Energy flow, pyramids and population growth

Last time

Before we start: last time, in Selective breeding, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

Where does the energy go?

A cow eats grass all day, yet most of the energy in that grass never becomes cow. Energy is lost at every step of a food chain, which is why there are few top predators. Today: energy flow, the pyramids that show it, and how populations grow.

Energy flow

The Sun is the principal source of energy for ecosystems. Producers transfer light energy to chemical energy by photosynthesis. Energy passes along the food chain as chemical energy in the organic nutrients that organisms eat. At each step, energy is lost, so less is passed on to the next trophic level.

Where the energy goes

Some energy is lost as heat from respiration, which the organism uses for movement and keeping warm. Some parts are not eaten, such as bones, roots or shells, and some are lost in waste and faeces. Because of this, only about ten percent of the energy usually passes to the next level.

Pyramid of energy

Here is a pyramid of energy. Each level is narrower than the one below, because less energy is passed on at each step.

Pyramids

A pyramid of numbers shows how many organisms there are at each trophic level. A pyramid of biomass shows the dry mass of living material at each level. A pyramid of energy shows the energy at each level, in kilojoules per square metre per year.

Population growth

When a few organisms enter a new habitat, growth is slow at first: the lag phase. Then growth speeds up. Births are far higher than deaths, so the population grows exponentially. Then it levels off, the stationary phase, because births equal deaths. Limited resources hold the numbers steady. If conditions worsen, deaths exceed births, and the population falls: the death phase.

A growth curve

Here is a population growth curve. It starts slowly, rises steeply, and then levels off at the carrying capacity.

What limits growth

Food, water and space can run out. Competition for them grows as the population grows. Predators and disease also rise with population density, so the death rate goes up. Waste products can build up and harm the population, as in a yeast culture.