Ecotone

An ecotone is the highly dynamic boundary between two disparate ecosystems: vegetation types and biomes. Ecotones can be very narrow and sharply defined, such as a terrestrial-aquatic boundary, or they can represent a broad transition between differing biomes, such as a gradual conversion between grassland and forest. Often, the flora and fauna found on either side of an ecotone will not be similar to one another, and species favoring one side of the ecotone will not fare as well on the other. Because of the variability in vegetation cover [Page 535]and abiotic factors characterized by ecotones, biodiversity across an ecotone tends to be higher than in relatively homogeneous habitats on either side of the ecotone. Ecotones can be formed by natural processes, such as floods, fires, and volcanic activity, but increasingly, human land use activities have created ecotones. Examples of anthropogenic ecotones are agricultural-pastoral boundaries, urban-rural spaces, and parks or protected land adjacent to lands used to meet human resource needs.

Ecotones typically favor certain types of vegetation and fauna over others. Species requiring a high degree of stability and habitat continuity will not be successful in or near an ecotone; species adapted to disturbances or boundaries can better exploit the resources found in these niches. Abiotic factors such as erosion, sediment deposition, snow accumulation, nutrient availability, salinity, and temperature are all affected by ecotones and tend to differ from one side of the boundary to another. Ecotones also can create microclimates, which further favors certain species over others. For example, a meadow surrounded by forest will be characterized by greater temperature extremes and more rapid changes in temperature than the surrounding forest. Additionally, direct sunlight reaching the ground will cause faster evaporation, and potentially dry meadow soils faster than those in the forest.

One concern that land managers express over ecotones is that they favor the success of invasive species. Human land uses such as road building or agricultural activities create ecotones with wholly different competition and predation regimes. Areas of undisturbed natural habitat are typically more resistant to invasive species, but patches where parts of previous ecosystems have been removed prove particularly prone to colonization by invasives, such as fire ants, kudzu, tumbleweed, or buffelgrass.

In areas characterized by naturally occurring habitat variability or “patchiness,” land managers can use certain techniques such as prescribed burns, tree harvesting, or grazing by animals to create desired ecotones. The mosaic of desert, grassland, riparian areas, and forests in the western United States are typical of this highly fragmented, ecotone-rich land cover. Because ecotones are a naturally occurring part of ecosystems, it is necessary to ensure the continued existence of threatened species. The maintenance of ecotones can be one aspect of human management in protected areas.

Ecotones are typically far from stable, and undergo spatial and temporal changes. Sea level changes impact the placement of the marine-terrestrial ecotone. Forest succession at an abandoned beaver pond represents a changing ecotone. Tree lines migrate up and down mountainsides as a result of changes in temperature and precipitation. Recent concern has focused on ecotone dynamics as a result of anthropogenic effects, including climate change. Ecotones are necessary for a certain amount of ecosystem function, which humans rely on. For example, sea level rise may obliterate many productive salt marshes, coral reefs, and estuaries, which provide nursery areas for countless species of marine, freshwater, and anadromous species, leading to a decline in productivity of fisheries and other human uses.

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