In Elton's account of the natural community as a
simplified economy he suggested the following four principles that
could describe the economy of nature as it operates everywhere on
earth.
Elton called first of his principles the "food
chain."
In every community, plants, through the
photosynthetic conversion of sunlight to food, form the first link
in a chain of nutrition. Food, one might say, is the essential
capital in the natural economic order. The remaining links- usually
no more than two or three, and almost never more than four- include
the herbivorous animals and their predators.
A typical food chain in North American oak woods
might link acorns, quail, and foxes, or acorns, mice, and weasels;
with some 200 species of birds and mammals alone feeding on the
oaks. The potential number of food chains is extraordinarily large.
They could be found in nature by the thousands, all showing a
common pattern but no two alike in every respect. And the bottom of
the chain, rather than the top, is the most important link: The
plants make the whole system possible.
Elton referred to the sum total of chains in any
community as the "food web", an exceedingly complex design of
crisscrossed lines of economic activity. Such webs are easiest to
analyze in the relatively unpopulated arctic zones and almost
impossible to untangle in the warm, humid tropics, where life forms
abound. In every food chain certain roles must be performed. The
plants, for example, are all "producers." Animals can be described
as either first- or second-order "consumers," depending on whether
they eat plants or other animals. Those animals that feed on the
most numerous plants in a habitat, like the bison on prairie
grasses, or copepods on diatoms in the sea, are the "key
industries" in those economies.
In 1926, August Thienemann had introduced the
terms "producer," "consumer," and "reducer," or "decomposer," which
describe economic roles of organisms in a specific ecological
setting; Elton now generalised them for every food chain in nature.
These labels emphasized the nutritional interdependence that binds
species together, and they became the boxes of resources with which
ecologists would increasingly build models of materials and energy
flows through ecosystems.
In general, an animal as large as an elephant
cannot survive on food as small and lively as insects. It would
exhaust itself in the chase and need more food than it could catch.
A more substantial, stationary, and unresisting kind of food, the
leaves, twigs and outer branches of trees and shrubs, is called
for.
The giant whale is able to live on tiny
crustaceans only because they are so numerous and easy to harvest.
The law of nature decrees that each species has an optimum food
size, and this law determines the structure of the food chain. "The
very existence of food-chains," Elton noted, "is due mainly to the
fact that any one animal can only live on food of a certain size.
Each stage in an ordinary food chain has the effect of making a
smaller food into a larger one, and so making it available to a
larger animal." The chain thus becomes an ascending scale of larger
and larger mouths and stomachs, except where a predator has the
advantage of such special weapons as poison, like the spider, or
group tactics, like the wolf pack. Even then, of course, the spider
cannot kill and consume an elephant.
The only species that ignores these rules is
civilised man, with his artificial techniques for more efficient
food gathering. He can kill the largest animals on earth, or he can
gather the smallest grain and seeds, and so eat lower on the food
chain. But for Elton, modern man is distinctly an outsider, not to
be confused with the mainstream economy of nature and its
workings.
To serve as food for organisms higher up on the
food chain, plants and animals near the bottom must be more
numerous and reproduce more rapidly. Their fertility is a function
of their position in the economic order: The smaller a creature,
the more common it is and the faster it reproduces. In contrast
predators at the top of the chain must reproduce more slowly than
their prey, or they will end up with nothing to eat. They must also
distribute themselves more sparsely across the land; hence every
tiger looks for his own hill and defends it against other tigers,
or he perishes. The demarcation and defence of private territories
for food or breeding insures that each successful individual will
have an adequate base for survival. Such territorialism goes on
where it must among the birds and mammals, which are constantly
required to adjust their density to their food supply.
The system of interacting populations resulting
from these different behavioural requirements Elton called the
"pyramid of numbers." A single plot of ground may provide a home
for millions of microscopic soil bacteria, thousands of grass
plants and insects, hundreds of trees, a few dozen rabbits,
sparrows, and squirrels and at the apex only a single hawk. As one
goes up the chain, the total weight of protoplasm at each level
(the "biomass") declines along with the numbers. One lion, for
instance, may eat fifty zebras in a year, so that the total
physical mass of the predator is but a small fraction of the
combined weight of its prey. The plants at the food chain's base
constitute by far the greatest part of the mass of living substance
on earth.