Ecosystems and Food Webs: Who Eats Whom?
A black-faced spoonbill feeding at Mai Po, a limpet grazing on a wet rock, a fallen leaf rotting on a country park trail – each one is part of a bigger system where living things depend on each other and on their surroundings.
In this chapter you will learn what habitats and ecosystems are, the jobs of producers, consumers and decomposers, how to build and read food chains and food webs, and how organisms interact through predation, competition and symbiosis. Take it one step at a time and you will soon "read" nature like a map!
1. Habitats and Ecosystems
What is a habitat?
A habitat is the environment in which an organism lives – its "home address". A good habitat provides food, water, shelter and a place to breed. Hong Kong has many habitats: mangroves, rocky shores, streams, grasslands, woodlands and even school gardens.
What is an ecosystem?
An ecosystem is made up of the organisms in an area, their environment, and the interactions between them. Organisms interact with each other (e.g. one eats another) and with their environment (e.g. plants take in water from the soil).
Living parts – plants, animals, fungi and bacteria.
Non-living parts – light, temperature, water, air, soil and rocks.
A population is all the organisms of one species in an area, e.g. all the fiddler crabs on one mudflat.
Three Hong Kong ecosystems
Mangroves at Mai Po – mangrove trees grow in salty mud that floods with the tides; crabs, mudskippers and shrimps live in the mud, and water birds come to feed.
Rocky shore – seaweeds, barnacles, limpets and crabs must survive strong waves, and drying out in the sun at low tide.
Country park woodland – trees and grasses give food and shelter to insects, spiders, frogs, birds and snakes.
Key takeaway: A habitat is where an organism lives. An ecosystem is the organisms plus their environment, all interacting together.
2. Producers, Consumers and Decomposers
Producers: the food makers
Producers make their own food using light energy. Green plants, seaweeds and other algae are producers (see the Photosynthesis chapter). All other organisms depend on them for food.
Consumers: the food eaters
Consumers cannot make their own food, so they eat other organisms. By what they eat:
Herbivores eat only plants or algae (e.g. grasshoppers, caterpillars, limpets).
Carnivores eat only animals (e.g. spiders, egrets, snakes).
Omnivores eat both (e.g. humans, many crabs, and bulbuls, which eat fruit and insects).
By their position in a food chain:
Primary consumers eat producers.
Secondary consumers eat primary consumers.
Tertiary consumers eat secondary consumers.
Decomposers: nature's recyclers
Decomposers are mainly bacteria and fungi (e.g. mushrooms and moulds). They break down dead organisms and animal waste, returning nutrients to the soil or water for plants to use again. Without them, dead leaves would pile up and nutrients would run out.
Examples from Hong Kong
Mai Po – producers: mangrove trees, algae; consumers: shrimps, crabs, mudskippers, egrets, black-faced spoonbills; decomposers: bacteria and fungi in the mud.
Rocky shore – producers: seaweeds; consumers: limpets, barnacles, whelks, crabs; decomposers: bacteria.
Woodland – producers: trees, grasses; consumers: caterpillars, frogs, birds, snakes; decomposers: fungi and bacteria in the leaf litter.
Memory aid: "Make it, Take it, Break it" – producers make food, consumers take (eat) it, decomposers break down what is left.
Key takeaway: Producers make food, consumers eat other organisms, and decomposers (bacteria and fungi) recycle nutrients from dead matter.
3. Food Chains: Following the Energy
A food chain shows how food, and the energy in it, passes from one organism to the next. It always starts with a producer:
grass \(\rightarrow\) grasshopper \(\rightarrow\) frog \(\rightarrow\) snake
Reading the arrows
Each arrow shows the direction of energy (food) flow, pointing from the food to the eater. "grass \(\rightarrow\) grasshopper" means "grass is eaten by the grasshopper". Memory aid: the arrow points to the mouth that is eating.
Naming the levels
Above, grass is the producer, the grasshopper the primary consumer, the frog the secondary consumer and the snake the tertiary consumer. A Mai Po example: algae \(\rightarrow\) shrimp \(\rightarrow\) small fish \(\rightarrow\) black-faced spoonbill.
Common mistake
Drawing the arrow from the eater to its food (snake \(\rightarrow\) frog), as if it meant "eats". Wrong! The arrow shows where the energy goes, so it points from the frog to the snake. And a food chain always starts with a producer, never with an animal.
Did you know? At each step most of the energy is used by the organism to stay alive or is lost as heat; only a small part passes on. That is why most food chains have only four or five links.
Key takeaway: A food chain starts with a producer, and its arrows point from the food to the eater.
4. Food Webs: Many Chains Joined Together
Most animals eat more than one kind of food and are eaten by more than one kind of animal, so food chains link up into a food web. A food web shows all the feeding relationships in an ecosystem.
A woodland food web
Caterpillars eat tree leaves. Birds eat tree fruits, caterpillars and grasshoppers. Grasshoppers eat grass. Frogs eat grasshoppers. Snakes eat frogs and birds. This web contains four food chains:
1. tree \(\rightarrow\) caterpillar \(\rightarrow\) bird \(\rightarrow\) snake
2. tree \(\rightarrow\) bird \(\rightarrow\) snake
3. grass \(\rightarrow\) grasshopper \(\rightarrow\) frog \(\rightarrow\) snake
4. grass \(\rightarrow\) grasshopper \(\rightarrow\) bird \(\rightarrow\) snake
One organism can be at more than one level. The bird (an omnivore) is a primary consumer when it eats fruit (chain 2) but a secondary consumer when it eats insects (chains 1 and 4). The snake is a secondary consumer in chain 2 and a tertiary consumer in the others.
How to construct a food web
Step 1: List the organisms and what each one eats.
Step 2: Put producers at the bottom, then primary, secondary and tertiary consumers higher up.
Step 3: Draw an arrow from each food to each organism that eats it.
Step 4: Check that no arrow points into a producer.
Step 5: Count the food chains by following every path from each producer to a top consumer.
Predicting changes in a food web
Everything is linked, so a change in one population affects others. If an animal decreases:
Its food is eaten less, so it tends to increase.
Its predators have less food, so they tend to decrease or eat more of their other foods.
Example: the frogs disappear
Grasshoppers are eaten less, so they increase and eat more grass, so grass may decrease. Snakes lose one food, so they may decrease or eat more birds, so birds may fall too. Real ecosystems are complicated, so we say "may" or "tends to". A web with many links is usually more stable, because consumers can switch foods.
Try it: set up a mini-ecosystem in a clear sealed jar with soil, pond water, water plants and a few small snails, and observe it for a few weeks.
Key takeaway: A food web is many linked food chains. To predict a change, ask: what does the organism eat, and what eats it?
5. How Organisms Interact
Predation
In predation, one animal (the predator) hunts, kills and eats another (the prey), e.g. an egret at Mai Po catching a mudskipper. When prey are plentiful, predators increase; then more prey are eaten and prey decrease; with less food, predators decrease too, and the cycle repeats.
Competition
Competition happens when organisms need the same limited resource, such as food, water, light, space or mates. They can be the same or different species. Examples: woodland trees compete for light; barnacles compete for space on rocks; egrets and spoonbills at Mai Po compete for small fish.
Symbiosis
Symbiosis is a close relationship between two different species that live together or depend closely on each other. In these examples, both benefit:
Clownfish and sea anemone – the clownfish hides among the stinging tentacles, safe from predators (a slimy coat protects it); it chases away fish that eat anemones and keeps the anemone clean.
Lichens – a fungus and an alga living together: the alga makes food by photosynthesis, the fungus gives shelter and holds water and minerals.
Bees and flowers – bees get nectar and pollen as food; they carry pollen between flowers, helping plants reproduce.
Not every close relationship helps both sides: a parasite, such as a flea on a dog, harms its host.
Did you know?
Many lichens are sensitive to air pollution, so lots of healthy lichens on rocks and tree trunks usually means clean air!
Quick check: a cat catching a mouse – predation; two seedlings shading each other – competition; a bee visiting a flower – symbiosis.
Key takeaway: Predation – one eats the other. Competition – both need the same limited resource. Symbiosis – two species live closely together, often both benefiting.
6. Invasive Species: Unwelcome Newcomers
An invasive species is brought by people into a new place, where it spreads quickly and harms the local ecosystem. It often has no natural predators there, so it out-competes or eats local species.
Hong Kong examples: Mikania (mile-a-minute weed) climbs over trees and blocks their light; apple snails eat crops and water plants; released pet red-eared slider turtles compete with local turtles. Control is difficult: pesticides may harm other organisms, and a new predator may eat local species instead.
Key takeaway: Invasive species upset food webs. Never release pets or aquarium plants and animals into the wild.
Quick Review
Habitat – where an organism lives.
Ecosystem – organisms and their environment (living and non-living parts) interacting.
Producers make food; consumers eat other organisms; decomposers (bacteria, fungi) recycle nutrients.
Food chain – starts with a producer; arrows point from food to eater (energy flow).
Food web – linked food chains; a change in one population affects others.
Predation, competition and symbiosis are key interactions.
Invasive species spread fast and harm local species.
Great work! Next time you visit a country park or the seashore, try to spot producers, consumers and decomposers, and imagine the food web linking them.