5.1 Communities and Ecosystems


5.1.1  Define species, habitat, population, community, ecosystem and ecology

Species:  A group of organisms that can interbreed and produce fertile, viable offspring

Habitat:  The environment in which a species normally lives or the location of a living organism

Population:  A group of organisms of the same species who live in the same area at the same time

Community:  A group of populations living and interacting with each other in an area

Ecosystem:  A community and its abiotic environment

Ecology:  The study of relationships between living organisms and between organisms and their environment

5.1.2  Distinguish between autotroph and heterotroph

Autotroph:  An organism that synthesises its organic molecules from simple inorgance substances (e.g. CO2 and nitrates) - autotrophs are producers

Heterotroph:  An organism that obtains organic molecules from other organisms - heterotrophs are consumers

5.1.3  Distinguish between consumers, detritivores and saprotrophs

Consumer:  An organism that ingests other organic matter that is living or recently killed

Detritivore:  An organism that ingests non-living organic matter

Saprotroph: An organism that lives on or in non-living organic matter, secreting digestive enzymes into it and absorbing the products of digestion

5.1.4  Decribe what is meant by a food chain, giving three examples, each with at least three linkages (four organisms)

A food chain shows the linear feeding relationships between species in a community

The arrows represent the transfer of energy and matter as one organism is eaten by another (arrows point in the direction of energy flow)

The first organism in the sequence is the producer, followed by consumers (1°, 2°, 3°, etc.)

Examples of Food Chains

5.1.5  Describe what is meant by a food web

A food web is a diagram that shows how food chains are linked together into more complex feeding relationships within a community

There can be more than one producer in a food web, and consumers can occupy multiple positions (trophic levels) 

5.1.6  Define trophic level  

An organism's trophic level refers to the position it occupies in a food chain

Producers always occupy the first trophic level, while saprotrophs would generally occupy the ultimate trophic level of a given food chain or food web

The trophic levels in a community are:

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5.1.7  Deduce the trophic levels of organisms in a food web and food chain

The trophic level of an organism can be determined by counting the number of feeding relationships preceding it  and adding one (producer always first) 

Trophic Level = Number of arrows (in sequence) before organism + 1

In food webs, a single organism may occupy multiple trophic levels

5.1.8  Construct a food web containing up to 10 organisms, using appropriate information

Hint:  When constructing a food web, always try to position an organism relative to its highest trophic level (to keep all arrows pointing in same direction)

Food web (trophic levels in red)

5.1.9  State that light is the initial energy source for almost all communities

  • All green plants, and some bacteria, are photo-autotrophic - they use light as a source of energy for synthesising organic molecules
  • This makes light the initial source of energy for almost all communities 
  • Some bacteria are chemo-autotrophic and use energy derived from chemical processes (e.g. nitrogen-fixating bacteria)

5.1.10  Explain the energy flow in a food chain

  • Energy enters most communities as light, where it is absorbed by autotrophs (e.g. plants) and converted into chemical energy via photosynthesis
  • Energy then gets passed to the primary consumer (herbivore) when they eat the plant, and then gets passed to successive consumers (carnivores) as they are eaten in turn
  • Only ~10% of energy is passed from one trophic level to the next, the rest is lost 
  • Because ~90% of energy is lost between trophic levels, the number of trophic levels are limited as energy flow is reduced at higher levels

Summary of Energy Flow in a Food Chain

5.1.11  State that energy transformations are never 100% efficient

  • When energy transformations take place in living organisms the process is never 100% efficient
  • Typically, energy transformations in living things are ~10% efficient, with about 90% of the energy lost between trophic levels
  • This energy may be lost as heat, be used up during cellular respiration, be excreted in faeces or remain unconsumed  as the uneaten part of food

5.1.12  Explain the reason for the shape of pyramids of energy

  • A pyramid of energy is a graphical representation of the amount of energy of each tropic level in a food chain
  • They are expressed in units of energy per area per time (e.g. kJ m2 year -1)
  • Pyramids of energy will never appear inverted as some of the energy stored in one source is always lost when transferred to the next source
  • This is an application of the second law of thermodynamics
  • Each level of the pyramid of energy should be approximately one tenth the size of the level preceding it, as energy transformations are ~10% efficient

5.1.13  Explain that energy enters and leaves ecosystems, but nutrients must be recycled

  • The movement of energy and matter through ecosystems are related because both occur by the transfer of substances through feeding relationships
  • However, energy cannot be recycled and an ecosystem must be powered by a continuous influx of new energy from an external source (e.g the sun)
  • Nutrients refer to material required by an organism, and are constantly being recycled within an ecosystem as food (either living or dead)
  • The autotrophic activities of the producers (e.g. plants) produce organic materials from inorganic sources, which are then fed on by the consumers
  • When heterotrophic organisms die, these inorganic nutrients are returned to the soil to be reused by the plants (as fertiliser)
  • Thus energy flows through ecosystems, while nutrients cycle within them

5.1.14  State that saprotrophic bacteria and fungi (decomposers) recycle nutrients

  • In order for organisms to grow and reproduce, they need a supply of the elements of which they are made 
  • The saprotrophic activity of decomposers (certain bacteria and fungi), free inorganic materials from the dead bodies and waste products of organisms, ensuring a continual supply of raw materials for the producers (which can then be ingested by consumers)
  • Thus saprotrophic bacteria and fungi play a vital role in recycling nutrients within an ecosystem