The Ecology Book

75

The desert locust (Schistocerca

gregaria) has to eat vast quantities

of carbon-rich plants in order to get

enough nitrogen and phosphorus

to maintain its C:N ratio.

See also: Ecophysiology 72–73 ■ The food chain 132–133 ■ Energy flow through

ecosystems 138–139 ■ The foundations of plant ecology 167

ECOLOGICAL PROCESSES

dioxide from the air and use the

sun’s energy to convert it into the

nutrients they require.

Higher up the food chain,

animals have largely fixed C:N:P

ratios, so they must deploy various

mechanisms to deal with any

imbalances of chemicals entering

the body. If an insect or animal

herbivore is getting too much carbon

from its plant diet, for instance, it

may adjust its digestive enzymes

and excrete it, store it as fats, or raise

its metabolic rate to burn it off,

breathing out the excess carbon as

CO 2. Overuse of such mechanisms

to redress a high imbalance can,

however, affect fitness, growth, and

reproduction. An animal that eats

other animals has less work to do,

because its prey’s C:N:P ratio

closely matches its own. However,

the size of its prey population is still

determined by the plants in its

environment because plants with a

high carbon ratio can only support

a small food chain of consumers.

Understanding our world

Food chains are one area of study;

ecological stoichiometry covers just

about everything and all the links

in between. By discovering how

the chemical content of organisms

shapes their ecology, scientists are

also learning how environments

can be better managed. Their

findings may significantly influence

the future of life on Earth. ■

The Growth

Rate Hypothesis

Cancer research is one area

where stoichiometry is now

being employed. Evidence is

growing for a theory called the

Growth Rate Hypothesis

(GRH), which may help explain

why some cancerous tumors

grow at faster rates than the

rest of the body.

The hypothesis states that

organisms with high C:P

(carbon:phosphorus) ratios,

such as fruit flies, have more

ribosomes in their cells, which

enables them to grow and

reproduce more rapidly.

Around half of all phosphorus

in an organism is in the form

of ribosomal RNA (rRNA); it is

present in every cell, creating

proteins to build new cells and

grow the body. Applying

biological stoichiometry,

James Elser and his team

have shown that fast-growing

tumors have a much higher

phosphorus content than

normal body tissue. Such

research may help scientists

understand how tumor growth

could be controlled.

Malignant lung tissue (seen

here) and cancerous colon tissue

both had the highest phosphorus

content in research exploring the

rapid growth rates of tumors.

Controlling ecological stoichiometry ratios

A locust eats grass that may contain six times

as much carbon as it needs. To get the right

balance, it excretes carbon or breathes it out

as CO 2. Locusts are widely used in research

because they are easy to breed.

LOCUSTS

5:1

GRASS

Carbon Nitrogen 33:1

KEY

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