GED Science Ecosystems and Energy Flow Summary

A producer makes its own food by photosynthesis. In a food web, producers have arrows going out to the animals that eat them, but no arrows coming in from food. Grass is the only producer in this web.

Wrong choices

• Hawk: The hawk eats other animals, so it is a consumer.
• Snake: The snake eats frogs and mice, so it is a consumer.
• Grasshopper: The grasshopper eats grass, so it is a consumer.

In this food web, the only arrow into the frog comes from the grasshopper. The frog has no other food, so if most grasshoppers die, the frog population is likely to drop first.

Wrong choices

• Snake: The snake also eats mice, so it still has food. It may be affected later, but the frog loses its only food.
• Grass: Grasshoppers eat grass, so fewer grasshoppers would most likely mean more grass, not less.
• Rabbit: Rabbits eat grass, not grasshoppers. With fewer grasshoppers eating grass, rabbits may even have more food.

Both partners benefit: the bee gets food (nectar), and the flower gets pollinated. A relationship where both sides benefit is called mutualism.

Wrong choices

• Parasitism: In parasitism, one organism is harmed. Here both the bee and the flower benefit.
• Predation: In predation, one animal kills and eats another. The bee does not eat the flower.
• Commensalism: In commensalism, one benefits and the other is not affected. Here both benefit.

A parasite lives on or in another organism (the host) and takes food from it, harming the host. The tick is the parasite and the deer is the host.

Wrong choices

• Commensalism: In commensalism, the host is not harmed. Here the deer is harmed.
• Competition: Competition is when organisms fight over the same resource, such as food. The tick is feeding on the deer itself.
• Mutualism: In mutualism, both benefit. The deer does not benefit.

An invasive species is not native to an area. It is brought in, often by people, and it spreads quickly, often because it has no natural predators there. It can crowd out native species and disrupt the ecosystem.

Wrong choices

• A species that has lived in an area for a very long time: A species that has always lived in an area is called a native species.
• A species that has gone extinct: An extinct species no longer exists, so it cannot spread.
• Any animal that eats only plants: An animal that eats only plants is an herbivore. That is not what invasive means.

Carrying capacity is the largest population an environment can support over time with its food, water, and space. When a population levels off and stays near one number, it has reached the carrying capacity.

Wrong choices

• The number of predators on the island: The number 500 is a count of deer, not of predators.
• The deer's birth rate each year: A birth rate is how many are born in a time period. The level-off number is a total population size.
• The number of deer first brought to the island: The population started small and grew. 500 is where it stopped growing, not where it started.

Predators keep prey numbers down. When the lynx population drops, fewer hares are eaten, so more hares survive and reproduce. The hare population is likely to grow until food or other limits slow it down.

Wrong choices

• It will stay exactly the same.: Populations change when predators change. A large drop in predators is likely to change the hare population.
• It will decrease.: With fewer predators, fewer hares are eaten, so the population is more likely to grow.
• It will die out completely.: Losing a predator does not wipe out the prey. More hares would survive.

Decomposers break down dead plants and animals. This releases nutrients back into the soil, where producers can use them again. Without decomposers, matter would not be recycled in the ecosystem.

Wrong choices

• They are primary consumers that eat living grass.: Primary consumers eat living plants. These organisms feed on dead leaves and logs.
• They are producers that make food from sunlight.: Producers make food by photosynthesis. Fungi and these bacteria get their food from dead material.
• They are top predators that control other populations.: Top predators hunt living animals. These organisms break down dead matter.

1. Grass → grasshopper: 10% of 20,000 = 2,000 kcal.
2. Grasshopper → frog: 10% of 2,000 = 200 kcal.

Check: 200 × 10 × 10 = 20,000. Most of the energy at each level is used for life processes or lost as heat.

Wrong choices

• 1,000 kcal: 1,000 kcal does not come from taking 10% at each step.
• 2,000 kcal: 2,000 kcal is the energy that reaches the grasshopper. One more 10% step is needed to reach the frog.
• 20 kcal: 20 kcal is one step too many. That would be the energy at the level after the frog.

1. In the second catch, 10 of 50 fish were tagged, which is 1/5 (20%).
2. If the 40 tagged fish are also 1/5 of the whole pond, the pond has 40 × 5 = 200 fish.

Check: 40 ÷ 200 = 0.2 and 10 ÷ 50 = 0.2.

Wrong choices

• 100: 100 does not match the proportion. With 100 fish, 40 tagged fish would be 40% of the pond, but only 20% of the second catch was tagged.
• 90: 90 is just 40 + 50. Some fish were caught twice, and many were never caught.
• 500: 500 comes from 50 × 10, which is not the correct proportion.