across these scales, because the same basic processes of energy transfer, nutrient cycling, and competition shape ecosystems whether they’re a square meter of grassland or an entire ocean basin.
How Energy Moves Through Ecosystems
Every ecosystem runs on energy, and nearly all of it starts with the sun. Plants, algae, and certain bacteria capture sunlight and convert it into food through photosynthesis. These are the producers, the foundation of almost every food chain on the planet. Herbivores eat the producers and become primary consumers. Carnivores that eat herbivores are secondary consumers, and those that eat other carnivores are tertiary consumers. At the top sit apex predators, which have no natural predators of their own.
Energy transfer between these levels is inefficient. Only about 10% of the energy at one level passes up to the next. The rest is lost as heat or consumed by decomposers: fungi, bacteria, and earthworms that break down dead organisms and recycle their nutrients back into the soil or water. This 10% rule is why food chains rarely have more than four or five levels, and why large predators like tigers need enormous territories to find enough food.
The Eight Major Land Ecosystems
Scientists classify Earth’s land environments into eight major biomes, each defined primarily by temperature range and rainfall.
- Tropical rainforests sit near the equator and are the most biodiverse ecosystems on land. Temperatures hold steady between 20°C and 34°C year-round, and annual rainfall ranges from 125 to 660 cm. Even their driest month can exceed the total annual rainfall of a desert.
- Savannas are tropical grasslands with scattered trees, shaped by seasonal drought and frequent fires.
- Subtropical deserts receive minimal rainfall and experience extreme temperature swings between day and night.
- Chaparral features dense, drought-resistant shrubs in Mediterranean-type climates with dry summers and mild, wet winters.
- Temperate grasslands have rich soils and moderate rainfall, but not enough to support dense forest.
- Temperate forests experience four distinct seasons with trees that drop their leaves in autumn.
- Boreal forests stretch across northern latitudes and have a simple two-layer structure: a canopy of conifers and a ground layer. Their needle-covered floors decompose slowly, leaving nutrient-poor soil.
- Arctic tundra is the coldest biome, with average winter temperatures around −34°C and a growing season of just 50 to 60 days. It exists in the far north and at high mountain elevations above the tree line.
Aquatic Ecosystems Cover Most of the Planet
Water covers roughly 71% of Earth’s surface, and the oceans hold about 96.5% of all the planet’s water. Aquatic ecosystems fall into three broad categories based on salt content: marine (saltwater), freshwater, and brackish (a mix of the two, found in places like estuaries where rivers meet the sea).
The ocean itself is divided into distinct zones. The sunlit upper layer, extending down about 200 meters, is where photosynthesis happens and where most marine life concentrates. Below that, light cannot penetrate, creating a permanently dark zone. The deepest reaches, at 4,000 meters or more, are the abyssal zone, home to organisms adapted to crushing pressure, near-freezing temperatures, and total darkness. Closer to shore, the intertidal zone is battered by waves and tides, while the neritic zone extends over the continental shelf and supports rich fisheries and coral reefs.
Freshwater ecosystems include lakes, ponds, rivers, streams, and wetlands. Wetlands alone encompass marshes, swamps, bogs, mudflats, and salt marshes, each with distinct communities of plants and animals. Though freshwater covers a small fraction of the planet’s surface, it supports a disproportionately high number of species. The southeastern United States, for example, is a global center for freshwater biodiversity thanks to its ancient geological terrain and more than 235,000 miles of waterways.
Where Biodiversity Concentrates
Species are not spread evenly across the planet. They cluster in biodiversity hotspots, regions where geography, climate, and evolutionary history have produced unusually high concentrations of life. Tropical rainforests top the list on land, packing more species into a single hectare than some entire countries contain. Coral reefs play a similar role underwater, supporting roughly a quarter of all marine species despite covering less than 1% of the ocean floor.
Geography creates biodiversity in surprising ways. California’s diversity stems from its extreme variability in landforms, climate, and soil types, all compressed by coastal mountains into a relatively small area. Hawaii’s isolation in the middle of the Pacific allowed species to evolve in directions found nowhere else on Earth. Alabama, not typically associated with ecological richness, is home to exceptionally diverse freshwater life because of its complex geological terrain and three major river basins. These examples illustrate that biodiversity hotspots aren’t always in the tropics.
Human-Made Ecosystems
Not every ecosystem is natural. Cities, farms, and managed landscapes are artificial ecosystems dominated by human structures and sustained by human-directed flows of energy and materials. A wheat field is an ecosystem, just a radically simplified one: a single crop species replaces hundreds of native plants, and fertilizer and irrigation substitute for natural nutrient cycles.
The most ambitious experiment in building an artificial ecosystem was Biosphere 2, a sealed glass structure in Arizona that enclosed a tropical forest, coral reef, desert, savanna, and agricultural area under one roof. Eight people lived inside for two years, testing whether a self-sufficient ecosystem could be engineered from scratch. The project revealed just how difficult it is to replicate the balance that natural ecosystems maintain on their own.
Ecosystems Under Pressure
Earth’s ecosystems are changing faster than at any point in recent history. Over the past 50 years, studied wildlife populations have declined by an average of 73%. More than 3,500 wild animal species are now threatened by climate change, and documented cases of climate-driven population collapses are increasing.
Forests are taking heavy losses. Global tree cover loss hit 29.6 million hectares in 2024, the second-highest figure on record. Fire-related destruction in tropical primary forests spiked 370% over the previous year, reaching 3.2 million hectares. That single year of primary forest loss released roughly 3.1 gigatons of CO₂-equivalent emissions, about 8% of total human-caused emissions for the year. By 2050, up to 47% of the Amazon rainforest could face compounding disturbances severe enough to trigger irreversible shifts in the ecosystem.
Oceans are absorbing the consequences too. Ocean heat content reached a record high, contributing to the largest coral bleaching event ever recorded, which affected 84% of the world’s reef area between January 2023 and May 2025. Ocean acidity also hit its highest level on record, with evidence suggesting a critical planetary boundary for ocean acidification was crossed in 2020. Coral reefs, already among the most sensitive ecosystems on Earth, face compounding threats from warming, acidification, and pollution simultaneously.