Ecological diagrams

Ecosystem: It’s Structure and Functions

An organism is always in the state of perfect balance with the environment. The environment literally means the surroundings.

The environment refers to the things and conditions around the organisms which directly or indirectly influence the life and development of the organisms and their populations.

“Ecosystem is a complex in which habitat, plants and animals are considered as one interesting unit, the materials and energy of one passing in and out of the others” – Woodbury.

Organisms and environment are two non-separable factors. Organisms interact with each other and also with the physical conditions that are present in their habitats.

“The organisms and the physical features of the habitat form an ecological complex or more briefly an ecosystem.” (Clarke, 1954).

The concept of ecosystem was first put forth by A.G. Tansley (1935). Ecosystem is the major ecological unit. It has both structure and functions. The structure is related to species diversity. The more complex is the structure the greater is the diversity of the species in the ecosystem. The functions of ecosystem are related to the flow of energy and cycling of materials through structural components of the ecosystem.

According to Woodbury (1954), ecosystem is a complex in which habitat, plants and animals are considered as one interesting unit, the materials and energy of one passing in and out of the others.

According to E.P. Odum, the ecosystem is the basic functional unit of organisms and their environment interacting with each other and with their own components. An ecosystem may be conceived and studied in the habitats of various sizes, e.g., one square metre of grassland, a pool, a large lake, a large tract of forest, balanced aquarium, a certain area of river and ocean.

All the ecosystems of the earth are connected to one another, e.g., river ecosystem is connected with the ecosystem of ocean, and a small ecosystem of dead logs is a part of large ecosystem of a forest. A complete self-sufficient ecosystem is rarely found in nature but situations approaching self-sufficiency may occur.

Structure of Ecosystem:

The structure of an ecosystem is basically a description of the organisms and physical features of environment including the amount and distribution of nutrients in a particular habitat. It also provides information regarding the range of climatic conditions prevailing in the area.

From the structure point of view, all ecosystems consist of the following basic components:

1. Abiotic components

2. Biotic components

1. Abiotic Components:

Ecological relationships are manifested in physicochemical environment. Abiotic component of ecosystem includes basic inorganic elements and compounds, such as soil, water, oxygen, calcium carbonates, phosphates and a variety of organic compounds (by-products of organic activities or death).

It also includes such physical factors and ingredients as moisture, wind currents and solar radiation. Radiant energy of sun is the only significant energy source for any ecosystem. The amount of non-living components, such as carbon, phosphorus, nitrogen, etc. that are present at any given time is known as standing state or standing quantity.

2. Biotic Components:

The biotic components include all living organisms present in the environmental system.

From nutrition point of view, the biotic components can be grouped into two basic components:

(i) Autotrophic components, and

(ii) Heterotrophic components

The autotrophic components include all green plants which fix the radiant energy of sun and manufacture food from inorganic substances. The heterotrophic components include non-green plants and all animals which take food from autotrophs.

So biotic components of an ecosystem can be described under the following three heads:

1. Producers (Autotrophic components),

2. Consumers, and

3. Decomposers or reducers and transformers

The amount of biomass at any time in an ecosystem is known as standing crop which is usually expressed as fresh weight, dry weight or as free energy in terms of calories/metre.

Producers (Autotrophic elements):

The producers are the autotrophic elements—chiefly green plants. They use radiant energy of sun in photosynthetic process whereby carbon dioxide is assimilated and the light energy is converted into chemical energy. The chemical energy is actually locked up in the energy rich carbon compounds. Oxygen is evolved as by-product in the photosynthesis.

This is used in respiration by all living things. Algae and other hydrophytes of a pond, grasses of the field, trees of the forests are examples of producers. Chemosynthetic bacteria and carotenoid bearing purple bacteria that also assimilate CO₂ with the energy of sunlight but only in the presence of organic compounds also belong to this category.

The term producer is misleading one because in an energy context, producers produce carbohydrate and not energy. Since they convert or transduce the radiant energy into chemical form, E.J. Kormondy suggests better alternative terms ‘converters’ or ‘transducers’. Because of wide use the term producer is still retained.

Consumers:

Those living members of ecosystem which consume the food synthesized by producers are called consumers. Under this category are included all kinds of animals that are found in an ecosystem.

There are different classes or categories of consumers, such as:

(a) Consumers of the first order or primary consumers,

(b) Consumers of the second order or secondary consumers,

(c) Consumers of the third order or tertiary consumers, and

(d) Parasites, scavengers and saprobes.

(a) Primary consumers:

These are purely herbivorous animals that are dependent for their food on producers or green plants. Insects, rodents, rabbit, deer, cow, buffalo, goat are some of the common herbivores in the terrestrial ecosystem, and small crustaceans, molluscs, etc. in the aquatic habitat. Elton (1939) named herbivores of ecosystem as “key industry animals”. The herbivores serve as the chief food source for carnivores.

(b) Secondary consumers:

These are carnivores and omnivores. Carnivores are flesh eating animals and the omnivores are the animals that are adapted to consume herbivores as well as plants as their food. Examples of secondary consumers are sparrow, crow, fox, wolves, dogs, cats, snakes, etc.

(c) Tertiary consumers:

These are the top carnivores which prey upon other carnivores, omnivores and herbivores. Lions, tigers, hawk, vulture, etc. are considered as tertiary or top consumers.

(d) Besides different classes of consumers, the parasites, scavengers and saprobes are also included in the consumers. The parasitic plants and animals utilize the living tissues of different plants and animals. The scavengers and saprobes utilize dead remains of animals and plants as their food.

Decomposers and transformers:

Decomposers and transformers are the living components of the ecosystem and they are fungi and bacteria. Decomposers attack the dead remains of producers and consumers and degrade the complex organic substances into simpler compounds. The simple organic matters are then attacked by another kind of bacteria, the transformers which change these organic compounds into the inorganic forms that are suitable for reuse by producers or green plants. The decomposers and transformers play very important role in maintaining the dynamic nature of ecosystems.

Function of Ecosystem:

An ecosystem is a discrete structural, functional and life sustaining environmental system. The environmental system consists of biotic and abiotic components in a habitat. Biotic component of the ecosystem includes the living organisms; plants, animals and microbes whereas the abiotic component includes inorganic matter and energy.

Abiotic components provide the matrix for the synthesis and perpetuation of organic components (protoplasm). The synthesis and perpetuation processes involve energy exchange and this energy comes from the sun in the form of light or solar energy.

Thus, in any ecosystem we have the following functional components:

(i) Inorganic constituents (air, water and mineral salts)

(ii) Organisms (plants, animals and microbes), and

(iii) Energy input which enters from outside (the sun).

These three interact and form an environmental system. Inorganic constituents are synthesized into organic structures by the green plants (primary producers) through photosynthesis and the solar energy is utilized in the process. Green plants become the source of energy for renewals (herbivores) which, in turn become source of energy for the flesh eating animals (carnivores). Animals of all types grow and add organic matter to their body weight and their source of energy is complex organic compound taken as food.

They are known as secondary producers. All the living organisms whether plants or animals in an ecosystem have a definite life span after which they die. The dead organic remains of plants and animals provide food for saprophytic microbes, such as bacteria, fungi and many other animals. The saprobes ultimately decompose the organic structure and break the complex molecules and liberate the inorganic components into their environment.

These organisms are known as decomposers. During the process of decomposition of organic molecules, the energy which kept the inorganic components bound together in the form of organic molecules gets liberated and dissipated into the environment as heat energy. Thus in an ecosystem energy from the sun, the input is fixed by plants and transferred to animal components.

Nutrients are withdrawn from the substrate, deposited in the tissues of the plants and animals, cycled from one feeding group to another, released by decomposition to the soil, water and air and then recycled. The ecosystems operating in different habitats, such as deserts, forests, grasslands and seas are interdependent on one another. The energy and nutrients of one ecosystem may find their way into another so that ultimately all parts of the earth are interrelated, each comprising a part of the total system that keeps the biosphere functioning.

Thus the principal steps in the operation of ecosystem are as follows:

(1) Reception of radiant energy of sun,

(2) Manufacture of organic materials from inorganic ones by producers,

(3) Consumption of producers by consumers and further elaboration of consumed materials; and.

(4) After the death of producers and consumers, complex organic compounds are degraded and finally converted by decomposers and converters into such forms as are suitable for reutilization by producers.

The principal steps in the operation of ecosystem not only involve the production, growth and death of living components but also influence the abiotic aspects of habitat. It is now clear that there is transfer of both energy and nutrients from producers to consumers and finally to decomposers and transformers levels. In this transfer there is a progressive decrease of energy but nutrient component is not diminished and it shows cycling from abiotic to biotic and vice versa.

The flow of energy is unidirectional. The two ecological processes—energy flow and mineral cycling which involve interaction between biotic and abiotic components lie at the heart of ecosystem dynamics. The principal steps and components of ecosystem are illustrated in Fig. 3.1.

ECOSYSTEMS

ECOSYSTEMS

·         The interaction between the living organism and the non-living environment is called ecosystem.

ECOSYSTEM – STUCTURE AND FUNCTION:

·         Interaction of biotic and abiotic components results in a physical structure that is characteristic of each type of ecosystem.

·         Identification and description of plant and animal species of an ecosystem gives its species composition.

·         Vertical distribution of different species occupying different levels is called stratification.

·         The components of the ecosystem are seen to function as a unit:

o    Productivity.

o    Decomposition.

o    Energy flow and

o    Nutrient cycle.

·         Description of pond as an ecosystem:

o    The abiotic components include all dissolved inorganic and organic substances and the rich soil deposit at the bottom of the pond.

o    The solar input, cycle of temperature, day length, regulates the rater of function of the entire pond.

o    The producer (autotrophic) includes phytoplankton, some algae and the floating, submerged and marginal plants found in edge of pond.

o     The consumers are represented by zooplankton, free swimming and bottom dwelling animals.

o    The decomposers are the fungi, bacteria especially abundant at the bottom of the pond.

·         Basic events (in terms of function) in an ecosystem:

o    Conversion of inorganic into organic material (photosynthesis) by producers.

o    Consumption of the autotrophs by heterotrophs.

o    Decomposition and mineralization of the dead organic matter to release them back for reuse by the autotrophs

o    There is unidirectional flow of energy towards the higher trophic levels and its dissipation and loss as heat to the environment.

PRODUCTIVITY:

·         Primary productivity:

o    The amount of biomass or organic matter produced per unit area over a time period by plants during photosynthesis.

o    It is expressed in terms of weight (g-2) or energy (kcal m-2)

o    The rate of biomass production is called productivity.

·         Gross primary productivity: (GPP) is the rate of production of organic matter during photosynthesis.

·         Net primary productivity:

o    A considerable amount of energy is utilized by plants in respiration.

o    Gross primary productivity minus respiration losses (R) is the net primary productivity.

o    GPP – R = NPP.

·         Net primary productivity is the available biomass for the consumption to heterotrophs (herbivore and decomposers.

·         Secondary productivity: is defined as the rate of formation of new organic matter by the consumer.

DECOMPOSITION:

·         Earthworm is said to be ‘friends’ of farmer:

o    Breakdown the complex organic matter.

o    Loosening of the soil helps in aeration and entry of root.

·         The decomposers break down complex organic matter into inorganic substances like carbon dioxide, water and nutrients, called decomposition.

·         Dead plant remains such as leaves, bark, flowers and dead remains of animals, including fecal matter, constitute thedetritus.

·         The process of decomposition completed in following steps:

o    Fragmentation  :  Break  down  of  detritus  into  smaller  particles  by  detritivore  (earthworm).

o    Leaching: Water soluble inorganic nutrients go down into the soil horizon and get precipitated as unavailable salts.

o    Catabolism  :  Bacterial  and  fungal  enzymes  degrade  detritus  into  simple  inorganic  substances.

o    Humification: Accumulation of dark coloured amorphous substances called humus.

Importance of humus:

o    Highly resistance to microbial action.

o    Undergo decomposition at an extremely slow rate.

o    Being colloidal in nature, it serves as reservoir for nutrients.

o    Mineralization: The  humus  is  further  degraded  by  some  microbes  and  release of  inorganic nutrients  occur.

Factor affects rate of decomposition:

·         Decomposition is largely an oxygen-requiring process.

·         Detritus rich in chitin and lignin has slow rate of decomposition.

·         Detritus rich in nitrogen and water-soluble substance like sugar has faster decomposition.

·         Temperature and soil moisture are most important climatic factor that regulate decomposition

·         Warm and moist environment favor decomposition.

Low temperature, dryness and anerobiosis inhibit decomposition.

ENERGY FLOW IN ECOSYSTEM:

• Except for deep sea hydrothermal ecosystem, sun is the only source of energy for all ecosystems on earth.
• Less than 50% of incident solar radiation is photosynthetically active radiations. (PAR).
• Plants capture 2-10 % of PAR and used in photosynthesis.
• All organisms depend on the producers, either directly or indirectly.
• Energy flow in the ecosystem is unidirectional i.e. energy transferred from producer to consumers.
• Energy transfer is not absolute, and spontaneous, unless energy is degraded it can not be transfer. When energy transferred from one trophic level to another, lot of energy lost in the form of heat to the environment.
• Only 10% of energy transferred from one trophic level to other.

Food chain:

• Grazing food chain: it extends from producers through herbivore to carnivore.
• Detritus food chain: Begins with dead organic matter (detritus) and pass through detritus feeding organism in soil to organisms feeding on detritus-feeders.
• In aquatic ecosystem GFC is the major conduit for energy flow.
• In terrestrial ecosystems a much larger fraction of energy flows through the detritus food chain than through GFC
• Different food chains are naturally interconnected e.g. a specific herbivore of one food chain may serve as food of carnivores of other food chains. Such interconnected matrix of food chains is called food web.
• Trophic level: A group of organism irrespective of their size having same source of energy or similar food habit constitute a trophic level.
• Standing crop: each trophic level has a certain mass of living material at a particular time called as the standing crop.
• The standing crop is measured as the mass of living organisms (biomass) or the number in a unit area.
• The number of trophic levels in a food chain is restricted by 10 % flow of energy, less amount of energy available to the last trophic level.

ECOLOGICAL PYRAMID:

• The base of the pyramid is broad and it narrows down at the apex. The similar shape is obtained when food or energy relationship between organisms at different trophic level.
• The relationship can be expressed in terms of number, energy or biomass.
• The base of the pyramid represented by producer and apex is the top consumer; other trophic levels are in between.
• In most ecosystems, all the pyramids, of number, of energy and biomass are upright.
• The pyramid of number in a tree ecosystem is inverted.
• The pyramid of biomass in sea also inverted because the biomass of fishes is far exceeds that of phytoplankton.
• Pyramid of energy is always upright, can never be inverted, because when energy flows from a particular trophic level to the next, some energy is always lost as heat at each step.

Limitations of ecological pyramids:

• It does not take into account the same species belonging to two or more trophic levels.
• It assumes a simple food chain, it never exits in nature.
• It dose not accommodate food web.
• Saprophytes are not given place in ecological pyramids.

ECOLOGICAL SUCCESSION:

• The gradual and fairly predictable change in the species composition of a given area is called ecological succession.
• Composition and structure of the community constantly change in response to changing environmental condition.
• This change is orderly and sequential, parallel with the changes in the physical environment.
• All the changes lead finally to a community that is in near equilibrium with the environment and that is called climax community.
• During succession some species colonize and area and their populations become more numerous, whereas populations of other species decline and even disappear.
• The entire sequences of communities that successively change in a given area are called sere.
• The individual transitional communities are termed as seral stages.
• In the successive seral stages there is a change in the diversity of species of organisms, in crease in number of species and total biomass.
• Primary succession: succession that starts where no living organisms are there- these could be areas where no living organism ever existed may be a bare rock or new water body.
• Secondary succession:  succession that starts in areas that somehow, lost all the living organisms that existed there.
• Primary succession occurs in:-
• newly cooled lava,
• bare rock,
• Newly created pond or reservoir.
• Secondary succession begins in areas where natural biotic communities have been destroyed such as
• In abandoned farm lands.
• Burned or cut forest,
• land that have been flooded
• Since some soil or sediment is present, secondary succession is faster than primary succession.

Succession in plants:

• Based on the nature of habitat – whether it is water or it is on very dry areas- succession of plants is called hydrarchor xerarch.
• Hydrarch succession takes place in water areas and the successional series progress from hydric to mesiccondition.
• Xerarch succession takes place in dry areas and the series progress from xeric to mesic conditions.
• Both hydrarch and xerarch successions lead to medium water conditions (mesic) – neither too dry (xeric) nor too wet (hydric)

Xerarch succession: Succession in bare rock:

• The species that invades bare area are called pioneer species.
• In primary succession on bare rock the pioneer species is the lichen.
• Lichen secretes acid to dissolve rock, helping in weathering and soil formation.
• The little soil, leads to growth of bryophytes (mosses).
• The mosses speed up the process of soil accumulation by trapping wind-blown particles.
• Lichen moss carpet provides suitable substratum for the germination of seeds of herbaceous plants.
• Gradually more soil is accumulated and herbaceous species make way for the invasion of shrubs followed by trees.
• The climax community is generally dominated by trees.

Hydrarch (succession in aquatic environment)

• In primary succession in water, the pioneer species arephytoplankton.
• Zooplanktons.
• Sub merged plant stage. (rooted hydrophytes)
• Sub merged and free-floating plant stage.
• Reed-swamp stage.
• Marsh-meadow stage.
• Shrub stage
• Trees
• The climax again would be the forest
• All the succession whether taking place in water or on land, proceeds to a similar climax community – the mesic.

NUTRIENT CYCLING:

• Organism needs constant supply of nutrients to grow, reproduce, and regulate various body functions.
• Standing state: the amount nutrients such as carbon, nitrogen, phosphorus, calcium etc. present in soil at any given time.
• Nutrient cycling: The movement of nutrient elements through the various component of an ecosystem is called nutrient cycling.
• Another name of nutrient cycling is biogeochemical cycle.
• Nutrient cycles are of two types:
• Gaseous cycle
• Sedimentary cycle.

• The reservoir for gaseous type of nutrient cycle (nitrogen, carbon) exists in the atmosphere.
• The reservoir for sedimentary cycle (sulphur, phosphorus) is Earth’s crust.
• Environmental factors like soil, moisture, pH temperature regulate the rate of release of nutrient into the atmosphere.
• The function of the reservoir is to meet the deficit which occurs due to imbalance in the rate of influx and efflux.

Ecosystem – Carbon cycle:

• Carbon constitutes 49 percent of dry weight of organism.
•  Out of total global carbon:
• 71 percent carbon found dissolved in ocean.
• About 1 percent in the atmosphere.

• 4 X 10¹³ kg of carbon is fixed in the biosphere by photosynthesis, annually.
• Large amount of carbon returned to the atmosphere as CO₂ through respiration of producers and consumers.
• Decomposers also return CO₂ to reservoir during decomposition process.
• Some amount of Carbon is lost to sediments and removed from circulation.

• Burning wood, forest fire, combustion of organic matter, fossil fuel, volcanic activities are additional sources for releasing CO₂ to atmosphere.

Influence of human activity on Carbon cycling.

• Rapid deforestation.
• Massive burning of fossil fuel for energy and transport
• Increased the rate of release of CO₂ into the atmosphere.

Ecosystem Phosphorus cycle:

• Phosphorus is a major constituent of biological membranes, nucleic acids and cellular energy transfer system(ATP)
• Animals need phosphorus to make shell, bones and teeth.
• Reservoir pool of phosphorus is the rock, which contain phosphorus in the form of phosphates.
• During weathering of rock small amount of phosphates dissolved in soil solution and are absorbed by the roots of the plants.
• Herbivore and other animals obtain organic form of phosphorus from plants.
• The waste product and dead organisms are decomposed by phosphate-solubilising bacteria releasing phosphorus.

How phosphorus cycle differs from carbon cycle?

• There is no respiratory release of phosphorus into atmosphere.
• Atmospheric inputs of phosphorus through rainfall are much smaller.
• Gaseous exchange of phosphorus between organism and environment are negligible.

ECOSYSTEM SERVICES:

• The products of ecosystem processes are named as ecosystem services.
• Healthy forest ecosystems purify air and water.
• Mitigate droughts and flood.
• Cycle nutrients.
• Generates fertile soil.
• Provide wildlife habitat.
• Maintain biodiversity.
• Pollinate crops.
• Provide storage site for carbon
• Provides aesthetic, cultural and spiritual values

PAR:        Photosynthetically Active Radiation
GAP:        Gross Primary Productivity
NPP:        Net Primary Productivity
DFC:        Detritus Food Chain
GFC:        Grazing Food chain