Food webs and ecosystems
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给教育者： 面向 Food webs and ecosystems（Australian Curriculum Science，Biology）的即用型课程幻灯片, 复习笔记, 图示——用在你的课堂上，或将该知识点作为学习者可实时游玩的互动课堂活动来运行。
课程笔记
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).
幻灯片
练习题
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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?
Easy- 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?
Easy- AFungi
- BGrass
- CRabbit
- DFox
7.In an energy pyramid, which trophic level has the greatest amount of energy?
Easy- 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?
Easy- APrimary consumers
- BProducers
- CSecondary consumers
- DTertiary consumers
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