What an Ecosystem Is
An ecosystem can be pictured as a functional unit of nature, a place where living organisms interact among themselves and also with their surrounding physical environment. It is not defined by a fixed size. An ecosystem can be as small as a pond or as large as a forest or a sea, and many ecologists regard the entire biosphere as one global ecosystem — a composite of all the local ecosystems on Earth.
Because the biosphere as a whole is far too big and complex to study at once, it is convenient to divide ecosystems into two basic categories.
Terrestrial, Aquatic and Man-Made
The two basic categories are terrestrial and aquatic. Forest, grassland and desert are examples of terrestrial ecosystems, while pond, lake, wetland, river and estuary are examples of aquatic ones.
Ecosystems need not be entirely natural. Crop fields and an aquarium are examples of man-made ecosystems — created and maintained by humans, but still functioning as units where organisms and their environment interact.
The Four Functions of an Ecosystem
What makes the components of an ecosystem behave as a single working unit is a set of four functions that tie them together:
- Productivity — the input, the capture of energy and building of biomass.
- Decomposition — the breakdown of dead matter back into simpler substances.
- Energy flow — the transfer of energy from one organism to the next.
- Nutrient cycling — the repeated reuse of nutrient elements.
The rest of the chapter follows this logic: first the input (productivity), then the transfer of energy through food chains and webs, and finally the output — degradation and the loss of energy.
Structure Then Function
To make sense of all this, it helps to look first at the structure of an ecosystem — its abiotic and biotic components and how they are arranged — and then at how energy actually flows through those components.
That is the plan for this chapter: understand the components as an integrated whole, then trace the input, transfer and output of energy, and the cycles, chains and webs that these energy flows create.
Quick Recap
- An ecosystem is a functional unit of nature where organisms interact with each other and their physical environment.
- Size ranges from a pond to the whole biosphere (the global ecosystem).
- Two basic categories: terrestrial (forest, grassland, desert) and aquatic (pond, lake, wetland, river, estuary); crop fields and aquaria are man-made ecosystems.
- The four functions that make it a unit are productivity, decomposition, energy flow and nutrient cycling.
- The chapter moves from structure to function — input (productivity), transfer (food chains/webs) and output (degradation, energy loss).
Solved Examples — Section 1
Q1. Define an ecosystem.
Answer: A functional unit of nature where living organisms interact among themselves and with their surrounding physical environment.
Q2. Name the two basic categories of ecosystems, with one example of each.
Answer: Terrestrial (e.g. forest) and aquatic (e.g. pond).
Q3. Give two examples of man-made ecosystems.
Answer: A crop field and an aquarium.
Q4. What is regarded as the global ecosystem?
Answer: The entire biosphere, seen as a composite of all local ecosystems on Earth.
Q5. List the four functions that make an ecosystem work as a unit.
Answer: Productivity, decomposition, energy flow and nutrient cycling.
Q6. In studying an ecosystem, what does 'input', 'transfer' and 'output' refer to?
Answer: Input is productivity, transfer is the movement of energy through food chains and webs (and nutrient cycling), and output is degradation and energy loss.