321IT HAS LONG BEEN ARGUED THAT ENERGETIC CONSIDERATIONS SET A LIMITMAXIMAL FOOD CHAINSTO THE NUMBER OF TROPHIC LEVELS THAT AN ENVIRONMENT CAN SUP-PORT

20.4.1 Productivity? Productive space? Or just space?

3

2

1

It has long been argued that energetic considerations set a limit

to the number of trophic levels that an environment can sup-

port. Of the radiant energy that reaches the earth, only a small

12

4

11

7

fraction is fixed by photosynthesis and made available as either

13

8

live food for herbivores or dead food for detritivores. Indeed,

the amount of energy available for consumption is considerably

5

9

less than that fixed by the plants, because of work done by the

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plants (in growth and maintenance) and because of losses due to

6

inefficiencies in all energy-conversion processes (see Chapter 17).

Thereafter, each feeding link amongst heterotrophs is character-

ized by the same phenomenon: at most 50%, sometimes as little

Figure 20.14 Community matrix for an exposed intertidal rocky

as 1%, and typically around 10% of energy consumed at one trophic

shore in Washington State, USA. The pathways of all possible

level is available as food to the next. The observed pattern of just

maximal food chains are listed. 1, detritus; 2, plankton; 3, benthic

three or four trophic levels could arise, therefore, simply because

algae; 4, acorn barnacles; 5, Mytilus edulis; 6, Pollicipes; 7, chitons;

a viable population of predators at a further trophic level could