Cambridge Lower Secondary · Science 0893 · Stage 8 · Biology

Ecosystems, Bioaccumulation and Invasive Species

Ecosystems around the Earth and the habitats inside them, how toxic substances build up along food chains, and how invasive species such as water hyacinth change whole ecosystems — with food-web practice in Checkpoint style.

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In the curriculum

Identify different ecosystems on the Earth and recognise that one ecosystem contains many different habitats (8Be.01)
Describe how toxic substances build up in living things (bioaccumulation) and the effect this has on an ecosystem (8Be.02)
Describe how a new or invasive species can affect other organisms and the whole ecosystem (8Be.03)

An ecosystem is all the living organisms in an area together with the non-living (physical) environment they live in, and all the ways they interact. The organisms depend on each other (for food, shelter, pollination) and on the non-living parts (water, light, temperature, soil, air). Because everything is linked, a change to one part — a poison in the water, or a new animal arriving — can affect the whole ecosystem.

Word / phrase	Meaning / use
ecosystem	النظام البيئي — the living organisms + their physical environment, interacting
habitat	الموطن — the place where an organism lives
microhabitat	موطن صغير — a very small habitat, e.g. under a stone
population	جماعة (عشيرة) — all the organisms of one species in an area
community	مجتمع حيوي — all the populations of different species in an area
producer / consumer	منتج / مستهلك — makes its own food / eats other organisms
toxic	سام — poisonous
bioaccumulation	التراكم الحيوي — a toxic substance building up in an organism and along a food chain
pesticide	مبيد حشري — a chemical used to kill pests
native species	نوع محلي (أصلي) — a species that lives naturally in an area
invasive species	نوع غازٍ — a species brought to a new area that spreads quickly and causes harm
biodiversity	التنوع البيولوجي — the variety of living things in an area

Ecosystems on the Earth

Ecosystems come in all sizes. The Red Sea coral reef is a huge ecosystem; a pond, a rotting log or even a school garden is a small one. Each ecosystem has its own conditions, and the organisms living there are adapted to them.

Ecosystem	Example place	Conditions	Some organisms	Habitats within it
Hot desert	Egypt's Western Desert (Sahara)	Very dry; very hot days and cold nights	camel, fennec fox, scorpion, desert lizard, acacia tree	sand dunes, under rocks, burrows, oases, rocky hills
Coral reef	Red Sea (e.g. Ras Mohammed)	Warm, clear, shallow, salty water; lots of light	corals, parrotfish, clownfish, sea turtles, sharks	reef crest, caves and cracks, seagrass beds, sandy sea floor, open water
Fresh water (river and lake)	The Nile; Lake Nasser	Fresh water; muddy bottom; reeds at the edges	tilapia, Nile perch, crocodile, heron, reeds	river bank, reed beds, open water, muddy bottom
Tropical rainforest	Amazon (South America), Congo (Africa)	Hot and wet all year	monkeys, parrots, frogs, insects, tall trees	canopy (tree tops), forest floor, rivers, under bark
Grassland (savannah)	East Africa	Grasses, few trees; a wet season and a dry season	zebra, lion, termites, acacia	open grass, termite mounds, waterholes
Polar	Arctic, Antarctica	Very cold; ice and snow; long dark winters	polar bear (Arctic), penguin (Antarctica), seals	sea ice, open sea, under the ice
Deep ocean	Ocean floor far below the surface	Dark, cold, very high pressure	deep-sea fish, squid, worms	ocean floor, hot-water vents

One ecosystem, many habitats. Inside a single ecosystem there are many different places to live, each with slightly different conditions. Different organisms live in each one.

A Red Sea coral reef is one ecosystem with many habitats: the beach, the seagrass bed, the reef crest, caves and cracks in the reef, the reef slope, open water and the sandy sea floor.

Habitat in the reef	Conditions	Organisms that live there (examples)
Seagrass bed	Shallow, calm, sandy	sea turtles and dugongs (graze on seagrass), young fish hiding
Reef crest	Very shallow, bright, strong waves	hard corals, sea urchins
Caves and cracks in the reef	Dark, sheltered	moray eels, octopus, fish resting by day
Open water above the reef	Bright, no shelter	shoals of fish, sharks, dolphins
Sandy sea floor	Soft sand, deeper	rays, crabs, worms that burrow

Microhabitats are tiny habitats: under a stone in the desert (cool and damp for a scorpion), inside a crack in a coral, under the bark of a tree. Exam tip: an ecosystem includes the non-living environment; a habitat is just the place where an organism lives.

النظام البيئي والموطن

النظام البيئي (ecosystem) = كل الكائنات الحية في منطقة + البيئة غير الحية (مياه، ضوء، حرارة، تربة) والعلاقة بينهم. الموطن (habitat) = المكان اللي الكائن عايش فيه. النظام البيئي الواحد فيه مواطن كتير: الشعاب المرجانية في البحر الأحمر فيها حشائش البحر، والكهوف والشقوق، والمية المفتوحة، والقاع الرملي.

Food chains and food webs: a quick reminder

Feeding links join the organisms of an ecosystem. In a food chain, the arrow means "is eaten by" and shows the direction energy (and anything in the food) moves. A producer (a plant or alga) makes its own food; consumers eat other organisms. The animal at the end of the chain is the top predator. A food web shows many food chains linked together. Because of these links, anything that changes one population affects others — and anything that is in the food (including poisons) is passed along the chain.

Bioaccumulation of toxic substances

Some toxic substances get into ecosystems from human activities: pesticides (such as DDT, used in the past to kill insects), heavy metals such as mercury and lead from factories and mining, and some industrial chemicals. The dangerous ones share two properties:

They are not broken down easily in the environment — they last for many years.
When an organism takes them in, its body cannot break them down or get rid of them, so they are stored (often in fat) and build up over its life. This is bioaccumulation.

A pesticide in a lake. The concentration in the water is tiny, but it increases at every step of the food chain. The fish-eating bird (top predator) has the highest concentration.

Tiny amounts of the pesticide wash into the water, and plankton (tiny organisms) take it in. It stays in their bodies.
Each small fish eats many plankton over its life. It keeps all the pesticide from all of them, so its concentration is higher.
Each large fish eats many small fish, and each fish-eating bird eats many large fish. At every step the pesticide becomes more concentrated.
So top predators have the highest concentration — high enough to harm or kill them, even though the water seemed only slightly polluted.

The impact on an ecosystem

Effect	Example
Top predators are poisoned — they become ill, die, or cannot reproduce	In the 1950s–60s, DDT built up in birds of prey such as peregrine falcons and eagles. It made them lay eggs with thin shells that broke when the parents sat on them, so few chicks hatched and their populations fell sharply. Many countries then banned DDT, and the birds slowly recovered.
Humans can be harmed — we are often top consumers	In Minamata, Japan (1950s), a factory released mercury compounds into the bay. Mercury built up in fish and shellfish, and people and cats that ate them were badly poisoned. Today pregnant women are often advised to limit large predatory fish such as swordfish, which can contain more mercury.
Knock-on effects through the food web	If the top predator population falls, the animals it ate may increase in number; they then eat more of their own food, so that population may fall. The balance of the whole ecosystem changes and biodiversity can fall.

How to reduce bioaccumulation: ban or limit long-lasting pesticides; use pesticides that break down quickly or biological pest control; treat factory waste before it reaches rivers and seas; test fish for mercury.

Exam technique — calculations: "How many times greater?" means divide the bigger concentration by the smaller one. For example, bird ÷ small fish = 25 ÷ 0.5 = 50 times.

New and invasive species

A native species lives naturally in an area. Sometimes a new species arrives that has never lived there. Humans often bring them — on purpose (as pets, farm animals, garden plants, fish for fishing, or to control pests) or by accident (in ships' ballast water, on cargo, through canals). Many new species die out or fit in without harm. But some become invasive: they spread quickly and harm native species and the ecosystem.

Why invasive species spread so quickly	How they affect other organisms and the ecosystem
No natural predators or diseases in the new area (these were left behind in their home area)	Competition: they compete with native species for food, space, light or nesting sites, and often win, so native populations fall
They reproduce quickly and in large numbers	Predation: they eat native species that have no defences against them
They are good competitors and can live in many conditions	Disease: they can bring diseases that kill native species
Native animals may not eat them (they may be poisonous or unfamiliar)	Habitat change: they can change the habitat itself (e.g. covering water, blocking light)
	Food web effects: changes in one population pass along the web; biodiversity falls; some native species may become extinct

Case studies

Invasive species	How it arrived	Effects
Water hyacinth in the Nile and Egyptian canals	A floating plant from South America, spread as a garden pond plant	Grows extremely fast and covers the water surface → blocks light, so water plants and algae below die → when they rot, microorganisms use up the oxygen, so fish die. It also blocks irrigation canals and boats, and loses a lot of water from its leaves.
Nile perch in Lake Victoria (East Africa)	Put into the lake in the 1950s to give fishermen a large fish to catch	A large predator with no predators of its own in the lake; it ate huge numbers of small native cichlid fish, and many of the hundreds of cichlid species disappeared.
Cane toad in Australia	Brought in 1935 to eat beetles that damaged sugar cane	It did not control the beetles, but bred quickly and spread over a huge area. Its skin is poisonous, so native predators (snakes, lizards, quolls) that eat it die.
Lionfish in the Caribbean and western Atlantic	Native to the Red Sea and Indian Ocean; released from home aquariums	No predators there; it eats huge numbers of small reef fish, so reef fish populations fall.
Grey squirrel in Britain	Brought from North America	Out-competes the native red squirrel for food and carries a disease that kills red squirrels; red squirrels have disappeared from most of England.

Controlling invasive species: prevention is best (checking ships and cargo, never releasing pets or aquarium fish into the wild). Others are removed by hand or machine (water hyacinth is cut and lifted out of canals), or controlled by a carefully tested natural enemy — for example, small weevils that eat only water hyacinth have been used in some lakes.

A lake food web after a large predatory fish was introduced. Arrows mean "is eaten by".

How to answer food web questions about a new species: (1) find what the new species eats — those populations will decrease; (2) find which animals ate the same food — they now face competition and may decrease; (3) follow the arrows one more step — the food of the animals that decreased may now increase. Always give the reason for each change.

التراكم الحيوي والأنواع الغازية

التراكم الحيوي: مادة سامة (زي مبيد DDT أو الزئبق) ما بتتكسرش وما بتخرجش من الجسم، فبتتخزن. وكل حيوان بياكل كتير من اللي قبله في السلسلة الغذائية، فالتركيز بيزيد في كل خطوة، وأعلى تركيز بيكون في المفترس الأخير (زي الطيور اللي بتاكل سمك). النوع الغازي: كائن جديد اتنقل لمكان مش مكانه، ومالوش أعداء طبيعيين هناك، فبيتكاثر بسرعة وينافس أو ياكل الأنواع المحلية — زي ورد النيل اللي بيغطي المية ويمنع الضوء فالسمك يموت.

Worked examples

Worked example 1 (ecosystem or habitat?) (a) Explain the difference between an ecosystem and a habitat. [2] (b) Name two different habitats within a coral reef ecosystem. [1]

(a) An ecosystem is all the organisms in an area and their physical (non-living) environment, interacting [1]; a habitat is the place where an organism lives [1].
(b) Any two: seagrass bed, reef crest, caves / cracks in the coral, open water, sandy sea floor, reef slope.
Common mistake: naming another ecosystem ("desert") when the question asks for a habitat within the reef.

Worked example 2 (bioaccumulation calculation) Use the pesticide diagram above. (a) How many times greater is the concentration in the large fish than in the plankton? [1] (b) Explain why the fish-eating bird has the highest concentration. [2]

(a) 2 ÷ 0.04 = 50 times
(b) The pesticide is not broken down / not excreted, so it stays in the body [1]; each bird eats many fish over its life, so it takes in the pesticide from all of them and it builds up to a high concentration [1].
Common mistake: "The bird drinks the polluted water." The water concentration is tiny; nearly all the pesticide comes from food.

Worked example 3 (impact on an ecosystem) A farmer sprays a long-lasting pesticide near a river. Twenty years later there are far fewer kingfishers (fish-eating birds) along the river. Suggest how the pesticide caused this and one other effect on the river ecosystem. [3]

The pesticide washed into the river and built up along the food chain (bioaccumulation) [1].
Kingfishers, as top predators, got the highest concentration, so many died or could not breed (e.g. thin eggshells) [1].
Other effect: with fewer kingfishers, the number of small fish may increase, so the animals they eat (e.g. insect larvae) may decrease / the balance of the food web changes [1].

Worked example 4 (invasive species in a food web) Use the lake food web above. The introduced fish spreads quickly. (a) Suggest what happens to the number of small fish. Explain your answer. [2] (b) Explain why the number of herons may fall. [2] (c) Suggest what may happen to the algae. Explain your answer. [2]

(a) Small fish decrease [1], because the introduced fish eats them (and it has no predators in the lake) [1].
(b) The heron also eats small fish, so there is less food for it / it competes with the introduced fish [1]; herons may eat more frogs, so frogs decrease too, and some herons starve or move away [1].
(c) Algae may increase [1], because fewer small fish and insect larvae are left to eat the algae [1].
Exam technique: use the word because, and name the organisms from the web.

Worked example 5 (water hyacinth) Water hyacinth covers the surface of a canal. Explain why fish in the canal die. [3]

The plant blocks light from reaching water plants and algae below. [1]
They cannot photosynthesise, so they die and rot; the microorganisms that decompose them use up oxygen (and less oxygen is made by photosynthesis). [1]
The fish do not get enough oxygen for respiration, so they die. [1]

Worked example 6 (why invasive species spread) Suggest two reasons why cane toads spread so quickly across Australia. [2]

Any two: no natural predators in Australia (predators that eat them die because the toads are poisonous); no diseases to control them; they reproduce quickly / lay many eggs; plenty of food; they can live in many habitats.
Common mistake: "because they are invasive" — that just repeats the question.

Common mistakes

Mixing up ecosystem (living + non-living, interacting) and habitat (the place an organism lives).
Saying the top predator gets poisoned because it "drinks the water". It gets the poison from the many organisms it eats.
Forgetting to say the substance is not broken down or excreted — that is why it builds up.
Writing "the population will change" without saying increase or decrease and why.
Thinking every new species is invasive. A species is invasive only if it spreads and causes harm.
Reading food web arrows backwards. The arrow points to the eater.
In "how many times" questions, subtracting instead of dividing.

How Checkpoint asks this

Food web questions: "An invasive species is added. Describe and explain what happens to population X." Give the change (increase/decrease) and the reason from the web.
Bioaccumulation data: a diagram or table of concentrations along a food chain. Calculate "how many times greater" and explain why the top predator has the most.
Match ecosystems to their conditions, or identify habitats within one ecosystem from a picture.
Suggest reasons: why an invasive species spreads (no predators, fast reproduction) or why a toxic substance is dangerous (not broken down).
Evaluate: "Was introducing species X a good idea?" Give an advantage and a disadvantage and reach a conclusion.

Practice

Remember

Ecosystem = living organisms + their non-living environment, interacting. Habitat = the place where an organism lives. One ecosystem has many habitats.
Examples of ecosystems: desert, coral reef, river or lake, rainforest, grassland, polar, deep ocean.
Bioaccumulation: a toxic substance that is not broken down or excreted builds up in an organism; its concentration increases at each step of a food chain, so top predators are harmed most.
Invasive species spread fast (no predators or diseases, fast breeding) and harm natives by competition, predation and disease; effects pass through the food web and biodiversity falls.
In food web questions, always state increase or decrease AND give the reason using the feeding links.

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