This unit is about how living things interact with each other and with their nonliving surroundings, from a single organism up to the whole biosphere. By the end, you should be able to classify ecological levels and species interactions, explain what limits a population, and use the 10% rule to track energy through a food web.
Read the overview first, then work through each topic's notes and checkpoint question before reviewing the common mistakes and key terms at the bottom.
Ecology is the study of how organisms interact with their environment, which includes both other living things and nonliving conditions. This unit builds a vocabulary for describing those interactions and then follows energy as it moves from producers to top predators. Expect a mix of classification questions, diagram reading and simple energy calculations.
Honors Biology Unit 17 covers three connected topics. The first topic covers levels of ecological organization, from organism to biosphere, and the difference between a habitat and a niche. The second covers biotic and abiotic factors, population limits, and species interactions such as competition, predation and symbiosis. The third follows energy through food webs and trophic levels. The 10% rule estimates that about a tenth of the energy at one level reaches the next.
A population is one species in one place, and a community is all the species in that place. An ecosystem adds the abiotic factors like sunlight, water and temperature. Knowing exactly what each level includes lets you name the scale of any ecological study.
Abiotic factors like water and temperature affect which organisms can live somewhere. Limited resources and other conditions can restrict the size of a population. Species also affect each other through competition, predation and symbiosis. You can classify each interaction by asking who benefits and who is harmed.
Most producers capture energy from sunlight, while some use chemical energy. Consumers get energy by eating. About 10% of the energy at one trophic level is typically stored in the next. Some energy leaves as heat, and some remains in uneaten material or waste. That loss explains why food chains are short and top predators are rare.
Organisms interact with one another and with their physical surroundings. Those interactions affect who eats whom, which species can share a space, and how large a population can grow. Energy flows one way through an ecosystem, and less is available at each successive trophic level. Most ecosystems get that energy from sunlight, but some rely on chemical energy.
Ecologists organize life into levels that describe increasingly broad scales, from an individual organism to the biosphere.
Biotic factors are the living parts of an ecosystem.
Energy enters almost every ecosystem as sunlight.
Open this guide for a closer review of the topic.
Ecologists organize life into levels that describe increasingly broad scales, from an individual organism to the biosphere. These levels are useful categories, though their boundaries do not always fit neatly inside one another. An organism is one individual, like a single largemouth bass. A population is all the individuals of one species in one area at the same time, like every bass in one lake. A community is all the populations of different species in that area, so the bass plus the frogs, algae and dragonfly larvae.
The jump from community to ecosystem is where abiotic factors enter. An ecosystem is the community plus the nonliving parts it depends on, such as water temperature, sunlight and dissolved oxygen. A biome is a large region with similar climate and similar kinds of communities, such as tundra, desert or tropical rainforest. Temperature and precipitation mostly decide which biome forms. The biosphere is every place on Earth where life exists.
Ecologists often describe populations by density, the number of individuals per unit of area or volume.
Worked example. You place five 1 m² quadrats at random in a 200 m² field and count dandelions in each one. You get 4, 6, 3, 7 and 5. That's 25 dandelions in 5 m², so density = per m². The estimated population is dandelions. Random placement helps reduce sampling bias, but five quadrats may not capture the variation across the whole field. More randomly placed quadrats can improve the estimate.
Check yourself: A biologist studies how water temperature affects every species living on a coral reef. Which level of organization is she working at? (Answer: Ecosystem, because the study includes an abiotic factor along with the whole community.)
Biotic factors are the living parts of an ecosystem. Abiotic factors are the nonliving parts, like temperature, light, water, pH and soil minerals. The two shape each other. A forest canopy (biotic) cuts the light reaching the ground (abiotic), which favors shade-tolerant plants.
A limiting factor is a resource or condition that restricts population growth. For example, a shortage of water can limit a desert population, while disease or extreme temperatures can also limit growth. Limiting factors affect carrying capacity (K), the population size an environment can sustain over time. K can change as conditions change. If a population exceeds K, it may decline through deaths or emigration.
Species also interact with each other. In competition, two organisms need the same limited resource, and both end up worse off. In predation, a predator kills and eats its prey. Symbiosis is a close, long-term relationship between two species. Its types are classified by whether each species benefits, is harmed or is unaffected. Mycorrhizal fungi help plant roots absorb water and minerals and get sugars in return, so both gain (mutualism). Barnacles riding on a whale get transportation while the whale is basically unaffected (commensalism). A tapeworm takes nutrients from its host's intestine, harming the host without usually killing it (parasitism).
Check yourself: Orchids grow on high tree branches to reach sunlight but take no nutrients from the tree. What type of relationship is this? (Answer: Commensalism, because the orchid benefits and the tree is unaffected.)
| Interaction | Species A | Species B | Example |
|---|---|---|---|
| Mutualism | Benefits | Benefits | Mycorrhizal fungi and plant roots |
| Commensalism | Benefits | Unaffected | Barnacles on a whale |
| Parasitism | Benefits | Harmed | Tapeworm in a mammal's intestine |
| Predation | Benefits | Harmed (killed) | Fox eating a rabbit |
| Competition | Harmed | Harmed | Two plant species shading each other |
Energy enters almost every ecosystem as sunlight. Producers (autotrophs) like plants, algae and cyanobacteria capture it through photosynthesis and store it in sugars. Some bacteria at deep-sea hydrothermal vents use chemosynthesis instead, getting energy from chemicals like hydrogen sulfide. Consumers (heterotrophs) get energy by eating. Primary consumers eat producers, secondary consumers eat primary consumers, and tertiary consumers eat secondary consumers. Decomposers like bacteria and fungi break down dead matter from every level and return nutrients to the soil.
A food chain shows one path, like grass → rabbit → fox → hawk. A food web shows many overlapping chains, which is more realistic because most animals eat more than one thing. Omnivores can sit at more than one trophic level. A bear eating berries is a primary consumer, but a bear eating salmon is a higher-level consumer.
About 10% of the energy at one trophic level is typically stored in biomass at the next level. Some energy leaves as heat during cellular respiration, while some remains in uneaten material or waste and can pass to decomposers. Energy moves one way and is not recycled, while nutrients do cycle.
Worked example. Producers in a meadow store 20,000 kcal in new biomass over a year. Using 10% transfer at each step for the same period, primary consumers store kcal, secondary consumers store kcal, and tertiary consumers store kcal. That's of the energy stored by producers. Less available energy helps explain why food chains are often short.
Check yourself: Secondary consumers store 500 kcal in new biomass over a year. Assuming 10% transfer at each step, about how much energy did producers store in new biomass over that period? (Answer: About 50,000 kcal, because you go back two levels: 500 × 10 × 10.)
| Pyramid type | What it measures | Can it be upside down? |
|---|---|---|
| Energy pyramid | Energy transferred to each trophic level per unit area over a period, such as kcal/m² per year | Never, because energy is lost at every transfer |
| Biomass pyramid | Total mass of living things at each level at one moment | Yes, in some oceans fast-reproducing phytoplankton weigh less than the zooplankton eating them |
| Pyramid of numbers | Count of individuals at each level | Yes, one oak tree can feed thousands of caterpillars |
| Term | Definition |
|---|---|
| Ecology | The study of how organisms interact with each other and with their physical environment. |
| Biotic factor | Any living part of an ecosystem, such as plants, animals, fungi or bacteria. |
| Abiotic factor | Any nonliving part of an ecosystem, such as temperature, sunlight, water, pH or soil minerals. |
| Biome | A large region with a similar climate and similar plant and animal communities, such as tundra or desert. |
| Niche | The role a species plays in its ecosystem, including its food, activity patterns and tolerated conditions. |
| Carrying capacity | The maximum population an environment can sustain over time, written as K. |
| Mutualism | A symbiotic relationship in which both species benefit. |
| Trophic level | The feeding position of an organism in a food chain or web. |
| Energy pyramid | A diagram showing the energy available at each trophic level, always widest at the producer base. |
Both describe organisms living in one place, so the words blur together. A population is always one species, like all the deer in a forest. A community is every species there, so deer plus oaks, wolves, fungi and everything else.
Habitat is where an organism lives, and niche is how it lives there. Hawks and owls share the same forest habitat, but hawks hunt by day and owls by night, so their niches differ. If a question asks about diet, timing or role, it's asking about niche.
Decomposers recycle nutrients like carbon and nitrogen back to producers, so it's tempting to assume energy gets recycled too. It doesn't. Energy leaves as heat at every level, so ecosystems need a continuing energy input. Most get it from sunlight, while some get it from chemicals.
Many people read the arrow as "eats," so they draw fox → rabbit. Arrows show the direction energy moves, from the food to the eater. The correct chain is rabbit → fox.