12.1 Ecosystems, Food Webs, and Energy Flow

Levels of Organization and Ecosystem Components

Ecology is organized in nested levels. From smallest to largest:

1. Organism — a single living individual.
2. Population — all individuals of one species in an area.
3. Community — all populations of all species in an area.
4. Ecosystem — the community plus its physical (abiotic) environment.
5. Biome — large regions defined by climate and dominant vegetation (desert, grassland, tropical forest).
6. Biosphere — all life and the parts of Earth that support it.

Every ecosystem has abiotic components — sunlight, water, temperature, soil, minerals, and oxygen — and biotic components — producers, consumers, and decomposers. A change in either set ripples through the whole system: a drought reduces producer growth, which reduces food for primary consumers, which reduces food for predators.

Food Chains, Food Webs, and Trophic Levels

A food chain shows one path of energy flow; a food web shows the interconnected feeding relationships of a community. Organisms occupy trophic levels:

• Producers (autotrophs) — capture energy, usually by photosynthesis (plants, algae, cyanobacteria).
• Primary consumers — herbivores that eat producers (grasshoppers, deer).
• Secondary consumers — carnivores that eat herbivores (frogs, snakes).
• Tertiary consumers — carnivores that eat other carnivores (hawks, large fish).
• Decomposers — bacteria and fungi that break dead organic matter and return nutrients to producers.

Energy Flow and the ~10% Rule

Energy enters an ecosystem as sunlight, is captured by producers, and flows upward through consumers. At each transfer most of it is lost as heat through metabolism and movement, so only about 10% of the energy at one trophic level becomes biomass at the next.

Because roughly 90% of the energy at each trophic level is lost as heat and metabolic work, an ecosystem's structure is constrained from the bottom up.

Three consequences follow, and each appears in exam items:

• Food chains are short. Four or five trophic levels is typically the maximum, because a fifth level would have too little energy remaining to support a viable population.
• Top predators are rare. A given area supports many grasshoppers, fewer snakes, and very few hawks.
• An energy pyramid is never inverted. Pyramids of numbers can be inverted — one oak tree supports thousands of insects — and pyramids of biomass occasionally are in aquatic systems where rapidly reproducing phytoplankton are consumed as fast as they grow. But energy always narrows upward, because the second law of thermodynamics does not permit otherwise.

Matter Cycles, Energy Flows

The single most important structural idea in ecology is that matter and energy behave differently.

This is why a sealed terrarium can recycle its water and nutrients indefinitely but still needs a light source, and it is the conceptual bridge from ecology to the carbon, nitrogen, and water cycles in Domain IV.

The Grazing Path and the Detritus Path

Classroom food webs almost always show only the grazing pathway: producer eaten by herbivore eaten by carnivore. Real ecosystems run a second route that usually carries more of the energy. The detritus pathway starts with dead leaves, roots, wood, waste, and carcasses, moves through detritivores such as earthworms, millipedes, and pillbugs that shred the material, and finishes with the bacteria and fungi that mineralize what is left.

The proportions surprise most people. Herbivores consume only a modest share of what producers make: in a mature forest, insects and browsers typically take somewhere between a few percent and a tenth of each year's leaf and wood production, and essentially all the rest reaches the ground as litter. Grasslands lose a larger fraction to grazers, but even there much of the root and shoot growth dies in place. The soil food web, not the visible chain of animals, therefore processes most of the energy an ecosystem captures.

Two teaching consequences follow. A food web drawn without decomposers is incomplete rather than merely simplified — it omits the larger pathway. And when a student asks where a fallen log's energy goes, the answer is a real trophic route with its own consumers, not "it disappears."

Energy Flow in Different Types of Ecosystems

Competency 015 asks teachers to analyze energy flow through various types of ecosystems, and the pattern is not identical everywhere. What differs is primary productivity — the rate at which producers convert captured energy into new biomass — and productivity sets the energy budget for every level above.

• Forests and grasslands are productive because light, water, and nutrients are all reasonably available; they support the familiar four-to-five-level chains used in most textbook examples.
• Deserts are limited by water, not light. Low producer growth means less energy entering the system, which is why desert food chains are short and desert predators hold large territories.
• The open ocean has low productivity per square meter because nutrients are scarce in surface water, yet its sheer area makes marine phytoplankton responsible for roughly half of all photosynthesis on Earth.
• Estuaries, salt marshes, and coral reefs are among the most productive systems per unit area anywhere, which is why they function as nurseries for so many species (see Section 13.2).
• Caves and deep-sea hydrothermal vents run without sunlight at all: chemosynthetic bacteria oxidize inorganic compounds such as hydrogen sulfide and take the producer role, as covered in Section 10.3.

The generalization students should carry away is that the number of trophic levels an ecosystem can support depends on how much energy its producers capture, not on how many kinds of animals live there. Texas makes the comparison concrete without leaving the state: Texas Parks and Wildlife's hunter education materials divide Texas into ten ecoregions, and the producers, productivity, and chain length of the Piney Woods in the east differ sharply from those of the Trans-Pecos desert in the west.

Reading a Food Web

A food chain is a single linear path; a food web shows the many overlapping chains that actually operate in a community. Arrows always point in the direction energy flows — from the organism being eaten toward the organism eating it. Reversing the arrows is the most common student error and a standard distractor.

Food webs make a community's interdependence testable. Remove the snakes from a Texas prairie web and grasshopper and rodent populations rise, grass biomass falls, and hawks lose a food source while gaining rodent prey. Asking students to predict two or three steps of consequence from a single removal is exactly the kind of systems reasoning Competency 015 targets.