U1.5 Food Chains and Food Webs
Master AP Environmental Science topic 1.11: food chains vs. food webs, keystone species removal effects, and how toxins biomagnify up trophic levels.
What you'll do in this lesson
A voice-first session with the Crimsora tutor on U1.5 Food Chains and Food Webs, then targeted practice and FRQs — with the tutor adapting to where you get stuck.
What this lesson covers
Energy and matter don't move through ecosystems in tidy straight lines. In this lesson you'll learn why a food web is a more realistic model than a single food chain, how one species can hold an entire community together, and why the top predators in an ecosystem often carry the highest loads of toxic chemicals like DDT and mercury.
These three ideas — trophic structure, keystone species, and biomagnification — show up constantly on the AP exam, often together in a single food-web diagram question. By the end you'll be able to trace energy through a web, predict a cascade of effects when a species disappears, and explain the difference between two concepts students frequently confuse: bioaccumulation and biomagnification.
These three ideas — trophic structure, keystone species, and biomagnification — show up constantly on the AP exam, often together in a single food-web diagram question. By the end you'll be able to trace energy through a web, predict a cascade of effects when a species disappears, and explain the difference between two concepts students frequently confuse: bioaccumulation and biomagnification.
Food Chains vs. Food Webs
A food chain is a single, linear pathway showing who eats whom, moving energy from one trophic level to the next. A simple chain might run grass to grasshopper to frog to snake to hawk. Each arrow points in the direction energy flows — from the organism being eaten toward the organism eating it. This is a common test trap: the arrow always points toward the consumer, not the food.
The problem with a food chain is that it oversimplifies. Real organisms eat many things and are eaten by many things. A food web connects multiple overlapping food chains into a realistic network. A hawk in a web might eat snakes, mice, and small birds, giving it several energy sources.
The key exam insight is stability. In a food web, if one prey species declines, predators can switch to alternate food sources, so the community absorbs the shock. In a single food chain, losing one link can collapse everything above it. When a question asks why food webs make ecosystems more resilient than food chains suggest, the answer is redundancy — multiple feeding pathways.
The problem with a food chain is that it oversimplifies. Real organisms eat many things and are eaten by many things. A food web connects multiple overlapping food chains into a realistic network. A hawk in a web might eat snakes, mice, and small birds, giving it several energy sources.
| Feature | Food chain | Food web |
|---|---|---|
| Structure | Single linear path | Interconnected network |
| Realism | Simplified | Realistic |
| Stability shown | Fragile | Resilient, with alternatives |
Trophic Levels and Energy Flow Recap
Although energy flow was covered in the previous lesson, food-web questions require you to correctly assign trophic levels. Producers (autotrophs) occupy the first level. Primary consumers (herbivores) are the second, secondary consumers the third, and tertiary consumers the fourth. Many organisms occupy more than one level — an omnivore that eats both plants and animals functions at several levels at once.
Remember the roughly 10 percent rule: only about 10 percent of the energy at one trophic level is passed to the next. This limits food chains to typically four or five links, because energy runs out. On a food web, this explains why top predators are relatively few in number — they sit atop a large energy pyramid.
This energy limitation also connects directly to biomagnification. Because it takes an enormous biomass of producers to support a small biomass of top predators, any toxin that doesn't break down becomes concentrated as it moves up. A top predator effectively eats the toxins accumulated across the entire pyramid beneath it. Keep the direction straight: energy decreases moving up trophic levels, while persistent toxins increase in concentration moving up.
Remember the roughly 10 percent rule: only about 10 percent of the energy at one trophic level is passed to the next. This limits food chains to typically four or five links, because energy runs out. On a food web, this explains why top predators are relatively few in number — they sit atop a large energy pyramid.
This energy limitation also connects directly to biomagnification. Because it takes an enormous biomass of producers to support a small biomass of top predators, any toxin that doesn't break down becomes concentrated as it moves up. A top predator effectively eats the toxins accumulated across the entire pyramid beneath it. Keep the direction straight: energy decreases moving up trophic levels, while persistent toxins increase in concentration moving up.
Keystone Species
A keystone species has an effect on its community that is disproportionately large relative to its abundance. It is not the most numerous species and not necessarily the top predator by biomass — it is the species whose presence maintains the structure and diversity of the whole ecosystem. Remove it, and the community reorganizes dramatically.
The classic example is the sea otter. Otters eat sea urchins; urchins eat kelp. When otters are removed, urchin populations explode and graze kelp forests down to barren rock, destroying habitat for countless species. Another example is the sea star Pisaster, whose removal in a famous experiment let mussels outcompete and crowd out other intertidal species, cutting diversity sharply.
A trophic cascade is the chain of indirect effects that ripples through an ecosystem when a keystone predator is added or removed. On the exam, you may be given a food web and asked to predict what happens after a keystone species is removed. Trace the effects step by step: the prey it controlled increases, then whatever that prey consumes decreases, and so on down the web. Do not confuse a keystone species with a dominant species (high biomass) or an indicator species (signals environmental conditions). The defining trait is large influence despite low abundance.
The classic example is the sea otter. Otters eat sea urchins; urchins eat kelp. When otters are removed, urchin populations explode and graze kelp forests down to barren rock, destroying habitat for countless species. Another example is the sea star Pisaster, whose removal in a famous experiment let mussels outcompete and crowd out other intertidal species, cutting diversity sharply.
A trophic cascade is the chain of indirect effects that ripples through an ecosystem when a keystone predator is added or removed. On the exam, you may be given a food web and asked to predict what happens after a keystone species is removed. Trace the effects step by step: the prey it controlled increases, then whatever that prey consumes decreases, and so on down the web. Do not confuse a keystone species with a dominant species (high biomass) or an indicator species (signals environmental conditions). The defining trait is large influence despite low abundance.
Bioaccumulation and Biomagnification
These two terms sound alike and are constantly mixed up. Bioaccumulation happens within a single organism: a persistent, fat-soluble toxin like DDT, mercury, or PCBs builds up in that individual's tissues over its lifetime because the body absorbs it faster than it can excrete it. Biomagnification happens across trophic levels: the toxin concentration increases at each higher level of the food chain.
Biomagnification occurs because a predator eats many contaminated prey, absorbing all their stored toxins, while excreting relatively little. Toxins that biomagnify share key traits: they are persistent (don't break down), fat-soluble (stored in tissue rather than excreted), and biologically active. The result is that top predators — bald eagles, ospreys, tuna, humans — carry the highest concentrations. DDT famously thinned the eggshells of predatory birds, nearly wiping out bald eagles and peregrine falcons before it was banned. A common exam question shows toxin concentrations at each level and asks you to identify the top predator (highest concentration) or explain why. The mechanism, not just the term, earns the point.
| Concept | Where it occurs | Direction |
|---|---|---|
| Bioaccumulation | Within one organism over time | Builds up in tissues |
| Biomagnification | Up the food chain, level to level | Increases each level |
Key terms
- Food chain.
- A single linear sequence showing energy transfer from one organism to the next, with arrows pointing toward the consumer.
- Food web.
- A network of interconnected food chains that models the multiple feeding relationships in a real ecosystem.
- Trophic level.
- A feeding position in a food chain or web, such as producer, primary consumer, or secondary consumer.
- Keystone species.
- A species with a disproportionately large effect on its community relative to its abundance; its removal restructures the ecosystem.
- Trophic cascade.
- A series of indirect effects rippling through an ecosystem when a species, often a keystone predator, is added or removed.
- Bioaccumulation.
- The buildup of a persistent toxin within a single organism's tissues over its lifetime.
- Biomagnification.
- The increasing concentration of a persistent toxin at each successive trophic level of a food chain.
Worked example
A lake food web has phytoplankton (0.01 ppm DDT) eaten by zooplankton, which are eaten by minnows, which are eaten by lake trout, which are eaten by ospreys. Explain what happens to DDT concentration up this web, name the process, and predict the effect on ospreys.
First, identify the trophic levels: phytoplankton are producers, zooplankton primary consumers, minnows secondary consumers, trout tertiary consumers, and ospreys the top predator.
DDT is persistent and fat-soluble, so it is not broken down or easily excreted. Within each organism it undergoes bioaccumulation. As each predator eats many contaminated prey, it takes in all the DDT stored across those prey while excreting little, so concentration rises at every step — this is biomagnification. Concentrations might climb roughly from ppm in phytoplankton to , then , then , then over ppm in ospreys.
Because ospreys sit at the top, they accumulate the highest concentration. DDT interferes with calcium deposition, thinning eggshells so eggs break during incubation. The predicted effect is reproductive failure and a declining osprey population.
The two-part exam skill here is naming the correct process (biomagnification up the web, bioaccumulation within each animal) and explaining the mechanism: persistence plus fat solubility plus predators consuming many prey.
DDT is persistent and fat-soluble, so it is not broken down or easily excreted. Within each organism it undergoes bioaccumulation. As each predator eats many contaminated prey, it takes in all the DDT stored across those prey while excreting little, so concentration rises at every step — this is biomagnification. Concentrations might climb roughly from ppm in phytoplankton to , then , then , then over ppm in ospreys.
Because ospreys sit at the top, they accumulate the highest concentration. DDT interferes with calcium deposition, thinning eggshells so eggs break during incubation. The predicted effect is reproductive failure and a declining osprey population.
The two-part exam skill here is naming the correct process (biomagnification up the web, bioaccumulation within each animal) and explaining the mechanism: persistence plus fat solubility plus predators consuming many prey.
Practice questions
In a kelp forest ecosystem, sea otters prey on sea urchins, which graze on kelp. If sea otters are removed, which outcome is most likely?
- Kelp increases because urchins can no longer reach it
- Urchins increase and overgraze the kelp, reducing habitat
- Urchin populations crash from lack of predators
- Kelp and urchins both remain unchanged
Answer: Urchins increase and overgraze the kelp, reducing habitat
Sea otters are a keystone species controlling urchin numbers. Remove the otters and urchins are released from predation, so their population grows and they graze kelp forests down to barren ground. This trophic cascade destroys habitat for many species. The other options reverse or ignore the predator-prey relationship.
Explain the difference between bioaccumulation and biomagnification, and describe two chemical properties a toxin must have to biomagnify in a food web.
Answer: Bioaccumulation is the buildup of a toxin within one organism over its lifetime; biomagnification is the increase in toxin concentration at each higher trophic level. To biomagnify, a toxin must be persistent (resistant to breakdown) and fat-soluble (stored in tissue rather than excreted).
Full credit requires distinguishing the scale of each process — bioaccumulation within an individual versus biomagnification across levels — and naming two correct properties. Persistence ensures the toxin isn't degraded, and fat solubility ensures it is stored and passed along rather than flushed out, so predators eating many prey accumulate ever-higher amounts.
Why does a food web make an ecosystem more resilient to disturbance than a single food chain would suggest?
Answer: A food web contains multiple overlapping feeding pathways, so if one species declines, consumers can switch to alternate food sources, allowing the community to absorb the loss instead of collapsing.
The key idea is redundancy. A linear food chain implies that losing one link collapses everything above it, but real ecosystems have interconnected paths. This alternative-prey buffering is why webs represent stability better than chains.
FAQ
- What is the difference between a keystone species and a dominant species?
- A keystone species has a large ecological effect despite low abundance — its removal restructures the community. A dominant species simply has high biomass or numbers. The defining feature of a keystone is disproportionate influence relative to how common it is.
- Do arrows in a food web point toward the predator or the prey?
- Arrows point toward the consumer (the predator), showing the direction energy flows. Energy moves from the organism being eaten to the organism eating it, so an arrow from grasshopper to frog means the frog eats the grasshopper.
- Why do top predators have the highest toxin concentrations?
- Because of biomagnification. Persistent, fat-soluble toxins concentrate at each trophic level. A top predator eats many contaminated prey and stores the combined toxins while excreting little, so it ends up with the highest concentration in the food web.
- How many trophic levels can a food chain usually have?
- Typically four or five, because only about 10 percent of energy transfers between levels. Energy runs out too quickly to support many more links, which is why top predators are relatively few in number.
Learn this with a teacher, not a page
The Crimsora tutor teaches U1.5 Food Chains and Food Webs live — explaining on a whiteboard, asking you questions, and adapting to where you get stuck.