Ecosystems

The direction that energy flows

HintFollow it from the meal to the diner — something is being handed on.

The whyThe arrow always points from the eaten to the eater, because that is the way the stored chemical energy travels: grass to rabbit to fox.

producer

HintIt makes its own food from sunlight instead of eating anything.

The whyThese are nearly always green plants or algae. They capture light energy and store it as chemical energy in glucose, which every other organism in the chain then depends on.

A feature that helps an organism survive where it lives

HintWhy does a polar bear have thick white fur, and a cactus spines?

The whyAdaptations may be structural (a camel hump), behavioural (migrating before winter) or functional (making very concentrated urine). They build up by natural selection over many generations.

A yes or no question

HintEach step splits the group in half, so exactly one branch can be true.

The whyEvery step offers two choices, each leading to another pair of choices or to a name. Good keys use features that do not change, such as leaf shape rather than colour.

Carbon dioxide

HintThe same gas you breathe out.

The whyPlants take it in through pores called stomata and combine it with water, using light energy, to make glucose. Oxygen is released as the waste product.

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.

Several food chains linked together

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.

They break down dead material and return nutrients to the soil

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 whySubstances such as DDT stay in body tissue, so a predator takes on much of what its prey had already stored. This nearly wiped out British peregrine falcons in the 1960s.