Biology 12

Chapter 13 Ecological Principles • MHR 445

Primary Productivity

Primary productivityis the amount of light energy

that autotrophs in an ecosystem convert to chemical

energy (and store in organic compounds) during a

specific period of time. It is commonly measured in

terms of energy per area, per year (J/m^2 /a). It can

also be expressed as biomass(mass) of vegetation

added to an ecosystem per area, per year (g/m^2 /a). It

is important to remember that primary productivity

is the rateat which organisms produce new

biomass, which is not the same as the total mass of

all photosynthetic autotrophs present in an area at

one time. For example, a forest has a very large

biomass — the mass of its vegetation is greater

than that of a grassland of equal size. But primary

productivity of the grassland may actually be

higher because animals are constantly eating the

plants and new ones are being produced. Thus,

new mass is being accumulated in the grassland at

a higher rate than in the forest.

The amount of primary productivity (as shown

in Figure 13.18) can vary significantly, both among

ecosystems and within an ecosystem over time.

The rate of productivity depends on many factors,

including the number of autotrophs present in the

ecosystem, the amount of light and heat present

(the process slows during winter), and the amount

of rainfall the system receives (since water is a

raw material of photosynthesis). In addition,

photosynthetic organisms need certain nutrients,

such as nitrogen, potassium, and phosphorus, to

grow. Even though many of these nutrients are not

directly involved in photosynthesis, they contribute

to limiting the rate of primary productivity by

affecting plant growth.

Energy Transfer at

Higher Trophic Levels

Not all solar energy captured by primary producers

is passed on to higher trophic levels. A substantial

portion of the energy captured by producers is used

in their own cellular respiration reactions. Another

large portion is simply never eaten by consumers

— which is why many ecosystems look green.

Only some of the total biomass eaten by

consumers is converted into the body tissues of the

organism that ate it. Figure 13.19 on the following

page shows what happens to the energy a herbivore

(caterpillar) obtains from the plant material it eats.

Approximately half the plant tissue is indigestible,

and the energy from this portion is expelled with

the caterpillar’s feces. Although some of this energy

will be consumed by decomposers and will

continue to be part of the ecosystem, it will not

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5 10 15 20 25

(^5001000150020002500)
open ocean
continental shelf
extreme desert, rock, sand, or ice
desert and semi-desert scrub
tropical rain forest
savanna
cultivated land
boreal forest (taiga)
temperate grassland
woodland and shrubland
tundra
tropical seasonal forest
temperate deciduous forest
temperate evergreen forest
swamp and marsh
land and stream
estuary
algal beds and reefs
upwelling zones
000
Figure 13.18Why do the oceans contribute such a high
proportion of Earth’s total primary productivity when their
average productivity is low compared with that of algal beds
and reefs? Net primary productivity is the total amount of
solar energy transformed to chemical energy by autotrophs
(called gross primary productivity), minus the amount used
by the autotrophs during cellular respiration.
Percentage (%) of Earth’s
surface area
A Average net primary
productivity (g/m^2 /a)
B Percentage (%) of Earth’s
net primary productivity

C