Food Chains and Food Webs
Trophic Relationships in Ecosystems
A food chain shows the linear path energy takes from one organism to the next. It's the simplest way to represent who eats whom. A typical chain looks like this:
Producer → Primary Consumer → Secondary Consumer → Tertiary Consumer
For example: grass → rabbit → fox → hawk.
At each step, energy transfers to the next level, but not efficiently. Only about 10% of the energy at one trophic level makes it to the next. The rest is lost as heat from metabolic processes. This is called the 10% rule, and it's one of the most important concepts in this unit.
A food web is a more realistic picture. Instead of a single chain, it shows the many overlapping feeding relationships in an ecosystem. Most organisms don't eat just one thing. A fox might eat rabbits, mice, and berries. A hawk might eat foxes, snakes, and songbirds. Food webs capture that complexity.
Trophic levels describe where an organism sits in the chain or web:
- First trophic level: Producers (autotrophs) convert sunlight into chemical energy through photosynthesis. These include plants, algae, and cyanobacteria.
- Second trophic level: Primary consumers (herbivores) feed directly on producers. Examples: rabbits, caterpillars, zooplankton.
- Third trophic level: Secondary consumers (carnivores) eat primary consumers. Examples: spiders, frogs, small fish.
- Fourth trophic level: Tertiary consumers (top predators) consume secondary consumers. Examples: hawks, wolves, sharks.
- Decomposers (bacteria, fungi) don't fit neatly into one trophic level. They break down dead organisms and waste at every level, recycling nutrients back into the ecosystem so producers can use them again.
One thing that trips students up: an organism can occupy different trophic levels depending on what it's eating. An omnivore like a bear acts as a primary consumer when eating berries and a secondary consumer when eating salmon. In a food web, that same species can appear at multiple levels.
Ecological Roles of Organisms
Producers (autotrophs) form the foundation of every ecosystem. They convert inorganic compounds into organic molecules that store chemical energy.
- Photosynthetic producers (plants, algae, cyanobacteria) use sunlight as their energy source. This is the most common form of primary production on Earth.
- Chemosynthetic producers use energy from chemical reactions instead of sunlight. Certain bacteria near deep-sea hydrothermal vents, for instance, oxidize hydrogen sulfide to build organic compounds. These organisms support entire ecosystems in places where sunlight never reaches.
Consumers (heterotrophs) get their energy by eating other organisms. The categories (primary, secondary, tertiary) describe what they eat, not how complex they are. Omnivores like humans, bears, and crows blur the lines because they feed at multiple trophic levels.
Decomposers are easy to overlook, but they're essential. Without bacteria and fungi breaking down dead matter, nutrients would stay locked in carcasses and waste. Decomposition releases those nutrients back into the soil and water, where producers absorb them. This keeps the entire cycle running.
Energy Flow and Biomass
Energy Pyramids and Trophic Levels
An energy pyramid is a diagram that shows how much energy is available at each trophic level. The base (producers) is always the widest, and each level above it gets narrower because energy is lost at every transfer.
Here's why the pyramid shape matters: if producers in a grassland capture 10,000 kcal of energy from the sun, primary consumers get roughly 1,000 kcal, secondary consumers get about 100 kcal, and tertiary consumers end up with only around 10 kcal. That dramatic drop explains why ecosystems can't support many trophic levels and why top predators are always relatively rare.
Biomass is the total mass of living organisms in a given area. A biomass pyramid works similarly to an energy pyramid, with producers usually having the greatest biomass and each higher level having less.
There's an important exception, though: in some aquatic ecosystems, the biomass pyramid can be inverted. Phytoplankton (the producers) reproduce and get consumed so rapidly that at any snapshot in time, their standing biomass is actually less than the zooplankton feeding on them. The pyramid looks upside down. This doesn't violate the energy rules; it just reflects the fast turnover rate of phytoplankton. Energy pyramids, by contrast, are never inverted.
Ecological Efficiency and Energy Transfer
Ecological efficiency is the percentage of energy successfully passed from one trophic level to the next. The average is about 10%, though it varies by ecosystem and organism type.
That 90% "lost" energy goes to several places:
- Heat from cellular respiration. Every organism burns energy to stay alive, and much of that energy escapes as heat.
- Metabolic costs. Movement, growth, reproduction, and maintaining body temperature all consume energy before it can be passed on.
- Undigested material. Not everything an organism eats gets absorbed. Bones, fur, cellulose, and other tough materials pass through as waste.
- Incomplete consumption. Predators rarely eat every part of their prey, and not every organism in a trophic level gets eaten at all.
This inefficiency is why most ecosystems top out at 4 or 5 trophic levels. There simply isn't enough energy left to support another level of predators beyond that.
Bioaccumulation
Accumulation of Toxins in Food Chains
Bioaccumulation is the buildup of a toxic substance in an organism's body over time. It happens when the organism takes in the toxin faster than it can break it down or excrete it. These substances tend to be stored in fatty tissues, where they persist for long periods.
Biomagnification takes this a step further. As toxins move up the food chain, their concentration increases at each trophic level. Here's how it works:
- A producer absorbs a small amount of a toxin (say, mercury) from the water.
- A primary consumer eats many producers over its lifetime, accumulating the mercury from all of them.
- A secondary consumer eats many primary consumers, concentrating the mercury even further.
- A top predator eats many secondary consumers and ends up with the highest concentration of all.
The classic example is DDT, a pesticide widely used in the mid-1900s. DDT accumulated in aquatic food chains and reached dangerously high concentrations in birds of prey like bald eagles and peregrine falcons. The toxin caused their eggshells to become so thin that they cracked during incubation, leading to severe population declines. This discovery was a major reason DDT was banned in the United States in 1972.
Common bioaccumulative substances include:
- Persistent organic pollutants (POPs) like DDT and PCBs
- Heavy metals like mercury and lead
The effects on organisms at the top of food chains can be severe: reproductive failure, organ damage, weakened immune systems, and increased mortality. This is why monitoring toxin levels in top predators (like tuna or eagles) serves as an indicator of ecosystem health.