Types of Succession

(1)    Primary succession: The succession process which starts in those base areas where there was no vegetation and animals earlier, is called primary succession. Such sites may be fresh lava flows, volcanic ash plains, newly formed sand dunes of flood plains, etc. it takes a very long period for the development of proper soil and arrival of pioneer vegetation communities. Slowly a vegetation community begins to develop passing through the seral stages such as herb community, scrub community, forest community or preclimax and finally climax community (climatic climax) . The vegetation supporting it is called climax vegetation. In the intervening stages all those rules of evolution such as intra and inter-specific competition, survival of fittest, natural selection, invasion of new plant and animal species, changes brought in by external forces etc work in full force.

(2)    Secondary succession: It refers to development of sequence of vegetation in those areas which had vegetation cover earlier but now have been rendered nude due to destruction of vegetation, either partly or completely, through either natural or manmade process e.g. Development of new forest on an abandoned land after shifting cultivation or jhum.

Ecosystem goods and Services / Ecological Services

Ecosystems provide many goods and services that are of vital importance for functioning of the biosphere, and provide basis for the delivery of tangible benefits to human society. These ecological services or benefits can be divided into following categories:

(1)    Supporting services:

Biotic Succession / Ecological Succession

The progressive replacement of one vegetation community in a given habitat by the other is called ecological succession. Succession implies sequential replacement of one plant community or the ecosystem by the other. A plant community is group of plants which are already adapted to a given habitat. In other words ecological succession is an evolutionary sequence of development of ecological community or ecosystem. The biotic succession continues through different phases called as ‘sere’ until a mature and equilibrium community called as climax community is formed.

Features of ecological succession

(1)    It has a particular direction.

(2)    It results from modification of physical environment by the community .Thus it is community controlled even though the environment sets the pattern and rate of change and often sets the limits as to how far development can go.

(3)    It ends in a stabilized ecosystem in which maximum biomass and symbiotic function between organisms are maintained per unit of available energy flow.

(4)    With succession following changes occur---

(a)    Diversity of species increases

(b)   Complex food chains develop

(c)    Biomass increases

(d)   Energy flow increases

(e)   New habitat niche are created

(f)     The climax or stable community controls or becomes buffer against physical forces like temperature, moisture, wind, light, etc.

The first organisms to become established in an ecosystem undergoing succession are known as pioneers. The stable community that is formed at the end of succession is called as climax community. The intermediate stages such as grassland, scrub, shrubs are called sere.

Ecosystem instability / Ecological Niche

Ecosystem instability refers to that state when an ecosystem is unable to adjust with environmental changes. The happens when the changes are continuous and enormous and these changes exceed the resilience or capacity of the ecosystem. The factors responsible for ecosystem stability or instability are always viewed in terms of ecosystem resilience. If the environmental changes exceed the ecosystem resilience, ecosystem instability is caused but when ecosystem is such that it can withstand environmental changes, ecosystem stability is maintained.

The ecosystem instability can occur due to natural factors such as massive volcanic eruption or climatic changes like ice age. Ecosystem instability is also induced due to manmade activities. For example,

(1)   Destruction of Himalayan ecosystem due to deforestation, overgrazing etc. leading to increased weathering and erosion of soil.

(2)   Replacement of natural vegetation and animal species by cultivation or urban land use.

(3)   Introduction of exotic plant species such as water hyacinth.

(4)   Changing the proportion of atmospheric gases, etc.

Ecological Niche

Ecological niche refers to the functional role and position of a species in relation to other species in the given ecosystem. In a natural ecosystem, several species of plant and animal communities perform different roles in getting food and thus each community is confined to certain locality having certain functions. Such locality having ideal environmental conditions for the survival of a species is called niche. The species of a given niche may not service in other niches. The ecosystem stability depends upon diversity of a niche. Greater the niche diversity, greater the stability of ecosystem. The ecosystem becomes unstable if one or more species are eliminated because then niche becomes empty and other species which were dependent on the eliminated species food now become vulnerable to elimination.

Ecological Stability

Ecosystem or ecological stability refers to capability of a natural ecosystem to apply self-regulating mechanisms so as to return to a steady state after an outside disturbance. Ecosystem stability meant there is a balance between production and consumption of each element in the ecosystem. The number of each species in a mature ecosystem generally remains constant.

The self regulatory mechanisms are also called homeostatic mechanisms .These are mainly negative and positive feedback mechanisms. For example. If the population of insects in a small ecosystem area increases greatly due to favorable climate, then food supply falls short of demand due to increased competition. Eventually some insects’ die of starvation and slowly the ecosystem returns to its original size.

Negative feedback occurs when the result of a process influences the operation of the process in such a way as to reduce changes. Negative feedback tends to make a system self-regulating. It can act as a stabilizing force and reduce the effect of fluctuations. Negative feedback loops where just the right amount of correction is applied in most timely manner can be very stable, accurate and responsive. Negative feedback controls the rate of a process to avoid accumulation of the product.

Oxygen Cycle

Similarly in the oxygen cycle the imbalances are likely to emerge due to industrial combustion since 1860’s industrial revolution and decreasing vegetation cover.

Hydrogen or Hydrological Cycle

It is also called hydrological cycle or water cycle as most of the hydrogen circulates in the biosphere in the form of H₂O- moisture or water. Hydrological cycle at global scale involves following sequential processes as Evaporation →Condensation → Precipitation (rainfall, snowfall) → Percolation into the ground and surface runoff, both eventually transferring to water bodies and oceans →Evaporation again.

Water is an important substance in the biosphere because

(i)                 It is able to dissolve almost all substances;

(ii)               It has great ability to store heat;

(iii)             It takes part in the nourishment of organisms and

(iv)              It helps in the circulation of elements in the biosphere etc

Any disturbance in the hydrological cycle through cumulative effects of decreased ground water storage due to urbanization, deforestation, decreased water level on the continents may have hazardous effects, though its full knowledge is not yet derived.

Carbon Cycle

The carbon circulates within biosphere in two cycles. In gaseous cycle it is present as free gas in the form of carbon dioxide (CO₂) and as a gas dissolved in the waters of sea and land (HCO₃⁻). In non-gaseous or solid cycle, it is present as biomass (carbohydrates i.e. - CH₂O compounds) in living/dead matter and as hydrocarbon compounds (petroleum, coal, natural gas) and as mineral carbonates in carbonate rocks (limestone, corals, i.e. - CaCO₃ compounds).

This carbon enters the biotic world through the action of autotrophy by the process of photosynthesis. The carbon then again returns back to the atmosphere and water by (I) respiration (CO₂); (ii) decay (producing CO₂ if oxygen is present methane (cH₄ if it is not).

The total amount of carbon in solid phase normally remains stored as it is. The carbon dioxide assimilated by plants is stored in the woody tissues of plants. This is called as organic reservoir of carbon. Forests are significant reservoirs of biological carbon of the biosphere (Carbon sink). The respiration by the biota releases CO₂ back into the atmosphere. Carbon is returned to the atmosphere at the same rate as it is removed, but in recent years the carbon concentration in the atmosphere has been increasing because of deforestation and burning of organic matter such as woods.

Simultaneously carbon from sedimentary (solid) phase is released into atmosphere mainly due to increased burning of fossil fuels and partly by weathering of rocks and volcanic eruption. In normal carbon cycle, the uptake and return of CO₂ occurs very slowly over a longer geological time scale. But the carbon cycle is being disturbed due to anthropogenic activities i.e. burning of fossil fuels, wood fuels and deforestation (forests are largest consumers of carbon dioxide). The gradual increase in concentration of carbon dioxide in the atmosphere would intensify green house effect of the atmosphere, resulting into global into global warming and climatic changes.

Biogeochemical Cycles

 One of the important manifestations of disturbances in the biogeochemical cycles is climatic extremes, recovery of ecosystems after extreme events or the climate change induced global warming itself. This gives the idea of importance of biogeochemical cycles. The circulation of matter (nutrients) in the biospheric ecosystem is accomplished in a series of cyclic pathways which are collectively known as biogeochemical cycle. A biogeochemical cycle is the cycling of chemical elements through the earth’s atmosphere, oceans and sediments as it is affected by the geological and biological cycles. It can be described as a series of compartments of storage reservoirs and pathways between these reservoirs. Biogeochemical cycles can be studied as cycles of individual elements such as carbon cycle. Hydrogen or water cycle, nitrogen cycle etc. Or broadly as Hydrological cycle, mineral cycle, etc.

Ecological Pyramid

This concept is often associated with description of food chains and food web. An ecological pyramid is an illustration of the reduction in energy as we move through each feeding (tropic) level in the ecosystem. The base of the pyramid is large since ecosystem’s energy factories (the plant producers) are converting solar energy into chemical energy through photosynthesis. A food chain can also depict a reduction in energy at each tropic level if the arrow drawn at different levels; continue to be reduced in size.

Ecological pyramid can be represented in three ways:

(1)   Pyramid of numbers: it can be generated by counting all the organisms at different feeding levels. This is a difficult task as we are not just identifying each species in the ecosystem but also counting how many of each species are present. Sometimes this may not work. For example, one tree (a producer) can represent an ecosystem and harbor numerous populations of herbivores and carnivores. Thus, the bottom of pyramid would be very small and not broad.

(2)   Pyramid of biomass: