Arrows represent energy flow! Energy flows in one direction through a food chain – from the sun to producers to consumers to decomposers. It does not cycle back. Unlike energy, matter (nutrients) cycles through ecosystems. A food chain is a simplification; in reality, most organisms eat multiple types of food, and food webs are more realistic. A food chain typically has 4-5 links because energy is lost at each transfer (about 90% is lost as heat, so only 10% is passed to the next level). The first trophic level (producers) contains the most energy. The loss of energy explains why there are more producers than consumers and why top predators are rare. Food chains can be of different lengths depending on the ecosystem. Aquatic food chains are often longer than terrestrial ones because the transfer efficiency may be higher. Detrital food chains (starting with dead organic matter) are important in decomposition. The concept of food chains was developed by Charles Elton in the 1920s. Food chains are used to model ecosystem dynamics and to understand the impacts of removing or adding species (cascading effects).

Grass is a producer! All green plants are producers. Algae (phytoplankton) are the main producers in the ocean, producing about 50-80% of Earth's oxygen. Chemosynthetic bacteria at deep-sea vents produce food using hydrogen sulfide (no sunlight needed). Producers form the base of every food web. The biomass of producers in an ecosystem is much greater than the biomass of consumers (biomass pyramid). Plants also produce oxygen as a byproduct of photosynthesis. Without producers, consumers could not survive. Producers are also called "autotrophs" (self-feeders). The rate of photosynthesis determines the amount of energy available to the rest of the food chain (primary productivity). Tropical rainforests have the highest primary productivity; deserts have the lowest. In the ocean, phytoplankton productivity is limited by nutrients (nitrogen, phosphorus, iron). Artificial "dead zones" (low oxygen) occur where excess nutrients cause algal blooms that then die and decompose, consuming oxygen.

A rabbit is a primary consumer! Herbivores have specialized teeth for grinding plants and long digestive tracts to break down cellulose. Ruminants (cows, deer) have a four-chambered stomach containing bacteria that digest cellulose. Some herbivores eat only specific plants (specialists); others eat a wide variety (generalists). The amount of energy transferred from producers to primary consumers is about 10% (the rest is used for metabolism or lost as heat). That is why there are fewer herbivores than plants. Overpopulation of herbivores (like deer) can lead to overgrazing and ecosystem damage. Primary consumers are often prey for secondary consumers. Their population size is controlled by predators, food availability, and disease. Herbivores also play a role in seed dispersal (eating fruits) and pollination (nectar-feeding animals). Some insects (caterpillars, aphids) are also primary consumers. In aquatic ecosystems, zooplankton (tiny animals) eat phytoplankton. Zooplankton are an important link between phytoplankton and fish. The world's largest herbivore is the African elephant (eats up to 300 pounds of vegetation per day). Termites are herbivores that eat wood (with the help of symbiotic bacteria). Primary consumers are essential for transferring energy from plants to the rest of the food web.

A hawk is often a tertiary consumer! The trophic level of an organism can vary depending on its diet. Humans are omnivores, occupying multiple trophic levels. Top predators (like lions, tigers, orcas) have no natural predators (except humans). They are keystone species – their presence influences the entire ecosystem. Removing top predators can cause trophic cascades: for example, removing wolves from Yellowstone allowed deer to overpopulate, which overgrazed vegetation, leading to erosion and loss of biodiversity. Reintroducing wolves in 1995 restored the ecosystem. Secondary and tertiary consumers receive even less energy than primary consumers (about 10% of 10% = 1% of the original energy). That is why there are so few top predators. Humans have a large impact on top predators through hunting, habitat destruction, and pollution (biomagnification: toxins concentrate at higher trophic levels). DDT caused eggshell thinning in eagles and hawks (secondary/tertiary consumers), leading to population declines. The ban on DDT (1972 in the US) allowed populations to recover. Apex predators are essential for ecosystem balance. They often prey on the sick and weak, keeping prey populations healthy. The trophic level of an organism is not always an integer (e.g., omnivores may have a trophic level of 2.5). Humans have an average trophic level of about 2.2 (similar to anchovies and pigs).

Decomposers recycle nutrients! They are essential for ecosystem health. When a plant or animal dies, decomposers break down the organic matter into inorganic nutrients (nitrates, phosphates, CO₂) that plants can absorb. Without decomposers, dead leaves, fallen trees, and animal carcasses would accumulate, and soil would become depleted of nutrients. Decomposers are also important in sewage treatment (breaking down waste) and composting (turning food scraps into fertilizer). Fungi are the primary decomposers of wood (they break down lignin and cellulose). Bacteria are the primary decomposers of animal carcasses. Earthworms ingest soil and dead leaves, aerate the soil, and produce nutrient-rich castings (worm poop). Detritivores (like millipedes, woodlice) physically break down dead matter, increasing surface area for bacteria and fungi. The rate of decomposition depends on temperature, moisture, and oxygen. Decomposition is faster in warm, moist conditions (tropical rainforests) and slower in cold, dry conditions (tundra, deserts). Decomposers also play a role in the carbon cycle (releasing CO₂) and nitrogen cycle (ammonification). In ecosystems without decomposers (e.g., certain deep-sea vents), chemosynthesis provides the energy. Some fungi form symbiotic relationships with plant roots (mycorrhizae), helping plants absorb water and nutrients in exchange for sugars. The largest living organism on Earth is a fungus (Armillaria ostoyae) in Oregon, covering 3.7 square miles (2,385 acres) and over 2,400 years old.

The 10% rule explains why there are so few top predators! For example, if grass contains 10,000 kcal of energy, rabbits (primary consumers) can only obtain about 1,000 kcal, snakes (secondary) about 100 kcal, and hawks (tertiary) about 10 kcal. That is why a single hawk requires a large territory to find enough food. Energy pyramids are always upright (unlike biomass pyramids, which can be inverted in some ecosystems). The 10% rule is an average; transfer efficiency can vary from 5-20% depending on the ecosystem and organism. Energy pyramids can be measured in kcal/m²/year or other units. Humans have reduced the length of food chains by consuming lower trophic levels (plants and herbivores) rather than top predators. Eating lower on the food chain is more energy-efficient and environmentally sustainable. Biomass pyramids show the mass of organisms at each level; they are also usually upright but can be inverted in aquatic ecosystems (where phytoplankton reproduce so quickly that the standing crop is small). Number pyramids can be inverted if the producer is large (e.g., a single oak tree supports many herbivores). The concept of trophic levels and energy transfer was developed by Raymond Lindeman (1942). Ecological efficiency varies by ecosystem: aquatic systems tend to have higher transfer efficiency (about 15-20%) than terrestrial systems (about 10%). Ectotherms (cold-blooded animals like fish, reptiles) are more energy-efficient than endotherms (warm-blooded animals like birds, mammals). That is why you find more predators in aquatic ecosystems. The "trophic pyramid" is another name for the energy pyramid. The loss of energy as heat is a consequence of the laws of thermodynamics (energy cannot be created or destroyed, only transformed). The second law of thermodynamics says that energy transformations increase entropy (disorder), resulting in heat loss.

A food web is more realistic! A food chain is a simplification, while a food web shows the complexity of an ecosystem. For example, a mouse might eat seeds and insects; it might be eaten by a snake, a hawk, or an owl. A food web would show all these connections. The complexity of a food web affects its stability: complex webs (with many connections) are more stable and resilient to species loss. Keystone species are species that have a disproportionately large effect on their ecosystem relative to their abundance. Their removal causes major changes (trophic cascade). The reintroduction of wolves to Yellowstone restored the food web and allowed vegetation to recover. Invasive species can disrupt food webs by outcompeting native species or introducing new predators. The number of trophic levels in a food web is limited by energy loss. Food webs can be used to study biomagnification: persistent pollutants (like DDT, mercury) become more concentrated at higher trophic levels. The Fukushima nuclear accident (2011) released radioactive cesium into the ocean; it was found in fish at higher trophic levels. Ecologists use food webs to model ecosystem dynamics and predict the effects of climate change. The concept of food webs was developed by Charles Elton (1927). Food webs can be analyzed mathematically using graph theory. Connectance measures the number of links in a food web. Omnivory (eating more than one trophic level) is common in nature. Predators that eat both herbivores and other predators blur trophic levels. Humans are omnivores and can occupy multiple trophic levels (eating plants, fish, beef). The human impact on food webs (overfishing, deforestation, pollution) is causing widespread ecosystem changes. Sustainable management requires understanding food web dynamics.

Biomagnification is the correct term! DDT caused eggshell thinning in bald eagles and peregrine falcons, nearly driving them to extinction. The ban on DDT (1972 in the US) allowed populations to recover. Mercury poisoning causes Minamata disease (neurological symptoms). The FDA recommends limiting consumption of large predatory fish (shark, swordfish, tuna) for pregnant women and children because of mercury. PCBs (polychlorinated biphenyls) are industrial pollutants that persist in the environment; they are linked to cancer and immune suppression. Microplastics can also carry persistent pollutants and are ingested by marine life. Biomagnification is a serious environmental problem because even low levels of pollution can cause harm to top predators. Polar bears have high levels of PCBs and other pollutants due to biomagnification (they eat seals, which eat fish, which eat plankton). Bioconcentration is the uptake of a toxin from water (not through the diet). Bioaccumulation is the general term for the build-up of toxins in an organism over time. Biomagnification is the increase in concentration up the food chain. The process is driven by the fact that persistent pollutants are not excreted easily and accumulate in fat tissue. Each higher trophic level consumes many organisms from the level below, so the concentration multiplies. For example, if each small fish has 1 ppm of mercury, and a large fish eats 10 small fish, the large fish may have 10 ppm (assuming no excretion). Humans are at the top of many food chains and therefore vulnerable to biomagnification. The Minamata disaster (Japan, 1950s-1960s) caused severe mercury poisoning from industrial wastewater; thousands died or were disabled. The cleanup took decades. The Stockholm Convention (2001) banned or restricted persistent organic pollutants (POPs), including DDT, PCBs, and dioxins. However, legacy pollution remains a problem. Biomonitoring uses animals (like mussels) to measure pollution levels. Understanding biomagnification is essential for setting safe levels of pollutants and for protecting public health.

Overfishing is a major threat! Other threats include habitat destruction (deforestation, wetland drainage, coral reef damage), pollution (pesticides, plastics, heavy metals), climate change (ocean acidification, warming, sea level rise), invasive species (introductions of non-native species that outcompete or eat native species), and hunting/poaching (of top predators). The passenger pigeon (once the most abundant bird in North America) was hunted to extinction in 1914. The dodo (Mauritius) was extinct by 1680 due to hunting and introduced predators. Many large predators (lions, tigers, wolves, sharks) are endangered due to habitat loss and human conflict. Trophic cascades can occur when a keystone species is removed. For example, the elimination of sea otters (due to hunting) led to an explosion in sea urchins, which overgrazed kelp forests. When sea otters were protected, kelp forests recovered. Sustainable fishing practices (catch limits, marine protected areas, bycatch reduction) help protect food chains. Aquaculture (fish farming) can reduce pressure on wild fish but has its own environmental problems (pollution, disease, escape of farmed fish). Bycatch (unwanted fish caught accidentally) kills millions of marine animals each year. Bottom trawling destroys seafloor habitats. The concept of maximum sustainable yield (MSY) is used to set fishing limits. Many fisheries are overfished or collapsed. Consumers can help by choosing sustainable seafood (look for MSC certification). Eating lower on the food chain (small fish like sardines, anchovies, herring) is more sustainable. Reducing food waste also reduces pressure on food chains. Habitat restoration (reforestation, wetland restoration, coral restoration) can rebuild damaged ecosystems. The Endangered Species Act (1973) protects species and their habitats in the US. The Convention on International Trade in Endangered Species (CITES) regulates wildlife trade. You can help by supporting conservation organizations, reducing your carbon footprint, and choosing sustainable products.

The grasshopper is the primary consumer! Producers: grass. Primary consumers (herbivores): grasshopper. Secondary consumers: frog (eats grasshopper). Tertiary consumers: snake (eats frog). Quaternary consumers (top predators): hawk (eats snake). This food chain has five trophic levels. The 10% rule means only about 0.1% of the original energy from the grass reaches the hawk (10% × 10% × 10% × 10% = 0.01%? Wait: 10% of 10% is 1%, 10% of that is 0.1%, 10% of that is 0.01% – correct. So from 10,000 kcal of grass, the hawk would get about 1 kcal. That is why top predators need large territories and are rare. Longer food chains are less stable because any disruption affects many levels. The number of trophic levels is limited by energy loss. In this chain, each arrow represents energy flow (from the eaten to the eater). The hawk is at the top; no other animal eats the hawk (except humans). Humans are often at the top of food chains. You have learned about producers, consumers, decomposers, energy flow, trophic levels, the 10% rule, food webs, biomagnification, and human impacts. You are now a food chain expert!