The Ecology Book

THE VARIETY OF LIFE 113

when conditions, such as sunlight

and nutrients, are right. Over time,

the mass of the phytoplankton will

far outweigh that of the fish.

Trophic pyramids

American ecologist Raymond

Lindeman proposed a pyramid of

energy, called the trophic pyramid,

showing the rate at which energy

is transferred from one level to the

next as herbivores eat plants, and

predators eat herbivores. An

organism’s trophic level is the

position it occupies in a food chain.

Plants and algae are at trophic level

1, herbivores at level 2, and the first

level of predators is at 3. It is rare

for there to be more than five levels.

Plants convert the sun’s energy

into stored carbon compounds,

and when a plant is eaten by a

herbivore, some of the energy

transfers to the animal. When

a predator eats the herbivore,

it receives a smaller amount of that

energy, and so on.

Published in 1942, Lindeman’s

Ten Percent Law explains that

when organisms are consumed,

only about 10 percent of the energy

transferred from them is stored as

flesh at the next trophic level. The

energy model creates a more

realistic picture of the condition

of an ecosystem. For example, if

the biomass of weed and fish in

a pond is the same, but the weed

reproduces twice as fast as the fish,

the energy of the weed would be

shown to be twice as large. Also,

there are no inverted pyramids—

there is always more energy in the

lowest trophic level than the one

above. Assessing energy transfer,

however, requires a lot of information

about energy intake, as well as the

number and mass of organisms.

Future thinking

Relationships between organisms

and their environment change

from place to place and through

time. Global climate change is one

example of environmental factors

that will increasingly affect animal

communities. Some changes have

already taken place, but one of the

challenges of ecological thinking

in the future is to forecast others. ■

Tench feed on snails, which graze

on periphyton—a mixture of microbial

organisms that cling to plants. By

reducing the number of snails, tench

increase the periphyton biomass.

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