Explain how energy flows one way through food chains and webs and is lost at each trophic level, and how matter (carbon and nitrogen) cycles through an ecosystem (NYSSLS LS2, energy and matter; using mathematics).

What this topic is asking

NYSSLS LS2 wants you to explain how energy flows through an ecosystem and how matter cycles. The Life Science: Biology Regents tests this with food chains and webs, the energy pyramid (including the "about 10 percent" rule, a using mathematics task), and the carbon and nitrogen cycles. The crosscutting concept is energy and matter.

Food chains and food webs

Each step in a food chain is a trophic (feeding) level: producers at the base, then primary consumers (herbivores), then secondary consumers, and so on. The arrows in a food chain point in the direction the energy flows (from the eaten to the eater).

Energy flows one way and is lost

This loss is why an energy pyramid (which shows the energy at each level) gets smaller toward the top, and why food chains rarely have more than four or five links: after a few levels there is too little energy left to support another. The "about 10 percent" figure lets you estimate energy down a pyramid, for example $10\,000 \text{ kJ} \rightarrow 1\,000 \text{ kJ} \rightarrow 100 \text{ kJ}$ across three levels.

Matter cycles

In contrast to energy, matter is conserved and cycles. The atoms of carbon, nitrogen and other elements pass from the environment into producers, along the food web, and back to the environment, to be used again:

• Carbon cycle. Carbon dioxide is taken in by producers in photosynthesis, passed along food chains, and returned to the air by respiration and by decomposers, and by burning fuels (see the cycling of energy and matter in cells).
• Nitrogen cycle. Nitrogen is converted into forms plants can use (by bacteria), built into proteins, passed along food chains, and released again by decomposers.

In both cycles, decomposers are crucial: by breaking down dead matter they release the locked-up nutrients so producers can reuse them.

Why energy must keep entering

Because energy is lost as heat at every transfer and cannot be reused, an ecosystem must receive a continuous input of energy (sunlight) to keep going. Matter, being recycled, does not need to be replaced in the same way. This contrast, energy flowing one way while matter cycles, is the central idea of the topic and a frequent exam point.

Try this

Q1. Explain why an energy pyramid gets smaller toward the top. [2]

• Cue. Only about 10 percent of the energy passes to each next level; the rest is used in life processes and lost as heat, so less energy is available higher up.

Q2. Explain why matter can be recycled in an ecosystem but energy cannot. [2]

• Cue. Matter is conserved and cycles (returned by decomposers and respiration); energy is lost as heat at each transfer and cannot be reused, so it must keep entering from the Sun.