NEET Biology Notes – Ecosystem

Ecosystem Introduction

According to Tansley (who also coined the term ecosystem), the sum total of interaction between living (biotic) and non-living (abiotic) components which is capable of independent existence is called an ecosystem.

Some other terms used for ecosystem are biocoenosis (by C. Mobius), microcosm (by Forbes), and geobiocoenosis (by Sukhachev).

Important Facts About Ecosystem

• The flow of energy is from producer to consumers.
• Cycling of matter takes place between biotic and abiotic components.
• There exists functional relationship between organisms and the environment.
• There is a great deal of biotic diversity.
• Homeostasis (self-regulation): The ecosystem has the capacity to overcome forces which tend to disturb the balance of the ecosystem (resilience).
• Incomplete ecosystem: An ecosystem lacking one or more structural components is called an incomplete ecosystem, e.g., deep sea and freshly formed rain water pond ecosystem.
• Artificial ecosystems: These are man-made ecosystems. Examples are modern agriculture, dams, zoo-logical parks, plantations, aquacultures, etc.
• The characteristics of artificial ecosystems are as follows:
  • Artificial ecosystems do not possess self-regulatory mechanism.
  • These have little diversity.
  • The food chain is simple.
  • Productivity is high.
  • There is little cycling of nutrients.

Structure And Function Of Ecosystem

Structure of Ecosystem

The structure of ecosystem depends upon the following components:

• Species diversity components of ecosystem
• Species composition
• Life cycle
• Stratification

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Function of Ecosystem

The edaphic functional components of ecosystem are studied with the following aspects:

• Productivity
• Mineral cycling
• Energy flow
• Food chain and web slopes
• Efficiency
• Biotic interrelationships
• Homeostasis
• Ecoregulation

Homeostasis in Ecosystem

Ecosystem maintains functional balance or homeostasis among different components. It is due to the following reasons:

• Carrying capacity
• Nutrient cycling
• Self-regulation
• Feedback

Stratification

Stratification is the structure or recognizable pattern in the spatial arrangement of the members of communities. More specifically, stratification represents vertical zonation in the community. For example, in grassland communities, there is subterranean floor containing basal portions of vegetation. However, stratification in a forest community is the most complicated where as many as five vertical subdivisions may be recognized. These vertical subdivisions are as follows:

• Subterranean
• Forest floor
• Herbaceous vegetation
• Shrubs
• Trees

Boundaries of Ecosystems

An ecosystem is generally regarded as a self-sufficient unit and a separate entity. However, it never operates in isolation. Boundaries between one ecosystem and another are indistinct and overlapping and all ecosystems on the earth are joined together to form a single global ecosystem known as biosphere, Some exchange of materials and energy always occurs between different ecosystems through geological, climatic, or biological processes.

Productivity Of Ecosystem

• Coral reefs, tropical rain forests, and sugarcane are most productive.
• Deserts and deep sea ecosystems are least productive.

Energy

An ecosystem is maintained by the flow of energy derived from the sun. Energy trapped by plants varies from ecosystem to ecosystem energy absorption at different levels.

• Gross primary productivity (GPP): It is the rate of organic matter synthesized by producers per unit area per unit time.
• Net primary productivity (NPP): It is the rate of organic matter built up or stored by producers in their bodies per unit time and area. Net productivity is equal to gross primary productivity minus loss due to respi- ration and other reasons.
  NPP = GPP- Respiratory loss
  The annual NPP of the whole biosphere is approximately 170 billion ton (dry wt.) of organic matter. Despite occupying about 70% of the surface, the productivity of oceans is only 55 billion ton.
• Secondary productivity: The rate of increase in energy containing organic matter or biomass by heterotrophs or consumers per unit time and area is known as secondary productivity.
• Community productivity: It is the rate of net synthesis of built-up of organic matter by a community per unit time and area.
• Ecological efficiency/trophic level efficiency: The percentage of energy converted into biomass by a higher trophic level over the energy of food resources available at the lower trophic level is called ecological efficiency.
• Photosynthetic efficiency = Gross primary productivity/Incident total solar radiation
• Net production efficiency = Net primary productivity/Gross primary productivity x 100

Decomposition

Decomposition is the process of physical and chemical breakdown of complex organic remains by organisms called decomposers, so as to produce inorganic raw materials (CO2, H2O, minerals, etc.) for recycling. The major site for decomposition is the upper layer of soil in terrestrial habitats and the bottom of water bodies. Freshly deposited organic matter constitutes raw material and is called litter. Detritus is degrading dead organic matter.

It is differentiated into above-ground and below-ground detritus. Above-ground detritus consists of dried plant parts (leaves, twigs, bark, and flowers), excreta, and dead remains of animals. Below-ground detritus is also called root detritus because it is mainly composed of dead roots. Underground organisms and their excreta also form a pan of below-ground detritus.

Decomposition Processes

Three types of processes occur simultaneously during the decomposition of detritus, viz. fragmentation, leaching, and catabolism.

• Fragmentation of detritus: Small invertebrate animals, called detrivores, feed on detritus, e.g., earthworms and termites. They bring about its frag- mentation. A part of detritus eaten by detrivores comes out in a highly pulverized state in their feces. Due to fragmentation during eating and pulverization in digestive tracts, detritus is changed into fine particles which have a large surface area.
• Leaching: Parts of soluble substances present in the fragmented and decomposing detritus (e.g., sugars and inorganic nutrients) get leached to the upper layers of soil by percolating water.
• Catabolism: It is carried out by saprotrophic bacteria and fungi. They secrete digestive enzymes over the fragmented detritus. The enzymes change complex organic compounds into simple compounds. Inorganic substances are also released in the process.

The rate of catabolic action or breakdown of different complex substances is different. This differential decomposition produces two substances: humus and inorganic nutrients in processes, respectively, called humification and mineralization, which occurs in soil.

• Humification: It is the process of particle decomposition of detritus to form humus. Humus is a dark-colored amorphous partially decomposed organic matter rich in cellulose, lignin, tannins, resin, etc., because of its highly resistant nature. It is slightly acidic, colloidal, and functions as a reservoir of nutrients.
• Mineralization: It is the release of inorganic substances (e.g., CO2, H2O, and minerals) from organic matter during the process of decomposition. These are formed along with simple and soluble organic substances when digestive enzymes are poured over organic matter by saprotrophic microbes.

Factors Affecting Decomposition

The rate of decomposition of detritus is controlled by a number of factors.

• Chemical nature of detritus: The decomposition of detritus is slow if it contains lignin, chitin, tannins (phenolics), and cellulose. It is rapid if detritus possesses more of nitrogenous compounds (such as proteins and nucleic acids) and reserves carbohydrates.
• Soil pH: Detrivores are fewer in acidic soils. Microbial activity is also low in such soils. Therefore, the rate of decomposition of organic matter is slow in acidic soils. Partially decomposed organic matter piles up over such soils. Detrivores are abundant in neutral and slightly alkaline soils while decomposer microbes are rich in neutral and slightly acidic soils.
• Temperature: At a temperature of more than 25°C, decomposers are very active in soils having good moisture and aeration. In humid tropical regions, it does not take more than 3-4 months for complete decomposition of detritus. However, under low temperature conditions (> 100°C) of soils, the rate of decomposition is very slow even if moisture and aeration are optimum.
• Moisture: Optimum moisture helps in quicker decomposition of detritus. Reduction in moisture reduces the rate of decomposition in areas of prolonged dryness such as tropical deserts where otherwise the temperature is quite high. Excessive moisture also impedes decomposition.
• Aeration: It is required for the activity of decomposers and detrivores. A reduced aeration will slow down the process of decomposition.

Energy Flow

• Food chain: It is a sequence of living organisms in which one organism consumes another due to interdependence.
• Key industry animals: These are herbivores who convert plant matter into animal matter.
  There are three types of industry animals:
  • Grazing food chain (GFC)/predator food chain major in aquatic ecosystems
  • Detritus food chain-major in terrestrial ecosystems
  • Parasitic food chain
• DFC
  • Source of energy is detritus not sun.
  • It is composed of a long chain of detritus-eating organisms (detritivores).
  • In some ecosystems (e.g., tropical rain forests), more energy flows in this chain than in the GFC.
• Food web: The interconnected food chains operating in an ecosystem are called food webs. It is, thus, a collection of food chains.
• Standing state or standing quality: It is the amount of inorganic substances present in an ecosystem per unit area at a given time.
• Standing crop: It is the amount of living material present in different trophic levels at a given time. It is commonly expressed as the number of organisms per unit area.

Ecological Pyramids

Ecological pyramids (Eltonian pyramids) were developed by Charles Elion in 1927. They are graphical representation of dif- ferent ecological parameters. In the pyramid, producers form the base and top carnivores the tip. The pyramid can be upright, inverted, or spindle shaped. These are of three types:

• Pyramid of number: Upright for grassland and pond and inverted for parasitic ecosystem.
• Pyramid of biomass: Upright for terrestrial habitats and inverted for aquatic habitats.
• Pyramid of energy: Always upright.

Ecological Succession (By Hult)

Every community undergoes a series of changes until a group of organisms is established which can live and reproduce most successfully in the area. This is called biotic succession. A biotic community normally undergoes continuous changes. The interactions among organisms in a community collectively constitute biotic factors; these influence the structure, composition, and function of a community.

Generally, a definite and orderly sequence of communities gradually appears in an area over a period of time. The first community which appears on an area is called pioneer community. A specific sequence of development of a community is related to a particular set of physical and chemical conditions. This is known as a sere, and is composed of a number of biotic communities replacing each other in the course of time called seral communities. The last community is called climax or a climatic climax (as it mostly depends upon climate).

• Primary succession: It occurs on biologically sterile area which has no record of any previous vegetation. The conditions are extreme and it takes very long time (1000 years) to complete, i.e., establishing climax community, e.g., newly created pond.
• Secondary succession: It occurs on an area that was previously occupied by vegetation and was later destroyed by fire, deforestation, over-grazing, volcanic eruptions, and floods. Humus is already present and environment is not so hostile. Therefore, it takes lesser time.
• Autogenic and allogenic succession: When a community replaces the other due to the modification of the environment by the community itself, the succession is called autogenic. On the contrary, when a community replaces the other largely due to the forces other than the effects of communities on the environment, the succession is said to be allogenic.
• Autotrophic and heterotrophic succession: Autotrophic succession is characterized by early dominance of autotrophic organisms and begins in predominantly inorganic environment. On the contrary, heterotrophic succession is characterized by early dominance of heterotrophs and begins in a predominantly organic environment.

Process of Succession

Major steps in a primary autotrophic succession are as follows:

• Nudation: An area is exposed.
• Migration: The process of dispersal of seeds, spores, and other structures of propagation of the species to bare area is known as migration.
• Germination: It occurs when conditions are favorable.
• Ecesis: Successful germination of propagules and their establishment in a bare area is known as ecesis.
• Colonization and aggregation: After ecesis, the individuals of the species increase in number as a result of reproduction.
• Competition and co-action: Due to limited resources, species show both inter and intraspecific competition. This results in the elimination of unsuitable and weaker plants.
• Invasion: Various other types of plants try to establish in the spaces left by the elimination of plants due to competition.
• Reaction: The newly arrived plants interrupt with the existing ones. As a result of reaction, the environment is modified and becomes unsuitable for the existing community which sooner or later is replaced by another community.
• Stabilization: It is the process when the final climax community becomes more or less stabilized for a longer period of time and can maintain itself in equilibrium with the climate of the area. As compared to the seral stage community, the climax community has larger size of individuals, complex organization, complex food chains and food webs, more efficient energy use, and more nutrient conservation.

Major Trends During Succession

• There is an increase in structural complexity.
• Diversity of species tends to increase.
• Biomass and standing crop increase.
• There is a decrease in net community production. There is an increase in non-living matter.
• Food chain relationship becomes complex.
• Niche becomes specific and narrower.
• Energy use and nutrient conservation efficiency increase.
• Stability increases.

Contents of Ecological Succession

• Lithosere (succession on desert/rock):
  • Lichen stage: Wind borne lichen propagules settle on the wet rock surface soon after rain or heavy dew. They develop attaching structure rhizenes. The pioneer lichens are usually crustose lichens, e.g., Graphis, Rhizocarpon, etc. They secrete lichen acids and carbonic acid. The acids slowly corrode rock surface and release minerals required for the proper growth of larger lichens, i.e., foliose lichens, e.g., Parmelia.
  • Moss stage: Foliose lichens growing on rocks make the conditions favorable for the growth of hardy mosses. Ultimately, the spot becomes suitable for invasion by the next stage (e.g., Hypnum and Bryum).
  • Annual grass stage: The mat formed by mosses on the partially fragmented rock becomes sufficiently moist during the rainy season for the germination of seeds of annual grasses and other hardy herbs, e.g., Aristida, Poa, and Eleusine.
  • Perennial grass stage: Annual grasses are replaced by perennial grasses due to increased moisture and soil in the rock crevices. The perennial grasses have runners and rhizomes which rapidly spread the grasses, e.g., Cymbopogon and Heteropogon.
  • Shrub stage: Seeds and rhizomes of xerophytic shrubs invade the area occupied by perennial grasses, e.g., Zizyphus, Capparis, Rhus, and Rubus. Shrubs are larger and their roots reach greater depth causing further cracks in the rocky substratum and, hence, helping in more soil formation.
  • Climax community: Several hardy and light demanding trees grow in the area occupied by shrubs. Slowly the environment becomes more moist and shadier so that plants. of climax community spread in the area. The type of climax community depends upon the climate. Therefore, it is also called climatic climax community.
• Hydrosere (succession on pond):
  • Plankton stage: It is the pioneer stage of hydrosere. Planktons reach the water body through wind or animals. The first to appear are minute autotrophic organisms called phytoplankton, e.g., diatoms; green flagellates; single-celled, colonial, or filamentous green algae; as well as blue green algae. They multiply rapidly. Soon a balance is created by the appearance of zooplankton which feeds on ohytoplankton.
  • Submerged stage: The bottom lined by soft mud having organic matter is favorable for the growth of submerged plants such as Hydrilla, Potamogeton, and Najas.
  • Floating stage: Floating leaved anchored plants (e.g., Nymphaea, Nelumbo, and Nuphar) appear where water becomes shallow. These plants have subterranean stems like rhizome and tuber. The plants make the water rich in mineral and organic matter. It becomes suitable for the growth of free floating plants such as Lemna, Spirodela, Wolffia, Azolla, and Eichhornia.
  • Reed swamp stage: Amphibious plants grow when the water body becomes shallow (0.3-1 m). Examples are Phragmites, Typha, Scirpus, and Sagittaria. The plants of swamp stage transpire nice quantities of water.
  • Sedge or marsh meadow stage: The shores built up in reed swamp stage are invaded by Carex (sedge); Cyperus; Juncus; grasses such as Themeda; and herbs such as Campanula, Caltha, and Polygonum. The plants transpire rapidly and add abundant humus.
  • Scrub/woodland stage: The periphery of sedge meadow stage is invaded by some rhizome bear- ing shrubby plants which can tolerate bright sunlight as well as water logged conditions, e.g., Cornus (bogwood), Cephalanthus (button brush); etc. They invite invasion by trees capable of bearing bright sunlight and water logging, e.g., Populus (cottonwood) and Alnus (alder).
  • Climax forest: New trees invade the area. They have shade-loving seed plants. These trees grow to greater heights.

Nutrient Cycling

Nutrient cycling is of three types:

• Gaseous: Examples are carbon (absorbed as CO2), nitrogen, oxygen (as vapors), and main reservoir pool (as atmosphere or water).
• Sedimentary: Biogenetic materials are nongaseous and have lithosphere as the main reservoir pool. Examples are P, Ca, and Mg. Both sedimentary and gaseous phases occur in case of sulfur.
• Mixed: Nutrients have both gaseous and nongaseous states, e.g., sulfur. Tundra consists of plains with snow, ice, and frozen soil (permafrost).

Aquatic Biomes

Five general categories of aquatic ecosystems are usually recognized:

• Marine pelagic areas within the water mass of oceans with water of high salinity
• Marine benthic areas on the ocean floor with water of high salinity
• Estuaries, bodies, or oceanic water near the mouths of freshwater rivers with intermediate salinity
• Freshwater lotic with flowing water of low salinity Freshwater lentic with non-flowing water of low salinity

Marine Biome

The marine environment is characterized by its high concentration of salt (about 3.5% in open sea) and mineral ions (mostly sodium and chloride followed by sulfur, magnesium, and calcium). All ocean basins are roughly of the shape of a wash basin (or an inverted hat).

From the coastline, a gradually sloping region extends to about 160 km into the sea. This zone, with a depth of 8-200 m, is called the continental shelf. The angle of the slope then abruptly steepens to form the continental slope which levels off into a more or less horizontal expanse forming the ocean floor.

The vertical zones of the ocean are determined on the basis of availability of light for photosynthesis.

• The well-lit upper 200 m forms the photic or euphotic zone.
• The next zone up to the depth 200-2000 m gets less light which is insufficient for photosynthesis and forms disphotic zone.
• Below 2000 m is the area of perpetual darkness-the abyssal zone/dark zone.

Freshwater Biomes (Lakes and Ponds)

Freshwater biomes are stationary water bodies. Ponds are small and shallow. Lakes are larger and deeper. Lakes develop in na- ture due to the following reasons:

• Glaciation
• Natural or man-made depressions
• Oxbow lakes which develop from the main stream of a river

Depending upon productivity, lakes are of three types:

• Oligotrophic lakes: These lakes are deep with rocky steep sides and with less circulation of nutrients. They have little biota but are rich in green algae. Brackish lakes are oligotrophic and also occur in arid areas, e.g., Sambhar Lake in Rajasthan.
• Eutrophic lakes: These lakes are rich in biota and have quick circulation of nutrients. These are shallow with abundant blue green algae, e.g., Dal Lake in Kashmir.
• Dystrophic lakes: These lakes are rich in humic acid and are less productive.

Life Forms

Raunkiaer (1934) has distinguished plants into five forms on the basis of size, shape, branching, crown, life span, and perennation.

• Phanerophytes: Perennial herbs, shrubs and trees, epiphytes, succulents, lianas, etc., where perennating buds occur at a height of 10 cm or more above the ground level.
• Chemaephytes: Small plants of cold areas where perennating buds or shoot apices lie at or above the ground level.
• Hemieryptophytes: Perennating structures occur at the ground level. Aerial shoots die with the onset of winter, e.g., rosette plants.
• Cryptophytes: Perennial plants with underground storage parts. These are of different types such as
  • Geophytes: These Subterranean perennating structure (root, root tuber, bulb, stem tuber, rhizome, corm).
  • Halophytes (marshy plants): Perennating structure embedded in mud.
  • Hydrophytes (aquatic plants)
  • Thorophytes: Plants which perennate in the form of seeds.

Ecosystem Services

The products of ecosystem processes are called ecosystern services, e.g., healthy forest ecosystems purify air and water, mitigate droughts and floods, cycle nutrients, generate fertile soils, provide wildlife habitat, maintain biodiversity, pollinate crops, provide storage site for carbon, and also provide aesthetic, cultural, and spiritual values.

Researchers like Robert Constanza et. al. have put an average price tag of US$ 33 trillion a year on these fundamental ecological services (i.e., nearly twice the value of a global GNP – US$ 18 trillion).

Out of total cost, soil formation accounts for about 50%, recreation and nutrient cycling less than 10% each, and climate regulation and habitat for wildlife account for about 6% each.

 

Assertion-Reasoning Questions

In the following questions, a statement of Assertion (A) is followed by a statement of Reason (R).

1. If both Assertion and Reason are true and the reason is the correct explanation of the assertion, then mark (1).
2. If both Assertion and Reason are true but the reason is not the correct explanation of the assertion, then mark (2).
3. If Assertion is true but Reason is false, then mark (3).
4. If both Assertion and Reason are false, then mark (4).

Question 1. Assertion: Agriculture and aquacultures are man-maintained ecosystems.

Reason: All biotic and abiotic factors are managed by humans in these ecosystems.

Answer. 1. In the following questions, a statement of Assertion (A) is followed by a statement of Reason (R).

Question 2. Assertion: Warm and moist environment can enhance the rate of decomposition.

Reason: Warm and moist climate leads to create anaerobic condition which promotes decomposition.

Answer. 3. If Assertion is true but Reason is false, then mark (3).

Question 3. Assertion: Detritus food chain begins with decomposers.

Reason: Detrivores, like fungi and bacteria, are major decomposers in such food chains.

Answer. 3. If Assertion is true but Reason is false, then mark (3).

Question 4. Assertion: Ecological pyramids cannot explain all vital functions of any ecosystem.

Reason: Pyramids actually do not explain the role of organisms working at more than one trophic level.

Answer. 2. If both Assertion and Reason are true and the reason is the correct explanation of the assertion, then mark (1).

Question 5. Assertion: Successional process starts only in those areas where no living organisms ever existed.

Reason: These areas are not supported by physical environmental conditions.

Answer. 4. If both Assertion and Reason are false, then mark (4)