Ecosystems

The direction that energy flows

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

为什么The 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

提示It makes its own food from sunlight instead of eating anything.

为什么These 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

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

为什么Adaptations 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

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

为什么Every 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

提示The same gas you breathe out.

为什么Plants 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

提示A rabbit does not turn every mouthful of grass into more rabbit.

为什么Only 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

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

为什么A 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

提示Without them, fallen leaves would pile up and the minerals inside stay locked away.

为什么Bacteria 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

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

为什么Random 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

提示There is less to eat, so fewer of them survive.

为什么The 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

提示Green plants do it whenever there is light.

为什么It 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

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

为什么Substances 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.