FUNCTIONS OF AN ECOSYSTEM Study Materials

Ecological Succession

Ecological succession or Ecosystem development

Ecological succession, also known as ecosystem development, is a dynamic process that describes the orderly and sequential changes in species composition and structure of a community over time in response to changing environmental conditions. This process ultimately aims to establish a stable and mature climax community, which represents the endpoint of succession and attains equilibrium within the ecosystem.

Key characteristics of climax succession include

• Increased Productivity: The climax community typically exhibits high levels of productivity, with efficient energy capture and utilisation within the ecosystem.

• Nutrient Cycling: There is a shift in nutrient dynamics, where nutrients are efficiently cycled within the ecosystem. This ensures a balanced nutrient availability for sustaining the biotic components of the community.

• Increased Diversity: The climax community harbors a greater diversity of organisms compared to earlier successional stages. This diversity arises from the establishment of various ecological niches, providing opportunities for a wide range of species to coexist.

• Complex Food Webs: As succession progresses towards climax, there is a gradual increase in the complexity of food webs within the ecosystem. This complexity arises from the interconnections and interactions among different trophic levels, leading to a more stable and resilient ecosystem.

These characteristics collectively contribute to the resilience and stability of the climax community, enabling it to persist in relative equilibrium until disturbed by external factors. Succession plays a fundamental role in shaping the present- day communities on Earth, as it has occurred over millions of years, influencing the distribution and abundance of species across various ecosystems.

Stages in Ecological Succession

Ecological succession progresses through distinct stages, each characterized by changes in species composition and environmental conditions. These stages include:

• Pioneer Community: This is the initial stage of succession, where the first species to colonize a barren or disturbed area establish themselves. These species, such as mosses and lichens, are often well-adapted to harsh

conditions and play a crucial role in soil formation and stabilization.

• Seral Communities: As succession proceeds, the pioneer community modifies the environment, creating conditions suitable for the establishment of new species. These transitional communities, known as seral communities, replace one another in a sequential manner as species composition and environmental conditions continue to change.

• Climax Community: The final stage of succession is the climax community, which represents a stable, mature, and diverse ecosystem. This community is well-adapted to prevailing environmental conditions and typically consists of dominant species, such as a well-developed forest in terrestrial ecosystems. The climax community persists in relative equilibrium until disrupted by a significant disturbance.

Primary Succession

Primary succession occurs in areas where no previous community has existed, such as newly formed rock surfaces or areas devoid of soil, like after a volcanic eruption or glacier retreat.

• Starting from Scratch: Pioneer species, like lichens and mosses, are the first to colonize these barren landscapes. They can survive with minimal soil and begin the process of breaking down rocks and forming soil.

• Building Soil and Habitat: As pioneer species grow and die, they contribute to soil formation. This soil gradually accumulates, allowing more complex plants, like grasses and shrubs, to take root and thrive.

• Transition to a Climax Community: Over time, as soil depth and nutrient levels increase, larger plants like trees can establish themselves. This leads to the development of a diverse and stable ecosystem, such as a forest.

• Secondary Succession: Once a climax community is established, if the area is disturbed by events like fires or logging, secondary succession occurs. This involves the re-establishment of vegetation in an area that already has soil, speeding up the process compared to primary succession.

So, primary succession is like nature starting from scratch, gradually building up soil and habitat until a mature ecosystem, like a forest, is formed.

Autotrophic succession refers to ecological succession where the initial stages are dominated by autotrophs, which are organisms capable of producing their own food through processes like photosynthesis. These autotrophs, such as plants, algae, and some bacteria, are able to harness energy from light, water, and carbon dioxide to synthesize organic compounds.

Heterotrophic succession, on the other hand, involves initial stages dominated by heterotrophs. Heterotrophs are organisms that cannot produce their own food and instead rely on organic matter produced by other organisms for their nutrition. Examples of heterotrophs include animals, fungi, and some bacteria.

Autogenic succession occurs primarily due to the actions of biotic components within the ecosystem. This includes processes such as plant growth, nutrient cycling, and the accumulation of organic matter like leaf litter, which gradually modify the environment over time.

In contrast, allogenic succession is driven by external abiotic factors such as natural disasters (volcanic eruptions, flooding, forest fires) or human interventions (deforestation, urbanization). These external forces can disrupt the existing ecosystem and initiate succession by altering environmental conditions and creating opportunities for new species to colonize the area.

So, in essence, autotrophic succession focuses on the dominance of self-sustaining organisms in the early stages, while heterotrophic succession emphasizes the reliance on external sources of organic matter. Autogenic succession is driven by internal biotic factors, whereas allogenic succession is triggered by external abiotic influences.

Secondary Succession

• Secondary succession is the sequential development of biotic communities after the complete or partial destruction of the existing community.

• As opposed to the primary succession, secondary succession is a process started by an event where a climax community or intermediate community is impacted by a disturbance (e.g., forest fire, harvesting, hurricane, floods etc.)

• This restarts the cycle of succession, but not back to the beginning because:

• Soil and nutrients are present

• Seeds, Roots etc. of plants may also be present.

Therefore, secondary succession is usually faster than primary succession.

Many factors can affect secondary succession, such as trophic interaction, initial composition, and competition- colonization trade-offs.

The secondary succession starts with growth of grasses and weeds. Next, small plants begin to grow and thereafter the sequence is similar to the primary succession.

Succession in Plants

• Succession in plants is based on the nature of the habitat i.e., whether it is water (or very wet areas) or it is on very dry areas.

Hydrarch succession: It is the succession that takes place in wetter areas and the successional series progress from hydric (condition of very high amount of moisture) to the mesic (condition of moderate amount of moisture) conditions.

Xerarch succession: It is the succession that takes place in dry areas and the series progress from xeric (condition of very low moisture) to mesic (condition of moderate amount of moisture) conditions.

• Hence, both hydrarch and xerarch successions lead to medium water conditions (mesic) – neither too dry (xeric) nor too wet (hydric).

Succession in Water

• In water, the pioneers are the small phytoplankton which are replaced with time by free floating angiosperms, then by rooted hydrophytes (aquatic plants), sedges (some monocotyledonous plants), grasses and finally the trees.

• The climax again would be a forest (with the passage of time, water body will be converted back into land).

• Phytoplankton stage

• Phytoplanktons (cyanobacteria), green algae (Spirogyra, Oedogonium), diatoms, etc. are the pioneer colonizers in the initial stage, starting from a water body, such as a pond.

• Their spores are carried by air to the pond.

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The phytoplankton are followed by zooplankton.

• They settle down to the bottom of the pond after death, and decay into humus that mixes with silt and clay particles brought into the basin by runoff water and wave action and form soil.

• As soil builds up, the pond becomes shallower and further environmental changes follow.

• Submerged stage

• As the water body becomes shallower, more submerged rooted species are able to become established due to increasing light penetration in the shallower water.

• This is suitable for growth of rooted submerged species such as Myriophyllum, Vallisneria, Elodea, Hydrilla, and Ceratophyllum. These plants root themselves in mud.

• Once submerged species colonize the successional changes are more rapid and are mainly autogenic as organic matter accumulates. Inorganic sediment is still entering the lake and is trapped more quickly by the net of plant roots and rhizomes growing on the pond floor. The pond becomes sufficiently shallow (2–5 ft) for floating species and less suitable for rooted submerged plants.

• Floating stage

• The floating plants are rooted in the mud, but some or all their leaves float on the surface of the water.

• These include species like Nymphaea, Nelumbo and Potamogeton.

• Some free-floating species also become associated with root plants. The large and broad leaves of floating plants shade the water surface and conditions become

unsuitable for growth of submerged species which start disappearing. The plants decay to form organic mud which makes the pond shallower yet (1–3 ft).

• Reed swamp stage

• The pond is now invaded by emergent plants such as Phragmites (reed-grasses), Typha (cattail), and Zizania (wild rice) to form a reed-swamp (in North American usage, this habitat is called a marsh). These plants have creeping rhizomes which knit the mud together to produce large quantities of leaf litter. This litter is resistant to decay and reed peat builds up, accelerating the autogenic change. The surface of the pond is converted into water- saturated marshy land.

• Sedge-meadow stage

• Water surface of a pond nearly covered by vegetation

• Successive decreases in water level and changes in substratum help members of Cyperaceae and Graminae such as Carex spp. (several species) and Juncus to establish themselves. They form a mat of vegetation extending towards the centre of the pond. Their rhizomes knit the soil further. The above water leaves transpire water to lower the water level further and add additional leaf litter to the soil. Eventually the sedge peat accumulates above the water level and soil is no longer totally waterlogged. The habitat becomes suitable for invasion of herbs (secondary species) such as Mentha, Caltha, Iris, and Galium which grow luxuriantly and bring further changes to the environment. Mesic conditions develop and marshy vegetation begins to disappear.

• Woodland stage

• The soil now remains drier for most of the year and becomes suitable for development of wet woodland.

• It is invaded by shrubs and trees such as Salix (willow), Alnus (alder), and Populus (poplar).

• These plants react upon the habitat by producing shade, lower the water table still further by transpiration, build up the soil, and lead to the accumulation of humus with associated microorganisms.

• This type of wet woodland is also known as carr. (Cotton rate of rotting).

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Climax stage

• Finally, a self-perpetuating climax community develops. It may be a forest if the climate is humid, grassland in case of sub-humid environment, or a desert in arid and semi- arid conditions. A forest is characterized by presence of all types of vegetation including herbs, shrubs, mosses, shade-loving plants and trees. Decomposers are frequent in climax vegetation.

The overall changes taking place during development of successional communities are building up of substratum, shallowing of water, addition of humus and minerals, soil building and aeration of soil. As the water body fills in with sediment, the area of open water decreases and the vegetation types moves inwards as the water becomes shallower. Many of the above-mentioned communities can be seen growing together in a water body. The center is occupied by floating and submerged plants with reeds nearer the shores, followed by sedges and rushes growing at the edges. Still further are shrubs and trees occupying the dry land

Homeostasis in Ecosystem

Homeostasis in Ecosystem

Homeostasis in ecosystems refers to their ability to maintain stable system properties despite external disturbances or changes. Ecosystems strive to maintain a state of equilibrium, where various factors such as species composition, population sizes, and functional processes remain relatively stable over time.

• Self-Regulation: Ecosystems have mechanisms for regulating their own species structure and functional processes. This capacity for self-regulation ensures that ecological communities can adapt to changes and maintain stability.

• Example in a Pond Ecosystem: In a pond ecosystem, if the population of zooplankton increases, they consume a large number of phytoplankton, causing food scarcity for zooplankton. This scarcity triggers a negative feedback mechanism: as zooplankton starve, their population decreases, allowing phytoplankton to proliferate again. This cycle continues, with populations fluctuating to maintain a balance.

• Limitations of Homeostasis: While ecosystems exhibit homeostatic capacity, it’s not unlimited. External disturbances beyond a certain threshold can disrupt equilibrium, leading to changes in species composition or functional processes. Additionally, not all aspects of an ecosystem are always well-regulated, and some fluctuations may occur.

• Comparison to Physiological Homeostasis: Homeostasis in ecosystems shares similarities with physiological homeostasis in organisms. Just as organisms maintain stable internal environments despite external fluctuations, ecosystems strive to maintain stability despite changes in external conditions.

Regulate

• Some organisms can maintain homeostasis by physiological (sometimes behavioral — migrating to tree shade) means which ensures constant body temperature, constant osmotic concentration, etc.

• It is a process where the organism moves away temporarily from the stressful habitat to a more hospitable area and return when a stressful period is over. For ex: the Amur falcon, a small sized raptor from the falcon family, breeds in Siberia and Northern China but migrates to the warmer climate of the Southern part of Africa via India every year.

• Almost all birds and mammals and a very few lower vertebrate and invertebrate species are indeed capable of such regulation (thermoregulation and osmoregulation).

Thermoregulation

• Thermoregulation is the process occurring inside the body that is responsible for maintaining the core temperature of the body.

• Thermoregulation works by the negative feedback loop where once the body temperature is either increased or decreased beyond its normal temperature, it is brought back to normal.

• Different homeostatic processes like sweating, dilation of blood vessels counteract the increased body temperature, whereas processes like contraction of blood vessels, and breakdown of adipose tissue to produce heat prevent the decreased body temperature.

• The process of thermoregulation is maintained by organs like skin and adipose tissue of the integumentary system and the hypothalamus of the brain.

Osmoregulation

Osmoregulation is the process of maintaining a constant osmotic pressure inside the body by balancing the concentration of fluids and salts.

During this process, excess water or ions or other molecules like urea are removed from the body to maintain the osmotic balance.

One classic example of this process is the removal of excess water and ions out of the blood in the form of urine to maintain the osmotic pressure of the blood.

The renin-angiotensin system and other hormones like antidiuretic hormones act as a messenger for the electrolytic regulation system of the body.

Conform

To “conform” means that an organism, like most animals and many plants, cannot maintain a stable internal environment. Instead, their body temperature or other bodily functions change in response to the surrounding environment.

• Temperature and Environment: Conformers, also known as ectotherms, rely on external sources like sunlight to regulate their body temperature. They don’t have the ability to maintain a constant internal temperature like warm-blooded animals do.

• Cold-Blooded Adaptation: These animals are often called “cold-blooded” because their body temperature adjusts to their surroundings. When it gets cold, they slow down, and when it’s warm, they become more active.

• Challenges for Small Animals: Small animals and birds have a harder time maintaining their body temperature because they lose heat more quickly due to their large surface area compared to their volume. This makes it energetically expensive for them to generate body heat, limiting their presence in extreme environments like polar regions.

• Adaptations for Heat: Some animals, like lizards and chameleons, bask in the sunlight to warm themselves up during colder weather. This behavior helps them regulate their body temperature within a limited range.

• Adapting to Stress: If the environment becomes too harsh, some organisms have evolved strategies like hibernation (sleeping through the cold) or aestivation (going dormant during hot, dry periods) to survive.

Energy Flow – Trophic Levels

• A trophic level (trophe = Nourishment) is the representation of energy flow in an ecosystem.

• The chemical energy of food is the main source of energy required by all living organisms. This energy is transmitted to different trophic levels along the food chain.

• Trophic structure of an ecosystem is defined as organization of biotic components based on their feeding/food relationship.

• This energy flow is based on two different laws of thermodynamics: First law of thermodynamics, that states that energy can neither be created nor destroyed, it can only change from one form to another.

• Second law of thermodynamics, that states that as energy is transferred more and more of it is wasted.

• The energy flow in the ecosystem is one of the major factors that support the survival of such a great number of organisms. For almost all organisms on earth, the primary source of energy is solar energy.

• It is amusing to find that we receive less than 50 per cent of the sun’s effective radiation on earth. When we say effective radiation, we mean the radiation which can be used by plants to carry out photosynthesis.

• Most of the sun’s radiation that falls on the earth is usually reflected back into space by the earth’s atmosphere. This effective radiation is termed as the Photosynthetically Active Radiation (PAR).

• Overall, we receive about 40 to 50 percent of the energy having Photosynthetically Active Radiation and only around 2-10 percent of it is used by plants for the process of photosynthesis.

• Thus, this percent of PAR supports the entire world as

plants are the producers in the ecosystem and all the other organisms are either directly or indirectly dependent on them for their survival.

• The energy flow takes place via food chain and food web. During the process of energy flow in the ecosystem, plants being the producers absorb sunlight with the help of the chloroplasts and a part of it is transformed into chemical energy and in the process of photosynthesis.

• This energy is stored in various organic products in the plants and passed on to the primary consumers in the food chain when the herbivores consume (primary consumers) the plants as food and convert chemical energy accumulated in plant products into kinetic energy, degradation of energy will occur through its conversion into heat. Then followed by the secondary consumers.

• When these herbivores are consumed by carnivores of the first order (secondary consumers) further degradation will occur. Finally, when tertiary consumers consume the carnivores, again energy will be degraded.

• Thus, the energy flow is unidirectional in nature. Based on the source of their nutrition or food, organisms occupy a specific place in the food chain that is known as their trophic level. Producers belong to the first trophic level, herbivores (primary consumer) to the second and carnivores (secondary consumer) to the third.

• The important point to note is that the amount of energy decreases at successive trophic levels. When any organism dies, it is converted to detritus or dead biomass that serves as an energy source for decomposers.

• Organisms at each trophic level depend on those at the lower trophic level for their energy demands. 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 biomass of a species is expressed in terms of fresh or dry weight.

• Moreover, in a food chain, the energy flow follows the 10 percent law. According to this law, only 10 percent of energy is transferred from one trophic level to the other; rest is lost into the atmosphere.

• 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

• Energy flows through the trophic levels from producers to subsequent trophic levels is unidirectional.

• Energy level decreases from the first trophic level upwards due to loss of energy in the form of heat at each trophic level.

Food Chain and Food Web

Food Chain

• A food chain is a linear network of links in a food web starting from producer organisms (such as grass or algae which produce their own food via photosynthesis) and ending at an apex predator species (like grizzly bears or killer whales), detritivores (like earthworms or woodlice), or decomposer species (such as fungi or bacteria).

• A food chain also shows how organisms are related to each other by the food they eat. Each level of a food chain represents a different trophic level.

• A food chain differs from a food web because the complex network of different animals’ feeding relations are aggregated and the chain only follows a direct, linear pathway of one animal at a time. Natural interconnections between food chains make it a food web.

• Transfer of food energy from green plants (producers) through a series of organisms with repeated eating and being eaten link is called a food chain. E.g., Grasses → Grasshopper → Frog → Snake → Hawk/Eagle.

• Each step in the food chain is called trophic level.

• A food chain starts with producers and ends with top carnivores

• The trophic level of an organism is the position it occupies in a food chain.

Types of Food Chains: 1) Grazing food chain and 2) Detritus food chain

Grazing food chain

Grazing Food Chain is a food chain in which the lowest trophic energy level is acquired from organisms that are involved in photosynthesis (plants, grass etc). These green plants are the primary producers in this food chain who prepare their own food with energy from the sun via photosynthesis. The food chain later moves from herbivores to carnivores. This grazing food chain is the primary source of energy flow in the ecosystem.

The consumers which start the food chain, utilising the plant or plant part as their food, constitute the grazing food chain.

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

In Aquatic ecosystem phytoplankton (primary producers) are eaten by zooplanktons which are eaten by fishes and fishes are eaten by pelicans.

Detritus food chain

• In biology, detritus is dead particulate organic material, as distinguished from dissolved organic material. Detritus typically includes the bodies or fragments of bodies of dead organisms, and fecal material. Detritus typically hosts communities of microorganisms that colonize and decompose (i.e. remineralize) it. In terrestrial ecosystems it is present as leaf litter and other organic matter that is intermixed with soil, which is denominated “soil organic matter”. The detritus of aquatic ecosystems is organic material that is suspended in the water and accumulates in depositions on the floor of the body of water; when this floor is a seabed, such a deposition is denominated “marine snow”.

• Detritus food chain is the type of food chain that starts with dead organic materials. The dead organic substances are decomposed by microorganisms. The organisms that feed on dead organic matter or detritus, are known as detritivores or decomposers. These detritivores are later eaten by predators.

• This type of food chain starts from organic matter of dead and decaying animals and plant bodies from the grazing food chain.

• Dead organic matter or detritus feeding organisms are called detrivores or decomposers. The detrivores are eaten by predators.

• In an aquatic ecosystem, the grazing food chain is the major conduit for energy flow.

• As against this, in a terrestrial ecosystem, a much larger fraction of energy flows through the detritus food chain than through the grazing food chain.

Food Web

A food web is the natural interconnection of food chains and a graphical representation of what-eats-what in an ecological community.

Another name for food web is consumer-resource system.

Ecologists can broadly lump all life forms into one of two categories called trophic levels:

The autotrophs, and

The heterotrophs.

To maintain their bodies, grow, develop, and to reproduce, autotrophs produce organic matter from inorganic substances, including both minerals and gases such as carbon dioxide. These chemical reactions require energy, which mainly comes from the Sun and largely by photosynthesis, although a very small amount comes from bioelectrogenesis in wetlands, and mineral electron donors in hydrothermal vents and hot springs.

These trophic levels are not binary, but form a gradient that includes complete autotrophs, which obtain their sole source of carbon from the atmosphere, mixotrophs (such as carnivorous plants), which are autotrophic organisms that partially obtain organic matter from sources other than the atmosphere, and complete heterotrophs that must feed to obtain organic matter.

The linkages in a food web illustrate the feeding pathways, such as where heterotrophs obtain organic matter by feeding on autotrophs and other heterotrophs.

The food web is a simplified illustration of the various methods of feeding that links an ecosystem into a unified system of exchange. There are different kinds of feeding relations that can be roughly divided into herbivory, carnivory, scavenging and parasitism. Some of the organic matter eaten by heterotrophs, such as sugars, provides energy. Autotrophs and heterotrophs come in all sizes, from microscopic to many tonnes - from cyanobacteria to giant redwoods, and from viruses and bdellovibrio to blue whales.

It represents all the possible paths of energy flow in an ecosystem.

• Significance of Food Web: If any of the intermediate food chains is 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.

Ecological Pyramids

Ecological Pyramids

• An ecological pyramid is known by different names such as Trophic Pyramid, Eltonian Pyramid, Energy Pyramid or Food pyramid.

• Ecological pyramids are used to compare different communities of the ecosystem by comparing trophic levels.

• It is a graphical representation designed to depict number of individuals, energy levels and the amount of biomass at different trophic levels of an ecosystem.

• Ecological pyramids begin with producers on the bottom (such as plants) and proceed through the various trophic levels (such as herbivores that eat plants, then carnivores that eat flesh, then omnivores that eat both plants and flesh, and so on). The highest level is the top of the food chain.

• The pyramid consists of a number of horizontal bars depicting specific trophic levels. The length of each bar represents the total number of individuals or biomass or energy at each trophic level in an ecosystem.

Pyramid of numbers

A pyramid of numbers shows graphically the population, or abundance, in terms of the number of individual organisms involved at each level in a food chain.

This shows the number of organisms in each trophic level without any consideration for their individual sizes/ biomass i.e., their weight.

Pyramid of number does not accurately define the trophic structure for an ecosystem as it is very difficult to count all the individuals.

Pyramid of numbers can be upright or inverted.

Pyramid of Numbers – Upright

The number of individuals decrease from lower level to higher trophic level. For e.g., Terrestrial or Aquatic ecosystem.

The primary producers (plants) occupy the lowest trophic level (base) because of their abundance. Next in the food chain is primary herbivores like a grasshopper. The individual number of grasshoppers is less than that of grass. The next in the food chain after primary herbivore is primary carnivore like rats. The number of rats is less than grasshoppers, because they feed on grasshoppers. The next higher trophic level is secondary carnivore like snakes. They feed on rats. The last in trophic level is the top carnivore like Hawk. They are least in numbers. Hence, with each higher trophic level, the number of individual decreases.

Pyramid of Numbers – Inverted

The number of individuals increase from lower level to higher trophic level. E.g., If we consider a single tree ecosystem.

If we look carefully the ecosystem of a single tree, we might observe a single tree host more than one herbivore i.e., plant eating birds. Then, in the next trophic level we have various parasites that derive their energy by feeding on primary herbivores. They are more in number than primary herbivores. Finally, at the top we have final consumers i.e., hyperparasites, which feed on parasites and derive their energies from them. In this case, number of individuals of hyperparasites are much greater number than parasites. Therefore, number of individuals increase from lower to higher trophic level.

• Spindle Shaped: Let us consider again example of ecosystem of a single tree, we might observe a single tree host more than one herbivore i.e., plant eating birds.

Then, in the next trophic level we have final consumer i.e., Hawk or Eagle that derive their energy by feeding on primary herbivores. In this case, initially number of individuals increase then it decreases. Hence, it forms a spindle shaped figure.

Pyramid of Biomass

A pyramid of biomass shows the relationship between biomass and trophic level by quantifying the biomass present at each trophic level of an ecological community at a particular time.

It is a graphical representation of biomass (total amount of living or organic matter in an ecosystem) present in unit area in different trophic levels. Typical units are grams per square meter, or calories per square meter.

Each trophic level has a certain mass of living material at a particular time called the standing crop.

Pyramid of biomass is usually determined by collecting all organisms occupying each trophic level separately and measuring their dry weight.

Measurement of biomass in terms of dry weight is more accurate. Why? - Dry weight is the exact mass of anybody which remains constant but wet mass or body with water have variable mass as per water present in the body. The moisture content of a biomass may vary depending upon environmental or physiological conditions.

Pyramid of Biomass – Upright

The upright pyramid of biomass is generally seen in Terrestrial ecosystems. Here, biomass (dry weight) of primary producers i.e., plants is maximum. The biomass of next trophic level i.e. primary consumers is less than the producers. The biomass of 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.

Why biomass decrease as we move up the trophic levels in a pyramid?

• Biomass can be lost between stages because not all of the matter eaten by an organism is digested. Some of it is excreted as waste such as solid faeces, carbon dioxide and water in respiration and water and urea in urine.

• Because only around 10% of the biomass at each trophic level is passed to the next, the total amount becomes very small after only a few levels. So, food chains are rarely longer than six trophic levels.

Pyramid of Biomass – Inverted

In contrast to terrestrial biomass, the pyramid of biomass in aquatic ecosystem may be “inverted”.

For example, in a pond ecosystem, the standing crop of phytoplankton, the major producers (bottom at trophic level), at any given point will be lower than the mass of the heterotrophs, such as fish and insects because the phytoplankton reproduce very quickly, but have much shorter individual lives.

Pyramid of Energy

A pyramid of energy or pyramid of productivity shows the production or turnover (the rate at which energy or mass is transferred from one trophic level to the next) of biomass at each trophic level.

Instead of showing a single snapshot in time, productivity pyramids show the flow of energy through the food chain. Typical units are grams per square meter per year or calories per square meter per year. As with the others, this graph show’s producers at the bottom and higher trophic levels on top.

When an ecosystem is healthy, this graph produces a standard ecological pyramid. This is because, in order for the ecosystem to sustain itself, there must be more energy at lower trophic levels than there is at higher trophic levels. This allows organisms on the lower levels to not only maintain a stable population, but also to transfer energy up the pyramid. The exception to this generalization is when portions of a food web are supported by inputs of resources from outside the local community. In small, forested streams, for example, the volume of higher levels is greater than could be supported by the local primary production.

Energy usually enters ecosystems from the Sun. The primary producers at the base of the pyramid use solar radiation to power photosynthesis which produces food. However most wavelengths in solar radiation cannot be used for photosynthesis, so they are reflected back into space or absorbed elsewhere and converted to heat.

Only 1 to 2 percent of the energy from the sun is absorbed by photosynthetic processes and converted into food. When energy is transferred to higher trophic levels, on average only about 10% is used at each level to build biomass, becoming stored energy. The rest goes to metabolic processes such as growth, respiration, and reproduction.

Advantages of the pyramid of energy as a representation

It takes account of the rate of production over a period of time.

Two species of comparable biomass may have very different life spans. Thus, a direct comparison of their total biomasses is misleading, but their productivity is directly comparable.

The relative energy chain within an ecosystem can be compared using pyramids of energy; also different ecosystems can be compared.

There are no inverted pyramids.

The input of solar energy can be added.

Disadvantages of the pyramid of energy as a representation:

The rate of biomass production of an organism is required, which involves measuring growth and reproduction through time.

There is still the difficulty of assigning the organisms to a specific trophic level. As well as the organisms in the food

chains there is the problem of assigning the decomposers and detritivores to a particular level.

Energy pyramid is most suitable to compare the functional roles of the trophic levels in an ecosystem.

An energy pyramid represents the amount of energy at each trophic level and loss of energy at each transfer to another trophic level.

Hence the pyramid is always upward, with a large energy base at the bottom.

Suppose an ecosystem receives 1000 calories of light energy on a particular day, most of the energy is not absorbed.

Some of it is reflected to space.

Of the energy absorbed only a small portion is utilised by green plants, out of which the plant uses up some for respiration; therefore only 100 calories are stored as energy-rich materials.

Now suppose an animal, say a deer, eats the plant containing 100 calories of food energy. The deer use some of it for its metabolism and stores only 10 calories as food energy.

A lion that eats the deer gets an 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.

Energy pyramid concept helps to explain the phenomenon of biological magnification — the tendency for toxic substances to increase in concentration progressively with higher trophic levels.

Bioaccumulation and Bio-magnification

Bioaccumulation and Bio-magnification

Pollutants mainly non-biodegradable ones build up in the organisms through two processes:

Bioaccumulation

Biomagnification

Bio persistence: It is defined as the time duration for which a waste material remains in the biosphere.

Bioaccumulation

It is the gradual accumulation of pollutants & substance, such as pesticide or other heavy chemicals, in an organism from all sources, including water, air and food.

It occurs when the rate of loss of the substance from the body of the organism through catabolism (breakdown of complex molecules in living organisms), or excretion is lower than the rate of accumulation of the substance.

For e.g., Rising mercury levels in fishes.

Biomagnification

It is the increase in concentration of a pollutant in the tissues of organism at successively higher levels in food chain.

In other words, it can be said that biomagnification refers to progressive bioaccumulation (increase in concentration) at each tropical level with the passage of time.

Consider for an example a water body is being polluted (let’s consider mercury for simplicity) by a factory discharge. Now if that water is being absorbed & accumulates in a Alage or directly into any marine organism – Bioaccumulation. But if we say that rising mercury levels are found in human body, then how do you justify that statement? – Here the process involved is biomagnification because directly the pollutant is not adding up in human body but through the food chain process. A pollutant must not be short lived or soluble in water as it will leave body through excretion and will not be biomagnified.

Nutrient Cycling

Nutrient cycling (Bio – Living; Geo – Rocks and Soils; Chemical – Process Involved)

• Every ecosystem has several interrelated mechanisms that affect human life. These are the water cycle, the carbon cycle, the oxygen cycle, the nitrogen cycle and the energy cycle. While every ecosystem is controlled by these cycles, in each ecosystem its abiotic and biotic features are distinct from each other. All elements in the earth are recycled time and again.

• Biogeochemical cycles refer to the flow of such chemical elements and compounds between organisms and the physical environment. Chemicals taken in by organisms are passed through the food chain and come back to the soil, air, and water through mechanisms such as respiration, excretion, and decomposition. As an element moves through this cycle, it often forms compounds with other elements as a result of metabolic processes in living tissues and of natural reactions in the atmosphere, hydrosphere, or lithosphere. Such cyclic exchange of material between the living organisms and their non- living environment is called Biogeochemical Cycle.

• Environmental factors, e.g., soil, moisture, pH, temperature, etc., regulate the rate of release of nutrients

into the atmosphere. The function of the reservoir is to meet with the deficit which occurs due to imbalance in the rate of influx and efflux.

Types of Biogeochemical Cycle

• Nutrient cycles are of two types:

Gaseous – The reservoir for gaseous type of nutrient cycle (e.g., nitrogen, carbon cycle) exists in the atmosphere

Sedimentary – The reservoir for sedimentary cycle is located in earth’s crust (Phosphorous, Sulphur etc.)

Carbon Cycle

• Carbon enters into the living world in the form of carbon dioxide through the process of photosynthesis as carbohydrates.

• These organic compounds (food) are then passed from the producers to the consumers (herbivores & carnivores).

• This carbon is finally returned to the surrounding medium by the process of respiration or decomposition of plants and animals by the decomposers.

• Carbon is also recycled during the burning of fossil fuels.

• Carbon is a minor constituent of the atmosphere as compared to oxygen and nitrogen.

• However, without carbon dioxide, life could not exist because it is vital for the production of carbohydrates through photosynthesis by plants and phytoplankton.

•
It is the element that anchors all organic substances from coal and oil to DNA (deoxyribonucleic acid: the compound that carries genetic information).

• Carbon is present in the atmosphere, mainly in the form of carbon dioxide (CO2).

• The carbon cycle involves a continuous exchange of carbon between the atmosphere and organisms.

• Carbon from the atmosphere moves to green plants and phytoplankton by the process of photosynthesis and then to animals.

• By the process of respiration and decomposition of dead organic matter, it returns to the atmosphere. It is usually a short-term cycle.

• Decomposition of decaying organic matter (carbohydrate, proteins and lipids, which are made up of carbon, oxygen, hydrogen and nitrogen) produces nitrogen and carbon dioxide.

• Some carbon also enters a long-term cycle. It accumulates as un-decomposed organic matter in the peaty layers of marshy soil or as insoluble carbonates in bottom sediments of aquatic systems, which take a long time to be released.

• Some carbon remains in the environment for a long time. It collects in places like the thick layers of decaying plants in wetlands or in solid forms like carbonate minerals at the bottom of lakes or oceans. These forms of carbon stay stored away for a while before eventually being released back into the environment.

• In deep oceans, such carbon can remain buried for millions of years till geological movement may lift these rocks above sea level.

• These rocks may be exposed to erosion, releasing their carbon dioxide, carbonates and bicarbonates into streams and rivers.

• Fossil fuels such as coals, oil and natural gas are organic compounds that were buried before they could be decomposed and were subsequently transformed by time and geological processes into fossil fuels.

• When they are burned, the carbon stored in them is released back into the atmosphere as carbon dioxide.

Nitrogen Cycle

Importance of Nitrogen Cycle

• Nitrogen is an essential component cells and tissues of all living organisms including human beings.

• It also helps plants to manufacture chlorophyll.

• It helps in enriching the soil & make it fertile.

• Our atmosphere contains nearly 79% of nitrogen but it can not be used directly by the majority of living organisms. Broadly like corbondioxide, nitrogen also cycles from gaseous phase to solid phase then back to gaseous phase through the activity of a wide variety of organisms. Cycling of nitrogen is vitally important for all living organisms. There are five main processes which essential for nitrogen cycle are elaborated below.

• Nitrogen fixation: This process involves conversion of gaseous nitrogen into Ammonia, a form in which it can be used by plants. Atmospheric nitrogen can be fixed by the following three methods: -

• Atmospheric fixation: Lightening, combustion and volcanic activity help in the fixation of nitrogen.

• Industrial fixation: At high temperature (400oC) and high pressure (200 atm.), molecular nitrogen is broken into atomic nitrogen which then combines with hydrogen to form ammonia.

• Bacterial fixation: There are two types of bacteria-

• Symbiotic bacteria e.g. Rhizobium in the root nodules of leguminous plants. (2) Freeliving or symbiotic for example: i). Nostoc ii). Azotobacter iii). Cyanobacteria can combine atmospheric or dissolved nitrogen with hydrogen to form ammonia.

• Nitrification: It is a process by which ammonia is converted into nitrates or nitrites by Nitrosomonas and Nitrococcus bacteria respectively.

Another soil bacteria Nitrobacter can covert nitrate into nitrite.

• Assimilation: In this process nitrogen fixed by plants is converted into organic molecules such as proteins, DNA, RNA etc. These molecules make the plant and animal tissue.

• Ammonification: Living organisms produce nitrogenous waste products such as urea and uric acid. These waste products as well as dead remains of organisms are converted back into inorganic ammonia by the bacteria This process is called ammonification. Ammonifying bacteria help in this process.

• Denitrification: Conversion of nitrates back into gaseous nitrogen is called denitrification. Denitrifying bacteria live deep in soil near the water table as they like to live in oxygen free medium. Denitrification is reverse of nitrogen fixation.

Apart from carbon, hydrogen and oxygen, nitrogen is the most prevalent element in living organisms.

Nitrogen is a constituent of amino acids, proteins, hormones, chlorophylls and many vitamins.

Plants compete with microbes for the limited nitrogen that is available in the soil. Thus, nitrogen is a limiting nutrient for both natural and agricultural ecosystems.

Nitrogen exists as two nitrogen atoms (N2) joined by a very strong triple covalent bond (N ≡ N).

In nature, lightning and ultraviolet radiation provide enough energy to convert nitrogen to nitrogen oxides (NO, NO2, N2O).

Industrial combustions, forest fires, automobile exhausts and power-generating stations are also sources of atmospheric nitrogen oxides.

Nitrogen Fixing – N2 to NH3

Nitrogen Fixing, the process of converting atmospheric nitrogen (N2) into usable forms like ammonia (NH3) or ammonium ions (NH4+), is crucial for life on Earth.

Abundant Nitrogen, but Unusable: The atmosphere has plenty of nitrogen, but it’s in a form that most organisms can’t directly use.

Nitrogen Fixation Process: Nitrogen must be “fixed” or converted into ammonia, nitrites, or nitrates before plants can absorb it. This process is part of the nitrogen cycle, where nitrogen is transformed from one form to another.

Methods of Nitrogen Fixation: Nitrogen fixation happens in three main ways: a) By microorganisms like bacteria and blue-green algae, b) Through human industrial processes like fertilizer production, and c) To a small extent, by natural atmospheric events like thunder and lightning.

Microbial Nitrogen Fixation: Certain microorganisms, called nitrogen-fixers, have the ability to convert atmospheric nitrogen into ammonia or ammonium ions. They do this using an enzyme called nitrogenase, which is found exclusively in prokaryotes (bacteria and archaea).

Types of Nitrogen-Fixing Microbes: These include:

• Free-living nitrogen-fixing bacteria found in soil, like Azotobacter and Clostridium.

• Symbiotic nitrogen-fixing bacteria, such as Rhizobium, which live in nodules on the roots of leguminous plants (like peas and beans).

• Cyanobacteria, like Nostoc and Anabaena, which are important nitrogen fixers in aquatic environments.

Leguminous Plants and Nitrogen Fixation: Leguminous plants have a special relationship with nitrogen-fixing

bacteria. These plants form nodules on their roots where nitrogen-fixing bacteria reside, providing the plant with a source of usable nitrogen.

In essence, nitrogen fixation is vital for making atmospheric nitrogen accessible to living organisms, and it occurs through the activity of specific microbes, industrial processes, and natural phenomena.

Nitrification – Ammonia to Nitrates

Nitrification is a crucial process in the nitrogen cycle that converts ammonia (NH3) into nitrites (NO2-) and then into nitrates (NO3-).

Ammonium to Nitrite: Specialized bacteria, like Nitrosomonas and Nitrococcus, oxidize ammonium ions to nitrite ions. This is the first step in nitrification.

Nitrite to Nitrate: Another group of bacteria, specifically Nitrobacter, further oxidizes the nitrite ions into nitrate ions. This completes the nitrification process.

Chemoautotrophic Bacteria: Nitrifying bacteria are chemoautotrophs, meaning they use inorganic chemicals as an energy source to produce organic compounds from carbon dioxide.

Plant Uptake: Plants absorb nitrates from the soil and transport them to their leaves. In leaves, nitrates are reduced to form ammonia, which is then used to build amino acids, the building blocks of proteins.

Role in Agriculture: Nitrification is essential in agriculture, where ammonia-based fertilizers are commonly used. Converting ammonia to nitrate makes nitrogen more available to plants. However, nitrate is more water-soluble than ammonia, leading to increased nitrogen leaching, which can pollute waterways.

Wastewater Treatment: Nitrification is also important in municipal wastewater treatment. Conventional treatment involves nitrification followed by denitrification, where nitrate is converted back into nitrogen gas and released harmlessly into the atmosphere.

Ammonification – Urea, Uric Acid to Ammonia

Ammonification is the process where nitrogen-rich waste products, like urea and uric acid, produced by living organisms, as well as dead organic matter, are broken down by bacteria into ammonia and ammonium ions. This ammonia can then either re-enter the atmosphere or be converted by soil bacteria into nitrate, a form of nitrogen that plants can use as a nutrient. Essentially, it’s nature’s way of recycling nitrogen from waste and dead organisms back into a form that can be used by living organisms again.

Denitrification – Nitrate to Nitrogen

Denitrification is a crucial step in the nitrogen cycle where nitrate, a form of nitrogen found in the soil or water, is converted back into atmospheric nitrogen gas.

• Special Bacteria: In both soil and oceans, specific bacteria, like Pseudomonas and Thiobacillus, carry out

denitrification. They transform nitrates or nitrites into elemental nitrogen.

• Returning Nitrogen to the Atmosphere: During denitrification, nitrogen escapes from the soil or water into the atmosphere, completing the nitrogen cycle loop.

• Nitrogen Cycle Recap: The nitrogen cycle consists of several steps: a) Nitrogen fixation converts atmospheric nitrogen into ammonia or ammonium ions. b) Nitrification transforms ammonia or ammonium ions into nitrites and then nitrates. c) Ammonification breaks down organic nitrogen (like urea and uric acid) from dead matter and animal waste into ammonia or ammonium ions. d) Denitrification converts nitrates back into nitrogen gas, closing the cycle.

• Human Impact: Human activities, especially industrial processes, have greatly increased the amount of nitrogen fixed into the environment. This excess nitrogen, when released into ecosystems, can become a pollutant, leading to problems like acid rain, eutrophication (excessive nutrients in water bodies), and harmful algal blooms.

Phosphate Cycle (Sedimentary Cycle)        

Phosphorus Cycle

Phosphorus is a major constituent of biological membranes, nucleic acids and cellular energy transfer systems. Many animals also need large quantities of this element to make shells, bones and teeth.

The natural reservoir of phosphorus is rock, which contains phosphorus in the form of phosphates.

When rocks are weathered, minute amounts of these phosphates dissolve in soil solution and are absorbed by the roots of the plants. Herbivores and other animals obtain this element from plants. The waste products and the dead organisms are decomposed by phosphate- solubilizing bacteria releasing phosphorus. Unlike carbon cycle, there is no respiratory release of phosphorus into atmosphere.

The other two major and important differences between carbon and phosphorus cycle are firstly, atmospheric inputs of phosphorus through rainfall are much smaller than carbon inputs, and, secondly, gaseous exchanges of phosphorus between organism and environment are negligible.

Water Cycle

About 2/3rd of the earth’s surface is covered with water. However, a very small portion of it is available to animals and plants as almost 95 % of the total water on the earth is chemically bound to rocks and does not cycle. Out of the remaining 5%, nearly 97.3% is in the oceans and 2.1% exists as polar ice caps. Thus only 0.6% is present as fresh water in the form of atmospheric water vapours, ground and soil water.

Source of Water on Earth: Precipitation (rain, snow, slush dew etc.)

Water received from the atmosphere on the earth returns back to the atmosphere as water vapour resulting from direct evaporation and through evapotranspiration the continuous movement of water in the biosphere is called water cycle (hydrological cycle).

The driving forces for water cycle are:

• Solar Radiation

• Gravity

• Evaporation and precipitation are two main processes involved in water cycle. These two processes alternate with each other Water from oceans, lakes, ponds, rivers and streams evaporates by sun’s heat energy. Plants also transpire huge amounts of water. Water remains in the vapour state in air and forms clouds which drift with wind. Clouds meet with the cold air in the mountainous.

• On an average 84% of the water is lost from the surface of the oceans by evaporation. While 77% is gained by it from precipitation. Water run-off from lands through rivers to oceans makes up 7% which balances the evaporation deficit of the ocean. On land, evaporation is 16% and precipitation is 23%.

Sulphur Cycle

The Sulphur cycle describes the movement of Sulphur through various reservoirs in the environment.

• Sulphur Reservoirs: Sulphur is primarily found in the soil and sediment, where it’s locked in organic deposits like coal, oil, and peat, as well as inorganic deposits such as pyrite and Sulphur rocks. These reservoirs contain Sulphur in forms like sulphates, Sulphides, and organic Sulphur.

• Release and Transport: Sulphur is released into the environment through processes like weathering of rocks, erosion runoff, and decomposition of organic matter. It’s then carried to terrestrial and aquatic ecosystems in salt solutions.

• Sedimentary Nature: The Sulphur cycle is mostly sedimentary, but two compounds, hydrogen Sulphide (H2S) and Sulphur dioxide (SO2), add a gaseous component to the cycle.

• Atmospheric Input: Sulphur enters the atmosphere from various sources, including volcanic eruptions, combustion of fossil fuels, and gases released during decomposition. Atmospheric hydrogen Sulphide is oxidized to Sulphur dioxide, which then returns to Earth dissolved in rainwater as weak Sulphuric acid.

• Plant Uptake and Incorporation: Plants take up Sulphur in the form of Sulphates and incorporate it into Sulphur- containing amino acids through metabolic processes. These amino acids are then used to build proteins in plant tissues and pass through the food chain.

• Return to Reservoirs: Sulphur bound in living organisms is returned to the environment through processes like excretion and decomposition. It returns to the soil, sediment, and water bodies, completing the cycle.

• Interconnected Cycles: The Sulphur cycle, like other biogeochemical cycles, interacts with other cycles in the

ecosystem, such as the carbon and nitrogen cycles, at various points.

How Human Activities are Affecting Matter Cycling on Earth?

• The human activities have altered the state of balance (natural equilibrium) in the cycles. Therefore, affecting the matter and biogeochemical cycles on earth.

For example: The excessive release (leading to global warming) and absorption of carbon dioxide by oceans (leading to ocean acidification) has created disequilibrium and ecological disbalance in the environment