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Cells, Genes & Evolution
From the inside of a cell to the logic of natural selection.
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What is generally considered the basic unit of life?
Ecosystems & Food Webs
Ecology is mostly about connections rather than individual species. These ten questions cover the ones that shape whole habitats.
About this topic
An ecosystem is a set of relationships more than a place. Energy enters as sunlight, gets captured by plants and algae, and then leaks away as heat at every step up the chain, which is why there are far fewer wolves than rabbits. Nutrients, unlike energy, go round and round: the nitrogen in a leaf has been through countless bodies before. The interesting part is what happens when a connection breaks. Take out one predator and the effects can run right down to the plants. This quiz covers the basic structure of food webs, the recycling that keeps them supplied, and a few of the cases where a single species turned out to be holding everything up.
Questions & answers
Ecosystems & Food Webs: all 10 questions and answers
Every question in this quiz is listed below with its correct answer and the reasoning behind it. Play first if you would rather not see the answers — or read through as a study sheet.
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1What is an organism that breaks down dead material called?
- A An apex predator
- B A decomposer Correct
- C A producer
- D A herbivore
Answer: B. A decomposer
Why: Fungi and bacteria do most of this work, releasing nitrogen, phosphorus and carbon back into the soil where plants can take them up again. Without decomposers, nutrients would stay locked inside dead tissue and the system would grind to a halt.
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2Roughly how much of the energy at one level of a food chain reaches the next?
- A About 10 per cent Correct
- B About 40 per cent
- C About 70 per cent
- D About 90 per cent
Answer: A. About 10 per cent
Why: Most of the rest is lost as heat or spent on moving, breathing and digesting, and some is simply never eaten. That steep loss is why food chains rarely run beyond four or five links and why top predators are so thinly spread.
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3How does a food web differ from a food chain?
- A It leaves out the plants at the bottom
- B It applies only to marine habitats
- C It excludes decomposers entirely
- D It links many overlapping chains within one ecosystem Correct
Answer: D. It links many overlapping chains within one ecosystem
Why: Almost every animal eats several things and is eaten by several others, so a single chain is a simplification. Because the connections overlap, losing one species tends to send effects sideways through the web rather than snapping a line.
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4What is a keystone species?
- A The most numerous species present
- B The first species to colonise bare ground
- C One whose loss changes an ecosystem out of all proportion to its numbers Correct
- D The largest predator in a habitat
Answer: C. One whose loss changes an ecosystem out of all proportion to its numbers
Why: Sea otters are the standard example: they eat sea urchins, urchins graze kelp, and where otters were hunted out the kelp forests disappeared. The term came from Robert Paine, who tested the idea in the 1960s by removing starfish from stretches of rocky shore.
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5A relationship in which both species benefit is called what?
- A Predation
- B Mutualism Correct
- C Commensalism
- D Parasitism
Answer: B. Mutualism
Why: Bees get nectar and flowers get pollinated, which is mutualism in its most familiar form. In commensalism only one partner gains while the other is neither helped nor harmed, as with barnacles riding on a whale.
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6Plants cannot use nitrogen gas directly. What converts it into a usable form?
- A Bacteria in the soil and in root nodules Correct
- B Sunlight striking the leaves
- C Earthworms digesting leaf litter
- D Rainwater dissolving it out of the air
Answer: A. Bacteria in the soil and in root nodules
Why: Nitrogen-fixing bacteria break the triple bond in nitrogen gas, and many live inside nodules on the roots of beans, peas and clover. Lightning fixes a small share as well, which is why farmers rotated legumes into their fields long before anyone knew why it worked.
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7What does an ecological niche describe?
- A The burrow or nest an animal lives in
- B The area a predator hunts across
- C The number of young a species produces
- D The role and conditions a species occupies in its habitat Correct
Answer: D. The role and conditions a species occupies in its habitat
Why: A niche takes in what a species eats, when it is active, what temperatures it tolerates and what eats it, not simply where it lives. Two species with identical niches in the same place cannot coexist indefinitely; one will displace the other.
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8Which organisms sit at the base of most open-ocean food webs?
- A Coral
- B Kelp
- C Phytoplankton Correct
- D Krill
Answer: C. Phytoplankton
Why: Microscopic drifting algae capture sunlight across the surface of the ocean and everything above them depends on that harvest. They also generate a large share of the oxygen in the atmosphere, comparable to all the forests on land.
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9What happens when fertiliser runs off farmland into a lake?
- A Nothing changes until the water freezes
- B Algae bloom, then decay and strip oxygen from the water Correct
- C The water becomes permanently clearer
- D Fish grow faster and populations rise
Answer: B. Algae bloom, then decay and strip oxygen from the water
Why: The process is called eutrophication, and the oxygen crash that follows the bloom can suffocate fish across a wide area. The dead zone that forms in the Gulf of Mexico each summer is fed this way by runoff carried down the Mississippi.
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10Which organisms typically colonise bare rock left behind by a retreating glacier?
- A Lichens Correct
- B Oak trees
- C Deep-rooted grasses
- D Bracken ferns
Answer: A. Lichens
Why: Lichens grip bare stone and slowly break it down with acids, producing the first thin traces of soil in what is called primary succession. Mosses follow, then grasses, and trees may take a century or more to arrive.
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