Ecosystems & ecology – understanding food chains and food webs for IB ESS

The IB Environmental Systems and Societies syllabus contains a large amount of material that you will need to revise and understand in order to be successful on your IB exam. Below you will find an excerpt from our IB Environmental Systems and Societies SL study guide written by author and IB teacher, Adrian Palmer. The excerpt explores communities and ecosystems from Topic 2 Ecosystems and Ecology by examining food chains and food webs and sharing some top tips to remember when taking your IB ESS exams.

Exploring communities and ecosystems

In ecology, the term ‘trophic’ is used to describe a feeding relationship. The following terms and definitions will help you understand the biotic components of ecosystems.

Trophic Level: The position that an organism occupies in a food chain, or a group of organisms in a community that occupy the same position in food chains.

• Example: Herbivore

Producers: Producers are also called ‘autotrophs’, meaning self-feeding. In most ecosystems these are green plants or algae, photosynthetic organisms that form the base of the food chain. In some deep-sea communities with no light, there are some chemosynthetic organisms – bacteria that feed on chemicals.

• Example: Lowland oak (Quercus robur)

Consumers: Any organism that eats or gains nutrition from another. They are also heterotrophs, which means they feed on another organism.

• Example: Any from the list below

Herbivores: Heterotrophs that are also termed primary consumers. They consume primary producers. ‘Herbivore’ literally means ‘eats grass’.

• Example: Cottontail rabbits (Sylvilagus floridanus)

Carnivores: Heterotrophs that are secondary consumers or greater. They divide into first order, second order, third order carnivores, and so on, up to a top carnivore at the end of the food chain. ‘Carnivore’ literally means ‘eats meat’.

• Example: Bluefin tuna (Thunnus thynnus)

Omnivores: These are heterotrophs that feed at any trophic level. They often have a varied diet. ‘Omnivore’ literally means ‘eats all’.

• Example: Badger (Meles meles)

Detritivores: Heterotrophic organisms (normally animals) that consume dead organic matter by ingestion. ‘Detritivore’ literally means ‘eats detritus’.

• Example: Earthworm (Lumbricus terrestris)

Decomposers: These organisms are fungi and bacteria that break food down outside their bodies by secreting enzymes into the environment. As this process is inefficient, they are important in recycling nutrients. Note they don’t ingest (eat) as animals do.

• Example: Penny bun fungus [cep] (Boletus edulis)

Trophic Structure through food chains and webs

Food chains, webs, and pyramids are all used to consider the feeding structure of communities. Here you’ll find a review of food chains and food webs. As you revise each, consider the use of each approach to understanding the feeding structure in different contexts.

Food Chains

Food chains show the sequence of organisms in successive trophic levels within a community. They are flow diagrams that show feeding relationships and, therefore, the movement of matter and energy.

Example 1: A tropical freshwater food chain for a lake with six trophic levels.

Example 2: South East Asia forest food chain with four trophic levels.

Example 3: An Arctic food chain with six trophic levels

Top Tip: Remember to put the arrows in the right direction. An arrow in a food chain or food web always moves in the direction of feeding. Reverse the arrows and the producers eat the consumers!

Applications and limitations of food chains

Food chains as simple as these examples are rare in nature, even in systems with low diversity of species. In the Arctic food chain given, there is alternate prey for each species feeding on a lower trophic level. The polar bear can choose from one of six species of seal, and may try to eat other mammals like walrus or people if it is really hungry.

Food chains are useful for analysing aspects of ecosystem function such as the response to persistent pollutants that pass on through food chains. They are also useful to study diseases that follow food chains, such as bovine spongiform encephalopathy (BSE, or mad cow disease).

Food Webs

Food webs show more complex and complete feeding patterns than food chains. You should learn one example of a food web like the one below, ideally from a local case study.

Application and limitation of food webs

Although food webs show complex feeding relationships, they are still not complete. They do not clearly show the differences in the quantity of living organisms in the different trophic levels. Detailed information on the feeding relationships of a wide variety of species is hard to come by. Species may even change their position during their life span. Each species would need a close examination of its diet, through field observations, or dissection of faeces or gut contents. Studies like this can be very useful to examine impact on feeding relations in a food web. For example, the impact of invasive species on native species through competition in a food web.

---

Continue your revision

The above definitions and descriptions should help you to strengthen your understanding of the complexities of food chains and food webs in preparation for your IB ESS exam. Remember that being successful in your exam preparation will require you to revise all topics within the syllabus. Be sure to check out our IB Environmental Systems & Societies study guide. With a thorough syllabus review, case studies, and practice questions, it can help you to structure your study and build your confidence.

For additional IB ESS resources visit our subject page where we regularly post helpful articles and study tools.