Energy Flow Through Ecosystems

Core Concepts

Primary Productivity

Gross primary productivity (GPP): The total amount of light energy converted to chemical energy (organic matter) by photosynthesis per unit area per unit time.

Net primary productivity (NPP): GPP minus the energy used by primary producers for their own cellular respiration (R). NPP = GPP − R. NPP represents the energy actually available to consumers — it is the new biomass added by producers.

Productivity varies dramatically among ecosystems. Tropical rainforests, estuaries, and coral reefs have the highest NPP per unit area. The open ocean has low NPP per unit area but, because it covers so much of the planet, contributes the largest share of total global NPP.

Factors limiting primary production include: light (especially in aquatic systems and forest understories), water (terrestrial systems), nutrients (nitrogen and phosphorus are most commonly limiting), and temperature.

Trophic Levels and Energy Transfer

Energy flows through ecosystems along trophic levels:

• Primary producers: Autotrophs (plants, algae, cyanobacteria).

• Primary consumers: Herbivores.

• Secondary consumers: Carnivores that eat herbivores.

• Tertiary consumers: Carnivores that eat other carnivores.

• Decomposers and detritivores: Organisms that consume dead organic matter and waste, returning nutrients to the soil or water.

Ecological efficiency is the percentage of energy transferred from one trophic level to the next. Typically, only about 10% is transferred (range: 5–20%). The remaining ~90% is lost because:

• Not all of the organism at the lower level is consumed.

• Of what IS consumed, not all is assimilated (feces contain undigested material).

• Of what IS assimilated, much is used for cellular respiration and lost as heat.

• Only the remainder is available for growth and reproduction (secondary production) — and thus available to the next trophic level.

This low transfer efficiency explains why food chains are rarely longer than 4–5 trophic levels — there is simply not enough energy to support another level. It also explains why top predators are rare and require large territories: a hawk needs many snakes, which need many mice, which need vast quantities of grass seed.

Ecological Pyramids

• Pyramid of energy: Always upright — energy decreases at each successive trophic level. This is a fundamental rule of ecosystem energetics.

• Pyramid of biomass: Usually upright (more producer biomass than herbivore biomass than carnivore biomass) but can be inverted in aquatic ecosystems where phytoplankton have very rapid turnover — a small standing crop of phytoplankton can support a larger biomass of zooplankton.

• Pyramid of numbers: Can be upright (many grass plants, fewer grasshoppers, fewer frogs, one hawk) or inverted (one large tree supports many herbivorous insects).

Energy Flows; Matter Cycles

Energy enters ecosystems as sunlight, is converted to chemical energy, transferred between trophic levels with substantial losses at each step, and ultimately dissipates as heat. Energy cannot be recycled — the Second Law of Thermodynamics dictates that energy conversions are never 100% efficient, and the unusable energy is lost as heat. Matter (carbon, nitrogen, phosphorus, water) cycles — atoms are reused by different organisms over time (Chapter 42).

Detrital Pathways

In many ecosystems, the majority of primary production is not consumed by herbivores but enters the detrital food web — it becomes dead organic matter consumed by decomposers (bacteria, fungi) and detritivores (earthworms, millipedes, some insects). The detrital pathway is often more important in terms of energy flow than the grazing pathway.

ELI-10

Energy in an ecosystem is like money in a bank account with terrible transfer fees. The sun deposits energy into plants (primary production). When a cow eats the plant, only about 10% of the plant’s energy becomes cow. When a human eats the cow, only about 10% of the cow’s energy becomes human. At every transfer, 90% is lost — burned as metabolic heat, left behind as undigested material, or never eaten in the first place.

This is why there are so few top predators. A lion needs wildebeest. Wildebeest need grass. To support one lion, you need many wildebeest and vast grasslands. The energy pyramid is always wide at the bottom and narrow at the top — there is simply not enough energy at the top to support many individuals.

This is also why energy FLOWS — it enters as sunlight, passes through the food web, and exits as heat. Energy cannot be recycled. You cannot capture the heat a cow loses and feed it back to the grass. Matter, on the other hand, CAN be recycled — the atoms in the grass become the cow, and when the cow dies and decomposes, those atoms return to the soil for new grass. Energy flows. Matter cycles. Those are the two cardinal rules of ecosystem function.

ELI Example

Think of an ecosystem’s energy flow as a series of buckets with leaky transfers. The first bucket (plants) is filled by a solar-powered hose. You pour water from the plant bucket to the herbivore bucket, but 90% splashes on the ground. You pour from the herbivore bucket to the carnivore bucket, and another 90% is lost. By the fourth or fifth pour, there is barely a trickle left — not enough to fill another bucket. That is why food chains are short. The splashed water (heat) cannot be gathered up and poured back — it is gone. But the bucket material (matter — carbon, nitrogen atoms) can be taken apart and reassembled into new buckets.

Do Not Confuse

• GPP vs. NPP: GPP = total photosynthesis. NPP = GPP − producer respiration = energy available to consumers. NPP is the ecologically relevant measure of productivity.

• Energy vs. Matter: Energy flows (one-way, sunlight → heat). Matter cycles (atoms are reused). This is a fundamental distinction.

• Pyramid of Energy vs. Pyramid of Biomass: Energy pyramid is always upright. Biomass pyramid can be inverted (aquatic systems with rapidly turning-over phytoplankton).

High-Yield Memory Anchors

• NPP = GPP − R. Energy available to consumers.

• ~10% ecological efficiency per trophic level. Limits food chains to 4–5 levels.

• Pyramid of energy always upright. Pyramid of biomass can be inverted (aquatic).

• Energy FLOWS (sunlight → heat). Matter CYCLES. Thermodynamic law.

Quick Check

Q1: If the NPP of a grassland is 10,000 kcal/m²/yr and the ecological efficiency is 10%, approximately how much energy is available to secondary consumers (third trophic level)?

A) 10,000 kcal/m²/yr

B) 1,000 kcal/m²/yr

C) 100 kcal/m²/yr

D) 10 kcal/m²/yr

Q2: Explain why pyramids of energy are always upright but pyramids of biomass can be inverted. Provide a specific example of an inverted biomass pyramid.

Q3: A new apex predator is introduced to an ecosystem. The food chain already has four trophic levels. Predict the likely fate of the introduced predator and explain your prediction using the principles of energy transfer.

Quick Check Answers

A1: C. 100 kcal/m²/yr. Producers (level 1): 10,000. Primary consumers (level 2): 10,000 × 0.10 = 1,000. Secondary consumers (level 3): 1,000 × 0.10 = 100 kcal/m²/yr.

A2: Energy pyramids are always upright because energy is always lost (as heat) at each transfer — the Second Law of Thermodynamics guarantees that the energy at level N+1 is less than at level N. Biomass pyramids can be inverted when the lower trophic level has a very high turnover rate — organisms reproduce and are consumed so rapidly that the standing biomass at any moment is low, even though total production is high. Example: in some open-ocean ecosystems, the phytoplankton biomass at any given moment is less than the zooplankton biomass because phytoplankton are consumed as fast as they reproduce. The phytoplankton’s high turnover rate supports the larger zooplankton standing crop, creating an inverted biomass pyramid.

A3: The introduced apex predator would likely fail to establish a viable population. With four existing trophic levels, the energy reaching the fifth level would be approximately 0.1⁴ × NPP = 0.01% of NPP. This tiny energy base could not support a breeding population. The predator would either starve, fail to reproduce, or require an impossibly large territory to capture enough prey. This is a direct consequence of the ~10% ecological efficiency per trophic level and explains why adding trophic levels is energetically constrained.

Chapter Summary

Energy enters ecosystems through photosynthesis (GPP), and the energy available to consumers is NPP (GPP − R). Only ~10% of energy transfers between trophic levels, limiting food chains to 4–5 levels and explaining the pyramid of energy. Ecological pyramids of biomass and numbers can be inverted under specific conditions. Energy flows one way through ecosystems (sunlight → heat); matter cycles. The low efficiency of energy transfer is a fundamental constraint on ecosystem structure.

Common Mistakes

• “The sun provides all the energy, so ecosystems have unlimited energy.” The sun provides abundant energy, but the rate of capture (NPP) is limited by water, nutrients, temperature, and leaf area. The energy transfers between trophic levels are also limited (~10% efficiency).

• “Energy cycles through ecosystems.” Energy flows, not cycles. It enters as light, is transformed through trophic levels, and exits as heat. Only matter cycles.