Ecosystem Dynamics: Energy Flow and Food Chains

• Energy flow and nutrient cycles are important aspects of ecosystem dynamics.

Energy Flow in an Ecosystem

• Energy flow refers to the movement of energy through an ecosystem.
• This flow occurs in a straight line.
• The sun is the fundamental source of energy.
• Producers, such as green plants and algae, capture only about 1% of solar energy.
• Producers convert solar energy into chemical energy through photosynthesis.
• Other organisms rely on the energy stored in producers.
• Feeding relationships in an ecosystem can be represented as food chains and food webs.

Food Chain

• A food chain is a linear representation of feeding relationships among organisms in an ecosystem.
• Each organism in a food chain derives energy from the preceding organism.
• Each feeding level is called a trophic level.
• Different species occupy different trophic levels.
• The first trophic level is occupied by producers/primary producers (autotrophs).
• All other levels are occupied by heterotrophs (consumers).
• The second trophic level is occupied by primary consumers (herbivores and detritivores)
• The third trophic level is occupied by carnivores feeding on herbivores (secondary consumers)
• The fourth trophic level is occupied by predators that feed on herbivores (higher carnivores).
• Examples of food chains include:
  • Green plants → Insects → Lizards → Snakes
  • Green Plants (Producers) → Deer (Herbivore) → Tiger (Carnivore)
  • Grassland: Grass → Grasshopper → Frog → Snake → Hawk
  • Freshwater Pond: Algae → Small animal → Small fish → Big fish → Bird
  • Plants → Deer → Lion
  • Plants → Worm → Bird → Cat
  • Algae → Small animal → Small fish → Big fish → Bird

Food Web

• In reality, organisms feed on more than one type of organism.
• In nature, an animal may feed in more than one food chain at the same time resulting in a food web.
• A food web consists of several food chains linked at different points.
• A food web has several alternative feeding pathways.
• Examples of food webs include grassland and forest ecosystems

Ecological Pyramids

• An ecological pyramid is a graphical representation of energy flow through the ecosystem.
• There is a disproportionate relationship between the numbers of producers, primary, secondary, and tertiary consumers in a community.
• The base of the pyramid is occupied by producers.
• The second level is occupied by primary consumers.
• The next level is occupied by secondary consumers and so on.
• There are three types of ecological pyramids:
  • Pyramid of numbers: Shows the number of individual organisms at each level.
  • Pyramid of biomass: Shows the amount of living tissue or total mass of organisms at each trophic level.
  • Pyramid of energy: Depicts the amount of energy at each trophic level.
• In a pyramid of numbers of a grassland, 7,500,000 Grasses support 12,050 Rats which support 580 Snakes which support 26 Hawks.

Nutrient Cycles

• Nutrient flow is the movement of nutrients through an ecosystem.
• Unlike energy, nutrients (chemical elements) are available only in limited supply in an ecosystem.
• Hence, nutrients have to be recycled (biogeochemical cycles).
• The flow of nitrogen is referred to as the Nitrogen Cycle.
• The flow of carbon is referred to as the Carbon Cycle.
• Water also moves in a cycle known as the hydrological cycle (Water Cycle).

Nitrogen Cycle

• Conversion of atmospheric nitrogen into nitrates is known as the Nitrogen Cycle.
• The major steps of the nitrogen cycle are:
  • Nitrogen fixation
  • Nitrification
  • Ammonification
  • Assimilation
  • Denitrification

Nitrogen Fixation

• This is the conversion of atmospheric nitrogen into nitrates.
• Nitrogen fixation can occur through:
  • Direct fixation by electrochemical means during lighting or thunderstorm.
  • Fixation by some microorganisms, especially bacteria, convert atmospheric nitrogen (N₂) into ammonia (NH3) or ammonium (NH₄⁺).
  • Examples include Azobacter, Clostridium, Nostoc, and Anabaena.
  • Rhizobium, in the root nodules of legumes like beans, groundnuts, and peas, also participate in nitrogen fixation.

Nitrification

• This is the process by which soil microorganisms convert ammonia (NH3) or ammonium (NH4+) to nitrate (NO3).
• The process is completed in two stages.
  • In the first stage, ammonia is oxidised to nitrites (NO₂) by bacteria such as Nitrosomonas, Nitrococcus, and Nitrospira.
  • In the second stage, nitrites (NO₂) are converted to nitrates (NO₃) by bacteria such as Nitrobacter and Nitrocystis.

Ammonification

• Proteins in dead organic matter are decomposed by microorganisms to produce amino acids and ammonia.
• The ammonia so formed is either released into the atmosphere or retained in the soil to be absorbed by plants.
• Under certain conditions, it may be oxidised to nitrates.
• Ammonification is the most common pathway for nitrogen to enter the soil.
• The organisms responsible for ammonification are mostly fungi (Actinomycetes) and bacteria such as species of Bacillus subtilis, B. mesenterilus.

Assimilation

• The process by which inorganic nitrogen substances are used to form organic nitrogen compounds.
• Organisms assimilate nitrogen by taking up NH3, NH4⁺, and NO3⁻ formed through nitrogen fixation and nitrification.
• Plant roots take up these forms of nitrogen, and animals assimilate nitrogen from plant tissues.

Denitrification

• The reduction of some nitrates (NO3) in the soil to gaseous nitrogen (N₂) or oxides of nitrogen or ammonia.
• Mostly under anaerobic conditions, the oxygen in nitrate molecules is used by denitrifying bacteria to oxidize carbohydrates.
• Examples of bacteria responsible for denitrification are Bacillus cereus, B. licheniformis, and Pseudomonas denitrificans.
• The process of denitrification delivers only a relatively small amount of nitrogen to the atmosphere.

Carbon Cycle

• Carbon cycle is the movement of carbon from the atmosphere into organisms and back to the atmosphere.
• The carbon cycle is complex because carbon can exist in different compounds in plants, animals, rocks, liquids, sediments, and air.
• Oceans are the major reservoir of CO₂, containing 50 times more than that of the air.
• Carbon is stored in oceans as bicarbonate mineral deposits on the ocean floor.
• The ocean regulates the CO₂ in the atmosphere.
• The carbon cycle operates through CO2 exchange among the atmosphere, biosphere, and the oceans.