Ecosystems

Deer numbers rise and they eat more of the saplings

HintTake away the hunter and the animal it hunted is left free to browse.

The whyA food chain works in both directions: the arrow shows energy passing up from the eaten to the eater, but each level also holds the level below it in check. Remove the top predator and the plant-eaters increase until they strip the vegetation — which is why the return of wolves to Yellowstone in 1995 was followed by willow and aspen recovering along the streams.

glucose

HintThe sugar a plant builds from carbon dioxide and water.

The whyProducers are nearly always green plants or algae. Every consumer in the chain, from the rabbit to the fox, depends on the energy the producer locked away in that sugar.

A behavioural adaptation

HintMigrating before winter and hibernating are both examples.

The whyAdaptations may be structural (a camel's hump), behavioural (migrating, hibernating) or functional (making very concentrated urine). All three build up by natural selection over many generations.

They vary between individuals of the same species

HintA young oak and a hundred-year-old one differ hugely in height, yet both are oaks.

The whyA good key uses features that stay fixed, such as leaf shape or the number of legs, so that any member of the species gives the same answer at every step. Each step still offers two choices, leading to another pair of choices or to a name.

Carbon dioxide

HintThe greenhouse air holds only about 0.04% of the gas that plants build their sugars from.

The whyPhotosynthesis has three main limiting factors: light, temperature and carbon dioxide. Whichever is in shortest supply sets the rate, so raising the others changes nothing. Once light, warmth and water are plentiful, commercial growers pipe extra carbon dioxide into the greenhouse and the crop grows faster.

Most is lost as heat, movement and waste

HintA rabbit does not turn every mouthful of grass into more rabbit.

The whyOnly about a tenth of the energy at one level reaches the next. That is why food chains rarely run past four or five links — there is too little left to support another.

A food web

HintMost animals eat more than one thing, so a single line becomes a network.

The whyA web shows the real feeding relationships in a habitat, and it shows why removing one species can affect many others that never ate it directly.

Decomposers

HintWithout them, fallen leaves would pile up and the minerals inside stay locked away.

The whyBacteria and fungi digest dead remains and droppings, releasing nitrates and other minerals for plants to absorb again. Without this step the nutrient cycles would stop.

So the sample is not biased

HintIf you picked the spots yourself, you might favour the patches that catch the eye.

The whyRandom coordinates stop the results reflecting your choices. Counting several quadrats, finding a mean and scaling up to the whole area then estimates the population.

It falls too

HintThere is less to eat, so fewer of them survive.

The whyThe two populations rise and fall in a repeating cycle, with the predator peak lagging behind the prey peak. Neither wipes the other out; each keeps the other in check.

Photosynthesis

HintGreen plants do it whenever there is light.

The whyIt is the major biological process that takes carbon dioxide out of the air; the oceans absorb a great deal too. Respiration, decay and the burning of fossil fuels all put it back.

The toxin builds up as it passes along the food chain

HintEach animal eats many of the one below it, and the poison is not easily broken down or excreted.

The whyPersistent pesticides such as DDT and dieldrin stay in body tissue, so a predator takes on much of what its prey had already stored. Both were sprayed or used as seed dressings on British farmland in the 1950s and 60s. Pigeons and other seed-eaters picked them up, peregrine falcons ate the pigeons, and by 1963 the peregrine had almost vanished from southern England.