Ecosystem processesDBITAMKB PDF
Ecosystem processesDBITAMKB PDF
Ecosystem processesDBITAMKB PDF
Ecosystem function is the capacity of natural processes and components to provide goods
and services that satisfy human needs, either directly or indirectly. Ecosystem functions
are conceived as a subset of ecological processes and ecosystem structures. Natural
processes, in turn, are the result of complex interactions between biotic (living organisms)
and abiotic (chemical and physical) components of ecosystems through the universal
driving forces of matter and energy.
Ecosystems use energy and cycle matter, and these processes define the basic
ecosystem functions. Energetic processes in ecosystems are usually described in terms of
trophic levels, which define the role of organisms based on their level of feeding relative to
the original energy captured by primary producers. As always, energy does not cycle, so
ecosystems require a continuous flow of high-quality energy to maintain their structure and
function. For this reason, all ecosystems are "open systems" requiring a net flow of energy
to persist over time. Energy input to ecosystems also drives the flow of matter between
organisms and the environment by nutrient cycling processes.
Energy enters, flows through, and exits an ecosystem, whereas chemical nutrients cycle
within it. Energy (orange arrows) entering from the sun as radiation is transferred as
chemical energy through the food web; each of these units of energy ultimately exits as
heat radiated into space. Most transfers of nutrients (blue arrows) through the food web
lead eventually to detritus; the nutrients then cycle back to the primary producers.
(figure3.1)
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Figure 3.1: Energy and nutrient dynamics in an ecosystem (Source: Pearson Education)
Ecosystem Productivity:
Gross primary productivity (GPP) is the rate at which an ecosystem’s producers convert
radiant energy into chemical energy. This energy is stored in compounds in their bodies. In
order to stay alive, grow, and reproduce, producers must use some of their stored
chemical energy for cellular respiration ( Ra).
Net primary productivity (NPP) is the rate at which producers use photosynthesis to
produce and store chemical energy, minus the rate at which they use some of this stored
chemical energy through cellular respiration. [ NPP = GPP – Ra ]
Net primary production is a measure of the rate at which producers make chemical energy
potentially available to the consumers in an ecosystem. On average, net primary
production is about one-half of gross primary production. Net primary production can be
expressed as energy per unit area per unit time [ J/(m2 · yr)] or as biomass added per unit
area per unit time [g/(m2 · yr)].
(Note that biomass is usually expressed in terms of the dry mass of organic material.)
Satellites provide a powerful tool for studying global patterns of primary production.
Ecosystems vary in their net primary production. For example, tropical rain forests have a
high net primary production and collectively are large contributors to Earth’s overall net
primary production. They have a great abundance and variety of plants to support a large
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biomass of consumers. By contrast, the open ocean has a low net primary production but
is more productive annually than any other ecosystem or life zone. This happens because
of the enormous volume of the ocean and its huge numbers of phytoplankton and other
producers.
Figure 3.2: Estimated annual average net primary productivity in major life zones and ecosystems.
(Source: Cengage Learning)
Only the plant matter represented by net primary production is available as nutrients for
consumers. Thus, the planet’s net primary production ultimately limits the number of
consumers (including humans) that can survive. When the most highly productive
ecosystems suffer destruction from human activities, Earth’s total productivity is reduced
and so is the total number of consumers the planet can support.
One way organisms in a community interact is by feeding on one another. Food chain and
food web shows how each organism gets its food from the other to gain energy for its
growth and survival. Energy, nutrients, and some compounds are thus transferred from
one organism to another along linear food chains and in complex cases called food webs.
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Figure 3.3: Food chains (i) grazing and detritus food chains (Source: Pearson Education)
Food chains are of two basic types (figure 3.3), and these are:
(i) Grazing food chain, which, starting from a green plant base, goes to grazing
herbivores and on to carnivores;
(ii) Detritus food chain, which goes from non-living organic matter into micro organisms
and then to detritus-feeding organisms (detritivores) and their predators.
Food web represents interlocking food chains that connect the organisms in an
ecosystem (figure 3.4).
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In an ecosystem, at each level of food chain there is energy transfer from one organism to
the other. Hence, the organism is assigned a trophic level based on the number of energy
transfer steps to that level. Trophic is derived from a Greek word “tropho” which means
“nourishment”. Trophic level also consists of all those organisms in a food web that have
same number of feeding levels away from original source of energy (figure 3.4).
Ecological pyramids are diagrams that shows the relative amounts of energy or matter or
numbers of organisms within each trophic level in a food chain or food web. The different
organisms in the pyramid are present in a sequence and include the producers at the base
followed by the carnivores at the top. The pyramids can be upright (means that the base is
larger in size and it decreases as we move upwards) , inverted (means that the base is
smaller in size and it increases as we move upwards) or spindle shaped (means that the
base is thin along with the top but the middle part is broad). But the energy pyramid is
always upright (figure 3.5).
Trophic efficiency is the percentage of energy transferred from one trophic level to the
next. Trophic efficiencies must always be less than production efficiencies (production
efficiency, is the percentage of energy stored in assimilated food that is used for growth
and reproduction, not respiration) because they take into account not only the energy lost
through respiration and contained in feces, but also the energy in organic material in a
lower trophic level that is not consumed by the next trophic level.
Trophic efficiencies range from roughly 5% to 20% in different ecosystems, but on average
are only about 10%. In other words, 90% of the energy available at one trophic level
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typically is not transferred to the next. This loss is multiplied over the length of a food
chain. The progressive loss of energy along a food chain limits the abundance of top-level
carnivores that an ecosystem can support. Only about 0.1% of the chemical energy fixed
by photosynthesis can flow all the way through a food web to a tertiary consumer. This
explains why most food webs include only about four or five trophic levels.
Biogeochemical cycles:
The elements and compounds that make up nutrients move continually through air, water,
soil, rock, and living organisms within ecosystems. Within the biosphere, this movement of
matter occurs in nutrient cycles, or biogeochemical cycles. Nutrient cycles involve both
biotic and abiotic components and hence are called biogeochemical cycles. Nutrient cycles
are driven directly or indirectly by energy from the sun and by Earth’s gravity. These cycles
include the hydrologic (water), carbon, nitrogen, and phosphorus cycles. They are
important parts of Earth’s natural capital. Yet, human activities are disrupting these cycles.
Although most ecosystems receive abundant solar energy, nutrients are available only in
limited amounts. Life therefore depends on the recycling of essential nutrients. Much of an
organism’s chemical stock is replaced continuously as nutrients are assimilated and waste
products are released. When the organism dies, the atoms in its body are returned to the
atmosphere, water, or soil by decomposers. By liberating nutrients from organic matter,
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decomposition replenishes the pools of inorganic nutrients that plants and other autotrophs
use to build new organic matter.
Ecological Succession:
An ecosystem is not static in nature. It is dynamic and changes its structure as well as
function with time.
Ecological succession is defined as an orderly process of changes in the community
structure and function with time. These changes are mediated through modifications in the
physical environment and ultimately culminating in a stabilized ecosystem.
The process of succession takes place in a systemic order of sequential steps as follows:
1. Nudation: It is a process of developing a bare area without any form of life for the
arrival of new species.
The causes of nudation may be:
Topographic: The existing community may disappear due to soil erosion
(by gravity, water or wind), land slide, volcanic activity etc.
Climatic: The existing community may be destroyed due to storm, fire,
frost, drought.
Biotic: The community may also be destroyed by anthropogenic activities
like destruction of forest, destruction of grass land etc. Besides, diseases
induced by bacteria and virus can also destroy the population.
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Ecosystems can recover naturally from most disturbances through the stages of ecological
succession. Sometimes, however, that recovery takes centuries, particularly when humans
have degraded the environment. Restoration ecologists seek to initiate or speed up the
recovery of degraded ecosystems. One of the basic assumptions is that environmental
damage is at least partly reversible. This optimistic view must be balanced by a second
assumptionthat, ecosystems are not infinitely resilient. Restoration ecologists therefore
work to identify and manipulate the processes that most limit recovery of ecosystems from
disturbances. Where disturbance is so severe that restoring all of a habitat is impractical,
ecologists try to reclaim as much of a habitat or ecological process as pos sible, within the
limits of the time and money available to them. The long-term objective of restoration is to
return an ecosystem as closely as possible to its predisturbance state through physical
reconstruction followed by biological restoration.
Review Questions:
1. Why is the transfer of energy in an ecosystem referred to as energy flow, not energy
cycling?
2. Explain why most food webs include only four or five trophic levels.
3. Explain how ecological succession plays an important role in restoration and
recovery of an ecosystem.