Ecosystem

Ecosystem NEET Notes | High-Yield Concepts & Ecosystem PYQs

Ecosystem - Structure and Function

Ecosystem: A functional unit of nature where living organisms interact among themselves and with the physical environment. The term was coined by A.G. Tansley in 1935 NEET 2016.
Biotic Community: An association of individuals of different species living in the same habitat and having functional interactions NEET 2015.
Forest Ecosystem: A natural terrestrial ecosystem that contains the maximum biomass NEET 2017.
Ocean Ecosystem: The most stable ecosystem globally NEET 1995.
Cropland Ecosystem: A man-made ecosystem characterized by the least genetic diversity and the absence of natural soil organisms and weeds NEET 2016.
Stability vs. Resilience: An ecosystem that can be easily damaged but recovers quickly if the damaging effect stops has low stability and high resilience NEET 2004.

Interaction of biotic and abiotic components results in a characteristic physical structure.
Stratification: The vertical distribution of different species occupying different levels. Example: Trees occupy the top vertical strata, shrubs the second, and herbs/grasses the bottom layers NEET 2015 2016.
Functional Components: The ecosystem functions as a unit via four primary processes: Productivity, Decomposition, Energy flow, and Nutrient cycling. Note: Stratification is a structural feature, not a functional unit NEET 2012.

Primary Production: The amount of biomass or organic matter produced per unit area over a time period by plants during photosynthesis NEET 2022 2023. Expressed in weight (g m⁻²) or energy (kcal m⁻²).
Productivity: The rate of biomass production, expressed as g m⁻² yr⁻¹ or (kcal m⁻²) yr⁻¹.
Gross Primary Productivity (GPP): The rate of production of organic matter during photosynthesis NEET 1998 2015. Tropical rain forests have the highest GPP and annual net primary productivity NEET 1997 2007.
Net Primary Productivity (NPP): The remaining biomass available for consumption by heterotrophs (herbivores and decomposers) NEET 2010.
Equation: NPP = GPP - R (where R = Respiration losses) NEET 2021 2024.
Crucial Detail: GPP is always more than NPP NEET 2020.
10% Law Application: If the NPP of the first trophic level is 100x, it becomes the GPP of the second trophic level (100x). The NPP of the second trophic level becomes 10x, which is the exact GPP of the third trophic level NEET 2016.

Secondary Productivity: The rate of formation of new organic matter by consumers (e.g., a rabbit in a grassland) NEET 2004 2012 2015.
The annual NPP of the whole biosphere is approximately 170 billion tons (dry weight) of organic matter.
Despite occupying about 70% of the Earth's surface, the productivity of the oceans is only 55 billion tons.

Raw material for decomposition is detritus (dead plant/animal remains, fecal matter). Plant decomposers are primarily Monera (bacteria) and fungi NEET 2001; they act as "nature's scavengers" NEET 1997.
If decomposers are completely removed from an ecosystem, mineral movement will be blocked NEET 1995.
All the following steps operate simultaneously on detritus:
- Fragmentation: Detritivores (e.g., earthworms) breakdown detritus into smaller particles NEET 2011 2022 2023.
- Leaching: Water-soluble inorganic nutrients go down into the soil horizon and precipitate as unavailable salts NEET 2013 2023.
- Catabolism: Bacterial and fungal enzymes degrade detritus into simpler inorganic substances NEET 2013 2022.
- Humification: Leads to the accumulation of a dark-colored, amorphous substance called humus. Humus is highly resistant to microbial action, undergoes decomposition at an extremely slow rate, and serves as a reservoir of nutrients NEET 2013.
- Mineralisation: The humus is further degraded by some microbes, releasing inorganic nutrients NEET 2023.

Decomposition is largely an oxygen-requiring process.
Slow Rate: Occurs if detritus is rich in lignin and chitin NEET 2020 2022, or if there is low moisture content NEET 2008, low temperature, or anaerobiosis.
Fast Rate: Occurs if detritus is rich in nitrogen and water-soluble substances like sugars NEET 2020 2022, in a warm and moist environment.

The Sun is the sole source of energy for all ecosystems, EXCEPT for deep-sea hydrothermal vents where primary producers are chemosynthetic bacteria NEET 2016. Most animals in deep oceanic waters are detritivores NEET 2015.
PAR (Photosynthetically Active Radiation): Less than 50% of incident solar radiation NEET 2011. Plants capture only 2-10% of PAR.
Energy flow is strictly unidirectional from producers to consumers.

Feature	Grazing Food Chain (GFC)	Detritus Food Chain (DFC)
Origin	Begins with living green plants (Producers).	Begins with dead organic matter (Detritus) NEET 1991.
Major Role	Major conduit of energy flow in aquatic ecosystems.	Major conduit of energy flow in terrestrial ecosystems.

Trophic Level: Based on the source of nutrition, organisms occupy a specific place in the food chain.
Standing Crop: The mass of living material (biomass) at a trophic level at a particular time NEET 2011 2015. Measured accurately as dry weight (fresh weight is not used for ecological pyramids NEET 2006).
10% Law: Only 10% of energy is transferred from a lower trophic level to the next higher level. The law was given by Lindemann NEET 1996. Because of massive energy loss, food chains are restricted to 3-4 levels NEET 2008.
Calculation Tip: If 20 J is trapped by a plant, the energy available to a peacock (Plant -> Mice -> Snake -> Peacock; T4) is 0.02 J NEET 2014. If T4 has 1000 J, T1 must have had 1,000,000 J NEET 2024.
Producers always outnumber primary consumers NEET 1988. Primary consumers are herbivores NEET 1995.
The dominant second trophic level in a lake ecosystem is zooplankton NEET 1994.
Exception: A single organism can occupy more than one trophic level simultaneously. Example: A sparrow eating seeds is a primary consumer (T2); eating insects makes it a secondary consumer (T3) NEET 2011. Similarly, fish in a pond ecosystem occupy multiple levels NEET 2009.

The base always represents producers (T1); the apex represents top carnivores.
Pyramid of Energy: Always upright, can never be inverted because energy decreases at successive trophic levels due to heat loss NEET 2011 2018 2019 2020 2024.
Pyramid of Biomass:
- Terrestrial/Forest: Upright.
- Aquatic/Sea: Generally inverted because the standing crop of phytoplankton is small but supports a large standing crop of zooplankton and fishes NEET 2015 2021.
Pyramid of Numbers:
- Grassland/Pond: Upright NEET 1993 2021.
- Single Tree Ecosystem: Inverted (one tree supports many insects, which support even more parasites).
- Forest Ecosystem: Spindle-shaped (Upright pyramid of number is absent because producers are large trees, fewer in number than the herbivores they support) NEET 2010 2012.

NEET 2015

It does not account for the same species belonging to two or more trophic levels.
It assumes a simple food chain, which almost never exists in nature.
It does not accommodate a food web.
Saprophytes (decomposers) are not given any place in ecological pyramids, despite playing a vital role in the ecosystem.

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