Energy Flow and Nutrient Cycles

Learning Objectives

1. Explain that energy flows because usable energy is always lost as heat in biological processes, while matter cycles because matter is conserved

2. Explain efficiency of energy transfer and its effect on the length of food chains

3. Compare and contrast biomass and energy pyramids in different ecosystems

4. Describe the major events in and interpret diagrams of the global cycling of water, carbon, and nitrogen

Energy flows but matter cycles

As enemies consume their victims in a community, they digest the matter of their victim and use some of it for energy for their own growth and reproduction. For instance, when the squirrel eats the conifer seed in the food web above, the transfer of energy is not efficient because squirrel tissue composition is very different from seed tissue composition (for example, plant cells have a cell wall, while animal cells don’t). Much of the potential energy in the seed is spent in chemically processing the plant tissue, or it remains in the seed tissue as it moves through the squirrels digestive tract and is excreted into the environment. For the average trophic interaction, roughly 90% of energy is lost at each trophic level transfer, and this loss of energy to the consumer limits the length of food chains within a food web.

All the matter in living organisms, made up mostly of carbon, hydrogen, oxygen, and nitrogen in organic molecules, is either incorporated into the enemy that consumes it or left behind in the environment (see Frog Energy Flow Figure). Each atom ends up somewhere, as described below in the nutrient cycles section, below. The energy obtained by each organism is:

• used for maintenance of the organisms

• used for growth and reproduction

• lost as heat or excreted waste from the organism

Frogs ingest energy that is used for metabolic processes (respiration), transformed into new frog biomass through growth and reproduction, or lost from the frog as feces. The energy flows from the frog into a predator, a parasite, or a detritovore. Energy is lost as it fuels the metabolic process that transform the energy and nutrients into biomass. (Source: “EnergyFlowFrog” by Thompsma – Own work. Licensed under CC BY-SA 3.0 via Commons)

This inefficient energy transfer from victim to enemy has population ecology implications. If only 10% of the energy makes it to the next trophic level, the population size of the top predator(s) remains small, while the population size and biomass of producers needs to be huge! In ecology, biomass is the combined mass of all the organisms of that species or group in the ecosystem. (Note that in the biofuel industry, the term biomass is used a little differently than in by an ecologist: ecologists refer to the entire organism, including roots and seeds, but biofuel biomass almost always refers to the mass of animal waste and harvested plant material used to make energy.)

While energy is transferred very inefficiently up a food chain, chemical toxins in the eaten organisms are incorporated into the consumer. Consumers eat many prey and retain all the toxins in those prey, accumulating higher toxin concentrations with each trophic position, a phenomenon called biomagnification (also called bioaccumulation). 

While the energy pyramid for any ecosystem always narrows as the trophic levels increase (see Biomass and Energy figure below), the biomass pyramid can sometimes invert if the population ecology of the producers includes rapid generation times and little investment in building a physical body. For example, a single-celled aquatic algal species potentially reproduces every day, while a whale species cannot breed for several years after birth. Contrast that marine system with an aquatic system such as Silver Springs, Florida (see figure) where plant tissue includes grasses with lignin (an indigestible plant polymer) and roots from which many primary consumers cannot gain energy.