Ecosystems & Energy Flow — Science

  1. Grade 6 — Ecosystems, Habitats & Biomes
  2. Grade 6 — Food Chains & Food Webs
  3. Grade 7 — Energy Flow & Food Webs
  4. Grade 7 — Energy Pyramids
  5. Grade 7 — Interactions & Ecosystem Stability
  6. Grade 10 — Populations & Communities
  7. Grade 10 — Energy Flow in Ecosystems
  8. Grade 12 — Ecosystem Structure & Energy Flow
  9. Grade 12 — Biodiversity & Ecosystem Services
  10. Grade 12 — Population Ecology
  11. Grade 12 — Biomes & Aquatic Systems

Apply it in a new setting

Problem 1

In a meadow, the producers (grasses) capture 60,000 kcal of energy per square meter each year. Grasshoppers (primary consumers) eat the grasses, and shrews (secondary consumers) eat the grasshoppers. Using the 10% rule for energy transfer between trophic levels, how much energy is available to the shrews?
Hints

Each step up a trophic level passes only about 10% of the energy to the next level.

Producers → primary consumers → secondary consumers is two energy transfers.

Multiply 60,000 by 0.10, then by 0.10 again.

Answer and explanation

600 kcal

Grasses → grasshoppers: 60,000 × 0.10 = 6,000 kcal reaches the primary consumers.

Grasshoppers → shrews: 6,000 × 0.10 = 600 kcal reaches the secondary consumers.

Energy available to the shrews is 600 kcal.

Problem 2

A hawk sits at the top of the food chain grass → mouse → snake → hawk. Ecologists estimate the hawks in an area need 30 kcal of energy. If only 10% of the energy passes to each higher trophic level, how much energy must the grasses (producers) originally capture to support that 30 kcal at the hawk level?
Hints

Count the number of energy transfers from the producers up to the hawk.

Each transfer keeps only 10%, so working backward you multiply by 10 at each step.

There are three transfers: multiply 30 by 10 three times.

Answer and explanation

30000 kcal

From grass to hawk there are three transfers: grass→mouse, mouse→snake, snake→hawk.

Working backward, each lower level holds 10 times as much energy: 30 → 300 → 3,000 → 30,000 kcal.

The grasses must capture 30000 kcal.

Problem 3

A wildlife biologist tracks a deer population that starts the year at 800. Over the year she records 240 births, 160 deaths, 30 deer immigrating in, and 50 deer emigrating out. What is the deer population at the end of the year?
Hints

Population change = (births + immigrants) − (deaths + emigrants).

Additions: 240 births + 30 immigrants. Removals: 160 deaths + 50 emigrants.

Add the net change to the starting population of 800.

Answer and explanation

860 deer

Additions: 240 births + 30 immigrants = 270.

Removals: 160 deaths + 50 emigrants = 210.

Net change = 270 − 210 = +60, so the population is 800 + 60 = 860 deer.

Problem 4

Under ideal conditions with unlimited resources, a population of 25 bacteria doubles every generation. If this exponential growth continues for 6 generations, how many bacteria will there be?
Hints

Doubling each generation means multiplying by 2 over and over.

After 6 generations the starting number is multiplied by 2 six times, a factor of 2^6.

2^6 = 64; multiply that by the starting 25.

Answer and explanation

1600 bacteria

With doubling, the population is multiplied by 2 once per generation, so after 6 generations it grows by a factor of 2^6.

2^6 = 64, so 25 × 64 = 1,600.

After 6 generations there are 1600 bacteria.

Problem 5

In a kelp forest, sea otters eat sea urchins, and sea urchins graze on kelp. A disease wipes out most of the sea otters. Which outcome is the most likely result?
  1. The kelp forest expands, because fewer otters means less disturbance to the kelp.
  2. Sea urchin numbers fall, because they depended on the otters for food.
  3. Sea urchin numbers rise and overgraze the kelp, shrinking the kelp forest.
  4. Nothing changes, because removing one species cannot affect the others.
Hints

Trace the chain: otters eat urchins, and urchins eat kelp.

With fewer otters, what happens to the urchin population?

Then ask what a larger number of urchins does to the kelp.

Answer and explanation

Sea urchin numbers rise and overgraze the kelp, shrinking the kelp forest.

Otters normally keep the urchin population in check.

With the otters gone, urchins increase and graze down far more kelp.

The kelp forest shrinks — the correct answer is C.

Problem 6

Most food chains have only 4 or 5 trophic levels and rarely more. Which statement best explains why?
  1. Top predators are simply too large physically to fit any more levels above them.
  2. About 90% of the energy is lost as heat and life processes at each transfer, so too little is left to support higher levels.
  3. There are not enough different species on Earth to build longer chains.
  4. Energy increases at each level, so the chain becomes unstable and collapses.
Hints

Think about how much energy actually passes from one level to the next.

Only about 10% moves up; the other ~90% is lost.

After several transfers, is there enough energy left to support yet another level?

Answer and explanation

About 90% of the energy is lost as heat and life processes at each transfer, so too little is left to support higher levels.

Each transfer passes only about 10% of the energy; roughly 90% is lost as heat and used for life processes.

After 4 or 5 levels, so little energy remains that it cannot support another level.

The energy lost at each step is the limit — the correct answer is B.

Problem 7

A population of rabbits introduced to an island grows rapidly at first, then its growth slows and the population levels off, staying near 500. Which statement best explains why the population stopped growing?
  1. The rabbits reached their maximum possible body size and so could no longer reproduce.
  2. Populations always stop growing after a fixed number of generations, regardless of resources.
  3. The rabbits ran out of genetic variation, which halted all births.
  4. As the population nears the island's carrying capacity, limited food and space slow reproduction and raise the death rate until births and deaths balance.
Hints

The value it levels off at (500) is a clue about the environment's limits.

What environmental factors get scarce as a population grows large?

Growth stops when the birth rate and death rate become equal.

Answer and explanation

As the population nears the island's carrying capacity, limited food and space slow reproduction and raise the death rate until births and deaths balance.

Resources such as food and space are limited, which sets a carrying capacity.

As the population approaches it, deaths rise and births fall until the two balance, near 500.

This is density-dependent limitation at the carrying capacity — the correct answer is D.

Browse all courses