4. HUMANS AND THE ENVIRONMENT
53
Using the energy from the Sun, they produce organic substances
from
mineral substances, water and carbon dioxide by means of
photosynthesis. The species within the other links of the food chain
are unable to use solar energy directly; they can only consume the
energy that is already enclosed in the matter of green plants. The
organic substances produced by plants and their accumulated energy
are first of all available to plant-eating insects, birds, mammals and
other animals living on plant food. Plants
are basically composed of
cellulose – an organic substance that is hard to digest. Herbivores
are able to digest cellulose with the help of micro-organisms that
inhabit their digestive tracts as symbionts. In this kind of symbiosis,
the animal provides shelter for the micro-organisms in its digestive
tract, while the micro-organisms help the animal digest the swallow-
ed plant food. Since cellulose has a low energetic value as food,
herbivores have to eat frequently and in large
quantities in order to
acquire the energy they need.
The ecological food chain represents the feeding hierarchy of
organisms in an ecosystem as well as the flow of substances and
energy in it. The ecological food chain ordinarily consists of three to
four trophic levels. The first level comprises autotrophic organisms
that make their body matter by using solar energy and the inorganic
substances taken from soil or water. All green plants are autotrophic,
including the algae growing in waterbodies. The green plants
produce their growth substances by means of photosynthesis. For
this reason, they are called producers.
There also are autotrophic
bacteria, though they are less widespread in ecosystems. They
synthesise their cell matter by means of chemical energy. All other
levels of the food chain consist of consumers. The second level
comprises herbivores that consume plants to obtain energy and the
needed substances. The third level comprises the primary predators
and parasites that use herbivores for food. In many ecosystems, the
primary
predators are also endangered, for they are eaten by the
secondary predators in turn. The amount of solar energy available
at each subsequent level of the ecological food chain is no more
than 10-15%. This is because most of this energy is used for the
metabolism, growth and reproduction of organisms at each level and
some energy is lost as heat.
For this reason, food chains are not long.
A decrease in the amount of energy in the food chain is reflected by
the number and total biomass of the organisms at each level. Usually
there are more plants than herbivores and more prey than predators.
The ecological pyramid of numbers shows this rule graphically
(Figure 4.10).
The food chain also includes decomposers. They obtain the
necessary energy and substances by decomposing the remains and
excrements of organisms accumulated within the entire food chain.
54
ENVIRONMENT, POLLUTION, DEVELOPMENT: THE CASE OF UZBEKISTAN
These organisms are called saprotrophs (from the Greek sapros –
‘rotten’, trophos – ‘feeding’). It is worth noting that the ecological
food chain is more of an abstract concept.
In reality, many species
use both plants and animals for food, for example, bears. Therefore,
one and the same species can subsist at several trophic levels.
Accordingly, a food web is a better way of representation of actual
trophic relations among species (Figure 4.11). Each node of the food
web
denotes a particular species, whereas the links with other nodes
indicate the trophic relations of these species with other species
within the ecosystem. Thus, one can say that ecological food chains
represent not so much relations among species but rather the flow of
energy and substances within an ecosystem in general.
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