This cheat sheet covers how ecosystems work, how organisms depend on each other, and why biodiversity matters. Students need these ideas to understand food chains, food webs, energy flow, and changes in populations. It connects living things with nonliving factors such as sunlight, water, soil, and temperature.
These concepts help explain how ecosystems stay balanced or become disrupted.
Key Facts
- Population density = number of individuals ÷ area, such as 120 deer ÷ 30 square kilometers = 4 deer per square kilometer.
- A food chain shows one path of energy flow, such as grass -> rabbit -> fox.
- A food web shows many connected food chains and gives a more realistic picture of feeding relationships.
- Only about 10% of energy passes from one trophic level to the next, so energy at next level = energy at current level × 0.10.
- Producers make their own food using sunlight, water, and carbon dioxide through photosynthesis.
- Consumers get energy by eating producers or other consumers, and decomposers recycle nutrients from dead organisms and waste.
- Biodiversity includes species richness, which is the number of different species, and species evenness, which is how balanced their populations are.
- A stable ecosystem usually has high biodiversity because more species can help maintain food web balance after a disturbance.
Vocabulary
- Ecosystem
- A community of living organisms interacting with each other and with nonliving parts of their environment.
- Biodiversity
- The variety of living things in an area, including different species, genes, and ecosystems.
- Producer
- An organism, such as a plant or algae, that makes its own food and forms the base of most food webs.
- Consumer
- An organism that gets energy by eating plants, animals, fungi, or other organisms.
- Decomposer
- An organism, such as a fungus or bacterium, that breaks down dead matter and returns nutrients to the ecosystem.
- Trophic Level
- A feeding position in a food chain or food web, such as producer, primary consumer, or secondary consumer.
Common Mistakes to Avoid
- Confusing food chains with food webs is wrong because a food chain shows one feeding path, while a food web shows many connected feeding paths.
- Drawing arrows toward what gets eaten is wrong because arrows show the direction energy moves, from the food source to the organism that eats it.
- Assuming all energy transfers to the next trophic level is wrong because only about 10% of energy is passed on and most is used or lost as heat.
- Ignoring decomposers is wrong because decomposers recycle nutrients that producers need to grow and keep the ecosystem functioning.
- Thinking high biodiversity only means many individuals is wrong because biodiversity depends on the number of species and how evenly individuals are spread among them.
Practice Questions
- 1 A pond has 240 frogs living in an area of 60 square meters. What is the frog population density?
- 2 If producers in an energy pyramid have 10,000 energy units, about how many energy units are available to the primary consumers?
- 3 In the food chain algae -> small fish -> heron, identify the producer, primary consumer, and secondary consumer.
- 4 A disease removes most of one plant species from a meadow. Explain why a meadow with higher biodiversity may recover better than a meadow with low biodiversity.
Understanding Ecosystems Biodiversity and Food Webs
Every organism has a niche, meaning its particular job and place in its habitat. A bee may pollinate flowers, eat nectar, and provide food for birds or spiders. Two species can live in the same area but use different resources.
This reduces direct competition. When species need the same limited food, water, shelter, or nesting space, competition becomes stronger. Predators can limit prey numbers.
Prey availability can limit predator numbers. Some relationships benefit both organisms, such as fungi living with plant roots.
Other relationships harm one organism, such as parasites feeding on a host. These links explain why a change to one species can affect many others.
Energy moves through living systems in one direction and is gradually lost as heat during life processes. Animals use much of the energy they take in for movement, body temperature, growth, and repair. This leaves less energy available to organisms that eat them.
For this reason, large predators are usually rare and food chains tend to have only a few steps. A field can support many insects, fewer small birds, and very few hawks. Biomass often follows the same pattern.
Biomass means the total mass of living material at a feeding level. Energy does not cycle back, but matter does. Carbon, water, nitrogen, and other materials return to soil, air, and water when organisms produce waste or die.
Population size changes because births, deaths, immigration, and emigration do not stay equal. Immigration means individuals move into an area. Emigration means they leave it.
A population may grow quickly when resources are plentiful. Growth slows when food runs short, diseases spread, predators increase, or space becomes crowded. The largest population an environment can support over time is called carrying capacity.
It is not a fixed number. A drought can reduce plant growth and lower the number of herbivores an area supports. A wet year may raise it for a while.
Students should notice that an average population density can hide important patterns. Deer may be clustered near a river rather than spread evenly across a whole forest.
Biodiversity is more than a long species list. A community can contain many species while being dominated by one of them. A more balanced community often has several species with useful overlapping roles.
If one pollinator declines, another may still pollinate some plants. If a disease affects one tree species, other trees may continue to hold soil and provide shade. This does not mean every loss has no effect.
Removing a key species can trigger a trophic cascade, where changes spread through several feeding levels. Sea otters, wolves, bees, and decomposers can have effects far beyond their numbers. Habitat loss, pollution, overharvesting, climate shifts, and introduced species can reduce biodiversity.
When studying a food web, track both direct feeding links and indirect effects. Look for which resources are limited, which organisms have few alternatives, and which changes could spread through the whole system.