The circle of life

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Australian Curriculum Level: Year 7

Strand: Science Understanding – Biological sciences

Content Descriptors: Interactions between organisms can be described in terms of food chains and food webs; human activity can affect these interactions (ACSSU112)

  • using food chains to show feeding relationships in a habitat
  • constructing and interpreting food webs to show relationships between organisms in an environment
  • classifying organisms of an environment according to their position in a food chain
  • recognising the role of microorganisms within food chains and food webs
  • investigating the effect of human activity on local habitats, such as deforestation, agriculture or the introduction of new species
  • exploring how living things can cause changes to their environment and impact other living things, such as the effect of cane toads

In reference to their savannah ecosystem Mufasa says to Simba in the movie, The Lion King, “Everything you see, exists together in a delicate balance.” While the explanation of the ‘circle of life’ is simplistic, it is not wrong.

A sustainable ecosystem, one that is not dramatically changing in nature or composition, is the result of a very delicate balance between the physical conditions of the habitat (abiotic factors) and every single organism (living thing) that lives there (biotic factors). It is easy to imagine that the climate and other physical conditions of a habitat determine the types of plants and animals that can live there, but the organisms also influence the abiotic factors. Microorganisms living in the soil can change the chemical composition of the earth, which in turn can change the types of plants that can grow in that area.

An Ecologist studies the interactions between individuals of a population, the interactions between different species and the interactions between the living community and the conditions of their habitat. Feeding relationships are an obvious example of interactions between different species. A food chain is simply the sequence of the different species that eat each other. If I were to give you a list of animals – bird, spider, cat and fly – most people would be comfortable saying that the spider would eat the fly, the bird would eat the spider and the cat would eat the bird. As a food chain, that feeding interaction would be represented like this:

The arrows in a food chain are really important. Students often draw them the wrong way around, having them pointing from the animal doing the eating, to the animal being eaten. However, food chains are actually a representation of how energy is passed through an ecosystem and so the arrows should always point from the organism being eaten towards the organism that is eating it.

Keeping in mind that food chains represent the transference of energy through the organisms of an ecosystem, the energy is most commonly passed between levels in the form of glucose – a simple sugar molecule. But this molecule doesn’t appear out of nowhere and so all food chains should start with a producer. A producer is an organism that can manufacture (or produce) its energy from the abiotic factors of its surroundings. All plants and many microscopic organisms use sunlight energy to power a chemical reaction called photosynthesis that produces glucose.

This heterotrophic spider gets energy from the insect it is consuming because of the glucose and other energy rich molecules that make up its body. 

Producers will use some of the energy they capture from the Sun to grow and reproduce, storing excess glucose and other energy-rich molecules in their cells. Producers are also classified as autotrophic (‘auto’ meaning self and ‘trophic’ meaning nourishment) because they have the ability to produce chemical energy for themselves. When the plant or other type of producer is eaten, this chemical energy is passed on to the organism that consumes the producer. And so, organisms that gain their energy from what they eat are called consumers or heterotrophs (‘hetero’ meaning other).

Consumers are further classified according to the trophic level (position in the food chain) in which they appear and the type of food they eat.

These toadstools are an example of a decomposer that helps to break down matter in basic compounds.

Most people are comfortable with terms like herbivore (plant eater), carnivore (meat eater) and omnivore (all eater – eats both plants and animals). However, a common group of organisms that are often overlooked are detritivores, those that consume dead and decaying matter. While perhaps not the sexiest of organisms in an ecosystem, detritivores are critical for breaking down dead bodies and helping to return nutrients to the soil, water and air of the habitat. They include organisms like worms, insects, crabs and sea urchins. Once detritivores have broken the decaying matter into small enough particles, decomposers break them down further into basic compounds and nutrients. The various types of fungi, bacteria and other microscopic organisms are responsible for this decomposition and the cycling of nutrients back into the ecosystem and appear at every trophic level except the first. Without these decomposers, the plants would not be able to continue to photosynthesise, the food chains would grind to a halt and the ecosystem would collapse as all the organisms eventually starved and died or moved away.

So a more accurate food chain (with classifications included) looks more like:

The above food chain features a feral cat. Regardless of how you feel about domestic cats in general, the extensive damage they cause to our Australian native fauna is undeniable. Cats were introduced to Australia by humans and have flourished here because they are accomplished hunters and have a wide variety of food sources available to them, with very few predators of their own. Without the feral cat in the food chain, the honeyeater would more likely have been eaten by a bird-of-prey. Introducing a new species into an ecosystem will affect the trophic levels of the food chain both above and below the introduced species. Will organisms in lower levels experience greater predation? Will organisms in higher levels have more prey options? How will organisms on the same level compete?

A more accurate representation of the feeding relationships within an ecosystem is a food web. It shows how there are usually multiple feeding options at every trophic level. It is very rare that a producer only has a single herbivore that eats it, just as it is unlikely that a predator only has one prey species. While more complicated, the detail of a food web shows just how interlinked the all species are within the ecosystem.

Humans have introduced a massive number of plant and animal species to Australia, both deliberately and accidentally and all have had some manner of impact on our native ecosystems. But introducing new species is not the only way human can change an ecosystem. Removing species can also have detrimental affects on the ecosystem. Deforestation and other forms of native habitat loss cause significant changes to ecosystems, often resulting in dramatic losses in population sizes and even extinction of entire species. A good example of this is the plight of the Orange-bellied Parrot (Neophema chrysogaster). Already struggling due to a variety of reasons, their likely extinction still looms despite the action plans for their recovery. One impact that is currently being examined is increased competition for nesting hollows with other natives like Sugar Gliders (Petaurus breviceps). Habitat loss is detrimental to both species and forcing their ranges to overlap far more than they would normally. Click here to read the Orange-bellied Parrot Recovery Plan.

There are far too many examples of both positive and negative changes that have happened to ecosystems as a result of human activities to go into here. But the key idea is that a food chain is only one strand in the very intricate and interconnected food web that in itself is only one part of the ecosystem. And to pull on even a single stand, will initiate vibrations throughout the whole ecosystem. Ecosystems exist in a delicate balance and, as Mufasa says, “You need to understand that balance and respect all the creatures from the crawling ant, to the leaping antelope.”