A biome is defined by its broad climate and vegetation structure, but within that broad category, local differences in soil, terrain, water, and disturbance history carve out dozens or even hundreds of distinct ecosystems. Think of a biome as a climate zone with a general “look,” like grassland or tropical forest. The ecosystems inside it are the specific, functioning communities of plants, animals, and microorganisms shaped by conditions on the ground.
Biomes and Ecosystems Operate at Different Scales
Ecologist Robert Whittaker defined a biome as a grouping of ecosystems on a given continent that share similar vegetation structure and major environmental features. That definition contains the answer to the question: a biome is, by design, a collection of ecosystems, not a single one. It’s a broad classification based on regional climate patterns like average temperature and rainfall, while an ecosystem is a much smaller unit defined by the specific organisms living there and the local conditions they depend on.
The World Wildlife Fund’s mapping system recognizes just 14 major habitat types (biomes) across the planet, but breaks the land surface into 867 distinct ecoregions within them. That’s an average of roughly 62 ecoregions per biome, and each ecoregion itself can contain multiple ecosystems. The tropical forest biome alone includes evergreen rainforest, seasonal deciduous forest, tropical cloud forest, and mangrove forest as distinct sub-types, each with its own species composition and ecological dynamics.
Terrain Creates Different Worlds in the Same Climate
Two locations 10 kilometers apart can sit in the same climate zone yet have completely different ecosystems because of topography. A south-facing slope receives more direct sunlight and stays warmer and drier than a north-facing slope just across a valley. A ridgetop is windswept and well-drained, while the valley floor collects water and cold air. These differences in elevation, slope angle, and orientation produce measurably different conditions for plant growth.
Research on watershed ecosystems has shown that a single river basin can be divided into distinct zones using a topographic position index that combines elevation and slope steepness. Areas with low, flat terrain support one set of land cover types, while steeper, higher-elevation areas support entirely different communities. In one studied watershed, the distribution of woodland, grassland, and cropland shifted dramatically across topographic zones, with woodland concentrated in steeper upper reaches and cropland dominating the flatter lowlands. Same regional climate, same biome classification, but the terrain alone sorted the landscape into distinct ecosystems.
Soil Differences Under the Same Sky
Even on perfectly flat ground, the underlying rock and soil can vary enough to support completely different plant communities. One patch of land might sit on limestone bedrock that produces alkaline, calcium-rich soil, while a neighboring patch sits on granite that weathers into acidic, nutrient-poor soil. These edaphic (soil-related) differences determine which plants can grow, which in turn determines which animals, fungi, and microbes establish themselves.
Soil drainage matters just as much as chemistry. Sandy soils drain fast and stay dry, favoring drought-tolerant species. Clay-heavy soils hold water and can become waterlogged, creating wetland conditions. Within a single temperate grassland biome, you can find dry upland prairie, wet meadow, and riparian woodland all within walking distance of each other, purely because of how water moves through different soil types. In the Arctic, the tundra biome contains diverse mosaics of communities structured by gradients in substrate, hydrology, and permafrost patterns. Steppe-tundras occur only where the substrate is calcareous and the climate is continental, while wet sedge meadows form where drainage is poor.
Climate Varies Locally, Not Just Regionally
The climate data that defines a biome is a regional average, but no organism actually lives in the regional average. Plants and animals experience microclimates: the temperature and humidity right at ground level, under a canopy, or beside a stream. Vegetation itself reshapes these conditions. Plants cool and humidify the air around them through evaporation from their leaves, a process that exchanges heat between the plant surface and surrounding air. A dense stand of trees can be several degrees cooler and significantly more humid than an open clearing 50 meters away.
These microclimate differences compound over time. A shaded, humid pocket supports shade-tolerant species and moisture-loving fungi that couldn’t survive in the sun-baked clearing next to it. The clearing, in turn, supports sun-loving grasses and the insects that depend on them. Both spots share the same biome classification, the same weather station data, but function as separate ecosystems because of the conditions organisms actually experience at ground level.
Water Availability Shapes Ecosystems at Every Scale
Precipitation is one of the primary factors that defines a biome, but water availability within a biome varies enormously based on proximity to rivers, lakes, and underground water sources. Global research has identified a critical relationship between rainfall and ecosystem function, with a turning point at roughly 671 millimeters of annual precipitation. Below that threshold, both climate and soil factors together drive ecosystem function. Above it, climate becomes the dominant force. This means that in drier biomes, small differences in soil moisture retention can have outsized effects, creating pockets of lush growth next to sparse, drought-stressed communities.
In forest ecosystems specifically, soil factors dominate ecosystem function in wetter regions, while climate factors dominate in drier ones. In grasslands and shrublands, climate is the primary driver regardless of precipitation level. These different responses mean that the same amount of variation in local conditions produces different patterns of ecosystem diversity depending on the biome, which is part of why some biomes contain more ecosystem variety than others.
Disturbance and Succession Keep Ecosystems Mixed
Fires, floods, storms, and landslides regularly reset patches of a biome to an earlier stage of development, and these recovering patches become their own temporary ecosystems. This process, called ecological succession, is one of the most powerful forces creating ecosystem diversity within a single biome.
When a patch of forest burns, it doesn’t immediately return to forest. It first becomes a field of bare soil colonized by grasses and wildflowers. Those early colonizers add nutrients to the soil and create conditions for shrubs to establish. The shrubs eventually give way to pioneer trees, and only after decades does the area return to mature forest. At any given moment, a fire-prone landscape contains patches at every stage of this progression, each functioning as a distinct ecosystem with its own species composition.
University of Chicago botanist Henry Chandler Cowles first documented this pattern in the 1890s along the shores of Lake Michigan. Sand dunes closest to the water supported only beach grass. Dunes further inland had cottonwoods. Behind those were pines, and further still were mature oak forests. The physical distance from the lake represented time: each dune community was a snapshot of a different successional stage. In areas where disturbance is frequent, like fire-prone regions of the western United States, mature forests exist alongside grassy meadows with scattered trees. The landscape never fully settles into one ecosystem type because new disturbances keep resetting different patches on different timelines.
Biological Feedbacks Add More Variety
Once different ecosystems establish themselves within a biome, the organisms in each one actively maintain the conditions that distinguish their patch. Trees in a forest ecosystem keep the understory cool and shaded, suppressing grass growth and favoring shade-tolerant seedlings. Grasses in an adjacent meadow ecosystem form dense root mats that prevent tree seedlings from establishing, keeping the area open. Beavers dam streams and create pond ecosystems where forest once stood. These biological feedbacks make ecosystem boundaries surprisingly stable even when the underlying climate and soil could theoretically support either community.
The result is that a single biome, defined by its broad climate envelope, becomes a patchwork of ecosystems shaped by terrain, soil, water, microclimate, disturbance history, and the organisms themselves. Each of these factors varies independently across the landscape, and every unique combination produces conditions suited to a different community of life. The biome sets the general rules. Everything else determines what actually grows where.