FUNCTIONS OF AN ECOSYSTEM

An ecosystem is a community of living organisms interacting with each other and their environment. The function of an ecosystem is broad, vast and completely dynamic. It can be studied under the following headings.

1. Energy flow
2. Nutrient cycling
3. Ecological succession or ecosystem development

1. Energy Flow 

1.1. Trophic Level Interaction

A Trophic level is the representation of, energy flow in an ecosystem. The Trophic level of an organism is the position it occupies in a food chain.

Trophic-level interaction deals with how the members of an ecosystem are connected based on nutritional needs.

                   Trophic levels  (trophe=nourishment)

• Energy flows through the trophic levels; From producers to subsequent trophic levels. This energy always flows from lower(producer) to higher(herbivore, carnivore)etc., trophic level. It never flows in the reverse direction that is from carnivores to herbivores to producers.
• There is a loss of some energy in the form of unusable heat at each trophic level so that the energy level decreases from the first trophic level.
• As a result there are usually four or five trophic levels and seldom more than six as beyond that very little energy is left to support any organism. Trophic levels are numbered according to the steps an Organism is away from the source of food or energy.

The trophic level interaction involves 3 concepts namely-

1. Food chain
2. Food web
3. Ecological pyramids

1.1.1. Food chain

Organisms in the ecosystem are related to each other through feeding mechanisms or trophic levels i.e., One Organism becomes food for the other. A sequence of organisms that feed on one another, form a food chain. A food chain starts with producers and ends with top carnivores.

The sequence of eaten and being eaten produces transfer of food energy and it is known as the food chain. The plant converts solar energy into chemical energy by photosynthesis.

Small herbivores consume the plant matter and convert them into animal matter. These herbivores are eaten by large carnivores.

Types of Food Chain

Usually, there are two types of food chains.

1. Grazing food chain
2. Detritus food chain

Grazing food chain- The consumers who start the food chain, utilizing the plant or plant products as their food, constitute the grazing food chain. This food chain begins from green plants at the base and the primary consumer is herbivores.

Grass—> Caterpillar—> lizard—> snake—> eagle

For example, in a terrestrial ecosystem, the grass is eaten up by Caterpillar, which in turn is eaten by a lizard and the lizard is eaten by a snake.

In aquatic ecosystems, phytoplanktons(primary producers) are eaten by zooplanktons which in turn are eaten by fish and the fish is eaten by Pelicans.

Detritus food chain- It starts from dead organic matter of decaying animals and plant bodies consumed by the microorganisms and then to detritus-feeding organisms called detrivores or decomposers and to other predators.

Litter—> earthworms—> chicken—> Hawk

The distinction between these two food chains is the source of energy for the first-level consumers. In the grazing food chain, the primary source of energy is living plant biomass while in the detritus food chain, the source of energy is dead organic matter or detritus. The two food chains are linked. The initial energy source For the detritus food chain is the waste materials and dead organic matter from the grazing food chain.

1.1.2. Food web

• A food chain represents, only one part of the food or energy flow through an ecosystem and implies a simple, isolated relationship, which seldom occurs in the ecosystems.
• An ecosystem may consist of several interrelated food chains. More typically, the same food resource is part of more than one chain, especially when that source is at the lower trophic levels.
• “A food web is, the natural interconnection of food chains and a graphical representation of what eats what in an ecological community.”
• A food web illustrates, all possible transfers of energy and nutrients among the organisms in an ecosystem, whereas a food chain traces only one pathway of the food.
• If any of the intermediate food chains are removed, the succeeding links of the chain will be affected largely. The food web provides more than one alternative for food to most of the organisms in an ecosystem and therefore increases their chance of survival.
• For example, grasses may serve food for rabbits grasshopper goats or cows. Similarly, a herbivore may be a food source for many carnivorous species.
• Also food availability and preferences of food of the organisms may shift seasonally.
• For example, watermelon is available in summer, and peaches in winter.
• Thus there are interconnected networks of feeding relationships that take the form of food webs.

1.1.3. Ecological pyramids

• A graphical representation designed to show the biomass or bio-productivity at each trophic level in a given ecosystem is called an ecological pyramid. The steps of trophic levels expressed in a diagrammatic way are referred to as ecological pyramids. The food producer forms the base of the pyramid and the top carnivore forms the tip.
• The number, biomass and energy of organisms gradually decrease with each step from the producer level to the consumer level and the diagrammatic representation assumes a pyramid shape.
• The pyramid consists of several horizontal bars depicting specific trophic levels which are arranged sequentially from primary producer level through herbivore, carnivore onwards. The length of each bar represents the total number of individuals at each trophic level in an ecosystem.

The ecological pyramids are of 3 categories-

1. Pyramid of numbers
2. Pyramid of biomass
3. Pyramid of energy or productivity

Pyramid of numbers:

This ecological pyramid deals with the relationship between the numbers of primary producers and consumers of different levels. It is a graphic representation of the total number of individuals of different species, belonging to each trophic level in an ecosystem.

Depending upon the size and biomass, the pyramid of numbers may not always be upright, and may even be completely inverted.

1. Pyramid of numbers- upright-
2. In this pyramid, the number of individuals is decreased from a lower level to a higher trophic level.
3. This type of pyramid can be seen in grassland ecosystems. The grass occupies the lowest trophic level(base) because of its abundance.
4. The next trophic level is primary consumer- herbivore- grasshopper.
5. The individual number of grasshoppers is less than that of grass. The next energy level is the primary consumer- the rat.
6. The number of rats is less than grasshoppers because they feed on grasshoppers. The next higher trophic level is a secondary carnivore that feeds on rat snakes.
7. The Next higher trophic level is the top carnivore-hawk.
8. With each higher trophic level, the number of individuals decreases.
9. Pyramid of numbers- inverted-
10. In this pyramid, the number of individuals is increased from a lower level to a higher trophic level.
11. A count in a forest would have a small number of large producers, like a few big trees.
12. This is because the tree(primary producer) is few and would represent the base of the pyramid and the dependent herbivores, In the next higher trophic level it is followed by parasites at the next trophic level. Hyper parasite being at higher trophic level represents a higher number.
13. The resulting pyramid is an inverted shape. A pyramid of numbers does not take into account the fact that the size of organisms being counted at each trophic level can vary.
14. It is very difficult to count all the organisms, in a pyramid of numbers so the pyramid of numbers does not completely define the trophic structure of an ecosystem.

 Pyramid of biomass

 A pyramid of biomass is a graphical representation of biomass present in a unit of the territory of different trophic levels.

 To overcome the shortcomings of the pyramid of numbers, primitive biomass is used. In this approach, individuals in each trophic level are weighed instead of being counted. This gives us a pyramid of biomass, i.e., the total dry weight of all organisms at each trophic level at a particular time.

 The pyramid of biomass is usually determined by collecting all organisms occupying each trophic level separately and measuring their weight. This overcomes the size difference problem because all kinds of organisms after trophic level are weighed. Biomass is measured in grams/cubic meters (g/m³).

1. Upward pyramid- For most ecosystems on land, the pyramid of biomass has a large base of primary producers with a smaller trophic level perched on top. The biomass of producers (autotrophs) is at maximum. The biomass of the next higher trophic level i.e., secondary consumers is less than the primary consumers. The top, high trophic level has very less amount of biomass.
2. Inverted pyramid- In many aquatic ecosystems, the pyramid of biomass may assume an inverted form. This is because the producers are tiny phytoplanktons that grow and reproduce rapidly. Here, the pyramid of biomass has a small base, with the consumer biomass at any instant exceeding the producer biomass and the pyramid assumes an inverted shape.

Pyramid of energy

• A graphical representation of the energy found within the trophic levels of an ecosystem is called a pyramid of energy. To compare the functional roles of the trophic levels in an ecosystem, an energy pyramid is most suitable. An energy pyramid reflects the laws of thermodynamics, with the conversion of solar energy to chemical energy and heat energy at each trophic level and with the loss of energy being depicted at each transfer to another trophic level. Hence the pyramid is always upward, with a large energy base at the bottom.
• For example, an ecosystem receives  1000 calories of light energy in a given day. Most of the energy is not absorbed;  Some is reflected back to space; of the energy absorbed only  A small portion is utilized by photosynthetic plants, out of which the plant uses some for respiration, therefore only 100 calories are stored as energy-rich materials.
• Suppose an animal, like a deer, eats a plant containing 100 calories of food energy. The deer uses some of it for its own metabolism and stores only 10 calories as food energy. A lion that eats the deer gets even smaller amount of energy. Thus usable energy decreases from sunlight to producer to herbivore to carnivore. Therefore, the energy pyramid will always be upright.
• The energy pyramid concept helps to explain the phenomena of biological magnification- the tendency for toxic substances to increase in concentration progressively at higher levels of the food chain.

Pollutants and trophic level 

Any undesirable particle that causes pollution is called a pollutant. Pollutants especially non-degradable ones move through the various trophic levels in an ecosystem.

Non-degradable pollutants are materials which cannot be metabolized by living organisms, like chlorinated hydrocarbons. Even the small concentrations of chemicals in the environment find their way into organisms in high enough dosage problems.

 The movement of these pollutants involves 2 main processes-

1. Bioaccumulation
2. Biomagnification

Bioaccumulation- It is the process of entry of pollutants into a food chain. In bioaccumulation, there is an increase in the concentration of a pollutant from the environment to the first organism in a food chain.

Biomagnification

• Biomagnification refers to the tendency of pollutants to concentrate as they move from one trophic level to the next.
• In biomagnification there is an increase in concentration of a pollutant from one link in a food chain to another.
• For biomagnification, the pollutant must be long-lived, mobile, soluble in fats, and biologically active.
• If a pollutant is short-lived, it will be broken down before it becomes dangerous.
• If it is not mobile, it stays in one place and is unlikely taken by organisms.
• If the pollutant is soluble in water, it will be excreted by the organisms.
• Pollutants that dissolve in fat, may retain for a long time.

 These pollutants can be measured in the fatty tissue of organisms like fish. In mammals, the milk produced by females is tested, since the milk has a lot of fat in it and is more susceptible to damage from toxins. A pollutant is not active biologically, it may biomagnify, but that is not dangerous(DDT).

Biotic interaction- Organisms living on the earth are interlinked to each other in one way or another. The interaction between the organisms is fundamental for the arrival and functioning of an ecosystem.

 Biotic interaction

 Types of biotic interaction

• Mutualism- In this type of interaction both species are benefited. Example- In pollination, the pollinator gets food( pollen, nectar) and the plant has its pollen transferred to other flowers for cross-fertilization.
• Commensalism- In this type of interaction, one species is benefited while  the other is unaffected. Example- Crocodile-plover bird; The bird gets into the crocodile's mouth and picks out the tiny bits of food stuck in his teeth. In this way, the bird gets food and the crocodile is unaffected.
•  Competition the species are harmed. Example- If the two species eat the same food, and there is not enough food for both, they may have access to less food than they would if alone. They both suffer a shortage of food.
•  Predation and parasitism- In this type of interaction one species benefits and the other is harmed. Example- predation- one fish kills and eats another fish.
•  Parasitism-  Mosquito is benefited by sucking blood; the host is harmed by losing blood.
•  Amensalism- In this type of biotic interaction, one species is harmed while the other is unaffected. Example- A large tree shades a small plant, retarding the growth of the small plant. The small plant does not affect the large tree.
• Neutralism- There is no net benefit or harm to either species. Perhaps in some interspecific interactions, the costs and benefits experienced by each partner are the same so that they sum to zero. It is not clear how often this happens in nature. Neutralism is also sometimes described as the relationship between two species inhabiting the same space and using the same resources, but that do not affect each other. In this case, one could argue that they are not interacting at all.

Biogeochemical cycle

• A biogeochemical cycle is a pathway by which a chemical substance cycles the biotic and abiotic components of the earth. The living world depends upon the energy flow and the nutrient circulation that occurs through the ecosystem. Both influenced the abundance of organisms, the metabolic rate at which they live, and the complexity of the ecosystem.
• Energy flows through the ecosystem enabling the organisms to perform various kinds of work and this energy is ultimately lost as heat forever in terms of the usefulness of the system. On the other hand, nutrients of food matter never get used up. They can be recycled again and again indefinitely.
• For example, When we breathe we may be inhaling several millions of atoms of elements that may have been inhaled by our ancestors or other organisms.
• Carbon, hydrogen, oxygen, nitrogen and phosphorus are elements and compounds that make up 97% of the mass of our bodies. In addition to these about 15-25 other elements are needed in some form for the survival and good health of plants and animals.
• These elements or mineral nutrients are always in circulation, moving from non-living to living and then back to the non-living components of the ecosystem in a more or less circular fashion. This circular fashion is known as biogeochemical cycling(bio-life; geo-atmosphere).

2. Nutrient cycling

• The nutrient cycle is a concept that describes how nutrients move from the physical environment to the living organisms and subsequently recycled back to the physical environment.
• This movement of nutrients from the environment into plants and animals again back to the environment is essential for life and it is the vital function of the ecology of any region. In any particular environment, to maintain its organism in a sustained manner, the nutrient cycle must be kept balanced and stable.
• Nutrient cycling is typically studied in terms of specific nutrients, with each nutrient in an environment having its own particular pattern of cycling. The carbon nutrient cycle and the nitrogen nutrient cycle are some of the most important. Both of these cycles make up an essential part of the overall soil nutrient cycle. Many other nutrient cycles are important in ecology, including a large number of trace mineral nutrient cycles.

Types of Nutrient Cycle

• Based on the replacement period a nutrient cycle is referred to as perfect or imperfect cycle.
• A perfect nutrient cycle is one in which nutrients are replaced as fast as they are utilized. Most gaseous cycles are generally considered as perfect cycles.
• In contrast, sedimentary cycles are considered relatively imperfect, as some nutrients are lost from the cycle and get locked into sediments and so become unavailable for immediate cycling.
• Based on the nature of the reservoir, there are two types of cycles namely gaseous and sedimentary cycles.

Gaseous cycles-  Some of the most important gaseous cycles are the water (hydrologic) cycle, carbon and nitrogen.

Water Cycle

• Water cycle also known as hydrological cycle is a biogeochemical cycle that describes the continuous movement of water of the earth.
• Water is a cyclic resource. It undergoes the cycle from ocean to land and land to ocean. The water cycle describes the movement of water on, in and above the earth.
• The water cycle has been working for billions of years and all the life on earth depends on it. The distribution of water on Earth is uneven.
• Many geographical areas have plenty of water while others have very limited quantity.
• Water cycle refers to the continuous exchange of water in different forms like liquid, solid and gaseous states between the oceans, atmosphere, lithosphere and organisms.

Carbon Cycle

• It is the way of reusing carbon atoms that travel from the atmosphere into organisms on Earth and then back into the atmosphere over and over again. Carbon forms the basic constitute of all the organic compounds.
• The carbon cycle is mainly the conversion of carbon dioxide. This process is initiated by the fixation of carbon dioxide from the atmosphere through photosynthesis.
• This results in the production of carbohydrates, and glucose that may be converted to other organic components such as sucrose, starch, celluloses etc. Here, some of the carbohydrates are utilized directly by the plant itself.
• During this process more carbon dioxide is generated and is released through its leaves or roots. The remaining carbohydrates which are not used by plant becomes part of plant tissue.
• The plant tissue is either eaten by herbivores or microorganisms during decomposition. Herbivores convert some of the carbohydrates into carbon dioxide in the process of respiration.
• The decomposers decompose the remaining carbohydrates after the animal dies. The carbohydrates that are decomposed by microorganism then get oxidized into carbon dioxide and are returned to the atmosphere.

Nitrogen Cycle

• Nitrogen is a major constituent of the atmosphere comprising about 78% of the atmospheric gases.
• It is an essential constituent of different organic compounds like amino acids, nucleic acids, proteins, vitamins and pigments.
• Certain species of soil bacteria and blue-green algae are capable of utilizing it in gaseous form directly. Generally, nitrogen is usable only after it is fixed.
• The main source of free nitrogen is the action of soil microorganisms and associated plant roots (symbiotic association) on atmospheric nitrogen found in pore spaces of the soil (Biological nitrogen fixation).
• Nitrogen can also be fixed in the atmosphere by lightning and cosmic radiation. In the oceans, some animals fix nitrogen. After atmospheric nitrogen is fixed in available form green plants can use it.
• Herbivores, feeding on plants will consume some of it. In dead plants and animals, the excretion of nitrogenous wastes are converted into nitrites by the action of bacteria present in soil. Some bacteria convert nitrites to nitrates or even free nitrogen by process denitrification.

Nitrogen fixation on earth is accomplished in three different ways:

1. By microorganisms (bacteria and blue-green algae)
2. By man using industrial processes (fertilizer factories)
3. To a limited extent by atmospheric phenomena such as thunder and lightning.

• Certain microorganisms fix atmospheric nitrogen into ammonium ions. These include free-living nitrifying bacteria(aerobic Azotobacter and anaerobic clostridium) and symbiotic nitrifying bacteria living in association with leguminous plants and symbiotic bacteria living in non-leguminous root nodule plants(Rhizobium) as well as blue-green algae (anabaena, spirulina). Nitrosomonas bacteria promote the transformation of ammonia into nitrite. Nitrate is then transformed into nitrate by the bacteria Nitrobacter.
•  In the soil as well as oceans there are special denitrifying bacteria( pseudomonas) which convert the nitrites/ nitrates to elemental nitrogen. This nitrogen escapes into the atmosphere, thus completing the cycle.

Sedimentary cycle: Phosphorus, calcium and magnesium circulate using the sedimentary cycle. The element involved in the sedimentary cycle normally does not cycle through the atmosphere but follows a basic pattern of flow through erosion, sedimentation, mountain building, volcanic activities and biological transport through that excreta of marine birds.

Phosphorus cycle

• Phosphorus plays a central role in aquatic ecosystems and water quality. Unlike carbon and nitrogen, which come primarily from the atmosphere, Phosphorus occurs in large amounts as a mineral in phosphate rocks and enters the cycle from erosion and mining activities. This is the nutrient considered to be the main cause of excessive growth of rooted and free-floating microscopic plants in lakes.
• The main storage for phosphorus is in the earth’s crust. On land, phosphorus is usually found in the form of phosphates. By the process of weathering and erosion phosphates enter rivers and streams that transport them to the ocean.
• In the ocean, the phosphorus accumulates on continental shelves in the form of insoluble deposits. After millions of years, the crustal plates rise from the sea floor and expose phosphates on land. Through the process of weathering the phosphates are released from rock and the cycle's geochemical phase begins.

• The sulfur reservoir is in the soil and sediments, where it is locked in organic and inorganic deposits in the form of sulphates, sulphide and organic sulfur. Organic deposits include coal, oil and peat;  inorganic deposits include pyrite rock and sulphur rock.
•  It is released by weathering of rocks, erosional runoff and decomposition of organic matter and is carried to  Terrestrial and aquatic ecosystems in a salt solution.
•  The sulfur cycle is mostly sedimentary except for two of its compounds- hydrogen sulfide(H₂S) and sulfur dioxide(SO₂) which add a gaseous component to its normal sedimentary cycle.
•  Sulfur enters the atmosphere from several sources like volcanic eruptions, combustion of fossil fuels,  from the surface of the ocean and gases released by decomposition. Atmospheric hydrogen sulfide also gets oxidized into sulfur dioxide. Atmospheric sulfur dioxide is carried back to the earth after being dissolved in rainwater as weak sulfuric acid.
•  Whatever the source, sulfur in the form of sulfates is taken up by plants and incorporated through a series of metabolic processes into sulfur-bearing amino acid which is incorporated in the proteins of autotroph tissues. It then passes through the grazing food chain. Sulfur bound in living organisms is carried back to the soil, to the bottom of ponds lakes and seas through excretion and decomposition of dead organic material.
•  The biogeochemical cycles discussed here are only a few of many cycles and they do not operate individually but interact with each other at some point other other.

3. Ecological succession

Ecological succession is the process of change in the species structure of an ecological community over time.  Succession is a  universal process of directional change in vegetation, on an ecological timescale.  Succession occurs when a series of communities replace one another due to large-scale destruction either natural or man-made. This process continues by replacing one community with the other until a stable, mature community develops.  Succession is a progressive series of changes which leads to the establishment of a relatively stable climax community.

 The first plant species to colonize an area is called the Pioneer community.

 The final stage of succession is called the climax community.

 The stage leading to the climax community is called the successional stage or sere.

 Succession is characterized by the following:

1. Increased productivity
2. The shift of nutrients from the reservoirs
3. Increase the diversity of organisms with increased niche development
4. Gradual increase in the complexity of food webs

Primary succession

• In primary succession on a terrestrial site the new site is first colonized by a few hardy pioneer species that are often microbes, lichens and mosses. The Pioneers over a few generations alter the habitat conditions by their growth and development.
•  These new conditions may be conducive to the establishment of additional organisms That may subsequently arrive at the site. The pioneers through their death and decay, leave patches of organic matter in which small animals can live.
•  The organic matter produced by these pioneer species produces organic acids during decomposition that dissolve and etch the substratum releasing nutrients to the substratum. Organic debris accumulates in pockets and crevices, providing soil in which states can become lodged and grow.
•  As the community of organisms continue to develop, it becomes more diverse and competition increases, but at the same time new niche opportunities develop.
•  The pioneer species Disappear as the habitat conditions change and the invasion of new species progresses, leading to the replacement of the preceding community.

Secondary succession

• Secondary succession occurs when plants organize an area in which the climax community has been disturbed. Secondary succession is the sequential development of biotic communities after the complete or partial destruction of the existing community.  A  mature or  intermediate community may be destroyed by natural events like floods, droughts, fire, storms or human interventions such as deforestation, agriculture, overgrazing etc.,
• This abandoned farmland is first invaded by hardy species of grasses that can survive in bare, sun-baked soil. These grasses may be joined by tall trees and herbaceous plants. These dominated the ecosystem for some years along with mice, rabbits, insects and seed-eating birds.
•  Eventually, some trees come up in this area, seeds of which may be brought by wind or animals. Over the years, a forest community has developed. Thus an abandoned farmland over some time becomes dominated by trees and is transformed into a forest.
• The difference between primary and secondary succession is, the second is that session starts on a well-developed soil already formed at the site. Thus succession is relatively faster as compared to primary succession which may often require hundreds of years.

 Autogenic and allogenic succession- When succession is brought about by living inhabitants of that community itself, the process is called autogenic succession. The change brought about by outside forces in a succession is called allogenic succession.

 Autotrophic and heterotrophic succession

• Succession in which, initially the green plants are much greater in quantity is known as autotrophic succession; and the ones in which the heterotrophs are greater in quantity are known as heterotrophic succession.
• Succession would occur faster in areas existing in the middle of the large continent. This is because, here all propagate or seeds of plants belonging to the different series would reach much faster, establish and ultimately result in the climax community.