A biome is a large-scale ecological unit, such as a tropical rainforest or desert, characterized by its specific climate and the plants and animals adapted to it. While maps often depict these biomes with sharp, defined lines, the reality on the ground is frequently more complex. This raises a fundamental question about how these massive biological communities meet: are the borders between them clear and abrupt, or do they blend into one another across vast transitional landscapes?
The Concept of an Ecotone
The transition area between two different biomes is known as an ecotone. Rather than an abrupt line, an ecotone is a zone of gradual change where the ecological characteristics of one biome slowly give way to those of another. These zones can vary tremendously in scale; an ecotone might be relatively narrow, like the space between a forest and a grassland, or it can be wide, stretching for many miles where a forest thins out into a desert. For instance, the transition from a dense forest to an open savanna involves a slow decrease in tree density and a corresponding increase in grasses and shrubs over a large area.
Ecotones are zones of tension where conditions are intermediate between the two adjacent ecosystems. This unique environment gives rise to a phenomenon called the “edge effect.” The edge effect describes how the boundary zone often supports a higher density and diversity of life than either of the neighboring biomes alone. This increased biodiversity is because the ecotone contains species from both adjacent communities, as well as specialist species that are adapted specifically to the transitional conditions. An example is the area where a river meets the land, which supports a mix of terrestrial and aquatic species, plus unique amphibians that thrive in that specific “edge” environment.
Factors That Create Gradual Transitions
The gradual nature of most biome boundaries is driven by slow changes in underlying environmental, or abiotic, factors. Climate is a primary driver, as temperature and precipitation patterns do not typically change abruptly. Instead, they shift across vast geographical distances in what is known as a climate gradient. As one travels from the equator towards the poles, for example, average temperatures steadily decrease, causing a slow transition from tropical, to temperate, and finally to polar biomes.
A similar gradient occurs with elevation. Hiking up a tall mountain will pass through several different vegetation zones that mirror the biome changes one might see over thousands of miles of latitude. This is because the temperature cools and atmospheric conditions change with increasing altitude, creating distinct layers of life on the mountainside. These transitions from deciduous forests at the base to coniferous forests, and then to alpine tundra near the peak, are almost always gradual.
Soil composition and topography also contribute to these blended boundaries. Soil type can change slowly across a landscape due to underlying geology and centuries of plant decomposition, influencing which plant communities can thrive. A region with sandy, well-drained soil will support different vegetation than an area with heavy, clay-rich soil, and the transition between these soil types is rarely a sharp line. The gradual shifts in these physical and chemical conditions are directly responsible for the corresponding slow change in the plant and animal communities that define a biome.
When Biome Boundaries Are Distinct
Although gradual transitions are common, sharp biome boundaries can and do exist. These abrupt changes are often caused by major geographic features or significant human activity. A dramatic example is a coastline, which forms a clear border between a terrestrial biome and a marine one.
Mountain ranges can also create sharp divides through a phenomenon known as the “rain shadow” effect. As moist air is forced up one side of a mountain, it cools and releases its precipitation, supporting a lush, wet biome like a temperate rainforest. By the time the air descends on the other side, it is dry, creating an arid desert or shrubland biome. This results in two starkly different biological communities existing in close proximity, separated by the mountain crest.
Human activity is an increasingly common cause of distinct biome boundaries. The creation of a large agricultural field right up to the edge of a forest or wetland creates an artificial, sharp border where a natural, gradual ecotone once existed. Similarly, clear-cutting a section of forest creates an abrupt, induced edge between the remaining forest and the cleared land. These anthropogenic boundaries can have significant impacts, affecting the microclimate and species movement in the remaining natural habitat.