Food webs and ecosystems

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Lesson notes

What is a food web?

A **food web** is the natural interconnection of food chains, showing 'what-eats-what' in an ecological community.
It is a simplified illustration of feeding pathways that link an ecosystem into a unified system of exchange.
Food webs map **trophic links** (feeding connections) between species or functional groups.
**Food chains** are linear, non-cyclic feeding pathways nested within a food web.

Trophic levels

Organisms are grouped into **trophic levels** based on their position in the food web.
**Basal species** (e.g., plants, algae) form the first trophic level; they are autotrophs that produce their own food.
**Primary consumers** (herbivores) eat producers; **secondary consumers** (carnivores) eat herbivores; **tertiary consumers** eat other carnivores.
**Apex predators** are at the top level with no natural predators.
**Omnivores** feed on more than one trophic level, causing energy to flow through multiple pathways.

Autotrophs and heterotrophs

**Autotrophs** (e.g., plants, cyanobacteria) produce biomass energy via photosynthesis or chemosynthesis.
**Heterotrophs** (e.g., animals, fungi) consume organic matter for energy and nutrients.
Some organisms (e.g., carnivorous plants) are **mixotrophs**, acting as both autotrophs and heterotrophs.

Energy flow in ecosystems

Energy flows through a food web from producers to consumers and decomposers.
Only about **10%** of energy at one trophic level is transferred to the next; the rest is lost as heat or used for metabolism.
This energy loss limits the number of trophic levels in an ecosystem (usually 4–5).

Abiotic and biotic factors

**Biotic factors** are living components (plants, animals, bacteria) that interact in the food web.
**Abiotic factors** are non-living physical and chemical components (sunlight, temperature, water, soil) that affect the ecosystem.
Changes in abiotic factors (e.g., drought) can alter food web structure and population sizes.

Impact of changes on food webs

Removing a species (e.g., a predator or prey) can cause **cascading effects** throughout the food web.
Habitat destruction reduces available resources, leading to population declines or extinctions.
Introducing a new species can disrupt existing feeding relationships and outcompete native species.

Decomposers and detritivores

**Decomposers** (bacteria, fungi) break down dead organic matter, releasing nutrients back into the ecosystem.
**Detritivores** (e.g., earthworms, millipedes) consume detritus and help in decomposition.
Decomposers are essential for nutrient cycling and are often included as a basal or separate trophic group.

Food web models and limitations

Food webs are **simplified representations** that often aggregate many species into **trophic species** (functional groups).
Ecologists use mathematical models to study food web dynamics and network properties.
Real ecosystems are more complex; food webs may omit parasites, microbes, or other important interactions.

Energy pyramid showing trophic levels and energy transfer (only ~10% passes to the next level).

A simple food web showing feeding relationships among grassland organisms.

Slides

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Practice questions

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1.What is the original source of energy for most food webs?
Easy
- AThe Sun
- BPlants
- CHerbivores
- DDecomposers
2.Which of the following is an abiotic factor in an ecosystem?
Easy
- ATemperature
- BGrass
- CRabbit
- DFungi
3.In a food chain, which trophic level do primary consumers occupy?
Easy
- ASecond trophic level
- BFirst trophic level
- CThird trophic level
- DFourth trophic level
4.Which of the following statements about food webs is correct?
Medium
- AFood webs show only one feeding pathway in an ecosystem.
- BFood webs are made up of interconnected food chains.
- CFood webs do not include decomposers.
- DFood webs only include animals.
5.What would most likely happen to the population of rabbits if all the foxes in a food web were removed?
Medium
- AThe rabbit population would initially increase, then may decrease due to food shortage.
- BThe rabbit population would decrease immediately.
- CThe rabbit population would remain unchanged.
- DThe rabbit population would become extinct.
6.Which of the following organisms is a decomposer?
Medium
- AFungi
- BGrass
- CRabbit
- DFox
7.In an energy pyramid, which trophic level has the greatest amount of energy?
Medium
- AProducers
- BPrimary consumers
- CSecondary consumers
- DTertiary consumers
8.In the energy pyramid shown, which level represents the organisms that obtain energy directly from the producers?
Medium
- APrimary consumers
- BProducers
- CSecondary consumers
- DTertiary consumers

Revision notes

## Food Webs and Ecosystems ### Key Concepts - **Food web**: A graphical representation of feeding relationships (who eats whom) in an ecological community. - **Food chain**: A linear, noncyclic feeding pathway from a base species to a top consumer. - **Trophic level**: Position in the food web (e.g., autotrophs, heterotrophs). ### Trophic Levels - **Autotrophs (producers)**: Produce biomass energy via photosynthesis or chemosynthesis. Example: plants, cyanobacteria. - **Heterotrophs (consumers)**: Obtain energy by consuming other organisms. Types: herbivores, carnivores, scavengers, parasites. - **Mixotrophs**: Organisms that can be both autotrophic and heterotrophic (e.g., carnivorous plants). - **Decomposers**: Break down dead organic matter (e.g., bacteria, fungi). ### Food Web Structure - **Basal species**: No prey; include autotrophs and detritivores. - **Top predators**: No natural predators (apex predators). - **Trophic links**: Feeding connections; connectance = number of links per consumer. - **Trophic species**: Functional groups with same predators and prey. ### Energy Flow - Energy flows from autotrophs to heterotrophs through feeding. - Only about 10% of energy transfers between trophic levels (energy pyramid). ### Predicting Changes - Removing a species can disrupt the web (e.g., loss of a predator may increase prey, affecting plants). - Habitat destruction reduces biodiversity and alters food webs. ### Key Terms - **Autotroph**: Organism that produces its own food. - **Heterotroph**: Organism that consumes others for energy. - **Trophic level**: Position in food chain. - **Food web**: Network of interconnected food chains. - **Decomposer**: Organism that breaks down dead matter.

Autotroph: An organism that produces its own food using sunlight or chemical energy (e.g., plants, algae).
Heterotroph: An organism that obtains energy by consuming other organisms (e.g., animals, fungi).
Trophic level: The position an organism occupies in a food chain or web (e.g., producer, primary consumer).
Food web: A network of interconnected food chains showing feeding relationships in an ecosystem.
Food chain: A linear sequence of organisms where each is eaten by the next in line.
Decomposer: An organism that breaks down dead organic matter, recycling nutrients (e.g., bacteria, fungi).
Producer: Another term for autotroph; an organism that makes its own food.
Consumer: An organism that eats other organisms; heterotroph.
Apex predator: A predator at the top of a food web with no natural predators.
Biomass: The total mass of living organisms in a given area or volume.

Flashcards

What is a food web?

A food web is the natural interconnection of food chains and a graphical representation of what-eats-what in an ecological community.

What is a trophic level?

A trophic level is the position an organism occupies in a food web, broadly classifying organisms as autotrophs or heterotrophs.

What are autotrophs?

Autotrophs are organisms that produce their own biomass energy, either by photosynthesis (capturing sunlight) or chemosynthesis (oxidizing inorganic compounds).

What are heterotrophs?

Heterotrophs are organisms that obtain organic matter by feeding on autotrophs or other heterotrophs; they consume rather than produce biomass energy.

What are mixotrophs?

Mixotrophs are organisms (e.g., carnivorous plants) that can act as both autotrophs and heterotrophs, obtaining organic matter from non-atmospheric sources in addition to photosynthesis.

What are the main types of consumer-resource interactions?

The main types are herbivory, carnivory, scavenging, and parasitism.

Who pioneered the concept of food cycles and food chains?

Charles Elton pioneered the concept of food cycles, food chains, and food size in his 1927 book 'Animal Ecology'.

What did Raymond Lindeman emphasize in his 1942 paper on trophic dynamics?

Lindeman emphasized the important role of decomposer organisms in a trophic system of classification.

What is a trophic species?

A trophic species is a functional group of species that have the same predators and prey in a food web.

What are basal species in a food web?

Basal species are those without prey; they can be autotrophs or saprophytic detritivores (decomposers).

What is a trophic link?

A trophic link is a feeding connection in a food web, representing who eats whom.

What is food web connectance?

Connectance is the number of trophic links per consumer in a food web.

How are food chains related to food webs?

Food chains are linear, noncyclic feeding pathways nested within the trophic links of food webs.

What are apex predators?

Apex predators are top-level organisms that no other species kills directly for food resource needs.

What are omnivores in a food web?

Omnivores are intermediate-level organisms that feed on more than one trophic level, causing energy to flow through multiple pathways.

What is the role of decomposers in a food web?

Decomposers break down dead organic matter and recycle nutrients, forming an important part of the basal species in some food webs.

How do ecologists use food web models?

Ecologists use quantitative models of trophic dynamics to measure and test patterns in food web structure, such as scaling laws relating predator-prey linkages to species richness.

What is the difference between a food web and a food chain?

A food web is a complex network of interconnected food chains, while a food chain is a single linear pathway of energy flow.

What abiotic factors affect ecosystems?

Abiotic factors include sunlight, temperature, water, soil, and nutrients; they influence the types and numbers of organisms in an ecosystem.

What biotic factors affect ecosystems?

Biotic factors include all living organisms and their interactions, such as predation, competition, and symbiosis.

How might removing a species from a food web impact the ecosystem?

Removing a species can disrupt energy flow, cause population changes in other species (e.g., prey increase or predator decline), and potentially lead to ecosystem instability.

How does habitat destruction affect food webs?

Habitat destruction reduces or eliminates food sources and shelter, leading to population declines, loss of biodiversity, and disruption of trophic interactions.

What is an example of a chemolithotroph?

The sulfur bacterium Thiobacillus, which lives in hot sulfur springs and obtains energy by oxidizing inorganic compounds.

What did Charles Darwin contribute to the concept of food webs?

Darwin used terms like 'entangled bank', 'web of life', and 'web of complex relations', and noted the role of earthworms in decomposition.

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