Environment Notes 02

Ecosystems

An ecosystem is a structural and functional unit of ecology where living organisms interact with each other and the surrounding physical environment. The term ‘ecosystem’ was coined by the English botanist Arthur Tansley in his 1935 paper “The use and abuse of vegetational concepts and terms”. He defined it as “the whole complex of organisms—both animals and plants—naturally living together as a sociological unit” along with their habitat. An ecosystem is an open system with a continuous but variable influx and loss of energy and matter.

Classification of Ecosystems

Ecosystems are broadly classified based on their primary environment.

1. Terrestrial Ecosystems (Land-based) These are ecosystems found on landforms. The nature of a terrestrial ecosystem is determined by factors like climate (temperature, rainfall), soil type, and topography.

• Forest Ecosystem: Characterized by a dense growth of trees and other woody vegetation. They are among the most complex and diverse terrestrial ecosystems.
  • Historical Context: Ancient civilizations often thrived near forests, which provided resources like timber, food, and medicinal plants. The Epic of Gilgamesh (c. 2100 BC) describes the great Cedar Forest of Mesopotamia.
  • Examples: Tropical Rainforests (Amazon, Congo Basin), Temperate Deciduous Forests (in Europe and North America), and Boreal Forests or Taiga (spanning Russia and Canada).
• Grassland Ecosystem: Dominated by grasses rather than large shrubs or trees. They occur in regions with moderate rainfall, insufficient for forests but more than that of deserts.
  • Examples: The Prairies of North America, Pampas of South America, Steppes of Eurasia, and Savannas of Africa. These ecosystems support large grazing mammals like bison, antelope, and zebra.
• Desert Ecosystem: Characterized by extremely low rainfall (typically less than 250 mm annually), high evaporation rates, and extreme temperature fluctuations. Organisms here are highly adapted to conserve water (xerophytes and specialized fauna).
  • Examples: Hot deserts like the Sahara in Africa and the Thar in India; Cold deserts like the Gobi in Asia and the Atacama in South America.
• Polar (Tundra) Ecosystem: A treeless biome found in the Arctic and on the tops of mountains (alpine tundra), where the climate is cold and windy, and rainfall is scant. The ground is often permanently frozen (permafrost).
  • Life Forms: Lichens, mosses, sedges, and dwarf shrubs. Fauna includes reindeer, musk ox, arctic fox, and polar bears.

2. Aquatic Ecosystems These are ecosystems present in a body of water. They are broadly classified based on their salt content.

a) Freshwater Ecosystem: Characterized by low salinity (less than 5 ppt).

• Lentic (from Latin lentus, meaning sluggish): Comprises standing or slow-moving water bodies.
  • Characteristics: They have distinct zones: littoral (near shore), limnetic (open, well-lit water), and profundal (deep, no light).
  • Examples: Ponds, lakes, swamps, and marshes. Lake Baikal in Siberia is the world’s oldest and deepest freshwater lake, a unique lentic ecosystem.
• Lotic (from Latin lotus, meaning washing): Comprises flowing water bodies.
  • Characteristics: The continuous flow of water creates a distinct environment with higher dissolved oxygen levels compared to lentic systems. Organisms are adapted to flowing conditions.
  • Examples: Rivers, streams, creeks. The Ganges river system is a vast lotic ecosystem supporting immense biodiversity and human populations.

b) Brackish Ecosystem: A mixture of freshwater and saline water, with salinity varying between 5 to 35 ppt. These are highly productive ecosystems.

• Estuarine Ecosystem: Formed where rivers meet the sea. They are characterized by fluctuating salinity and temperature due to tidal action and river flow.
  • Examples: The Sundarbans, the world’s largest mangrove forest, is a massive estuarine ecosystem at the mouth of the Ganges, Brahmaputra, and Meghna rivers.
• Lagoons: A shallow body of water separated from a larger body of water (usually the ocean) by a barrier such as a reef, sandbar, or barrier island.
  • Context: Lagoons can be brackish if fed by freshwater streams or marine (hypersaline) if evaporation is high and freshwater input is low.
  • Examples: Chilika Lake in Odisha is the largest brackish water lagoon in India. The Venice Lagoon in Italy is another famous example.
• Backwaters: A network of interconnected canals, rivers, lakes, and inlets, a labyrinthine system formed by the action of waves and shore currents creating low barrier islands across the mouths of the many rivers flowing down from the Western Ghats.
  • Examples: The Kerala backwaters, locally known as Kayals, are a prime example. Vembanad Lake is the largest of these backwaters.

c) Marine Ecosystem: Characterized by high salinity (over 35 ppt) and cover more than 70% of the Earth’s surface.

• Oceanic Zones:
  • Photic (Euphotic) Zone: The surface layer of the ocean that receives sunlight, allowing photosynthesis to occur. It extends to a depth of about 200 meters. This zone has the highest concentration of life, including phytoplankton, which form the base of the marine food web.
  • Aphotic Zone: The portion of the ocean where sunlight does not penetrate. It lies below the photic zone. Life here relies on energy from chemical reactions (chemosynthesis) or organic matter sinking from above. It is further divided into:
    • Littoral Zone: The intertidal zone on the continental shelf.
    • Bathyal Zone: The ‘midnight zone’ on the continental slope, from 200 to 2,000 meters.
    • Abyssal Zone: The ‘abyss’ on the abyssal plains, from 2,000 to 6,000 meters, characterized by immense pressure and near-freezing temperatures.
• Marine Life Forms:
  • Planktons: Microscopic organisms that drift with water currents.
    • Phytoplankton: Plant-like (e.g., diatoms, algae) that are primary producers.
    • Zooplankton: Animal-like (e.g., krill, copepods) that are primary consumers.
  • Nektons: Actively swimming organisms that can move against currents.
    • Examples: Fish, whales, dolphins, squids.
  • Benthos: Organisms that live on or in the seafloor.
    • Examples: Crabs, sea stars, clams, sponges, sea anemones. They are found from the shallow littoral zone to the deep abyssal zone.

3. Artificial Ecosystems These are man-made or anthropogenic ecosystems, designed and managed by humans to derive specific benefits. They are characterized by low species diversity, simple food chains, and high dependence on external energy and nutrient inputs.

• Also Known As: Cultural ecosystems.
• Examples:
  • Agroecosystems: Farmlands, crop fields.
  • Aquaculture ponds: For fish or shrimp farming.
  • Urban ecosystems: Cities, industrial parks.
  • Man-made reservoirs and dams.

Structure of Biotic Components

The living parts of an ecosystem are classified based on their mode of nutrition.

1. Producers (Autotrophs) Organisms that produce their own food from simple inorganic substances.

• Photoautotrophs: Use sunlight as their energy source for photosynthesis. This includes all green plants, algae, and cyanobacteria. They form the base of most food chains.
• Chemoautotrophs: Derive energy from the oxidation of inorganic chemical compounds. They are typically bacteria and archaea found in environments devoid of sunlight, such as deep-sea hydrothermal vents. For example, sulfur-oxidizing bacteria.

2. Consumers (Heterotrophs) Organisms that obtain energy by feeding on other organisms.

• Primary Consumers (Herbivores): Feed directly on producers (e.g., cow, deer, grasshopper).
• Secondary Consumers (Carnivores): Feed on primary consumers (e.g., frog, fox, snake).
• Tertiary Consumers (Top Carnivores): Feed on secondary consumers (e.g., lion, hawk, shark).
• Omnivores: Consume both plants and animals (e.g., humans, bears, crows).

3. Decomposers (Saprotrophs) Organisms that break down dead organic matter (detritus), releasing essential nutrients back into the ecosystem for producers to use.

• Detritivores: Ingest dead organic matter directly. They are a crucial link in the detritus food chain.
  • Examples: Earthworms, millipedes, vultures, dung beetles.
• Saprotrophs (Decomposers proper): Primarily bacteria and fungi. They do not ingest food but secrete digestive enzymes onto the dead organic matter and then absorb the resulting soluble organic compounds. This process is known as external digestion.

Trophic Structures The concept of trophic levels was central to the work of Raymond Lindeman in his seminal 1942 paper, “The Trophic-Dynamic Aspect of Ecology”. A trophic level represents the position an organism occupies in a food chain.

• Trophic Level 1 (T1): Producers (Plants, Algae).
• Trophic Level 2 (T2): Primary Consumers (Herbivores).
• Trophic Level 3 (T3): Secondary Consumers (Carnivores).
• Trophic Level 4 (T4): Tertiary Consumers (Top Carnivores/Omnivores).
• An ecosystem typically has a maximum of four to five trophic levels because of the substantial energy loss at each successive level.

Energy Pathways This refers to the flow of energy through an ecosystem’s trophic levels. The primary source of energy for almost all ecosystems is solar energy.

• The 10% Rule: Proposed by Raymond Lindeman (1942), this rule states that during the transfer of energy from one trophic level to the next, only about 10% of the energy is stored as biomass and becomes available to the next level. The remaining 90% is lost, primarily as metabolic heat during respiration, or is unavailable (e.g., indigestible parts).
• Unidirectional Flow: Energy flow in an ecosystem is unidirectional and non-cyclical. It flows from the sun to producers, then to consumers, and is eventually lost as heat. It cannot be recycled.

Food Chain A linear sequence of organisms where nutrients and energy are transferred from one organism to another. It illustrates who eats whom in an ecosystem.

• Grazing Food Chain: Starts with living green plants (producers), proceeds to herbivores (primary consumers), and then to carnivores (secondary/tertiary consumers).
  • Example: Grass → Grasshopper → Frog → Snake → Hawk.
• Detritus Food Chain: Starts with dead organic matter (detritus), which is consumed by detritivores and decomposers. This food chain is crucial for nutrient cycling.
  • Example: Dead Leaves → Earthworm → Bird.

Food Web A food web is a more realistic representation of feeding relationships in an ecosystem. It consists of multiple interconnected food chains.

• Significance: Food webs demonstrate that most organisms have multiple food sources and are preyed upon by multiple predators. This complexity provides greater stability to an ecosystem. If one food source becomes scarce, a consumer can switch to an alternative, preventing a population crash. Simple ecosystems with linear food chains are highly vulnerable to disturbances.

Ecological Pyramids

The concept of ecological pyramids was developed by the British ecologist Charles Elton in his 1927 book “Animal Ecology”. They are graphical representations of the trophic structure of an ecosystem.

• Pyramid of Numbers: Represents the total number of individual organisms at each trophic level.

  • Upright: In most ecosystems, like a grassland, the number of producers (grass) is far greater than the number of herbivores (deer), which in turn is greater than the number of carnivores (tigers).
  • Inverted: Can occur in a tree ecosystem where a single large producer (one tree) supports a vast number of primary consumers (insects, birds), which are in turn consumed by a smaller number of secondary consumers.

• Pyramid of Biomass: Represents the total dry weight (biomass) of all organisms at each trophic level.

  • Upright: In most terrestrial ecosystems, the total biomass of producers is greater than that of consumers.
  • Inverted: Common in aquatic ecosystems (e.g., a pond or ocean). The biomass of producers (phytoplankton) at any given time is very small compared to the biomass of consumers (zooplankton, fish). This is because phytoplankton have a very short lifespan and high turnover rate; they reproduce rapidly and are consumed quickly, supporting a larger biomass of consumers.

• Pyramid of Energy: Represents the total amount of energy flow at each trophic level over a specific period.

  • Always Upright: This pyramid can never be inverted because the flow of energy between trophic levels is always unidirectional and follows the 10% rule. There is a progressive loss of energy at each successive trophic level, so the energy available at the producer level will always be the highest.

Prelims Pointers

• Ecosystem: Term coined by A.G. Tansley in 1935.
• Lentic Ecosystem: Standing or slow-moving freshwater (e.g., lakes, ponds).
• Lotic Ecosystem: Flowing freshwater (e.g., rivers, streams).
• Brackish Water: Salinity is a mix of fresh and saline water (e.g., estuaries, lagoons).
• Kayals: Local term for backwaters in Kerala.
• Photic Zone: The upper layer of the ocean (up to 200m) that receives sunlight.
• Aphotic Zone: The ocean zone below 200m where sunlight does not penetrate.
• Benthos: Organisms living on the sea floor (e.g., crabs, sponges).
• Nekton: Actively swimming organisms in water (e.g., fish, whales).
• Plankton: Microscopic organisms that drift with water currents.
  • Phytoplankton: Plant-like (producers).
  • Zooplankton: Animal-like (primary consumers).
• Autotrophs: Organisms that produce their own food (e.g., plants, chemosynthetic bacteria).
• Heterotrophs: Organisms that feed on others for energy.
• Detritivores: Animals that feed on dead organic matter (e.g., vultures, earthworms).
• Saprotrophs: Organisms (bacteria, fungi) that decompose dead matter through external digestion.
• Trophic Levels: Concept elaborated by Raymond Lindeman (1942).
• 10% Rule of Energy Transfer: Proposed by Raymond Lindeman.
• Food Web: An interconnection of multiple food chains, providing stability to an ecosystem.
• Ecological Pyramids: Concept developed by Charles Elton (1927).
• Pyramid of Energy: Is always upright.
• Pyramid of Numbers: Can be upright or inverted (e.g., inverted in a tree ecosystem).
• Pyramid of Biomass: Can be upright (terrestrial) or inverted (aquatic).

Mains Insights

1. Ecosystem Stability and Complexity:

   • Cause-Effect: The complexity of a food web is directly related to the stability of an ecosystem. A more complex web with multiple interconnected food chains offers alternative food sources for organisms. This resilience allows the ecosystem to better withstand disturbances, such as the decline of a particular species, without collapsing.
   • Relevance (GS-III): This principle is fundamental to biodiversity conservation. Loss of biodiversity simplifies food webs, making ecosystems more fragile and vulnerable to climate change, invasive species, and human pressures. For instance, protecting a wide range of pollinator species ensures the stability of an agroecosystem, even if one pollinator species declines.

2. Anthropogenic Impact on Ecosystems:

   • Analysis: Human activities fundamentally alter ecosystem structures. Agricultural practices (monoculture) create highly simplified, artificial ecosystems with low diversity and linear food chains, making them susceptible to pests and diseases. Dam construction converts lotic (river) ecosystems into lentic (reservoir) ecosystems, drastically changing aquatic life and downstream hydrology.
   • Consequence: Such alterations disrupt energy flow and nutrient cycles. Eutrophication in lakes due to fertilizer runoff is a classic example of human activity overloading an ecosystem’s capacity to process nutrients, leading to algal blooms and dead zones.

3. The Invisible Engine: The Detritus Food Chain:

   • Historiographical Viewpoint: Early ecological studies, as noted by scholars like Eugene Odum, often focused heavily on the grazing food chain. However, it is now understood that in many ecosystems, especially forests and estuaries, a larger portion of energy flows through the detritus pathway.
   • Significance: This highlights the critical, often-overlooked role of decomposers and detritivores in nutrient cycling. Without them, nutrients would remain locked in dead organic matter, and ecosystem productivity would halt. Policies on waste management, soil health, and forest conservation must recognize and protect this “invisible” component of biodiversity.

4. Bioaccumulation and Biomagnification: A Consequence of Trophic Structure:

   • Concept: While not explicitly in the summary, this is a critical analytical insight. Non-biodegradable toxins (like DDT, heavy metals) that enter an ecosystem get absorbed by organisms at the lowest trophic level. Since these toxins are not metabolized, they accumulate in tissues (bioaccumulation). As energy moves up the food chain, these toxins become more concentrated at each successive trophic level (biomagnification).
   • Impact (GS-III/IV): This process has devastating effects on top predators, such as eagles and tigers, leading to reproductive failure and population decline. The classic case of DDT thinning the eggshells of birds of prey, documented in Rachel Carson’s “Silent Spring” (1962), brought this issue to global attention. It raises ethical questions about industrial pollution and its disproportionate impact on non-target species.