Ecosystem Notes Class 12 Biology Chapter 12

Class 12 CBSE Biology Chapter 12- Ecosystem: Living organisms interact with one another and their physical surroundings in a functioning ecosystem. The size of ecosystems can vary, from little ponds to enormous forests or seas. According to some ecologists, the entire biosphere is made up of all the regional ecosystems on Earth. It is reasonable to divide the biosphere into two primary categories: terrestrial and aquatic ecosystems because of its enormous size and complexity. Forests, grasslands, and deserts are examples of terrestrial ecosystems, while ponds, lakes, wetlands, rivers, and estuaries are examples of aquatic ecosystems. Additionally, man-made ecosystems such as crop fields and aquariums can also be considered. In this article, we will look at Chapter 12- Ecosystem of Class 12 to gain a better understanding of ecosystems. 

Ecosystem: Structure and Function

The interaction between living (biotic) and non-living (abiotic) elements in an ecosystem leads to the development of a unique physical structure specific to each ecosystem type. By identifying and counting the plant and animal species within an ecosystem, we can determine its species composition. The arrangement of different species in vertical layers within an ecosystem is known as stratification. For instance, in a forest, trees typically occupy the uppermost vertical layer, shrubs occupy the second layer, and herbs and grasses are found in the lower layers.

Types of Ecosystem

1. Terrestrial ecosystems are land-based and include various types such as forests, grasslands, tundra, and deserts. Forests consist of trees, animals, and microorganisms that interact with the environment. Grasslands are dominated by grasses and herbs. Tundra systems lack trees and are found in cold climates. Deserts have little rainfall and limited vegetation.
2. Aquatic ecosystems exist in bodies of water and can be further divided into freshwater ecosystems and marine ecosystems. Freshwater ecosystems include lakes, ponds, rivers, streams, and wetlands, with no salt content. Marine ecosystems encompass seas and oceans, with higher salt content and greater biodiversity compared to freshwater ecosystems

The components of the ecosystem function as a unit by the following: Productivity; Decomposition; Energy flow; and Nutrient cycling.

Productivity 

• Solar energy is an essential requirement for the functioning and sustainability of any ecosystem. 
• Primary production refers to the amount of biomass or organic matter produced by plants through photosynthesis within a given area and time period.
• Productivity refers to the rate of biomass production and is expressed in grams per square meter per year or kcal per square meter per year, allowing for comparisons between different ecosystems. 
• Productivity can be divided into two components: gross primary productivity (GPP) and net primary productivity (NPP). GPP represents the total organic matter produced by plants through photosynthesis, while a portion of GPP is consumed by plants in respiration. NPP is calculated by subtracting the losses due to respiration (R) from GPP, and it represents the available biomass for consumption by heterotrophs such as herbivores and decomposers. 
• Secondary productivity, on the other hand, refers to the rate at which consumers generate new organic matter.

Decomposition

• Decomposers are organisms that break down complex organic matter into inorganic substances like carbon dioxide, water, and nutrients through a process called decomposition. Dead plant material, such as leaves, bark, flowers, and animal remains including fecal matter, make up the detritus, which serves as the raw material for decomposition. 
• The process of decomposition involves several important steps, including fragmentation, leaching, catabolism, humification, and mineralization.
• Fragmentation refers to the breaking down of detritus into smaller particles by detritivores like earthworms. 
• Leaching involves the downward movement of water-soluble inorganic nutrients into the soil, where they become precipitated as unavailable salts.
•  Bacterial and fungal enzymes then degrade the detritus, converting it into simpler inorganic substances in a process known as catabolism. 

• All of these decomposition steps occur simultaneously on the detritus. Humification and mineralization occur specifically in the soil during decomposition. Humification results in the accumulation of a dark-colored amorphous substance called humus, which is highly resistant to microbial action and decomposes at an extremely slow rate. Humus, being colloidal in nature, serves as a nutrient reservoir. Some microbes further degrade humus through mineralization, releasing inorganic nutrients.
• Decomposition is primarily an aerobic process, meaning it requires oxygen. The rate of decomposition is influenced by the chemical composition of the detritus and climatic factors. In a particular climate, decomposition is slower when detritus contains high levels of lignin and chitin, while it is faster when detritus is rich in nitrogen and water-soluble substances like sugars. Temperature and soil moisture are the most crucial climatic factors that regulate decomposition by affecting the activities of soil microbes. Warm and moist environments promote decomposition, while low temperatures and anaerobic conditions inhibit decomposition, leading to the accumulation of organic materials.

Energy Flow

• Except for deep-sea hydrothermal ecosystems, the sun is the primary source of energy for all ecosystems on Earth. The flow of solar energy captured by plants through different organisms in an ecosystem is unidirectional, from the sun to producers and then to consumers.
• This flow of energy from the sun to producers and consumers is in accordance with the first law of thermodynamics, which states that energy cannot be created or destroyed but can only be converted from one form to another. In this case, solar energy is converted into chemical energy by producers and then transferred to consumers.
• Ecosystems are also subject to the second law of thermodynamics, which states that the entropy, or disorder, of a closed system, tends to increase over time. Ecosystems require a constant supply of energy to counteract this tendency towards increasing disorderliness and to synthesize the molecules they need.
• In ecological terminology, green plants are referred to as producers. In terrestrial ecosystems, herbaceous and woody plants serve as the major producers, while in aquatic ecosystems, various species such as phytoplankton, algae, and higher plants act as primary producers.
• Food chains and webs exist in nature, where organisms are interdependent on each other for their food needs. These chains or webs start with plants as the producers, and animals feed on plants or other animals, forming a complex network of interactions. Energy flow within these chains or webs is unidirectional, as energy is transferred from producers to consumers. When an organism dies, the detritus food chain or web begins.
• All animals, whether they feed directly or indirectly on plants, are considered consumers or heterotrophs. Primary consumers are herbivores that feed on producers, while secondary consumers are animals that eat other animals which, in turn, eat plants or their produce. There can also be tertiary consumers. The primary carnivores or secondary consumers are carnivores that feed on herbivores. This forms a simple grazing food chain.
• The detritus food chain starts with dead organic matter and consists of decomposers, such as fungi and bacteria, which derive their energy and nutrients from breaking down detritus. Decomposers, also known as saprotrophs, secrete digestive enzymes to break down dead and waste materials into simple inorganic materials, which they absorb.
• In aquatic ecosystems, the grazing food chain is the major conduit for energy flow. In contrast, in terrestrial ecosystems, a larger fraction of energy flows through the detritus food chain than the grazing food chain. These two chains can be interconnected at certain levels, with some organisms from the detritus food chain being prey to animals in the grazing food chain. Some animals, like cockroaches and crows, are omnivores, further adding to the complexity of the food web.
• Human beings can be classified within the consumer category as omnivores, as they consume both plant and animal-based food.
• Organisms occupy specific trophic levels in a food chain or web based on their feeding relationships with other organisms. Producers belong to the first trophic level, herbivores (primary consumers) to the second, and carnivores (secondary consumers) to the third. It is important to note that the amount of energy decreases at each successive trophic level. When an organism dies, it is converted to detritus or dead biomass, which serves as an energy source for decomposers. Organisms at each trophic level depend on those at the lower trophic levels for their energy demands.
• Each trophic level has a standing crop, which refers to the mass of living material at a particular time in a given area. The standing crop can be measured in terms of biomass (the mass of living organisms) or the number of organisms. Measurement of biomass in terms of dry weight is more accurate because it excludes the variability caused by water content, which can fluctuate.
• The number of trophic levels in a grazing food chain is limited due to the 10 percent law, which states that only about 10 percent of the energy is transferred from one trophic level to the next. This means that as energy moves up the food chain, each higher trophic level receives only a fraction of the energy from the level below it. This restriction on energy transfer explains why natural food chains typically have a limited number of levels, such as producers, herbivores, primary carnivores, and secondary carnivores.

Ecological Pyramids

The pyramid shape is commonly used to illustrate the relationship between organisms in terms of food or energy. It resembles a pyramid with a broad base and a narrowing apex. This shape remains consistent whether we express the relationship using numbers, biomass, or energy. In this representation, the base of the pyramid represents the producers or the first trophic level, while the apex represents the tertiary or top-level consumer. There are three main types of ecological pyramids that are typically studied: the pyramid of numbers, the pyramid of biomass, and the pyramid of energy.

• Calculations involving energy content, biomass, or numbers must include all organisms within a specific trophic level to accurately represent the relationships. It is important not to make generalizations based on a few individuals at each trophic level. Additionally, a single organism can occupy multiple trophic levels simultaneously. It’s essential to understand that trophic levels represent functional roles rather than specific species. For instance, a sparrow can be a primary consumer when consuming seeds, fruits, and peas, and a secondary consumer when consuming insects and worms.
• Determining the number of trophic levels at which human beings function in a food chain would require an analysis of their dietary habits and position within the ecosystem.
• In most ecosystems, the pyramids of number, energy, and biomass are upright, meaning that there are more producers in terms of number and biomass than herbivores and more herbivores than carnivores. Additionally, energy decreases as it moves up the trophic levels.
• However, there are exceptions. For example, if we count the number of insects feeding on a large tree, the resulting pyramid would not follow the typical pattern. If we also consider the number of small birds depending on the insects, as well as the number of larger birds preying on the smaller birds, the shape of the pyramid would differ.
• The inverted pyramid of biomass in the sea, where the biomass of fishes exceeds that of phytoplankton, may appear paradoxical. The explanation lies in the fact that phytoplankton reproduce rapidly and are consumed by zooplankton, which in turn become prey for small fishes. The high reproductive rate of phytoplankton compensates for their low individual biomass, leading to a larger overall biomass of fish.
• The pyramid of energy cannot be inverted. This is because energy is lost as heat at each trophic level as it flows from one level to the next. Each bar in the energy pyramid represents the amount of energy present at a specific trophic level during a given time or annually per unit area.
• Despite their usefulness, ecological pyramids have certain limitations. They do not account for the same species occupying multiple trophic levels or accommodate complex food webs. Additionally, saprophytes, which play a crucial role in ecosystems, are not represented in ecological pyramids.

 Ecological Succession

Ecological succession refers to the gradual and predictable change in the species composition of a particular area over time. During succession, certain species colonize the area and their populations become more abundant, while populations of other species decline or disappear. Succession occurs in areas where no living organisms exist initially, such as bare rock, which is known as primary succession. It can also occur in areas where all existing organisms have been lost, termed secondary succession.

Primary succession takes place when new land is created or when bare rock is revealed, offering a habitat that can be inhabited for the first time.

Secondary succession occurs when a previously inhabited area is once again colonized after a disturbance that results in the death of a significant portion of the entire community

Nutrient Cycling

Nutrient cycling, also known as biogeochemical cycles, refers to the movement of nutrient elements within ecosystems. It is a process where nutrients, which are essential for living organisms, are continuously recycled indefinitely. Nutrient cycling involves the transfer and transformation of nutrients through various components of the ecosystem.

Ecosystem Services

Ecosystems play a crucial role in providing a wide range of economic, environmental, and aesthetic benefits. These benefits are known as ecosystem services. For instance, healthy forest ecosystems contribute to air and water purification, drought and flood mitigation, nutrient cycling, soil fertility, wildlife habitat, biodiversity conservation, crop pollination, and carbon storage, as well as aesthetic, cultural, and spiritual values. While it is challenging to determine the exact value of these biodiversity services, it is reasonable to consider them highly valuable.

Recently, Robert Constanza and his colleagues attempted to assign a monetary value to nature’s life-support services. Their research estimated an average annual price tag of $33 trillion for these essential ecosystem services, which are often taken for granted as they are freely available. This value is nearly twice the global gross national product (GNP) of $18 trillion.

Soil formation accounts for approximately 50% of the total cost of various ecosystem services, while other services such as recreation and nutrient cycling contribute less than 10% each. Climate regulation and wildlife habitat each represent about 6% of the overall cost.

Function of Ecosystem

• Ecosystem functions refer to the natural processes and energy exchanges that occur among different plant and animal groups across the world’s biomes.
• For instance, green leaves play a role in food production through photosynthesis, while roots absorb nutrients from the soil. Herbivores consume the leaves and roots, and predators feed on these herbivores. Decomposers have the important task of breaking down complex organic materials into simpler inorganic compounds that can be utilized by producers.
• Essentially, ecosystem functions involve the flow of energy and nutrients within the food chain. These interactions are crucial for sustaining plant and animal life on Earth, as well as facilitating the decomposition of organic matter and the creation of biomass.

FAQs on Ecosystem

Q1: Briefly write the difference between detritivores and decomposers. 

Answer:

Detritivores are organisms that consume detritus and break it down into smaller particles, like earthworms. Decomposers, on the other hand, are organisms that chemically break down complex organic matter into simpler inorganic substances through the release of enzymes.

Q2: What is ‘R’ in the given equation for ecosystem productivity? 

• NPP = GPP – R

Answer:

R’ represents the energy utilized by plants or producers in respiration. It is also referred to as respiratory losses

Q3: Why primary productivity is different in different types of ecosystems?

Answer:

Primary productivity varies in different types of ecosystems due to two main factors:

• Variation in plant species (producers) and their photosynthetic capacity within each ecosystem.
• Dependence on environmental factors and nutrient availability