Ecosystems — Answer Key
Part A: Multiple Choice
Circle the best answer for each question.
1. A desert ecosystem and a tropical forest ecosystem both have producers, consumers, and decomposers. What is the main reason the desert has fewer organisms?
A) Deserts have no sunlight for producers to use for photosynthesis.
B) Deserts receive very little rainfall, which limits how many organisms can survive.
C) Deserts have no soil, so decomposers cannot return nutrients to the ground.
D) Deserts are too close to the equator for most organisms to live there.
Rainfall is the key limiting factor in deserts. Little water means few plants, which limits the entire food chain.
2. In an ocean ecosystem, tiny organisms called phytoplankton produce most of the oxygen. What would happen if phytoplankton populations crashed?
A) Ocean consumers would simply switch to eating seaweed instead.
B) Oxygen levels in the atmosphere would drop and ocean food webs would collapse.
C) Only deep-sea organisms would be affected because phytoplankton live on the surface.
D) Land ecosystems would not be affected because they have their own producers.
Phytoplankton produce ~50% of Earth's oxygen and are the base of ocean food chains. Their loss would be catastrophic.
3. A grassland ecosystem has grass, mice, snakes, and hawks. Which statement best explains why there are more mice than hawks?
A) Mice reproduce faster than hawks, so their numbers are always higher.
B) Energy decreases at each level of the food chain, so fewer top predators can be supported.
C) Hawks eat less food than mice, so they do not need a large population.
D) Mice are smaller and need less space, so more of them fit in the ecosystem.
The 10% energy rule means each higher level has less energy to support fewer individuals. Top predators always have the smallest population.
4. Two ponds are next to each other. Pond A has many different species of fish, insects, and plants. Pond B has only three species. Which pond would recover better after a drought and why?
A) Pond B, because fewer species means less competition for the remaining water.
B) Pond A, because greater biodiversity means more species can fill different roles during recovery.
C) Both ponds would recover equally because droughts affect all ecosystems the same way.
D) Pond B, because the three species would have evolved to survive droughts better.
Biodiversity provides resilience. If one species is lost, others can fill its role. Low-diversity ecosystems are fragile.
Part B: Fill in the Blank
Write the correct answer on each line.
1. A biome is a large area with a specific climate and set of organisms, like a desert or forest.
A biome is defined by climate (temperature and precipitation) and the characteristic plant and animal communities.
2. Biodiversity means having many different species of organisms in an ecosystem.
Biodiversity = variety of species. High biodiversity means more resilient, stable ecosystems.
3. Organisms develop physical and behavioral adaptations to survive in their specific ecosystem.
Adaptations are traits evolved over generations that help organisms survive and reproduce in their environment.
4. The tundra biome is very cold and has frozen soil called permafrost that stays frozen year-round.
Permafrost is permanently frozen subsoil found in tundra and polar regions, limiting what can grow.
5. Coral reefs are ocean ecosystems with high biodiversity that support thousands of species.
Coral reefs are among Earth's most biodiverse ecosystems, often called the 'rainforests of the sea.'