Ecology, Populations & Biodiversity — Chapter Practice, Grade 12

Use food webs, population models, habitat patterns, and representative sampling.

Problem 1

In an energy pyramid, which trophic level has the LEAST energy available to it?
  1. Primary consumers
  2. Secondary consumers
  3. Top predators
  4. Producers
Answer and explanation

Top predators

Because only ~10% of energy transfers at each step, the top predators at the peak have the least energy available.

Problem 2

A wetland's producers have a GPP of 18,000 kcal/m²/yr and spend 7,000 kcal/m²/yr on respiration. What is the net primary productivity (NPP)?
Answer and explanation

11000 kcal/m²/yr

NPP = GPP - respiration = 18,000 - 7,000 = 11,000 kcal/m²/yr.

Problem 3

Producers store 80,000 kcal/m²/yr as NPP. Using the 10% rule, how much energy is available to the secondary consumers (two levels up)?
Answer and explanation

800 kcal/m²/yr

Primary consumers: 80,000 × 0.10 = 8,000; secondary consumers: 8,000 × 0.10 = 800 kcal/m²/yr.

Problem 4

Net primary productivity (NPP) is best described as
  1. the energy lost as heat by top predators
  2. the energy released by decomposers
  3. the total energy producers capture from the sun before any is used
  4. the energy producers store as biomass after paying their own respiration cost
Answer and explanation

the energy producers store as biomass after paying their own respiration cost

NPP = GPP - respiration, the energy left as biomass and available to consumers.

Problem 5

Explain how limited trophic-transfer efficiency can constrain higher-level production. Does it establish a universal maximum of four or five trophic levels?
Answer and explanation

Production generally decreases along successive transfers because respiration, uneaten material, and other pathways limit transfer into new biomass. Actual efficiencies and food webs vary; the10% estimate does not establish a fixed universal maximum number of trophic levels. Organism size, production, turnover, and feeding relationships also matter.

Problem 6

A producer level has 4000 kJ and efficiency 20%. Find next-level production. Show the reasoning, not just the conclusion.
Hints

Supplied efficiencies are conditional model values.

Answer and explanation

800 kJ.

Problem 7

Why should decomposers appear connected to several trophic levels?
Hints

State what the supplied observations or conditions establish, and separate that from any assumption.

Answer and explanation

They process dead material and wastes from producers and multiple consumer levels.

Problem 8

Energy transfer and matter cycling need separate accountsEnergy transfer and matter cycling need separate accountsSunlight inputBiologicaltransfersHeat tosurroundingsMatter can cycle through reservoirs.Energy flows through; elements are recycled through processes.
Original qualitative energy flow and material-cycle distinction; no numerical fluxes are implied.
Interpret the supplied visual. Can decomposers recycle the heat lost by respiration into the original food energy without a new energy source?
Answer and explanation

No. Nutrients can be reused, but restoring chemical energy requires an energy input and appropriate processes.

Energy enters many ecosystems as sunlight and eventually disperses as heat; it is not recycled through a closed biological energy loop. Matter such as carbon, nitrogen and water moves among reservoirs through physical, chemical and biological processes. Decomposers transform organic material and return accessible nutrients, while their respiration also transfers energy. Draw separate energy arrows and element inventories so that a material cycle is not mistaken for unlimited energy recovery.

No. Nutrients can be reused, but restoring chemical energy requires an energy input and appropriate processes.

Response criteria

  • Interpret labels, structures, axes, units and directions correctly.
  • Explain the physical or environmental relationship or mechanism.
  • Respect stated assumptions and distinguish observation from inference.

Problem 9

A habitat has GPP12000 kJ/m²/yr and producer respiration 5000. Consumers produce 840 kJ/m²/yr. Find NPP and the NPP-to-consumer production efficiency.
Answer and explanation

NPP=7000 kJ/m²/yr. Efficiency=840/7000=12%. The fraction uses NPP, not GPP, and does not imply every intermediate process has 12% efficiency.

NPP=7000 kJ/m²/yr.

Efficiency=840/7000=12%.

The fraction uses NPP, not GPP, and does not imply every intermediate process has 12% efficiency.

Response criteria

  • Use the stated measurements or conditions, with correct units.
  • Connect the result to the physical or environmental mechanism or evidence relationship.
  • Identify the assumption, model limit or further evidence requested.

Problem 10

Which of these is an example of genetic diversity?
  1. The variety of alleles for coat color within a single wolf population
  2. The range of ecosystems found across a continent
  3. The total biomass of producers in a field
  4. The number of different bird species in a forest
Answer and explanation

The variety of alleles for coat color within a single wolf population

Genetic diversity is variation within a species or population, such as different coat-color alleles in one wolf population.

Problem 11

A wetland that naturally filters pollutants out of water is providing which type of ecosystem service?
  1. Cultural
  2. Regulating
  3. Genetic
  4. Provisioning
Answer and explanation

Regulating

Purifying water is a regulating service — the ecosystem stabilizes and improves environmental conditions.

Problem 12

A pond sample has two fish species with 30 and 70 individuals (N = 100). Calculate Simpson's Diversity Index, D = 1 - Σ(n/N)².
Answer and explanation

0.42

(0.30)² + (0.70)² = 0.09 + 0.49 = 0.58; D = 1 - 0.58 = 0.42.

Problem 13

A sea star preys on mussels that would otherwise crowd out other shoreline species. Removing the sea star causes local diversity to collapse. The sea star is best described as a
  1. invasive species
  2. decomposer
  3. keystone species
  4. producer at the base of the food web
Answer and explanation

keystone species

Its impact is far greater than its abundance, and its removal reshapes the whole community — the definition of a keystone species.

Problem 14

Explain how high genetic diversity within a species can make that species more likely to survive a new disease.
Answer and explanation

With many different alleles present, it is more likely that at least some individuals happen to carry genes that resist the disease. Those individuals survive and reproduce, so the population persists, whereas a genetically uniform population could be wiped out all at once.

Problem 15

Why compare evenness as well as richness in communities? Show the reasoning, not just the conclusion.
Hints

Use a service-specific measure and sampling design.

Answer and explanation

Equal richness can hide strong dominance differences and altered functioning.

Problem 16

A service improves after restoration. What evidence helps attribute the improvement?
Hints

State what the supplied observations or conditions establish, and separate that from any assumption.

Answer and explanation

Baseline and comparison data, consistent methods and a mechanism linking restoration to the measured service.

Problem 17

State the exact diversity-index convention before calculatingState the exact diversity-index convention before calculatingSpeciespi in Xpi²A0.250.0625B0.250.0625C0.250.0625D0.250.0625D=1−Σpi²=1−0.25=0.75
Original index calculation from illustrative proportions, with explicit formula and no finite-sample correction.
Interpret the supplied visual. Find D for two species with fractions 0.8 and 0.2.
Answer and explanation

D=1−(0.64+0.04)=0.32 under the stated convention.

For this course, the selected Simpson diversity convention is D=1−Σpi², with pi the observed fraction in species i. Larger values mean greater diversity under this convention. Other publications use the sum or its reciprocal, so the name alone is ambiguous. For four equally common species D=0.75. The result is an estimate from sampled counts and does not identify habitat quality, extinction risk or causal mechanisms by itself.

D=1−(0.64+0.04)=0.32 under the stated convention.

Response criteria

  • Interpret labels, structures, axes, units and directions correctly.
  • Explain the physical or environmental relationship or mechanism.
  • Respect stated assumptions and distinguish observation from inference.

Problem 18

Counts are 6,3,1 for three species. Find richness and D=1−Σpi², then state a sampling limitation.
Answer and explanation

Richness=3. Fractions 0.6,0.3,0.1 give D=1−(0.36+0.09+0.01)=0.54. The sample may miss rare or hard-to-detect species; matched effort and replication matter.

Richness=3.

Fractions 0.6,0.3,0.1 give D=1−(0.36+0.09+0.01)=0.54.

The sample may miss rare or hard-to-detect species; matched effort and replication matter.

Response criteria

  • Use the stated measurements or conditions, with correct units.
  • Connect the result to the physical or environmental mechanism or evidence relationship.
  • Identify the assumption, model limit or further evidence requested.

Problem 19

A population reproducing with unlimited resources, doubling again and again, produces which growth curve?
  1. An S-shaped logistic curve
  2. A flat horizontal line
  3. A bell-shaped curve
  4. A J-shaped exponential curve
Answer and explanation

A J-shaped exponential curve

Constant per-capita growth with no limits accelerates into a J-shaped exponential curve.

Problem 20

For N=200, r=0.5/year, and K=1000, find the instantaneous rate dN/dt=rN(1−N/K), in individuals/year.
Answer and explanation

80 rabbits/yr

(1,000 - 200)/1,000 = 0.8; 0.5 × 200 × 0.8 = 80. This is an instantaneous rate in the continuous model, not an exact finite-year increase.

Problem 21

A population of 4,000 has 240 births and 40 deaths in a year (ignore migration). What is the annual growth rate, as a percent?
Answer and explanation

5 %

Net change = 240 - 40 = 200; 200/4,000 = 0.05 = 5%.

Problem 22

Which of these is a density-DEPENDENT limiting factor?
  1. Disease spreading through a crowded population
  2. A volcanic eruption
  3. A hurricane that floods the habitat
  4. A sudden hard freeze
Answer and explanation

Disease spreading through a crowded population

Disease spreads faster as density rises, so it intensifies with crowding — a density-dependent factor. The others strike regardless of population size.

Problem 23

Explain why a population may temporarily overshoot its carrying capacity and then crash.
Answer and explanation

If a population grows rapidly, it can climb past K before limiting factors fully catch up. Once there are more individuals than the environment can support, resources run short and deaths spike, so the population crashes back down below K before stabilizing.

Problem 24

For dN/dt=rN with r=0.10/year and N=200, find instantaneous growth. Show the reasoning, not just the conclusion.
Hints

Rates, net numbers and total population are different quantities.

Answer and explanation

20 individuals/year in the exponential model.

Problem 25

In logistic growth, why does growth slow as N approaches K?
Hints

State what the supplied observations or conditions establish, and separate that from any assumption.

Answer and explanation

The model's factor 1−N/K reduces per-capita growth; real mechanisms and K can change with conditions.

Problem 26

Carrying capacity is conditional and can changeCarrying capacity is conditional and can changeLogistic population model N02505007501000036912Time (years)K=1000 here is a fixed model parameter, not an eternal limit.
Calculated logistic curve with N₀=100,r=0.6 yr⁻¹,K=1000; not field observations.
Interpret the supplied visual. For r=0.4 yr⁻¹,K=1000 and N=500, find the instantaneous rate.
Answer and explanation

0.4×500×(1−0.5)=100 individuals/year.

The logistic model uses dN/dt=rN(1−N/K), with K a carrying-capacity parameter under specified conditions. Growth slows as N approaches K in this fixed-parameter model. Real resources, habitat, climate, interactions and disturbance can change K, and delayed responses can produce overshoot or oscillations outside the simple curve. Maximum total logistic growth occurs at N=K/2 for fixed r,K; it is not the same as the highest per-capita growth rate.

0.4×500×(1−0.5)=100 individuals/year.

Response criteria

  • Interpret labels, structures, axes, units and directions correctly.
  • Explain the physical or environmental relationship or mechanism.
  • Respect stated assumptions and distinguish observation from inference.

Problem 27

For continuous logistic growth with r=0.5 yr⁻¹,K=800 and N=200, find dN/dt. Compare with N=400.
Answer and explanation

At200:0.5×200×0.75=75 individuals/year. At400:0.5×400×0.5=100/year. Total growth rises even while per-capita growth falls.

At200:0.5×200×0.75=75 individuals/year.

At400:0.5×400×0.5=100/year.

Total growth rises even while per-capita growth falls.

Response criteria

  • Use the stated measurements or conditions, with correct units.
  • Connect the result to the physical or environmental mechanism or evidence relationship.
  • Identify the assumption, model limit or further evidence requested.

Problem 28

The two climate factors that most determine which terrestrial biome forms in a region are
  1. altitude and soil color
  2. temperature and precipitation
  3. wind speed and day length
  4. latitude and longitude
Answer and explanation

temperature and precipitation

Average temperature and annual precipitation are the master controls that define terrestrial biomes.

Problem 29

A biome with very cold temperatures, a short growing season, and a layer of permanently frozen soil (permafrost) is the
  1. tropical rainforest
  2. hot desert
  3. tundra
  4. temperate grassland
Answer and explanation

tundra

Permafrost and a short, cold growing season are defining features of the tundra.

Problem 30

A tropical rainforest has an NPP of about 2,000 g/m²/yr; a hot desert about 80 g/m²/yr. About how many times greater is the rainforest's productivity?
Answer and explanation

25 times

2,000 ÷ 80 = 25 times greater, driven mainly by abundant water and warmth.

Problem 31

Estuaries, where rivers meet the sea, are among the most productive ecosystems on Earth mainly because
  1. they are extremely deep and dark
  2. they have very high salinity that excludes most life
  3. they never exchange water with the open ocean
  4. they receive abundant nutrients from the river and plenty of sunlight in shallow water
Answer and explanation

they receive abundant nutrients from the river and plenty of sunlight in shallow water

River-borne nutrients plus sunlit shallow water fuel very high productivity, making estuaries rich nurseries.

Problem 32

Explain how a warming climate could shift the location of a biome, and why some species may struggle to keep up.
Answer and explanation

Because biomes are defined by temperature and precipitation, warming shifts those conditions toward the poles and higher elevations, so a biome's suitable zone moves. Species must migrate to track their climate, but many cannot move fast enough, are blocked by cities or mountains, or run out of cooler ground, so their populations decline or the community reorganizes into a new ecosystem.

Problem 33

Why can deep water limit photosynthetic production even when nutrients are available? Show the reasoning, not just the conclusion.
Hints

Use multiple variables at relevant spatial scales.

Answer and explanation

Light availability declines with depth; energy input can become limiting.

Problem 34

How can seasonal rainfall matter despite equal annual totals?
Hints

State what the supplied observations or conditions establish, and separate that from any assumption.

Answer and explanation

Long dry intervals and timing relative to growth affect water availability and survival.

Problem 35

Link a trait to a constraint without inventing a purposeLink a trait to a constraint without inventing a purposeEnvironmentalconditionWater limitationTrait / processReduced leaf area /water lossPerformancetradeoffPossible growth costAn adaptation explanation needs a mechanism and context.A useful trait is not evidence of conscious evolutionary intent.
Original trait–constraint example; no claim that every dryland plant uses the same strategy.
Interpret the supplied visual. Why might a water-saving trait be less favorable in a shaded, wet setting?
Answer and explanation

Its reduced light capture or other costs can outweigh water savings under different constraints.

An adaptation is a heritable feature shaped by evolutionary processes that can improve performance in a relevant environment. Reduced leaf area can reduce water loss but also limit light capture; traits have tradeoffs and depend on context. Acclimation within an individual’s life is different from inherited evolutionary change across generations. Explain how a trait affects exchange, survival or reproduction and what comparison could test that mechanism, rather than saying organisms simply decided what they needed.

Its reduced light capture or other costs can outweigh water savings under different constraints.

Response criteria

  • Interpret labels, structures, axes, units and directions correctly.
  • Explain the physical or environmental relationship or mechanism.
  • Respect stated assumptions and distinguish observation from inference.

Problem 36

A well-mixed 1000 L tank gains 600 mg oxygen/day and loses 900 mg/day with no water-volume change. Find mass and concentration changes per day; state a model limit.
Answer and explanation

Mass change=−300 mg/day; concentration change=−0.30 mg/L/day. The result assumes the stated constant volume, uniform mixing and continuing fluxes; real fluxes can respond to changing conditions.

Mass change=−300 mg/day; concentration change=−0.30 mg/L/day.

The result assumes the stated constant volume, uniform mixing and continuing fluxes; real fluxes can respond to changing conditions.

Response criteria

  • Use the stated measurements or conditions, with correct units.
  • Connect the result to the physical or environmental mechanism or evidence relationship.
  • Identify the assumption, model limit or further evidence requested.

Problem 37

20 individuals in 5 m² gives what density?
Hints

Write the relationship first; substitute with consistent units.

Answer and explanation

4 individuals/m²

20/5=4.

Problem 38

Why randomize quadrat placement?
Answer and explanation

To reduce selection bias and improve representativeness for the defined habitat.

Response criteria

  • State a scientifically accurate conclusion.
  • Show the calculation or mechanism that supports it.
  • Include units, conditions, or limits where relevant.

Problem 39

Two species have fractions .4 and .6. Find D=1−Σpi².
Hints

List the given quantities, select a relationship, and check units before substituting.

Answer and explanation

0.48

1−(.16+.36)=.48.

Problem 40

Why is one large total count not a complete sampling description?
Answer and explanation

Area, effort, detectability, locations, and timing determine its interpretation.

Response criteria

  • State a scientifically accurate conclusion.
  • Show the calculation or mechanism that supports it.
  • Include units, conditions, or limits where relevant.

Problem 41

Five equal 2 m² quadrats contain 30 plants total. Find density. Show the reasoning, not just the conclusion.
Hints

Patchiness and selection method affect uncertainty and representativeness.

Answer and explanation

30/10=3 plants/m².

Problem 42

Why should a mark–recapture estimate state assumptions?
Hints

State what the supplied observations or conditions establish, and separate that from any assumption.

Answer and explanation

Movement, survival, marking effects and mixing can bias estimates if the model's conditions fail.

Problem 43

Density divides total count by the actual sampled areaDensity divides total count by the actual sampled areaQuadratAreaIndividualsA0.25 m²3B0.25 m²5C0.25 m²4Density=12/0.75=16 individuals/m²Mean count per quadrat is 4, not 4 individuals/m².
Illustrative quadrat records with explicit areas; no extrapolated field total is asserted.
Interpret the supplied visual. If the same 12 individuals came from three 1 m² quadrats, what density would result?
Answer and explanation

4 individuals/m²; the sampled area changes the denominator.

Density is individuals per area or volume, not merely an average count per container. Three 0.25 m² quadrats totaling 12 individuals cover 0.75 m², giving 16 individuals/m². A whole-field abundance estimate multiplies by a relevant area only if representativeness is justified. Use total sampled area when quadrat sizes differ, and avoid treating a handful of plots as an exact census. Report variability and selection limits alongside the estimate.

4 individuals/m²; the sampled area changes the denominator.

Response criteria

  • Interpret labels, structures, axes, units and directions correctly.
  • Explain the physical or environmental relationship or mechanism.
  • Respect stated assumptions and distinguish observation from inference.

Problem 44

Four 0.50 m² quadrats contain 2,4,6,4 individuals. Find pooled density and an abundance estimate for 100 m² if sampling is representative; state why it is an estimate.
Answer and explanation

Total 16 over 2.0 m² gives 8 individuals/m². Extrapolated abundance≈800. Spatial variation, finite sampling and imperfect detection make it an estimate rather than a census.

Total 16 over 2.0 m² gives 8 individuals/m².

Extrapolated abundance≈800.

Spatial variation, finite sampling and imperfect detection make it an estimate rather than a census.

Response criteria

  • Use the stated measurements or conditions, with correct units.
  • Connect the result to the physical or environmental mechanism or evidence relationship.
  • Identify the assumption, model limit or further evidence requested.

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