4.1 Essential ideas

4.1.1 Species, community and ecosystems

Ecology is the study of how organisms interact with one another and with their environments. Practically, this involves the study of ecosystems, which are sustainable systems consisting of biotic (living) and abiotic (non-living) components.

Important concepts related to the structure of ecosystems are described in the table below.

Concept Description
Species A group of organisms that can interbreed to produce fertile offspring. These may be reproductively isolated (i.e. geographically) or live together in populations.
Population Individuals of the same species living together in time and space
Community Populations (of different species) that interact with each other
Autotroph  A species that is able to produce its own food, usually through photosynthesis
Producer An autotroph
Heterotroph A species that feeds on plant or animal material (i.e. cannot make its own food)
Consumer A heterotroph that feeds on other animals or plants by ingesting them
Detritivore  A heterotroph that obtains nutrients by ingesting detritus (See Figures 4.1.1a-c)
Saprotroph  A heterotroph that obtains nutrients by externally digesting dead organisms (See Figure 4.1.1f)

Note: Both detritivores and saprotrophs are also known as decomposers, because they play an important role in recycling nutrients back into an ecosystem.

Many small animals, especially insects and earthworms, are detritivores. Shown here are a wood louse, a millipede and a dung beetle, all members of phylum Arthropoda.

woodlouseFigure 4.1.1a – Wood louse
millipedeFigure 4.1.1b – Millipede
dung beetleFigures 4.1.1c – Dung beetle

Communities consist of all the biotic components of an ecosystem. Parts of the community interact with each other through:

  • competition – both intraspecific (within a single species) and interspecific (between species) – for food and resources
  • trophic relationships, for example predation, herbivory, parasitism
  • symbiosis, which means ‘living together’; types of symbiotic relationship are described in the following table.




Commensalism One organism benefits while the other is unharmed Spiders build webs on tree trunks
Mutualism Both organisms benefit from the relationship Bees pollinate flowers while collecting nectar
Parasitism One organism benefits while the other is harmed (See image 4.1.1g-h) Strangler figs grow around other trees, preventing their growth

Interaction of biotic and abiotic components

A community forms an ecosystem by its interactions with the abiotic environment. Abiotic factors can be quantitatively observed.


Examine the picture of a freshwater ecosystem shown in Figure 4.1.1d and do the following:

  • Identify examples of autotrophs, heterotrophs, populations, communities.
  • Suggest names for species of saprotrophs and detritivores (common names are acceptable) that might be found in this ecosystem. Indicate on the image where they might be found.
  • Outline the role of the following abiotic factors in the functioning of this ecosystem: pH, humidity, temperature, carbon dioxide concentration, oxygen concentration, salinity, precipitation, soil composition, air composition.
  • In groups of two or three, discuss methods that you might use to measure how one of the abiotic factors above influences the structure of this ecosystem.

freshwater ecosystemFigure 4.1.1d – A freshwater ecosystem

Sustainability of ecosystems

A mature ecosystem has the potential to be sustainable over long periods of time. In ecological terms, sustainability refers to an ecosystem’s ability to maintain a level of biodiversity and productivity that does not consume resources or cause destruction of the ecosystem’s structure. Consider the following facts:

  • Autotrophs obtain inorganic nutrients from the abiotic environment, and synthesise food for heterotrophs.
  • The supply of inorganic nutrients is maintained by nutrient cycling. Nutrients are cycled when bacteria and other decomposers return inorganic nutrients to the system.

the amount of nutrients cycling through an ecosystemFigure 4.1.1e – The amount of nutrients cycling through an ecosystem is constant (unless there is disturbance)

In Figure 4.1.1e we see that nutrients cycle through the system, and energy is lost as heat. No new mass is being added – it is simply being transformed between one trophic level and another. Sustainable ecosystems do not become more or less productive.

saprotroph funghiFigure 4.1.1f – Saprotroph
Most fungi are are saprotrophs – they obtain nutrients by digging long hyphae into the woody bark of trees and digesting cellulose. Some species of bacteria are also saprotrophic.

Language tools

  • The word ‘species’ is both singular and plural.
  • Students often confuse the terms detritivore and saprotroph. The important distinction is the location of digestion: internal digestion for detritivore, external digestion for saprotroph.
  • Large scavengers such as vultures and hyenas are not considered detritivores, since they consume most of the organic matter, rather than return it to the soil.

Course link

Learn more about biotic interactions, the ecological role of different species, and sustainability in Option Topic C: Ecology and Conservation.

Language tools

Most ecological terms can be deconstructed into their Latin or Greek roots. For example, autotroph comes from the Greek suffix auto-, meaning ‘self’ and the word trophos meaning ‘food’. If you can remember a few root words, it will help you to understand some of the new terms you encounter. 

strangler treeFigure 4.1.1g – Parasitism by strangler trees
Many members of the genus Ficus grow around, and eventually kill, host trees.

parasitic plantFigure 4.1.1h – Hyobanche sanguinea is a parasitic plant that requires a host plant to get its nutrition

International mindedness

Keeping the productivity of ecosystems in mind:

  • How can we increase food production (i.e. to alleviate world hunger) and promote sustainability at the same time?
  • What are some methods to promote sustainability in other important human activities such as forestry, sewage and water treatment, or the manufacturing of consumer goods?

In the lab


In biology textbooks from the early 20th century and up until about 1980, the ‘balance of nature’ was often mentioned in reference to ecosystems. More recently, ‘dynamic equilibrium’ is the term used to describe the resilience of ecosystems to outside disturbances. Is this a case of old wine in a new bottle?

Nature of Science

Scientists are always looking for patterns and trends. New scientific knowledge comes out of discrepancies in the trends. For example, some plants are heterotrophic!