Ecosystems, Year 9: People and the environment

Bioaccumulation

HintThe first part of the word means life; the rest means piling up.

The whySome poisons cannot be broken down or excreted, so they stay in an animal's tissues and the store grows throughout its life. Each animal also eats many of the organisms below it, so the amount stored is larger at every step of a food chain, and the animals at the top carry the most.

They are not broken down or excreted, so they stay in the body

HintAsk where a substance ends up if an animal can neither digest it away nor get rid of it.

The whyA toxin that is broken down, or passed out in urine, never has the chance to collect. The dangerous ones last: DDT, for example, dissolves in body fat and stays there for years, so every contaminated meal adds to the store.

200

HintNone of it leaves after each meal, so every fish adds to the running total.

The whyEach fish hands over its whole store and the heron keeps all of it. That is why a top predator can carry hundreds of times more toxin than the animals it eats, even when the water holds only a trace.

Through their food — the birds they hunted carried it

HintPeregrines catch pigeons, and pigeons feed on farmland grain.

The whySeed-eating birds picked up small doses from treated grain, and each peregrine ate many of those birds. The pesticide built up in the falcons until their eggshells grew thin and broke. Numbers recovered after these pesticides were banned.

Greys outcompete them for food and carry a virus that kills reds

HintThe newcomers are bigger, eat the same nuts and seeds, and arrived with something invisible.

The whyAn introduced species can upset a food web that took thousands of years to settle. Grey squirrels take the food red squirrels need and carry squirrelpox, which does them no harm but is deadly to reds. Red squirrels now survive mainly where greys are rare or absent.

It falls, because the parents cannot find enough food

HintFollow the link in the web: take away what the chicks are given to eat.

The whyRemoving one species from a food web affects everything that feeds on it. Seabirds such as puffins and kittiwakes depend on sandeels, which is why large-scale sandeel fishing was closed in the English North Sea and in Scottish waters in 2024.

They lost their nesting sites, shelter and food

HintThink about everything a dense, berry-covered hedge offers a small bird.

The whyA hedgerow is a habitat: it supplies berries, seeds and insects to eat, cover from predators and safe places to nest. Changing how land is used removes habitats, and the species that depended on them decline even though nobody harmed them directly.

Few ladybirds are left to eat the aphids

HintThink about which of the two insects was keeping the other in check.

The whyAphids reproduce extremely quickly, so the few that survive the spray can rebuild their numbers within weeks. Their predators recover far more slowly. With little left to hunt them, the pests return in greater numbers, which is one reason farmers are encouraged to protect natural predators.

Young fish escape and survive to breed

HintAsk what size of animal slips through a bigger hole, and what it can go on to do.

The whyIf fish are caught before they are old enough to reproduce, the population cannot replace itself and it collapses. Letting the small ones through leaves enough breeding adults for the future, so fishing can continue without wiping the stock out.

Nature conservation

HintThe second word is also used for looking after old paintings and historic buildings.

The whyConservation covers anything done to stop species and habitats being lost: protecting land as nature reserves, controlling hunting and fishing, breeding rare animals and storing seeds. The aim is to keep populations large enough to survive on their own.

Protecting a habitat protects every species living in it

HintCount how many kinds of plant, insect, bird and fungus share a single wood.

The whyA zoo can hold a few hundred kinds of animal; a woodland holds thousands of species, most of them small and unnoticed, along with all the feeding links between them. Species cannot survive in the wild without somewhere suitable to live, so saving habitats comes first.

Reintroduction

HintIt means bringing a species back to a place where it used to live.

The whyRed kites were wiped out across most of Britain by persecution. Releasing birds into suitable habitat, and protecting them by law, let breeding populations build up again. The method only works once the original cause of the decline has been removed.

To keep each species' genetic material safe in case it dies out in the wild

HintThink of it as a backup copy, stored somewhere the original cannot be damaged.

The whyA gene bank stores seeds, eggs, sperm or tissue so that the hereditary material of a species is not lost. If a plant becomes extinct in the wild, stored seeds can be grown to bring it back, and breeders can use them to add useful features to crops.

endangered

HintIt is the stage a species reaches before the word used for the dodo.

The whyScientists assess how many individuals are left and how fast numbers are falling. Species judged to be at high risk are listed as endangered, which helps governments and charities decide where conservation effort and money are most urgently needed.