Ecotone and Energy Flow: Ecology and Ecosystem


Ecotone, a transitional area of vegetation between two different plant communities, such as forest and grassland. It has some of the characteristics of each bordering biological community and often contains species not found in the overlapping communities. An ecotone may exist along a broad belt or in a small pocket, such as a forest clearing, where two local communities blend together.

The ecotones may be narrow (1 km) or very wide, extending to large areas up to 100 kms.
In other way, a zone of transition between two or more adjacent communities is known as an ecotone. The common examples of ecotone are following:
• the border between a forest and a grassland,
• area between a soft-bottom and a hard-bottom marine substrate,
• the bank of a stream running through a meadow,
• estuary lying between river and sea, etc.
The word was coined from a combination of eco(logy) plus -tone, from the Greek tonos or tension – in other words, a place where ecologies are in tension.

Biotic Aspects of the Ecotones
In the ecotone, the environmental conditions are intermediate between the two adjacent communities, but the structure of ecotone is strikingly different from its adjacent communities. Ecotones offer more food and shelter than its main communities. Due to this, ecotone has higher species diversity and in many cases higher population than either of the main communities.
This diversity that is not directly controlled by the climate or fundamental environmental factors, but because of the migrations of individuals of different species populations from both communities in search of food and other resources. Further, a number of special populations can become adapted specifically to the ecotone.
As a rule ecotones contains more species and denser population that either of the neighboring communities. This is called Edge Effect. There are certain species which are completely restricted to the ecotone and they are characterstic of any ecotone and are celled Edge Species.
A common example of the edge effect in action can be seen in those species of owl that live in ecotone between forest and grassland. They depend on forest trees for nesting, and they do their hunting in the grassland, where they deepend on field rodents for food.
In manmade communities such as agricultural field the ecotone between the field and the forest may act as a refuse for animal species formarly found in the plowed area, as well as for other plants such as weeds. Ecotone of this type is also the prime habitat of many species of insects, game birds and mammals.
Ecotones are particularly significant for mobile animals, as they can exploit more than one set of habitats within a short distance.

Physical Factors behind the Creation of Ecotones
Changes in the physical environment create ecotones. Such changes in the physical environment may produce a sharp boundary, as in the example of a shoreline or the interface between areas of forest and cleared land.
Alternatively, a more gradually blended interface area will be found, where species from each community will be found together as well as unique local species. Mountain ranges often create such ecotones, due to the wide variety of climatic conditions experienced on their slopes. They may also provide a boundary between species due to the obstructive nature of their terrain; Mont Ventoux in France is a good example, marking the boundary between the flora and fauna of northern and southern France. Most wetlands are also ecotones.

Energy Flow: Ecology and Ecosystem

The transfer of energy from one trophic level to another level is called energy flow. The flow of energy in an ecosystem is unidirectional. It flows from producer level to the consumer level and never in the reverse direction. Hence energy can be used only once in the ecosystem. A large amount of energy is lost at each trophic level. It is estimated that 90% of the energy is lost when it is transferred from one trophic level to another. Hence the amount of energy available decreases from step to step.
Ecosystem requires a constant input of energy as every component of an ecosystem is regularly dissipating energy. Two laws of thermodynamics govern this flow of energy.
1. According to first law of thermodynamics energy can be transferred as well as transformed but is neither created nor destroyed.
2. According to second law of thermodynamics every activity involving energy transformation is accompanied by dissipation of energy. Except for deep hydrothermal ecosystems.

Models for Energy Flow
1. Single Channel Energy Flow Models
Energy flow through an ecosystem was explained by E. P. Odum stating that as the flow of energy takes place, there is a gradual loss of energy at every level, thereby resulting in less energy available to next trophic level. The flow of energy takes place in a unidirectional manner through a single channel of green plants to herbivores and carnivores. The energy which is passed to the next trophic level is never reverted back to the lower level. Thus organisms at higher trophic level always depend upon the organisms at lower trophic level for their energy requirements. Thus due to one way flow of energy the ecosystem will collapse, if the primary source (the sun) is not there.


2. Y-shaped Model of Energy Flow

It is also called double channel model, given by H. T. Odum. In nature, both grazing and detritus food chains operate in the same ecosystem. However, sometimes grazing food chain (open sea ecosystem) or detritus food chain (in forest ecosystem) predominates. In nature, these two chains seem to be separated, but are not so. Some dead animals which were a part of grazing food chain become incorporated in the detritus food chain like the faces of grazing animals. When shown in the form of diagram, this interdependence looks like the letter Y. This double channel model shows the passage of energy through these two food chains. This model is more real and practical than single channel model, as it separate the grazing and detritus food chains in both time and space.


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