Unit 2: Ecosystem

An ecosystem is the structural and functional unit of ecology where living organisms interact with each other and with their surrounding physical environment. The term was first coined by A.G. Tansley in 1935.

1. Components and Types of Ecosystems

Components of an Ecosystem

Every ecosystem is composed of two major interlinked categories of components: Abiotic (non-living) and Biotic (living).

Ecosystem: A self-regulating biological community of interacting organisms and their physical environment.

Sub-categories of Biotic Components

• Producers (Autotrophs): Organisms that synthesize their organic nutrients from inorganic molecules using light (photosynthesis) or inorganic chemical reactions (chemosynthesis).
• Consumers (Heterotrophs): Organisms that must ingest other organisms to obtain organic nutrients. Under this, we have:
  • Primary Consumers (Herbivores): Feed directly on producers. Example: Grasshoppers, deer, zooplankton.
  • Secondary Consumers (Primary Carnivores): Feed on primary consumers. Example: Frogs, small fish, birds.
  • Tertiary Consumers (Secondary Carnivores): Feed on secondary consumers. Example: Snakes, larger fish, wolves.
  • Quaternary Consumers (Apex Predators): Occupy the top of the food chain with no natural predators. Example: Lions, eagles.
• Decomposers (Saprotrophs/Detritivores): Organisms that decompose organic matter from dead plants and animals. They secrete enzymes to break down organic structures externally, then absorb the nutrients. Example: Mushrooms, soil bacteria.

Types of Ecosystems

Ecosystems can be classified based on their origin and habitat medium into natural and artificial categories.

Common Mistake: Students often mistake agricultural fields for natural terrestrial ecosystems. Because human intervention, artificial fertilizers, and selected crops define agricultural fields, they are structurally and functionally classified as artificial (anthropogenic) ecosystems.

2. Food Chain, Food Web, and Energy Flow

Food Chain

A food chain is a linear sequence of organisms through which nutrients and energy pass as one organism eats another. Each step in a food chain represents a distinct trophic level.

Trophic Level: The specific feeding position or level that an organism occupies in a food chain based on its source of nutrition.

There are two fundamental types of food chains in ecosystems:

1. Grazing Food Chain (GFC): Starts from living green plants (producers) at the first trophic level and progresses through herbivores to carnivores.

   Example: Grass (Producer) -> Grasshopper (Primary Consumer) -> Frog (Secondary Consumer) -> Snake (Tertiary Consumer) -> Eagle (Quaternary Consumer).

2. Detritus Food Chain (DFC): Starts with dead organic matter (detritus) that is consumed by detritivores and decomposers, which are subsequently eaten by predators.

   Example: Fallen Leaves (Detritus) -> Earthworm (Detritivore) -> Sparrow (Primary Carnivore) -> Falcon (Secondary Carnivore).

Food Web

In nature, simple linear food chains rarely exist in isolation. Many organisms eat more than one type of food, creating interconnected networks. This network of multiple food chains is called a food web.

Food Web: An interconnected, complex network of multiple food chains demonstrating the diverse feeding relationships and pathways of energy transfer within an ecosystem.

Energy Flow through the Ecosystem

Energy enters the biosphere primarily as solar radiation. This energy is captured by plants and transferred through the food web. The behavior of energy flow is governed by the Laws of Thermodynamics:

• First Law of Thermodynamics: Energy cannot be created or destroyed; it can only be transformed from one form to another. Solar energy is converted into chemical energy (organic matter) by producers.
• Second Law of Thermodynamics: During any energy transformation, some energy is lost as unusable heat, increasing the entropy of the system. Thus, no biological energy transfer is 100% efficient.

Because of these thermodynamic losses, energy flow through an ecosystem is strictly unidirectional. Energy lost as heat to the environment can never be recycled back into the biological pathway or recaptured by the sun.

The Ten Percent Law

Proposed by Raymond Lindeman in 1942, this law governs trophic efficiency:

Ten Percent Law: Only about 10% of the energy present at any given trophic level is transferred to the next higher trophic level. The remaining 90% is lost during transfer as heat, used during respiration, or remains unconsumed as waste.

Example: If producers trap 10,000 Joules of energy from the sun:

• Primary consumers receive: 1,000 Joules
• Secondary consumers receive: 100 Joules
• Tertiary consumers receive: 10 Joules
• Apex predators receive: 1 Joule

3. Ecological Pyramids, Efficiencies, and Productivity

Ecological Pyramids

Ecological pyramids are graphical models designed to represent the structural and functional parameters (numbers, biomass, or energy) of successive trophic levels in an ecosystem.

• Pyramid of Numbers: Represents the total number of individual organisms at each trophic level.
  • Upright: Common in grassland and pond ecosystems, where millions of microscopic phytoplankton support fewer zooplankton, which support even fewer fish.
  • Inverted: Seen in parasitic food chains (e.g., one oak tree supporting thousands of herbivorous insects, which host millions of hyper-parasites) or a single tree supporting many herbivorous birds.
• Pyramid of Biomass: Represents the total dry weight of living organic matter (biomass) at each trophic level per unit area.
  • Upright: Common in terrestrial ecosystems (forests, grasslands) where the biomass of producers (trees, grasses) is far greater than the biomass of herbivores and carnivores.
  • Inverted: Characteristic of aquatic ecosystems (oceans, lakes). The standing crop biomass of producers (phytoplankton) at any single moment is much lower than that of the consumers (zooplankton, fish) because phytoplankton reproduce and get consumed rapidly.
• Pyramid of Energy: Represents the total amount of energy accumulated per unit area per unit time at each trophic level.
  • Always Upright: The pyramid of energy can never be inverted in any ecosystem. Because of the Second Law of Thermodynamics and the 10% law, energy is progressively lost as heat at each higher level, meaning the base must always contain the maximum energy.

Ecological Efficiencies

Ecological efficiency refers to the efficiency with which energy is transferred from one trophic level to the next. Several parameters define these ratios:

• Photosynthetic Efficiency: The percentage of solar radiation falling on a plant's leaves that is converted into chemical energy.

  Photosynthetic Efficiency = (Gross Primary Productivity / Incident Solar Radiation) * 100

• Consumption Efficiency (CE): The percentage of available energy produced by one trophic level that is consumed by the next higher level.

  CE = (Ingestion at Level n / Production at Level n-1) * 100

• Assimilation Efficiency (AE): The percentage of ingested energy that is digested and absorbed by the consumer, with the rest excreted as feces.

  AE = (Assimilation at Level n / Ingestion at Level n) * 100

• Production Efficiency (PE): The percentage of assimilated energy that is converted into new biomass, with the rest lost as respiratory heat.

  PE = (Production at Level n / Assimilation at Level n) * 100

• Lindeman/Ecological Efficiency: The product of consumption, assimilation, and production efficiencies, representing total energy transfer efficiency.

  Ecological Efficiency = (Energy Assimilated at Level n / Energy Assimilated at Level n-1) * 100

Productivity

Productivity refers to the rate of generation of biomass in an ecosystem per unit area over a specified time interval (typically expressed as g/m²/year or kcal/m²/year).

Important Observation: Marine ecosystems, despite covering nearly 70% of the Earth's surface, contribute less than one-third of the global Net Primary Productivity, primarily because large open ocean areas lack essential nutrients like iron and nitrogen.

4. Biogeochemical Cycles

Biogeochemical cycles represent the movement of chemical elements (nutrients) through the biological (biotic) and geological (abiotic) components of the biosphere.

There are two primary categories of biogeochemical cycles:

• Gaseous Cycles: The primary reservoir of nutrients is the atmosphere or hydrosphere. These cycles are highly rapid and dynamic. Examples: Nitrogen Cycle, Carbon Cycle.
• Sedimentary Cycles: The primary reservoir of nutrients is the lithosphere (Earth's crust). These cycles are slower and can suffer long-term structural immobilization. Example: Phosphorus Cycle.

1. The Nitrogen Cycle

Nitrogen is a vital component of amino acids, proteins, enzymes, and nucleic acids (DNA and RNA). While atmospheric air contains 78% nitrogen gas (N2), plants and animals cannot use it in this gaseous form. It must be converted into soluble forms (ammonium and nitrates) through a five-step biological cycle.

1. Nitrogen Fixation: The process of converting inert gaseous nitrogen (N2) into reactive nitrogen compounds like ammonia (NH3) or ammonium (NH4+).
   • Biological Nitrogen Fixation: Done by symbiotic bacteria (e.g., Rhizobium living in root nodules of leguminous plants) and free-living bacteria (e.g., Azotobacter, Clostridium, and blue-green algae like Anabaena).
   • Atmospheric Fixation: Occurs through lightning or cosmic radiation, which breaks the N2 triple bond to form nitrates.
   • Industrial Fixation: Done through the industrial Haber-Bosch process to produce fertilizers.
2. Nitrification: The two-step biological conversion of ammonia/ammonium into plant-absorbable nitrates (NO3-).
   • First, ammonia is converted into nitrites (NO2-) by soil bacteria like Nitrosomonas and Nitrosococcus.

     2NH3 + 3O2 -> 2NO2- + 2H+ + 2H2O

   • Second, nitrites are converted into nitrates (NO3-) by bacteria like Nitrobacter.

     2NO2- + O2 -> 2NO3-

3. Assimilation: Plant roots absorb nitrates (NO3-) or ammonium (NH4+) from the soil and incorporate them into plant proteins, nucleic acids, and amino acids. Herbivores assimilate this nitrogen by eating plants.
4. Ammonification: When plants and animals die or excrete nitrogenous waste, decomposers (bacteria and fungi) break down the organic nitrogen compounds back into inorganic ammonia (NH3) or ammonium (NH4+).
5. Denitrification: The conversion of soil nitrates (NO3-) back into gaseous nitrogen (N2), which is released back into the atmosphere. This is carried out by anaerobic denitrifiers (e.g., Pseudomonas and Thiobacillus denitrificans) typically in waterlogged or oxygen-depleted soils.

2. The Carbon Cycle

Carbon is the structural backbone of all organic molecules. The carbon cycle is a gaseous cycle that coordinates the flow of carbon between the atmosphere, biosphere, oceans, and lithosphere.

• Photosynthesis: Autotrophs absorb gaseous CO2 from the atmosphere and fix it into carbohydrate molecules using sunlight.

  6CO2 + 6H2O + Light Energy -> C6H12O6 + 6O2

• Respiration: Plants, animals, and decomposers break down carbon compounds (glucose) to produce metabolic energy (ATP), returning CO2 to the atmosphere.

  C6H12O6 + 6O2 -> 6CO2 + 6H2O + Metabolic Energy

• Decomposition: Decomposers break down dead organic matter, releasing carbon as CO2 through cellular respiration, or depositing it into soil reserves.
• Combustion: The rapid burning of organic biomass, wood, and fossil fuels (coal, oil, natural gas) releases locked carbon back into the atmosphere as carbon dioxide.

3. The Phosphorus Cycle

Phosphorus is a critical element for energy storage and transfer (ATP), genetic materials (DNA, RNA), cell membrane components (phospholipids), and vertebrate bone and teeth structures. The phosphorus cycle is a sedimentary cycle with no active atmospheric gaseous phase.

1. Weathering of Rocks: The primary reservoir of phosphorus is phosphate rock. Physical and chemical weathering of rocks releases phosphate ions (PO4^3-) into soil water and runoff.
2. Absorption by Plants: Terrestrial plants absorb inorganic phosphates from the soil through their roots and assimilate them into organic molecules.
3. Consumption: Animals acquire phosphorus by eating plants or feeding on other herbivores.
4. Decomposition and Excretion: Decomposers (bacteria and fungi) break down dead organisms and animal wastes, returning organic phosphorus back into the soil as inorganic orthophosphate (mineralization).
5. Sedimentation and Geological Uplift: Soluble phosphorus in aquatic ecosystems eventually binds with calcium, iron, or aluminum and precipitates into sedimentary layers on the ocean floor. Over millions of years, tectonic processes and geological uplift expose these oceanic sediments back to land, allowing weathering to restart the cycle.

Comparison of Gaseous vs. Sedimentary Cycles