Ecosystem

• Ecosystem

  • Varies greatly in size from a small pond to a large forest or a sea
  • Ecologists regard the entire biosphere as a global ecosystem, as a composite of all local ecosystems on Earth
  • Divided into two basic categories: terrestrial and aquatic
• Terrestrial ecosystems

  • Forest
  • Grassland
  • Desert
• Aquatic ecosystems

  • Pond
  • Lake
  • Wetland
  • River
  • Estuary
• Man-made ecosystems

  • Crop fields
  • Aquarium
• Ecosystem structure and function includes: productivity, decomposition, energy flow, and nutrient cycling
• Pond ecosystem


  A shallow water body in which all the basic components of an ecosystem are well exhibited
• Components of a pond ecosystem

  • Abiotic (water, dissolved substances, soil)
  • Autotrophs (phytoplankton, algae, floating/submerged/marginal plants)
  • Consumers (zooplankton, free-swimming/bottom-dwelling organisms)
  • Decomposers (fungi, bacteria, flagellates)
• Pond ecosystem performs all the functions of any ecosystem and of the biosphere as a whole: conversion of inorganic to organic material by autotrophs, consumption of autotrophs by heterotrophs, decomposition and mineralisation of dead matter
• Primary production

  Amount of biomass or organic matter produced per unit area over time by plants during photosynthesis
• Gross primary productivity (GPP)


  Rate of production of organic matter during photosynthesis
• Net primary productivity (NPP)


  GPP minus respiration losses, available biomass for consumption by heterotrophs
• Annual net primary productivity of the whole biosphere is approximately 170 billion tons (dry weight) of organic matter, with oceans contributing only 55 billion tons
• Decomposition


  Breaking down of complex organic matter into inorganic substances like carbon dioxide, water and nutrients by decomposers
• Steps in decomposition

  1. Fragmentation

  2. Leaching

  3. Catabolism

  4. Humification

  5. Mineralisation
• Decomposition is largely an oxygen-requiring process, and its rate is controlled by chemical composition of detritus and climatic factors like temperature and soil moisture
• Sun is the only source of energy for all ecosystems on Earth, and plants and photosynthetic bacteria capture only 2-10% of the photosynthetically active radiation
• Producers


  Green plants that capture solar energy and synthesize organic compounds. They are also known as transducers
• Consumers

  Animals that depend on plants (directly or indirectly) for their food needs, classified as primary, secondary, and tertiary consumers
• Decomposers


  Heterotrophic organisms, mainly fungi and bacteria, that degrade dead organic matter or detritus
• In a terrestrial ecosystem, a much larger fraction of energy flows through the detritus food chain than through the grazing food chain
• Organisms in a food chain

  • Grass (Producer)
  • Goat (Primary Consumer)
  • Man (Secondary Consumer)
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• Detritus food chain (DFC)

  Begins with dead organic matter, made up of decomposers which are heterotrophic organisms, mainly fungi and bacteria, that degrade dead organic matter or detritus
• Saprotrophs

  Decomposers that secrete digestive enzymes to break down dead and waste materials into simple, inorganic materials
• In an aquatic ecosystem

  GFC is the major conduit for energy flow
• In a terrestrial ecosystem

  A much larger fraction of energy flows through the detritus food chain than through the GFC
• Detritus food chain may be connected with the grazing food chain at some levels</b>
• Natural interconnection of food chains make it a food web
• Trophic level


  The place an organism occupies in the food chain based on its feeding relationship with other organisms
• Trophic levels

  • Producers (first trophic level)
  • Herbivores (primary consumers, second trophic level)
  • Carnivores (secondary consumers, third trophic level)
• The amount of energy decreases at successive trophic levels
• When any organism dies it is converted to detritus or dead biomass that serves as an energy source for decomposers
• Organisms at each trophic level depend on those at the lower trophic level for their energy demands
• Standing crop

  The mass of living material at a particular trophic level at a particular time
• Measurement of biomass in terms of dry weight is more accurate than fresh weight
• The number of trophic levels in the grazing food chain is restricted due to the 10 per cent law - only 10 per cent of the energy is transferred to each trophic level from the lower trophic level
• Ecological pyramid

  A graphical representation of the food or energy relationship between organisms at different trophic levels, expressed in terms of number, biomass or energy
• Types of ecological pyramids

  • Pyramid of numbers
  • Pyramid of biomass
  • Pyramid of energy
• In most ecosystems, all the pyramids of number, energy and biomass are upright
• Pyramid of energy is always upright, can never be inverted, because energy is lost as heat at each trophic level