Ecosystem Energy Flow and Food Chains

• Energy flow refers to the movement of energy through an ecosystem
• Energy flows in an ecosystem in a straight line
• The sun is the source of energy for ecosystems
• Producers, such as green plants and algae, trap about 1% of solar energy and convert it into chemical form via photosynthesis
• Other organisms rely on the food stored by producers

Food Chains

• A food chain is a linear representation of feeding relationships among living organisms in an ecosystem
• Each organism derives energy from the preceding organism in a food chain
• Each feeding level in a food chain is a trophic level
• Different species occupy different trophic levels
• The first trophic level is occupied by producers/primary producers (autotrophs)
• Heterotrophs (consumers) occupy all other levels
• The second trophic level is occupied by primary consumers (herbivores and detritivores)
• Carnivores that feed on herbivores (secondary consumers) occupy the third trophic level
• Predators that feed on herbivores (higher carnivores) occupy the fourth trophic level

Food chain examples

• Green plants -> insects -> lizards -> snakes
• Green plants (producers) -> deer (herbivore) -> tiger (carnivore)
• Grasses (producers) -> grasshopper (consumer of the first order) -> frog (consumer of the second order) -> snake (consumer of the third order) -> hawk (top carnivore)
• Algae -> small animal -> small fish -> big fish -> bird
• Plants -> deer -> lion
• Plants -> worm -> bird -> cat
• Grass -> grasshopper -> frog -> snake -> hawk

Food Webs

• A food chain depicts each organism feeding on only one type of organism
• Organisms may feed on more than one organism at the same time in reality
• An animal may feed in more than one food chain simultaneously, resulting in a food web in nature
• A food web consists of multiple linked food chains
• A food web has several alternative feeding pathways

Ecological Pyramids

• An ecological pyramid is a graphical depiction/representation of energy flow through the ecosystem
• A disproportionate relationship between the numbers of producers, primary, secondary, and tertiary consumers exists in a community
• These relationships can be represented graphically as a triangular ecological pyramid
• Producers occupy the base, primary consumers the second level, and secondary consumers and so on follow
• There are three types of ecological pyramids:
  • Pyramid of numbers: Shows the number of individual organisms at each level
  • Pyramid of biomass: Indicates the amount of living tissue or total mass of organisms within each trophic level
  • Pyramid of energy: Depicts the amount of energy at each trophic level

Nutrient Cycles

• Nutrient flow refers to the movement of nutrients through an ecosystem
• Unlike energy from the sun, nutrients (chemical elements) are available only in limited supply in an ecosystem
• Nutrients have to be recycled in biogeochemical cycles
• The flow of nitrogen is the nitrogen cycle
• Carbon in the carbon cycle
• Water moves in the hydrological cycle (water cycle)

Nitrogen cycle

• The major pathways are:
  • Nitrogen fixation
  • Nitrification
  • Ammonification
  • Assimilation
  • Denitrification

Nitrogen Fixation

• This is the conversion of atmospheric nitrogen into nitrates
• Direct fixation via electrochemical means during lighting or thunderstorms can cause nitrogen fixation
• Fixation by some microorganisms, especially bacteria, utilise hydrogenase enzyme to convert atmospheric nitrogen (N₂) into ammonia (NH3) or ammonium (NH₄⁺):
  • Examples include Azobacter, Clostridium, Nostoc, and Anabena
  • Rhizobium is an important group of nitrogen-fixing bacteria living in root nodules of legumes, like beans, peas, and groundnuts, in mutualistic association

Nitrification

• 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, bacteria such as Nitrosomonas, Nitrococcus, and Nitrospira oxidise ammonia to nitrites (NO₂-)
• In the second stage, bacteria such as Nitrobacter and Nitrocystis convert nitrites (NO₂-) to nitrates (NO₃-)

Ammonification

• A group of microorganisms decompose proteins in dead organic matter to produce amino acids and ammonia
• Ammonia released into the atmosphere or retained in the soil is absorbed by plants
• Ammonia may be oxidised to nitrates under certain conditions
• Ammonification is the most common pathway for nitrogen to enter the soil, as many soils lack adequate nitrifying bacteria
• Mostly fungi (Actinomycetes) and bacteria, such as species of Bacillus subtilis and B. mesenterilus, are responsible for ammonification

Assimilation

• The process by which inorganic nitrogen compounds are used to form organic nitrogen compounds
• Organisms assimilate nitrogen by taking up NH3, NH4+, and NO3- formed through nitrogen fixation and nitrification, incorporating them into other molecules
• Plant roots take up these forms of nitrogen through their roots, and animals assimilate nitrogen from plant tissues

Denitrification

• The reduction of nitrates (NO3) in the soil to gaseous nitrogen (N₂) or oxides of nitrogen or ammonia
• Denitrifying bacteria oxidise carbohydrates, using oxygen in nitrate molecules, mostly under anaerobic conditions
• Some important bacteria for denitrification are Bacillus cereus, B. licheniformis, and Pseudomonas denitrificans
• Denitrification delivers a relatively small amount of nitrogen to the atmosphere

Carbon Cycle

• The movement of carbon from the atmosphere into organisms and back
• Very complex because carbon can exist in different compounds in plants, animals, rocks, liquids, sediments, and air
• Oceans are the major CO₂ reservoir, containing 50 times more CO₂ than the air
• Stored in oceans as bicarbonate mineral deposits on the ocean floor
• The ocean regulates CO₂ in the atmosphere
• Operates in the form of CO₂ exchange among the atmosphere, biosphere, and the oceans