Natural environments rarely have sharp boundaries, instead blending into one another across transitional areas. These zones of biological overlap often represent dynamic and complex habitats. Understanding these areas is fundamental to ecology because they reveal how different communities interact and how life adapts to shifting physical conditions.
Defining the Ecotone
The term ecotone refers to a zone of junction, or transition area, that exists between two distinct ecological communities or biomes. This concept was first formally introduced in 1904, combining the Greek root eco- (ecology) with -tone, derived from tonos, meaning tension.
An ecotone differs from the core of an ecosystem by featuring species from both adjacent environments. It can be a very narrow, clearly defined boundary, or it can stretch across a broad geographical area. Unlike a sharp, arbitrary edge, the ecotone represents a measurable gradient of environmental and biological change. This transitional zone is distinct from an ecocline, which refers only to a gradual shift in a single physical factor, such as a change in salinity or temperature over distance.
Distinctive Characteristics
Ecotones are characterized by a unique set of measurable biological and physical traits that distinguish them from the surrounding ecosystems. The most celebrated of these traits is the “edge effect,” an ecological phenomenon where the population density and species richness are greater within the ecotone than in either of the communities it separates. This increased diversity occurs because the zone provides access to resources from two different habitats simultaneously.
These zones support unique organisms known as edge species, which are specially adapted to the fluctuating, intermediate conditions of the ecotone. Edge species may not survive in the core habitats on either side. The environment within the ecotone often exhibits sharp environmental gradients, meaning physical factors like moisture, light, or temperature can change rapidly over a short distance.
Types of Ecotones and Real-World Examples
Ecotones occur across all major environmental classifications, connecting terrestrial biomes, aquatic systems, and interfaces between the two. A common and highly productive example is the estuary, which forms where a freshwater river meets the saltwater ocean. This aquatic ecotone is defined by a steep salinity gradient and supports unique communities, such as mangrove forests or salt marshes, adapted to brackish water.
Riparian zones, the strips of vegetation found along a riverbank, represent a terrestrial-aquatic ecotone. These zones transition from the fully saturated conditions of the river to the drier upland soil, supporting a distinct mix of water-tolerant trees and plants.
On a larger scale, the transition between a forest and a grassland is a classic terrestrial ecotone. This boundary, which can be thousands of kilometers wide, is often determined by regional differences in rainfall and fire frequency.
Vertical ecotones are also prominent, particularly the montane ecotones found on mountain slopes. The timberline, or tree line, is a clear example, marking the upper limit where environmental conditions become too harsh for continuous forest growth. Human activity can also create artificial ecotones, such as the sharp boundary between a natural forest and an adjacent agricultural field.
Importance in Ecology
Ecotones maintain landscape-level ecological processes and biodiversity. They act as essential corridors, facilitating the movement of species and the exchange of genetic material between separate populations. This gene flow is important for the resilience of species facing environmental changes. Ecotones are often considered biodiversity hotspots, contributing disproportionately to regional species counts. They also function as natural buffer zones that protect core ecosystems; for instance, coastal ecotones like mangrove forests absorb storm energy, shielding inland areas.