11.2 Energy flow and nutrient cycling in ecosystems

Energy Flow in Nutrient Cycling in Ecosystems

Energy flow and nutrient cycling are the two fundamental processes that keep ecosystems functioning. Energy enters as sunlight, passes through living organisms, and eventually dissipates as heat. Nutrients, by contrast, are recycled repeatedly between living and nonliving parts of the system. Understanding how these two processes work together is central to Earth Systems Science.

Energy Flow in Ecosystems

Food Chains and Food Webs

A food chain is a linear sequence showing how energy transfers from one organism to the next: plants → herbivores → carnivores. It's a useful simplification, but real ecosystems are messier than that.

A food web captures that messiness. It's a network of interconnected food chains showing all the feeding relationships in an ecosystem. A hawk might eat a mouse or a snake, and both of those eat insects. Multiple overlapping pathways make food webs a far more realistic picture of how energy actually moves through a community.

This interconnectedness also matters for stability. If one species declines, organisms that feed on multiple prey sources can shift to alternatives, which helps buffer the ecosystem against collapse.

Trophic Levels and Energy Transfer

Trophic levels are the feeding positions organisms occupy in a food chain or web:

• Primary producers (autotrophs) form the first trophic level. They convert light energy into chemical energy through photosynthesis (plants, algae, cyanobacteria).
• Primary consumers (herbivores) sit at the second level and get energy by eating producers (rabbits, caterpillars, zooplankton).
• Secondary consumers (carnivores) occupy the third level and eat herbivores (foxes, frogs, small birds).
• Tertiary consumers (top predators) sit at the fourth level and eat other carnivores (wolves, eagles, sharks).

The critical concept here is the 10% rule: only about 10% of the energy at one trophic level transfers to the next. The rest is lost as heat through cellular respiration, used for the organism's own life processes, or left as undigested waste.

Here's what that looks like with real numbers:

This steep energy loss explains why ecosystems support far fewer top predators than herbivores, and why food chains rarely extend beyond four or five levels.

Primary Production, Secondary Production, and Decomposition

Primary production is the rate at which producers convert light energy into chemical energy (glucose) through photosynthesis. Two terms matter here:

• Gross primary production (GPP) is the total energy captured by photosynthesis.
• Net primary production (NPP) is what remains after subtracting the energy producers use for their own respiration. NPP represents the energy actually available to the rest of the food web.

Secondary production is the rate at which consumers convert the energy from their food into their own new biomass (growth and reproduction). It depends on how much an organism eats and how efficiently it converts that food into body mass.

Decomposition is the breakdown of dead organic matter by decomposers like bacteria and fungi. Decomposers release nutrients locked in dead tissue back into the soil, water, or atmosphere, making them available for producers to use again. Without decomposition, nutrients would stay trapped in dead matter and primary production would grind to a halt.

Nutrient Cycling in Ecosystems

Unlike energy, which flows through an ecosystem in one direction and is eventually lost as heat, nutrients are recycled. They move between living organisms and the nonliving environment through biogeochemical cycles.

Carbon Cycle

The carbon cycle describes how carbon moves through the atmosphere, biosphere, oceans, and geosphere.

• Photosynthesis pulls $CO_2$ out of the atmosphere and locks it into organic molecules like glucose and cellulose.
• Respiration by producers, consumers, and decomposers releases $CO_2$ back into the atmosphere.
• Oceans absorb atmospheric $CO_2$, acting as major carbon sinks. However, excessive absorption lowers ocean pH, causing ocean acidification that harms shell-building organisms like corals and mollusks.
• Fossil fuel combustion and deforestation release stored carbon back into the atmosphere faster than natural processes can absorb it, driving the increase in atmospheric $CO_2$ that causes climate change.

The carbon cycle connects the biosphere directly to the climate system, which is why it gets so much attention in Earth Systems Science.

Nitrogen Cycle

The nitrogen cycle describes how nitrogen transforms and moves through ecosystems and the atmosphere. Although $N_2$ makes up about 78% of the atmosphere, most organisms can't use it in that form. It has to be converted first.

The key steps of the nitrogen cycle:

1. Nitrogen fixation: Atmospheric $N_2$ is converted into ammonia ($NH_3$). This is done primarily by nitrogen-fixing bacteria, including rhizobia that live in symbiotic root nodules on legumes like soybeans and alfalfa. Lightning can also fix small amounts.
2. Nitrification: Soil bacteria convert ammonia into nitrites ($NO_2^-$) and then into nitrates ($NO_3^-$), the form most plants can absorb.
3. Assimilation: Plants take up nitrates through their roots and incorporate nitrogen into organic compounds like amino acids and proteins.
4. Ammonification: When organisms die, decomposers break down their nitrogen-containing organic compounds and release ammonia back into the soil.
5. Denitrification: Anaerobic bacteria in waterlogged soils convert nitrates back into atmospheric $N_2$, completing the cycle.

Phosphorus Cycle and Biogeochemical Cycles

The phosphorus cycle differs from the carbon and nitrogen cycles in one major way: phosphorus has no significant atmospheric (gas) phase. It moves almost entirely through rock, soil, water, and organisms.

• Weathering of phosphate-containing rocks releases inorganic phosphate ($PO_4^{3-}$) into soil and water.
• Plants absorb inorganic phosphate through their roots and build it into essential molecules like ATP, DNA, and phospholipids.
• Decomposers break down dead organisms and release inorganic phosphate back into the soil and water.
• Because phosphorus cycles slowly and has no atmospheric shortcut, it is often the limiting nutrient for primary production, especially in freshwater ecosystems.

Biogeochemical cycles is the broader term for all the pathways by which essential nutrients (carbon, nitrogen, phosphorus, sulfur, water) move through the biotic and abiotic parts of Earth's systems. These cycles are driven by biological processes (photosynthesis, decomposition), geological processes (weathering, volcanism), and chemical reactions.

Human activities disrupt these cycles in measurable ways:

• Burning fossil fuels adds excess carbon to the atmosphere.
• Heavy fertilizer use introduces excess nitrogen and phosphorus into waterways, causing eutrophication (algal blooms that deplete oxygen and kill aquatic life).
• Deforestation reduces the biosphere's capacity to absorb and cycle nutrients.

These disruptions connect directly to major environmental problems, which is why understanding nutrient cycling is so important for understanding Earth as a system.