Māris kļaviņŠ, azamat azizov, JĀnis zaļoksnis environment, pollution, development: the case of uzbekistan

Figure 4.8. 
Lichen is a 
symbiotic association of 
alga and fungus
Figure 4.9. 
Ecological 
food chain

---

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.