
243
ECOSYSTEM
biomass or organic matter produced per unit area over a time period by
plants during photosynthesis. It is expressed in terms of weight (gm
–
2
) or
energy (kcal m
–
2
). The rate of biomass production is called productivity.
It is expressed in terms of gm
–2
yr
–1
or (kcal m
–
2
) yr
–1
to compare the
productivity of different ecosystems. It can be divided into gross primary
productivity (GPP) and net primary productivity (NPP). Gross primary
productivity of an ecosystem is the rate of production of organic matter
during photosynthesis. A considerable amount of GPP is utilised by plants
in respiration. Gross primary productivity minus respiration losses (R),
is the net primary productivity (NPP).
GPP – R = NPP
Net primary productivity is the available biomass for the consumption
to heterotrophs (herbiviores and decomposers). Secondary productivity
is defined as the rate of formation of new organic matter by
consumers.
Primary productivity depends on the plant species inhabiting a
particular area. It also depends on a variety of environmental factors,
availability of nutrients and photosynthetic capacity of plants. Therefore,
it varies in different types of ecosystems. The annual net primary
productivity of the whole biosphere is approximately 170 billion tons
(dry weight) of organic matter. Of this, despite occupying about 70 per
cent of the surface, the productivity of the oceans are only 55 billion tons.
Rest of course, is on land. Discuss the main reason for the low
productivity of ocean with your teacher.
14.3 DECOMPOSITION
You may have heard of the earthworm being referred to as the farmer’s
‘friend’. This is so because they help in the breakdown of complex organic
matter as well as in loosening of the soil. Similarly, decomposers break
down complex organic matter into inorganic substances like carbon
dioxide, water and nutrients and the process is called
decomposition.
Dead plant remains such as leaves, bark, flowers and dead remains of
animals, including fecal matter, constitute detritus, which is the raw
material for decomposition. The important steps in the process of
decomposition are fragmentation, leaching, catabolism, humification and
mineralisation.
Detritivores (e.g., earthworm) break down detritus into smaller particles.
This process is called fragmentation. By the process of leaching, water-
soluble inorganic nutrients go down into the soil horizon and get precipitated
as unavailable salts. Bacterial and fungal enzymes degrade detritus into
simpler inorganic substances. This process is called as catabolism.
It is important to note that all the above steps in decomposition operate
simultaneously on the detritus (Figure 14.1). Humification and
mineralisation occur during decomposition in the soil. Humification leads