Ecosystems, Matter & Energy — Chapter Practice, Grade 7

Five focused units, a three-session investigation, chapter review, and chapter quiz.

Problem 1

Why is a decomposer not outside the food system?
Answer and explanation

It obtains and transfers organic matter and energy from dead material and wastes within connected pathways.

Problem 2

Which way should an arrow point from an eaten seed to a seed-eating bird?
Answer and explanation

From seed to bird, because the bird receives matter and stored energy from the seed.

Problem 3

Why might a consumer persist after one food source declines?
Answer and explanation

It may use other resources, migrate, or respond over time; the web shows possible connections rather than an immediate fixed outcome.

Problem 4

Do decomposers recycle all usable energy back to plants?
Answer and explanation

No. They recycle matter into other forms while energy transfers and much dissipates as heat.

Problem 5

Illustrative web: grass feeds insects and rabbits; insects and seeds feed mice; mice and rabbits feed foxes. An insect decline is recorded, but other resource counts are missing. Trace one affected pathway and explain why the exact mouse or fox change cannot be determined.
Answer and explanation

The insect decline can affect the mouse pathway and indirectly foxes, but mice also have seeds and foxes also have rabbits. The web does not give dependence strengths, alternative resource changes, movement, or timing. It supports a conditional mechanism, not an exact population forecast.

Follow insect→mouse→fox under the stated arrows.

Identify seeds as another mouse resource and rabbits as another fox resource.

Request counts, rates, and interval information.

Response criteria

  • Explain the relevant biological process accurately.
  • Use the supplied evidence and show calculation steps with units where needed.
  • Connect a labeled model or mechanism with the claim.
  • State an assumption, evidence limitation, or useful next observation.

Problem 6

Why can one organism have more than one feeding position?
Answer and explanation

It can eat resources from different positions, such as both producers and other consumers. The web must show its actual resource relationships.

Response criteria

  • Explain the relevant biological process accurately.
  • Use the supplied evidence and show calculation steps with units where needed.
  • Connect a labeled model or mechanism with the claim.
  • State an assumption, evidence limitation, or useful next observation.

Problem 7

In the stated convention, grass→rabbit means what?
  1. Grass hunts rabbits
  2. Rabbit receives food matter and energy from grass
  3. Rabbits create sunlight
  4. Grass becomes a whole rabbit instantly
Answer and explanation

Rabbit receives food matter and energy from grass

The arrow tracks transfer from the eaten resource toward the consumer.

Problem 8

Name information missing from a web that prevents an exact population prediction.
Answer and explanation

Feeding rates, dependence on each resource, initial counts, movement, and environmental conditions are examples of needed information.

Problem 9

Why can a few trees support many insects without contradicting energy flow?
Answer and explanation

Individual count differs from energy amount; organisms vary greatly in size and available energy.

Problem 10

If 200 of 1,000 units transfer, what percentage is modeled?
Answer and explanation

200/1,000×100=20%, rather than an assumed universal ten percent.

Problem 11

Does lower energy at the next position violate conservation?
Answer and explanation

No. Energy has other transfers and destinations; it is not all captured as next-level food.

Problem 12

Can a 10% energy transfer identify exact predator count?
Answer and explanation

No. Count depends on each organism’s needs and other ecological information not supplied by that fraction.

Problem 13

Illustrative energy model: producers provide 2,000 units; first consumers receive 300; second consumers receive 60 during the same interval. Calculate both transfer percentages and explain why the ten-percent shortcut and an energy-destruction claim are wrong.
Answer and explanation

The two modeled transfers are 15% and 20%. They are not both ten percent. Lower energy reaching the next position does not mean energy was destroyed; organisms use energy and transfer it into other pathways and heat. The model does not give the full separate budget or exact organism counts.

300/2,000×100=15%.

60/300×100=20%.

Identify other energy pathways and the stated boundary.

Response criteria

  • Explain the relevant biological process accurately.
  • Use the supplied evidence and show calculation steps with units where needed.
  • Connect a labeled model or mechanism with the claim.
  • State an assumption, evidence limitation, or useful next observation.

Problem 14

A diagram gives 500 energy units and 100 animals. Can those labels be treated as the same measurement?
Answer and explanation

No. Energy amount and individual count are distinct quantities. A relationship between them requires further biological information.

Response criteria

  • Explain the relevant biological process accurately.
  • Use the supplied evidence and show calculation steps with units where needed.
  • Connect a labeled model or mechanism with the claim.
  • State an assumption, evidence limitation, or useful next observation.

Problem 15

Which quantity belongs on an energy pyramid?
  1. Energy per stated boundary and interval
  2. Only individual names
  3. An uncalibrated image width
  4. A genetic allele letter
Answer and explanation

Energy per stated boundary and interval

Define energy units and comparable spatial and time boundaries.

Problem 16

A stipulated 10% transfer starts with 800 units. How many reach the next level?
Answer and explanation

80 energy units

0.10×800=80 under the stipulated rule.

Problem 17

Does sunlight supply the carbon atoms in sugar?
Answer and explanation

No. Carbon dioxide supplies carbon matter in the plant model; light supplies energy.

Problem 18

Does a cycle mean no reservoir can change?
Answer and explanation

No. Inputs and outputs can differ over an interval, changing a reservoir’s stored amount.

Problem 19

Does evaporation destroy water molecules in this model?
Answer and explanation

No. It changes state and location while the water remains matter in the larger system.

Problem 20

Why can a pond lose water while matter is conserved?
Answer and explanation

Water can leave its chosen boundary through evaporation, flow, or other transfers and remain elsewhere in the larger system.

Problem 21

Illustrative pond interval: rain adds 40 L; runoff adds 20 L; evaporation removes 25 L; outflow removes 15 L. Other pathways are excluded. Calculate storage change, label inputs and outputs, and explain the conservation distinction.
Answer and explanation

The modeled pond gains 20 L. Rain and runoff are inputs, while evaporation and outflow cross outward. The changing pond amount is consistent with matter conservation because the pond is only one boundary. The result assumes the stated pathways are complete for the interval and does not predict future weather or every real pond exchange.

Inputs: 40+20=60 L.

Outputs: 25+15=40 L.

Change: 60−40=+20 L.

Response criteria

  • Explain the relevant biological process accurately.
  • Use the supplied evidence and show calculation steps with units where needed.
  • Connect a labeled model or mechanism with the claim.
  • State an assumption, evidence limitation, or useful next observation.

Problem 22

Why does a carbon reservoir not necessarily return all its carbon to air each day?
Answer and explanation

Pathways have different rates and storage durations. Some carbon remains in tissues, soils, sediments, or other reservoirs over longer periods.

Response criteria

  • Explain the relevant biological process accurately.
  • Use the supplied evidence and show calculation steps with units where needed.
  • Connect a labeled model or mechanism with the claim.
  • State an assumption, evidence limitation, or useful next observation.

Problem 23

Which contributes carbon matter to photosynthetic sugar production?
  1. Carbon dioxide
  2. The color of sunlight
  3. Only a thermometer
  4. A graph title
Answer and explanation

Carbon dioxide

Carbon dioxide supplies carbon atoms; light provides energy.

Problem 24

A water store receives 30 L and loses 18 L in one stated interval. Find storage change.
Answer and explanation

12 L

30−18=+12 L under the stated complete budget.

Problem 25

Why are spaces in soil relevant?
Answer and explanation

They can hold air and water and affect movement and root conditions, rather than being biologically meaningless empty areas.

Problem 26

Do decomposers return usable energy as sunlight?
Answer and explanation

No. They change matter pathways while energy transfers through metabolism and heat; continued energy input is still needed.

Problem 27

Why compare dry masses rather than wet masses alone?
Answer and explanation

Changing water content can alter wet mass without an equivalent change in the organic material being studied.

Problem 28

Why is “more water always improves soil conditions” unsupported?
Answer and explanation

Water can occupy air spaces and interact with other conditions; the effect depends on the organism and setting.

Problem 29

Illustrative dry-material study over the same interval: matched samples each start at 20 g; A ends at 14 g and B at 18 g. Moisture treatment differs; other conditions are stated matched. Calculate mass losses and explain the supported pattern with a boundary limitation.
Answer and explanation

A shows 6 g loss and B 2 g, so A lost 4 g more in this illustrative comparison. This is consistent with a treatment-related decomposition question under the stated matching. It does not identify every organism or destination of matter. Check replication, dry-mass method, and material transfers before generalizing to every soil.

A loss: 20−14=6 g.

B loss: 20−18=2 g.

Compare dry-mass method and missing destinations.

Response criteria

  • Explain the relevant biological process accurately.
  • Use the supplied evidence and show calculation steps with units where needed.
  • Connect a labeled model or mechanism with the claim.
  • State an assumption, evidence limitation, or useful next observation.

Problem 30

A leaf bag is weighed wetter at the end and gains mass. Does that prove organic material was created?
Answer and explanation

No. Added water or other material can change the measured mass. Define the boundary and compare dry material if that is the intended quantity.

Response criteria

  • Explain the relevant biological process accurately.
  • Use the supplied evidence and show calculation steps with units where needed.
  • Connect a labeled model or mechanism with the claim.
  • State an assumption, evidence limitation, or useful next observation.

Problem 31

Which could confound wet-mass decomposition measurements?
  1. Changed water content
  2. Correct units alone
  3. A stated interval alone
  4. A labeled table alone
Answer and explanation

Changed water content

Water differences affect wet mass independently of dry organic material loss.

Problem 32

Dry material changes from 25 g to 19 g. How much dry mass is lost?
Answer and explanation

6 g

25−19=6 g, without identifying every destination.

Problem 33

Why should a count include area and effort?
Answer and explanation

Without them, equal or different counts might reflect different survey scope rather than biological change.

Problem 34

Does an increased count prove more births?
Answer and explanation

No. Immigration or detection differences can also increase the recorded number.

Problem 35

Why can carrying capacity change?
Answer and explanation

Resources, habitat, and environmental conditions change, altering what the modeled setting can support.

Problem 36

Why might a lower observed count not mean the same number died?
Answer and explanation

Movement or lower detectability can affect counts; births and deaths are not the only pathways.

Problem 37

Illustrative interval: population begins at 50; births add 12; immigration adds 5; deaths remove 8; emigration removes 4. No other pathways are stipulated. Calculate ending count and explain why a before-after count alone cannot identify all pathways.
Answer and explanation

The modeled population ends at 55, a net increase of five. The same net change could result from other combinations of births, deaths, and movement. Before-after counts alone do not identify the mechanism; pathway observations, boundaries, and detection methods are needed. This model does not establish a permanent habitat capacity.

Gains: 12+5=17.

Losses: 8+4=12.

End: 50+17−12=55.

Response criteria

  • Explain the relevant biological process accurately.
  • Use the supplied evidence and show calculation steps with units where needed.
  • Connect a labeled model or mechanism with the claim.
  • State an assumption, evidence limitation, or useful next observation.

Problem 38

A field count rises after a survey team doubles its observation time. Can the increase be assigned directly to population growth?
Answer and explanation

No. More effort can detect more individuals. Use comparable survey effort and methods before interpreting the difference as true growth.

Response criteria

  • Explain the relevant biological process accurately.
  • Use the supplied evidence and show calculation steps with units where needed.
  • Connect a labeled model or mechanism with the claim.
  • State an assumption, evidence limitation, or useful next observation.

Problem 39

Which is an abiotic factor?
  1. Temperature
  2. A predator
  3. A decomposer
  4. A competing plant
Answer and explanation

Temperature

Temperature is a physical condition, while the others involve organisms.

Problem 40

A population of 40 gains 10 and loses 7 under a complete model. Find ending count.
Answer and explanation

43 individuals

40+10−7=43.

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