5. New Environment Notes 02

Elaborate Notes

ECOLOGICAL PYRAMID

An ecological pyramid is a graphical model used to illustrate the quantitative relationships between different trophic levels in an ecosystem. The concept was first developed by the British ecologist Charles Elton in his seminal work Animal Ecology (1927). These pyramids are constructed with producers forming the base, and successive trophic levels of consumers (primary, secondary, tertiary) occupying the subsequent tiers.

• Trophic Level: This term refers to the specific position an organism occupies in a food chain or food web. It is a functional classification, not a taxonomic one. For instance, a single species like a black bear (Ursus americanus) can occupy multiple trophic levels, acting as a primary consumer when eating berries and a secondary or tertiary consumer when eating fish or deer fawns.

• Structure: Each bar or tier of the pyramid represents a distinct trophic level. The width or area of the bar is proportional to the quantity (number, biomass, or energy) of the organisms at that level per unit area or volume. The pyramid’s structure illustrates the flow of energy from producers to top carnivores.

• There are three primary types of ecological pyramids:

  a) PYRAMID OF BIOMASS

  • This pyramid represents the total amount of living organic matter (biomass) present at each trophic level in an ecosystem at a given time.
  • Biomass is typically measured as the mass of organisms per unit area, often expressed in units like grams per square meter (g/m²) or kilograms per hectare (kg/ha). To ensure consistency, it is measured as dry weight, which removes the variable water content from organisms.
  • The biomass at a specific trophic level at a particular time is termed the standing crop. It represents a static snapshot of the ecosystem’s biomass distribution.
  • Upright Pyramid of Biomass: In most terrestrial ecosystems, such as grasslands and forests, the pyramid of biomass is upright. The total biomass of producers (e.g., grasses) is significantly greater than the biomass of primary consumers (e.g., herbivores like deer), which in turn is greater than the biomass of secondary consumers (e.g., carnivores like wolves).
  • Inverted Pyramid of Biomass: This is characteristic of certain aquatic ecosystems. For example, in a pond or an open ocean, the biomass of producers (phytoplankton) at any given moment can be much lower than the biomass of primary consumers (zooplankton). This occurs because phytoplankton have a very high rate of reproduction and a short lifespan (high turnover rate). They are consumed rapidly by zooplankton, but their rapid production can sustain the larger biomass of the next trophic level.

  b) PYRAMID OF NUMBER

  • This pyramid illustrates the total number of individual organisms at each trophic level per unit area.
  • Upright Pyramid of Number: This is the most common type, found in ecosystems like grasslands where the number of individual producers (grasses) is vast, followed by a smaller number of herbivores (e.g., grasshoppers), and an even smaller number of carnivores (e.g., frogs, snakes). A similar upright pyramid is observed in the polar ecosystem near Antarctica, where millions of phytoplankton support a smaller number of krill, which in turn support an even smaller number of whales and seals.
  • Inverted Pyramid of Number: This occurs in ecosystems where a single large producer supports numerous smaller consumers. A classic example is a tree ecosystem, where one large tree (producer) can support thousands of insects (primary consumers), which in turn support a smaller number of birds (secondary consumers).

  c) PYRAMID OF ENERGY

  • This pyramid, also known as the trophic pyramid, is the most fundamental and accurate representation of an ecosystem’s functional nature. It depicts the total amount of energy flow at each trophic level over a specific period, usually a year. The units are typically kilocalories per square meter per year (kcal/m²/yr).
  • It is always upright. This is a direct consequence of the Second Law of Thermodynamics, which states that during any energy transfer, some energy is lost as heat and becomes unavailable for work.
  • Lindeman’s 10% Rule: Based on his “Trophic-Dynamic Concept” (Raymond Lindeman, 1942), it is generally accepted that only about 10% of the energy from one trophic level is incorporated into the biomass of the next higher trophic level. The remaining 90% is used for metabolic processes (respiration, movement, reproduction) or is lost as heat.
  • This progressive loss of energy at each successive level explains why food chains are typically limited to 4 or 5 trophic levels. There is simply not enough energy remaining at the top to support another level.
  • Significance of Pyramid of Energy:
    • It provides a clear understanding of ecological productivity, showing the rate at which biomass is generated at different levels.
    • It allows for the calculation of the efficiency of energy transfer (trophic level transfer efficiency) between levels.
    • It is a crucial tool for assessing the environmental impact of human activities, as it can illustrate how disruptions at the producer level (e.g., deforestation, pollution) can have magnified effects up the food chain.

ECOLOGICAL PRODUCTIVITY

• Ecological productivity is the rate at which biomass is generated in an ecosystem. It is essentially the rate of conversion of solar energy into chemical energy by producers and the subsequent transfer of this energy through different trophic levels.
• It is measured in units of mass per unit area (or volume) per unit time, such as grams per square meter per year (g/m²/yr) or kilocalories per square meter per year (kcal/m²/yr).
• Primary Productivity: This is the rate at which energy is captured and stored as organic substances by photosynthetic or chemosynthetic autotrophs (producers). Examples include the productivity of green plants in terrestrial ecosystems and phytoplankton in aquatic ecosystems.
  • Gross Primary Productivity (GPP): This is the total rate of photosynthesis, or the total amount of solar energy fixed by producers into chemical energy in an ecosystem.
  • Net Primary Productivity (NPP): This is the rate at which producers create new biomass. It is the GPP minus the energy producers lose through their own respiration (R). NPP is the energy that is available to the consumers in the ecosystem. The relationship is: NPP = GPP - R.
• Secondary and Tertiary Productivity: This refers to the rate of assimilation of energy and creation of new biomass by consumers. Secondary productivity is at the level of primary consumers (herbivores), and tertiary productivity is at the level of secondary consumers (carnivores).
• Factors Affecting Productivity: The rate of productivity is influenced by several environmental factors, including:
  • Sunlight: The amount of solar radiation available for photosynthesis.
  • Water: Availability of water is crucial for plant growth.
  • Nutrients: The presence of essential nutrients like nitrogen and phosphorus in the soil or water.
  • Temperature: Affects the rate of metabolic processes, including photosynthesis.
• Productivity across Ecosystems:
  • High Productivity Regions: Tropical rainforests, coral reefs, estuaries, and wetlands exhibit very high NPP due to optimal conditions of sunlight, water, and nutrients. Tropical rainforests can have an NPP of over 2000 g/m²/yr.
  • Low Productivity Regions: Deserts, tundra, and deep oceans have very low NPP due to limiting factors like water scarcity (deserts), low temperatures (tundra), or lack of sunlight and nutrients (deep oceans). Open oceans, despite their vast size, have low productivity per unit area, comparable to deserts (< 200 g/m²/yr).

ECOLOGICAL SUCCESSION

• Ecological succession is the orderly and predictable process of change in the species structure of an ecological community over time. This concept was extensively developed by American ecologist Frederic Clements in the early 20th century.

• Sere: The entire sequence of communities that replace one another in a given area is called a sere.

• Seral Stage (or Seral Community): Each intermediate community or developmental stage within the sere is known as a seral stage.

• Pioneer Species: These are the first species to colonize a barren or disturbed environment. They are typically hardy, fast-growing, and have excellent dispersal mechanisms. Lichens and mosses are common pioneer species in primary succession.

• Climax Community: This is the final, relatively stable stage of succession. The community is in equilibrium with its environment and is characterized by high species diversity and complex food webs.

• TYPES OF SUCCESSION

  • a) Primary Succession: This occurs in an area that is essentially lifeless, where no soil or life existed before. The process starts from bare rock, sand, or a newly formed volcanic island.
    • Example: The colonization of land created by the eruption of the Krakatoa volcano in 1883 is a classic, well-documented case of primary succession. Another example is succession on a glacial moraine after a glacier retreats. The process is extremely slow as it requires the formation of soil, which can take hundreds or thousands of years.
  • b) Secondary Succession: This occurs in an area where a pre-existing community has been removed or disturbed by events like forest fires, floods, or human activities such as logging or farming.
    • Example: The regeneration of a forest after a fire, or the process of an abandoned agricultural field gradually returning to a forested state (old-field succession). Since soil and some life forms (like seeds and roots) are already present, secondary succession is significantly faster than primary succession.
  • c) Autogenic Succession: This is succession driven by the biotic components of the ecosystem itself. The organisms within the community modify the environment (e.g., by changing soil composition, light availability), making it less favorable for themselves and more favorable for other species to invade and replace them.
    • Example: In a forest, taller trees grow and create shade, which prevents the growth of their own sun-loving seedlings but allows shade-tolerant species to thrive, leading to a change in the dominant species over time.
  • d) Allogenic Succession: This is succession driven by external, abiotic factors. Changes in environmental conditions, such as climate change, soil erosion, or volcanic eruptions, cause the replacement of the existing community.
    • Example: A prolonged drought can cause a grassland to be replaced by a desert scrub community. The retreat of glaciers due to climate warming clears land, allowing new forests to grow where ice once was.

FUNCTIONS OF ECOSYSTEM (ECOLOGICAL NICHE)

• The concept of an ecological niche refers to the unique functional role and position of a species within its ecosystem. It is a multidimensional concept that includes not only the physical space an organism occupies (its habitat) but also its interactions with other species and its role in the flow of energy and nutrients.

• The term was first used by Joseph Grinnell (1917) to describe an organism’s spatial distribution, later defined functionally by Charles Elton (1927), and formalized by G. Evelyn Hutchinson (1957) as an “n-dimensional hypervolume,” where dimensions are environmental conditions and resources.

• A niche encompasses:

  • The physical conditions it requires (e.g., temperature, pH, humidity).
  • The resources it consumes (e.g., food, nutrients).
  • Its interactions with other species (predators, prey, competitors).
  • Its role in the ecosystem (e.g., pollinator, decomposer).

• Competitive Exclusion Principle: Propounded by Russian biologist G.F. Gause (1934), this principle states that two species competing for the same limiting resources cannot coexist at constant population values, if other ecological factors remain constant. One species will inevitably have a slight advantage that will lead to the elimination of the other. Thus, no two species can have the exact same niche.

• Specialist vs. Generalist Species:

  • Specialist Species: These species have a narrow or limited niche. They thrive only in a specific habitat or feed on a very limited range of food. This makes them highly efficient in their environment but very vulnerable to change.
    • Examples: The Giant Panda, which feeds almost exclusively on bamboo; the Koala of Australia, dependent on eucalyptus leaves; the Snow Leopard, adapted to high-altitude cold deserts; the Lion-tailed Macaque of the Western Ghats.
  • Generalist Species: These species have a broad niche. They can live in a wide variety of environments and consume a diverse range of food. This adaptability makes them more resilient to environmental changes.
    • Examples: Goats, Rats, Raccoons, Human beings, and the House Sparrow.

• SIGNIFICANCE OF NICHE CONCEPT:

  • Ecosystem Functioning: Helps in understanding how species coexist and contribute to the stability and functioning of an ecosystem.
  • Conservation: Understanding the niche of a species is critical for its conservation. It helps in identifying its specific habitat requirements and potential threats (e.g., Project Tiger in India, which focuses on conserving the tiger’s entire niche, including its prey base and habitat).
  • Biodiversity Management: Explains how a high number of species can coexist in a habitat by partitioning resources and occupying different niches.
  • Invasive Species Management: Helps predict which species might become invasive and how they might impact native species by outcompeting them for their niche.
  • Evolutionary Ecology: The niche concept is central to understanding how species adapt and evolve over time through processes like natural selection.

BIOTIC INTERACTION

Biotic interactions are the effects that organisms in a community have on one another. These interactions can be positive (+), negative (-), or neutral (0) for the species involved.

S.No	INTERACTION	SPECIES A	SPECIES B	DESCRIPTION & EXAMPLE
1.	Mutualism	+	+	An obligatory interaction where both species benefit. They are interdependent and cannot survive without each other. Example: Coral polyps and Zooxanthellae (algae); the algae photosynthesize and provide food for the coral, which in turn provides a protected environment.
2.	Commensalism	+	0	One species benefits, while the other is neither harmed nor benefited. Example: Epiphytic plants like orchids growing on trees. The orchid gets access to sunlight without harming the tree. Suckerfish attaching to sharks.
3.	Proto-Cooperation	+	+	A non-obligatory interaction where both species benefit but can survive independently. Example: The Cattle Egret and grazing cattle. The egret feeds on insects stirred up by the cattle, while the cattle may benefit from the removal of pests.
4.	Amensalism	0	-	One species is inhibited or harmed, while the other is unaffected. A common form is allelopathy. Example: A large Banyan tree shading out smaller plants beneath it, inhibiting their growth. The Black Walnut tree releases a chemical (juglone) that harms nearby plants.
5.	Parasitism	+	-	One organism (the parasite) benefits at the expense of another (the host). The parasite lives on or in the host. Example: Tapeworms in the intestines of humans; the parasitic plant Cuscuta (dodder) on a host plant.
6.	Predation	+	-	One organism (the predator) hunts and kills another organism (the prey) for food. Example: A lion hunting a deer; a spider catching a fly.
7.	Competition	-	-	Both species are harmed as they compete for the same limited resources (food, water, space, mates). Example: Lions and Cheetahs competing for prey like gazelles in the African savanna.
8.	Cannibalism	+	-	A form of predation where the predator and prey are of the same species. Example: Female praying mantis eating the male after mating.

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Prelims Pointers

• Trophic Level: An organism’s functional position in a food chain.
• Ecological Pyramid: A graphical representation of trophic structure, first conceptualized by Charles Elton (1927).
• There are three types of ecological pyramids: Pyramid of Number, Pyramid of Biomass, and Pyramid of Energy.
• Standing Crop: The total biomass of organisms in a trophic level at a specific point in time. Biomass is measured as dry weight.
• Pyramid of Biomass (Inverted): Found in aquatic ecosystems (e.g., phytoplankton and zooplankton).
• Pyramid of Number (Inverted): Found in tree ecosystems (e.g., one tree supporting many insects).
• Pyramid of Energy: Always upright due to energy loss at each trophic level.
• Lindeman’s 10% Rule: Only about 10% of energy is transferred from one trophic level to the next.
• Ecological Productivity: Rate of biomass production (g/m²/yr).
• GPP (Gross Primary Productivity): Total rate of photosynthesis.
• NPP (Net Primary Productivity): GPP minus Respiration (R). This is the energy available to consumers.
• High Productivity Regions: Tropical rainforests, coral reefs, estuaries.
• Low Productivity Regions: Deserts, open oceans, tundra.
• Ecological Succession: Predictable change in community structure over time.
• Sere: The entire sequence of communities in succession.
• Seral Stage: An intermediate stage in succession.
• Pioneer Species: First species to colonize a barren area (e.g., lichens, mosses).
• Climax Community: The stable, final stage of succession.
• Primary Succession: Occurs on lifeless areas with no soil (e.g., volcanic island, glacial moraine).
• Secondary Succession: Occurs in a disturbed area where soil is already present (e.g., abandoned field, burnt forest).
• Ecological Niche: The functional role of a species in its ecosystem.
• Specialist Species: Have a narrow niche (e.g., Panda, Koala, Lion-tailed Macaque).
• Generalist Species: Have a broad niche (e.g., Rat, Goat, Human).
• Biotic Interactions:
  1. Mutualism (+,+): Both benefit, obligatory (Coral and Zooxanthellae).
  2. Commensalism (+,0): One benefits, other unaffected (Orchids on trees).
  3. Proto-Cooperation (+,+): Both benefit, non-obligatory (Cattle and Egret).
  4. Amensalism (0,-): One unaffected, other harmed (Banyan tree shading smaller plants).
  5. Parasitism (+,-): Parasite benefits, host is harmed (Tapeworm in host).
  6. Predation (+,-): Predator hunts and kills prey (Lion and deer).
  7. Competition (-,-): Both are harmed by competing for a resource (Lions and Cheetahs).

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Mains Insights

Cause-Effect Relationships and Analytical Perspectives

1. Limitations of Ecological Pyramids:

   • Ecological pyramids provide a simplified view. They do not account for species that operate at multiple trophic levels (e.g., omnivores).
   • They do not represent decomposers (saprophytes), which play a vital role in nutrient cycling, even though they derive energy from all trophic levels.
   • A pyramid of numbers or biomass represents a static snapshot (standing crop) and does not indicate the rate of turnover or productivity, which can lead to misleading inverted pyramids. The pyramid of energy is thus the most reliable model.
   • These pyramids can lead to the concepts of bioaccumulation (accumulation of a substance in an organism) and biomagnification (increasing concentration of a substance in tissues of organisms at successively higher levels in a food chain). This is a critical insight for GS Paper III (Environment).

2. Debates in Ecological Succession:

   • Clements vs. Gleason: The classical view by Frederic Clements saw succession as a deterministic, predictable process leading to a single, stable climax community (monoclimax theory). In contrast, Henry Gleason’s individualistic concept proposed that community composition is a result of chance and the individual responses of species to environmental conditions, leading to multiple potential outcomes.
   • Modern Synthesis: Current understanding incorporates both views. While there are predictable patterns (as Clements suggested), the exact path and final state of a community are also influenced by stochastic (random) events and species’ individual characteristics (as Gleason argued). This complexity is crucial for ecosystem restoration projects.

3. The Niche Concept and Conservation Strategy (GS Paper III):

   • Fundamental vs. Realized Niche: A species’ fundamental niche is the full range of conditions and resources under which it can survive and reproduce. Its realized niche is the portion of the fundamental niche that it actually occupies, constrained by competition and other biotic interactions.
   • Application in Conservation: Understanding this distinction is vital. A conservation plan for a species like the tiger must not only protect its habitat (fundamental niche) but also its prey base and manage competition from other predators (realized niche).
   • Invasive Species: Invasive species often succeed because they occupy an empty niche in the new environment or are superior competitors that displace native species from their realized niches. Understanding niche dynamics is key to predicting and managing invasions.

4. Productivity and Global Environmental Issues (GS Paper I & III):

   • Geographical Distribution: The global patterns of NPP directly correlate with the distribution of major biomes. This links physical geography (climate, soil) with ecology. For instance, high insolation and rainfall in equatorial regions lead to high NPP and the formation of tropical rainforests.
   • Climate Change Impact: Climate change can alter the factors affecting productivity (temperature, water availability), thus shifting the productivity of entire biomes. For example, desertification reduces NPP, while warming in the Arctic might temporarily increase it.
   • Carbon Sequestration: High-productivity ecosystems like forests and oceans act as major carbon sinks. Their degradation not only reduces biodiversity but also hampers the planet’s ability to absorb CO₂, exacerbating global warming. This connects ecological productivity to international climate policy.