IB DP Biology: SL复习笔记4.1.7 Skills: Species, Communities, Ecosystems & Energy Flow

Identification of Trophic Level

Classifying species as autotrophs, consumers, detritivores, or saprotrophs

  • A species can be classified as an autotroph, consumer, detritivore, or saprotroph on the basis of its mode of nutrition


  • An autotroph synthesises, or produces, its own organic molecules from simple inorganic substances in its environment
    • Photosynthetic organisms use light energy to convert carbon dioxide from the air into organic molecules such as carbohydrates
    • Some autotrophs use energy from the oxidation of inorganic compounds instead of light energy
    • Autotrophs that use light energy are known as photoautotrophs, while those that use energy from oxidation of chemicals are known as chemoautotrophs
  • Because autotrophs make their own organic molecules without relying on other organisms, they are known as producers
  • Most green plants are autotrophs, with few exceptions
    • Some unusual plants are parasitic, gaining their nutrients from the roots of host plants, or via networks of fungi in the soil


  • Heterotrophic organisms gain their organic molecules from other organisms
  • There are several types of heterotroph, including consumers, detritivores, and saprotrophs


  • Consumers gain their organic molecules by ingesting the tissues of other living or recently dead organisms
  • The consumers that eat plants are known as herbivores, and are the primary consumers in a food chain
  • The consumers that eat other animals are carnivores, and those that eat the primary consumers are secondary consumers
  • Carnivores that eat secondary consumers are tertiary consumers


  • Detritivores gain organic molecules by ingesting the tissues of dead organisms or ingesting animal waste
  • Detritivores digest their food inside their bodies
  • Examples of detritivores include earthworms, woodlice and dung beetles


  • Saprotrophs also gain their organic molecules from dead matter, but they digest their food externally
  • Saprotrophic organisms secrete enzymes onto dead matter, and these enzymes break down the food before nutrients are absorbed
  • Saprotrophs include fungi and bacteria

Classifying Species as Autotrophs, Consumers, Detritivores, or Saprotroph Table


Exam Tip

The two main modes of nutrition are autotrophism and heterotrophism, and within those modes are different types e.g. a heterotroph can be a consumer, a detritivore, or a saprotroph depending on its food source and its method of digestion

Practical 5: Mesocosm - Sustainable Ecosystem

Reasons for building mesocosms

  • A mesocosm is an experimental container in which a naturally occurring ecosystem is simulated
  • Mesocosms can be used to study the response of an ecosystem to changes in specific factors such as nutrient and light levels
  • Unlike a real ecosystem, it is possible in a mesocosm to control all of the factors other than the variable being studied
  • Mesocosms can be set up in many different ways for many purposes
    • Water tanks can be set up on land to study the effect of sewage pollution on ponds or lakes
    • Underwater enclosures can be built in coastal waters or lakes to study the effect of temperature change or dissolved carbon dioxide on ocean ecosystems
    • Trees can be planted in large greenhouse-like buildings to replicate a rainforest to investigate the passage of carbon through this ecosystem
  • Mesocosm experiments can be considered unrealistic due to their enclosed nature and the level of control that can be achieved
    • Realism can be improved by designing large mesocosms that share more of the features of a real ecosystem e.g. enabling mixing of layers of water in a large ocean mesocosm

Building a mesocosm in the lab

  • It is possible to build small mesocosms in the laboratory
  • Factors to consider:
    • The container should be transparent to enable sunlight to reach producers inside the mesocosm
    • Autotrophs should be included so that light energy can be converted into chemical energy inside the mesocosm
    • Small primary consumers such as zooplankton or other small invertebrates could be included, but it is important to consider whether the mesocosm is likely to be large enough to support them
    • Do not include secondary consumers in a mesocosm because there will not be enough energy in the food chain to sustain them for long, and it could be considered unethical to allow the primary consumers to be eaten in this way
  • Mesocosms can be set up as open systems, i.e. without a lid, but sealed systems are more controlled, and therefore more useful for experimental purposes
    • Sealed systems prevent organisms and substances from entering or leaving
  • In the lab, a mesocosm can be set up and then a known factor can be altered to assess its effect
    • E.g. different light levels, different temperatures etc.
  • In order to assess the impact of changing one factor, a control mesocosm must be set up at the same time
    • A control mesocosm will be exactly the same as the experimental mesocosm, but the altered variable will not be changed
    • The purpose of this is to demonstrate that any change in the mesocosm is due to the altered factor and not another factor

Terrestrial mesocosm

  • Place drainage material such as gravel in the bottom of a clear container
  • Add a layer of charcoal on top of the drainage layer; this can help to prevent the growth of mould
  • Place a layer of sphagnum moss or filter paper on top of the charcoal to provide separation between the base layers and the organic matter above
  • Add a layer of soil or compost above the separation layer; this provides organic material and micro-organisms to aid with nutrient cycling
  • Plant slow-growing producers such as healthy mosses and ferns in the growth medium
  • Water the growth medium before sealing the container with a lid
    • The mesocosm may need watering while it establishes, but avoid excessive watering; once the mesocosm has stabilised, the plants should release enough water vapour during respiration to maintain moisture levels
  • Place the container in a light location, and ensure that the temperature is stable

Aquatic mesocosm

  • The base layer of the mesocosm should consist of organic substrate from the bottom of a lake or pond; this will provide naturally occurring nutrients and microorganisms
  • Add lake or pond water; this ensures that it contains the required microscopic organisms and avoids chemicals from tap water
  • Add healthy aquatic plants to produce carbohydrates and oxygenate the water
  • Small aquatic organisms such as water fleas or water snails can be added, but not more organisms than the size of mesocosm can support
  • Place the container in a light location, and ensure that the temperature is stable


Building a terrestrial or aquatic mesocosm