Ecosystems as Open Systems (HL IB Biology)

• An ecosystem can be defined as:

A group of organisms interacting with each other and with the non-living parts of the environment

• There are both biotic components and abiotic components within an ecosystem
• Ecosystems vary greatly in size and scale
  • Both a small pond in a back garden and the open ocean could be described as ecosystems
• Ecosystems vary in complexity:
  • A desert is a relatively simple ecosystem
  • A tropical rainforest is a very complex ecosystem

Ecosystem example

• An ocean is an example of a complex ecosystem
• There is a large community of organisms, including fish, crustaceans, corals, algae, plants and microorganisms
• The abiotic components of the ecosystem include the salinity, pH, temperature, light intensity, and mineral availability
• The abiotic components of the ecosystem influence the community of organisms, e.g. by providing habitat, nutrients and other resources organisms need in order to survive and reproduce

Ecosystems include communities and their interactions with the abiotic environment

Ecosystems as open systems

• Ecosystems are open systems, meaning that both energy and matter can enter and exit the system
• Energy can enter and exit as follows: 
  • Energy enters in the form of sunlight, and flows through an ecosystem in the form of stored chemical energy
  • Energy can also enter an ecosystem stored in the tissues of any organisms that migrate into the system
  • When energy is stored in the molecules of an organism and that individual leaves the ecosystem, that stored energy is removed
• Matter can enter and exit as follows:
  • Matter enters an ecosystem when an organism arrives, in the form of all of the molecules of its cells and tissues, e.g. when a bird migrates into an ecosystem
  • Matter is removed when an organism leaves an ecosystem, e.g. dead plant matter could be washed away by the waves on a beach and carried to a new ecosystem, or trees are cut down and the timber removed
• It is worth noting that most of the organisms in an ecosystem remain inside the system throughout their lives, and the matter and energy stored in their tissues is recycled within the ecosystem when an individual dies
  • While ecosystems are open, they are considered to be largely self-contained

• Open systems are different to closed systems
  • In a closed system:
    
    • Matter can only be recycled within the system and cannot enter or leave
    • Energy can enter and leave
  • Earth is an example of a closed system; energy enters and leaves but matter is recycled

• The sun is the initial source of energy for most food chains
  • Light energy from the sun is converted by producers into chemical energy stored in the tissues of plants during the process of photosynthesis
  • Chemical energy stored in the tissues of plants passes to primary consumers when they ingest plants, and on to secondary consumers when the primary consumers are themselves ingested
• There are a few unusual exceptions to this, such as:
  • Food chains located in deep sea volcanic vents and underground caves where no light can penetrate; these rely on bacteria gaining energy from chemical processes
    
    • Note that some caves may receive energy stores from the earth's surface that originally gained their energy from the sun, e.g. if water flows through the cave

NOS: Laws in science are generalised principles, formulated to describe patterns observed in nature. Unlike theories, they do not offer explanations, but describe phenomena

• Laws describe patterns that occur in nature; they are developed by scientists after carrying out observation and they do not seek to explain why something is happening
  • E.g. the first law of thermodynamics states that energy cannot be created or destroyed
• Theories go further than this; they are explanations of phenomena observed in nature
  • E.g. a theory known as 'systems theory' seeks to explain open and closed systems
• Both laws and theories can be used to make predictions, e.g. about the effect of change on a system

Useful generalisations

• The statement that 'the sun is the initial source of energy for food chains' is an example of a useful generalisation; this is because it is true for most food chains, but there are examples of food chains that do not directly rely on the sun as a source of energy