An ecosystem is all the living things in an area plus the nonliving things they depend on. Living parts, such as plants, animals, fungi and bacteria, are biotic factors. Nonliving parts, such as sunlight, water, temperature and soil, are abiotic factors. Energy enters most ecosystems through producers, which use sunlight to make food. Consumers eat producers or other consumers, and decomposers break down dead matter so nutrients return to the soil. A food chain shows one path of energy; a food web shows the many overlapping chains in a real ecosystem.
Only about 10 percent of the energy at one level of a food chain passes to the next, because most is used for life processes or lost as heat. That is why an energy pyramid narrows toward the top and why there are fewer top predators than plants. The NGSS performance expectations MS-LS2-1, MS-LS2-2 and MS-LS2-3 cover how resources affect populations, the patterns of interactions between organisms and how matter cycles and energy flows. The questions below include two quick 10 percent calculations.
Choose an option or type your answer, then press Check. Capital letters and extra spaces do not matter in typed answers, and units are optional.
Answer: D) Grass
Grass makes its own food using sunlight, so it is the producer at the base of the chain.
Answer: decomposers (also accepted: decomposer)
Decomposers break down dead plants and animals and return nutrients to the soil.
Answer: B) Water temperature
Abiotic means nonliving. Water temperature is nonliving; algae, frogs and bacteria are all living (biotic).
Answer: 1000 (also accepted: 1,000, 1000 units, 1,000 units)
10 percent of 10,000 is 10,000 times 0.1 = 1,000 units.
Answer: A) the direction energy flows
Each arrow points from the organism being eaten to the one eating it, showing how energy moves.
Answer: 100 (also accepted: 100 units)
Two steps up: 10,000 times 0.1 = 1,000, then 1,000 times 0.1 = 100 units.
Answer: C) mutualism
When both species benefit, the relationship is mutualism.
Answer: parasitism (also accepted: parasitic, parasitic relationship)
The tick benefits and the dog is harmed, so it is parasitism.
Answer: B) Their population will decrease.
With less food available, fewer foxes can survive and raise young, so the fox population falls.
Answer: herbivore (also accepted: herbivores)
Herbivores eat plants. They are primary consumers in a food chain.
Answer: B) commensalism
One species benefits and the other is unaffected, which is commensalism.
Answer: web (also accepted: food web)
A food web shows all the feeding relationships, since most animals eat more than one thing.
Answer: D) Most energy is lost at each level, so little is left at the top.
About 90 percent of the energy is lost at each step, so the top level can only support a small number of animals.
Arrows point in the direction energy flows, from the food to the eater: grass → rabbit means the rabbit eats the grass.
Decomposers recycle nutrients from every level. Without them, dead material would pile up and soil would run out of nutrients.
Matter cycles through an ecosystem, but energy flows one way and is lost as heat at each level. New energy keeps arriving from the sun.
In mutualism both species benefit. In commensalism one benefits and the other is not helped or harmed.
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Open the student portal →On average only about 10 percent of the energy at one trophic level is passed to the next. The rest is used for movement, growth and body heat, or is not eaten.
No. Mushrooms are fungi, which are decomposers. They cannot photosynthesize and get their food by breaking down dead material.