3 Environmental Science : Ecosystem part 3
(2) Again 21 per cent of this energy, or 23 gcal/cm2/yr (show on the bottom as respiration)
is consumed in metabolic reactions of autotrophs for their growth, development,
maintenance and reproduction.
(3) 15 gcl/cm2/yr are consumed by herbivores that graze of feed on autographs-this
figure amounts to 17 per cent of net autotroph production.
(4) Decomposition is 3 gcal/cm2/yr which amount to be 3, 4 per cent of net production.
(5) The remainder of the plant material, 70 gcal/cm2/yr of 79.5 per cent production, is
not utilised. It becomes part of the accumulating sediments. Apparently much more
energy is available for herbivory than is consumed.
We may conclude the following conclusions
(1) Various pathways of loss are equivalent to and account for total energy capture of
the autotrophs i.e. gross production.
(2) The three upper ‘fates’ i.e. decomposition, herbivory and not utilized collectively are
equivalent to net production.
(3) Of the total energy which is incorporated at the herbivory level, i.e. 15/ gcal/cm2yr,
30 percent of 4.5 gcal/cm2/yr is used in metabolic reactions.
(4) In this way more energy is lost via respiration by herbivores (30 percent) than by
autotrophs (21 percent),
(5) Considerble energy is available for the carnivores, namely 10.5 gcal/cm2/yr
or 70-per cent. It is not entirely utilized, merely 3.0 gcal/cm2/or 28.6 per cent
of net production passes to the carnivores. This utilization of resources is
evidently more efficient than the one, which occurs at autotroph-herbivore
transfer level.
(6) At the carnivore level the consumption in metabolic activity is about percent of the
carnivores energy intake.
(7) The remainder becomes part of the un-utilized sediments;
(1) There is Noe-way Street along which energy moves (unidirectional flow of energy.
(a) The energy that is captured by the autotrophs does not revert back to solar
input.
(b) The energy which passes does not pass back to the autotrophs. It moves
progressively through the various trophic levels. As such, it is no longer
available to the previous level. Since there is one-way flow of energy, the
system would collapse in case the primary source, the sun, were cut off.
(2) Secondly, progressive decrease in energy level is seen at each trophic level. This
decrease is accounted as under:
(i) By the energy dissipated as heat in metabolic activities.
(ii) Measured here as respiration coupled with unutilized energy.
Below is a figure after Epodum (1963)
Trophic levels
G reen plants
producers
Consum ers
2
Herbivores
NU NA
3
Carnivores
Total light
1 and L
heat
3000 — 15000 15 1.5 0.3
R R R
L
A PN P 2 P 3
K cal/m /da 2 y
L
P
N 1 A
A
P 1 P
1
P or
G
Fig. 3.2 Energy flow diagram
This is a simplified energy flow diagram
(1) The diagram depicts three trophic levels. Boxes numbered 1, 2, 3 in a leaner food
chain exhibit these.
(2) L. shows total energy input (3000).
(3) LA shows light absorbed by plant cover (1500).
(4) P.G. shows gross Primary production.
(5) A shows total assimilation.
(6) Pn shows net primary production.
(7) P shows secondary (consumer) production.
(8) Nu shows energy not used (stored or exported).
(9) NA shows energy not assimilated by consumers (egested).
(10) R shows respiration.
Some more elucidation of the figure is as under:
(1) The ‘boxes’ represent the trophic levels
(2) The ‘pipes’ depict the energy flow in and out of each level.
Energy inflows balance outflows
The first law of thermodynamics requires it. The energy transfer is accompanied by
dispersion of energy into unavailable heat (i.e. respiration). The second law requires it.
It is very simplified energy flow model of three trophic levels
Apparently the energy flows is greatly reduced at each successive trophic level from
producers to herbivores and then to carnivores. It is reflected that at each transfer of energy
from one level to another, major part of energy is lost as heat or other form. The energy flow
is reduced successively. We may consider it in either term as under:
(1) In terms of total flow (i.e. total energy input and total assimilation).
(2) In terms of secondary production and respiration components.
In this way of the 3,000 Kcal of total light, which falls upon the green plants,
approximately 50 per cent (1500 Kcal) is absorbed. Only 1 per cent (15 Kcal) of it is converted
at first trophic level. Thus net primary production comes to be at 15 Kcal. Secondary
productivity (P2 and P3 in the diagram) is about 10 percent at successive consumer trophic
levels in other words at the levels of herbivores and the carnivores. However, efficiency may
be sometimes higher as 20 per cent, at the carnivore level as shown (or P3=0.3 Kcal) in the
diagram.
It may be concluded from the above studies as under:
(1) There is a successive reduction in energy flow at successive trophic levels. Thus
shorter the food chain, greater would be the available food energy. The reason is
with an increase in the length of food chain, there is a corresponding more loss of
energy.
(2) With a reduction in energy flow (shown as ‘pipes’ in the diagram) at each successive
trophic level, there is also a corresponding decrease in standing crop or biomass
(shown as ‘boxes’ in the diagram). However, it does not mean that there exists any
correlation between the biomass and energy. Indeed energy as taken here represents
rate functions or production rates. The relationships between biomass and content
are not fixed. They may differ according to the situations. For example, one gram
of an algae (lesser biomass) may be equal to many grams (more biomass) of a forest
tree leaves as the rate of production of the algae is higher than that of tree leaves.
Y-shaped energy flow model-Two channel energy flow model
Following the example of Lindeman, several authors described energy flow modes for
different kinds of ecosystems. Two illustrations are here:
(1) Teal (1957) prepared an energy flow diagram of Root Spring in U.S.A.
(2) H.T. Odum (1957) prepared energy flow model for Silver Springs, Florida, U.S.A.
(3) 30, 810 Kcal/m2 y remained for net production.
In model given by Teal (1957) for Root Springs, most of the energy rich material eaten
by heterotrophs entered the systems as plant debris. On the other hand in the model given
by H.T. Odum (1957) for Silver Spring, most of the heterotroph’s food in food chain was
produced by green with in some systems heterotrophs consume living plants while in others
they feel on dead plant parts (detritus).
(1) In Root Springs, the chain began with dead plant parts.
(2) In Silver Springs the chain began with live plant parts.
On the basis of the studies E.P. Odum pointed out that in nature there are present two
basic food chains in any system:
(1) The grazing food chain beginning with green plant base going to herbivores and
then to carnivores, and
(2) The detritus food chain beginning with dead organic matter acted by microbes,
then passing to detritivores and their consumers (predators).
The figure given below present one of the first published energy flow models as pioneered
by H.T. Odum in 1956.
P
1
P
P3 1
P
P N
Fig. 3.3 First Energy Flow Model (1956)
The above figure illustrates energy flow in a community with a large import and
smaller export of organic matter.
P indicates gross primary production; PN indicates net primary production. P2.
P
2...............P5 indicate secondary production at the shown levels.
Gross Primary production GPP = Total photosynthetic C fixation Autotrophic Respiration,
RA = GPP-NPP
Net primary Production, NPP-RA
Heterotrophic Respiration, RH = respiration of consumers and decomposers.
Ecosystem Production, NEP = GPP-RE
The three major steps in energy flow correspond to:—
(a) Exploitation efficiency,
(b) Assimilation efficiency,
(c) Net production efficiency.
The product of the assimilation net production efficiencies gives gross production
efficiency i.e. by the fraction of the eaten material eventually transformed into consumer
biomass. The whole food web may be taken to be the product of the gross production
efficiency and the exploitation efficiency. The various kinds of energetic efficiencies can be
defined as under:
Exploitation efficiency = Ingestion of food/prey production.
Assimilation efficiency = Assimilation/ingestion;
Net production efficiency = Production/assimilation.
Ecological efficiency = Exploitation efficiency × Assimilation efficiency × Net
production efficiency;
= Consumer production/prey production;
= Production/ingestion.
Gross production = Production/ingestion.
In animals, rate of production appears to depend on body mass. Per unit body mass,
small animals are found more productive than big animals. Again invertebrates are less
productive than mammals. Molluscs, annelids, isopods, and insects are invertebrates of
intermediates size between copepods and echinoids.
Some conclusions regarding energy flow in the ecosystem are as under:
There is no quantitative relationship between the production of a certain trophic level
and the production of the next lower trophic level (both in calorific terms) except for the very
high or very low values of the former. This applies to the “phytoplankton-filter feeders” and
as well as “filtrators-invertebrate predators” trophic links in the plankton food chain.
The utilization of primary production in pelagic zone often depends on the nature of
dominant species of producers and consumers. In a system containing phyto-planktonic
algae-macroconsumers effective utilization occurs mostly via grazing. In the case of larger
algae and smaller consumers, primary production is mainly utilized via bacterial detritus
medium.
The energy transfer efficiency from the filtrator’s trophic level to their invertebrate
predators is often higher than from phytoplankton to filtrators.
ECOLOGICAL SUCCESSION-MEANING AND TYPES
Meaning of Succession
Biotic communities are not static. Instead they change through time. This change can
be understood on several levels. The simplest level is the growth, interaction and death of
individual organisms as they pass through their life cycles, affected by the cycles of seasons
and other natural phenomena. Some other levels of community change act over longer time
spans and that account for much larger changes in community composition and structure.
These include ecological succession and community evolution.
It is evident from the above said that the term succession denotes a sequence of changes
in the species composition of a community, which is generally associated with a sequence
of changes in its structural and functional properties. The term is generally used for temporal
sequence (in terms of years, decades or centuries) of vegetation on a site; although only
short term changes can be observed directly and the long term ones are inferred from
spatial sequences.
The changes associated with succession are usually progressive or directional. This fact
enables one to predict which species are likely to replace other in the course of a succession.
Sucession tends to continue until the species combinations best suited to the regional climate
and the particular site are established.
Historical Background
The basic idea of succession was in the beginning forwarded by Anon Kerner (1863) in
his book “Plant Life of the Danube Basin” during the description of the regeneration of a
swamp forest. The term ecological succession was first of all used by Hult (1885) in the study
of communities of Southern Sweden. H.C. Cowles held that communities are not static but
dynamic. This changed understanding be visualized as an orderly, directional and predictable
phenomenon. It was added that succession is autogenic i.e. regulated by biotic interactions
within the community. The central foundation of the classical theory was that early
communities alter the environment to their detriment and favour later successional
communities. It was revealed by the later studies that allogenesis was perhaps more common
and dominant than autogenesis; allogenesis means the control of community dynamics by
factors originating outside the community boundaries.
The succession of animals on these dunes was studied by ‘Shelford (1913). Later on,
Olson (1958) restudies the ecosystem development on these dunes and has given us an
updated information about it. Federick Clements (1907-1936) elaborates the principles and
theory of succession. He proposed the monoclimax hypotesis of succession. During the later
years certain other hypotheses were proposed by various ecologists to explain the nature of
climax communities: for example, polyclimx hypothesis by Braun-Blanquet (1932) and Tansley
(1939): climax pattern hypothesis by Whittaker (1953), Mac intosh (1958) and Sellack (1960):
and stored energy theory of information theory by Fosberg (1965, 1967) and Odum (1969).
Odum (1969) defined succession in terms of 3 parameters, viz.:
(1) Succession is an orderly process of reasonable directional and fairly predictable
community development;
(2) Succession results from modification of the physical environment by a community,
i.e. succession is largely community controlled.
(3) Succession culminates in a stabilized ecosystem in which maximum biomass and
symbiotic function between organisms are maintained per unit of available energy
flow. Whittaker (1975), held that through the course of succession community
production, height and mass, species-diversity, relative stability, and soil depth and
differentiation generally all tend to increase. The culminating point of succession
is a climax community of relatively stable species composition and steady-state
function, It is adapted to its habitat. It is permanent in its habitat if it is not
disturbed.
Illustrations
Ecological succession can be explained with the help of illustrations as under: -
1. Lake
When a lake fills with silt it changes gradually from a deep to a shallow lake of pond,
then to a marsh, and beyond this, in some cases, to a dry-land forest.
2. Crop field
When a crop field is deserted or a forest is severely burned over, it is just like a plot
of bare ground and a series of plant communities grow up there and replace on another -
firest annual weeds, then perennial weeds and grasses, then shrubs, and trees-until a forest
ends the development.
In this way, ecological succession is an orderly and progressive replacement of one
community by another until a relatively stable community, called the climax community,
occupies the area.
(1) In the first example the principal cause of the change in the community was
physical process-the filling in of the lake with silt.
(2) In the second example, a principal cause was the growth of plants on an existing
soil.
Development
Ecological succession develops as under:
1. Pioneers
The first organisms to become established in an ecosystem undergoing succession are
called pioneers; the stable community that ends the succession is termed the climax
community.
2. Sere
The whole series of communities which are involved in the ecological succession at a
given area. For example, from grass to shrub to forest, and which terminates in a final
stable climax community, is called as sere.
3. Seral stage
Each of the changes that take place is a seral stage.
4. Community
Each seral stage is a community, although temporary, with its own characteristics. It
may remain for a very short time or for many years.
Classification of Seres
Seres are sometimes classified according to the predominant force that is bringing them
about. These forces are biotic, climatic, physiographic, and geologic. Their resultant seres
are commonly called bioseres, cliseres, eoseres and geoseres.
Types of Succession
The succession may be of the following two types:
1. Primary Succession
Primary Succession is the process of species colonization and replacement in which the
environment is initially virtually free of life. In the other words the process starts with base
rose or sand dune or river delta or glacial debris and it ends when climax is reached. The
sere involved in primary succession is called presere.
2. Secondary Succession
Secondary succession is the process of change that occurs after an ecosystem is disrupted
but not totally obliterated. In this situation, organic matter and some organisms from the
original community will remain; thus the successional process does not start from scratch.
As a result, secondary succession is more rapid than primary. It is seen in areas burned by
fire or cut by farmers for cultivation. The sere involved in secondary succession is called
subsere.
Types of Succession
The primary and secondary successions may be of three types. The classification is on
the basis of the moisture contents:
(a) Hydrach of Hydrosere
The succession when starts in the aquatic environment such as ponds, lakes, streams,
swamps, bogs, etc. is called hydrach or hydrosere.
(b) Mesarch
The succession when begins in and area, where adequate moisture is present, is called
mesarch.
(c) Xerach or Exerosere
The succession when starts in xeric or dry habitat having minimum amounts of moisture,
such as dry deserts, rocks, etc. is called xerach. A temporary community in an ecological
succession on dry as sterile habitat is called xerosere. It may be of three types as under:-
(1) Iithosere-succession initiating on sand;
(2) Psammosere-succession initiating on sand;
(3) Halosere-succession starting on saline water or soil.
Autogenic Community
Autogenic community is the succession progressing entirely as a result of interactions
of the organisms and their environment (i.e. “driving force” is internal to the community)
for example succession on sand dunes.
Allegonic Community
Allegonic community is the succession moving under the influence of external factors,
as input of nutrients, succession in a small pond or bog.
Autotrophic And Heterotrophic Succession
Sometimes, succession is classified as autotrophic and heterotrophic on the basis of
community metabolism:
(1) Autotrophic succession is characterized by early and continued dominance of
autotrophic organisms like green plants. It begins in a predominantly inorganic
environment. In it the energy flow is maintained indefinitely.
(2) Heterotrophic succession is characterized by early dominance of heterotrophs, such
as bacteria, actinomycetes, fungi and animals. This sort of succession begins in a
predominantly organic environment and there is a progressive decline in the energy
content.
Serule
The miniature succession of micro-organic environment and different types of fungi on
the fallen logs of the decaying wood, tree bark, etc. is called serule.
Drury and Nisbet (1973) classified succession into three main types:
(a) Category I includes many classical types of secondary succession and some primary
successions. It involves temporal sequences on one site with climate and
physiography mostly remaining stable.
(b) Category II includes many primary successions (especially those in ponds and
lakes) and a few secondary successions. In this, temporal sequences on site with
the local environment changes under the influence of such external factors as
climate, erosion, drainage, nutrient inputs, etc.
(c) Category III includes those changes, which take place over long (geological) time
scale, and cover spatial sequences on adjacent sites.
Common attributes of Ecological Succession
Some Common attributes of ecological succession are as under:
Table 3.3 Ecosystem Attributes
Ecosystem attributes Development Stage Mature Stages
Community Energetics More or less than 1 About 1 (or
Gross production/community respiration approaches 1)
Gross production/standing crop biomass High Low
Biomass supported/unit energy flow Low High
Net community production (yield) High Low
Food Chains Mainly grazing; detrital Predominantly linear
web like
B. Community Structure
Total organic matter Less More
Inorganic nutrients Extrabiotic Intrabiotic
Species diversity Low High
Biochemical diversity Low High
Spatial heterogeneity and stratification Poor Well-organized
(Contd.
C. Life History
Niche specialization Broad Narrow
Organism size Small Big
Life Cycle Simple & short Complex, Long
D. Nutrient Cycling
Mineral cycles Open Closed
Nutrients exchange rate Rapid Slow
Role of detritus in nutrient regeneration
E. Selection
Growth For rapid growth “I” Mainly for feedback
selection control (k-selection
Quantity Quality
F. Overall Homeostasis
Internal symbiosis Undeveloped Developed
Nutrient conservation Poor Good
Stability (resistance to external perturbations Poor Good
Entropy High Low
SUCCESSION: GENERAL PROCESS, CLIMAX
General Process
The process of succession being with a bare area or nudation formed by several reasons,
such as volcanic eruption, landslide, following sequential steps.
1. Nudation
The process of succession begins with a bare area or nudation formed by several reasons,
such as volcanic eruption, Landslide, flooding, erosion, deposite, fire, disease, or other
catastrophic agency. Man also may be reason of formation of new lifeless bare areas for
example, walls, stone quarrying, burning, digging, flooding large land areas under reservoirs,
etc.
2. Invasion
The invasion means the arrival of the reproductive bodies or propagules of various
organisms and their settlement in the new or bare area. Plants are the first invaders
(pioneers) in any area the animals depend on them for food. The invasion includes the
following three steps:
(a) Dispersal or migration: The seeds, spores or other propagules of the species
reach the bare area through air, water or animals.
(b) Ecesis: Ecesis is the successful establishment of migrated plant species into the
new area. It includes germination of seeds or propagules, growth of seedlings and
starting of reproduction by adult plants.
(c) Aggregation: In this stage, the successful immigrant individuals of a species
increase their number by reproduction and aggregate in large population in the
area. As a result individuals of the species come close to one another.
QUESTIONS1. Explain the concept of an ecosystem with their structure and function.
2. Write the procedures of ecosystem in your own words.
3. Draw an energy flow in the ecosystem.
4. What is Ecological Succession, and food chain ? Explain in your own words.
5. Write short notes on:
(a) Ecological pyramids,
(b) Types and characteristics,
(c) Structure and functions of ecosystems.
is consumed in metabolic reactions of autotrophs for their growth, development,
maintenance and reproduction.
(3) 15 gcl/cm2/yr are consumed by herbivores that graze of feed on autographs-this
figure amounts to 17 per cent of net autotroph production.
(4) Decomposition is 3 gcal/cm2/yr which amount to be 3, 4 per cent of net production.
(5) The remainder of the plant material, 70 gcal/cm2/yr of 79.5 per cent production, is
not utilised. It becomes part of the accumulating sediments. Apparently much more
energy is available for herbivory than is consumed.
We may conclude the following conclusions
(1) Various pathways of loss are equivalent to and account for total energy capture of
the autotrophs i.e. gross production.
(2) The three upper ‘fates’ i.e. decomposition, herbivory and not utilized collectively are
equivalent to net production.
(3) Of the total energy which is incorporated at the herbivory level, i.e. 15/ gcal/cm2yr,
30 percent of 4.5 gcal/cm2/yr is used in metabolic reactions.
(4) In this way more energy is lost via respiration by herbivores (30 percent) than by
autotrophs (21 percent),
(5) Considerble energy is available for the carnivores, namely 10.5 gcal/cm2/yr
or 70-per cent. It is not entirely utilized, merely 3.0 gcal/cm2/or 28.6 per cent
of net production passes to the carnivores. This utilization of resources is
evidently more efficient than the one, which occurs at autotroph-herbivore
transfer level.
(6) At the carnivore level the consumption in metabolic activity is about percent of the
carnivores energy intake.
(7) The remainder becomes part of the un-utilized sediments;
(1) There is Noe-way Street along which energy moves (unidirectional flow of energy.
(a) The energy that is captured by the autotrophs does not revert back to solar
input.
(b) The energy which passes does not pass back to the autotrophs. It moves
progressively through the various trophic levels. As such, it is no longer
available to the previous level. Since there is one-way flow of energy, the
system would collapse in case the primary source, the sun, were cut off.
(2) Secondly, progressive decrease in energy level is seen at each trophic level. This
decrease is accounted as under:
(i) By the energy dissipated as heat in metabolic activities.
(ii) Measured here as respiration coupled with unutilized energy.
Below is a figure after Epodum (1963)
Trophic levels
G reen plants
producers
Consum ers
2
Herbivores
NU NA
3
Carnivores
Total light
1 and L
heat
3000 — 15000 15 1.5 0.3
R R R
L
A PN P 2 P 3
K cal/m /da 2 y
L
P
N 1 A
A
P 1 P
1
P or
G
Fig. 3.2 Energy flow diagram
This is a simplified energy flow diagram
(1) The diagram depicts three trophic levels. Boxes numbered 1, 2, 3 in a leaner food
chain exhibit these.
(2) L. shows total energy input (3000).
(3) LA shows light absorbed by plant cover (1500).
(4) P.G. shows gross Primary production.
(5) A shows total assimilation.
(6) Pn shows net primary production.
(7) P shows secondary (consumer) production.
(8) Nu shows energy not used (stored or exported).
(9) NA shows energy not assimilated by consumers (egested).
(10) R shows respiration.
Some more elucidation of the figure is as under:
(1) The ‘boxes’ represent the trophic levels
(2) The ‘pipes’ depict the energy flow in and out of each level.
Energy inflows balance outflows
The first law of thermodynamics requires it. The energy transfer is accompanied by
dispersion of energy into unavailable heat (i.e. respiration). The second law requires it.
It is very simplified energy flow model of three trophic levels
Apparently the energy flows is greatly reduced at each successive trophic level from
producers to herbivores and then to carnivores. It is reflected that at each transfer of energy
from one level to another, major part of energy is lost as heat or other form. The energy flow
is reduced successively. We may consider it in either term as under:
(1) In terms of total flow (i.e. total energy input and total assimilation).
(2) In terms of secondary production and respiration components.
In this way of the 3,000 Kcal of total light, which falls upon the green plants,
approximately 50 per cent (1500 Kcal) is absorbed. Only 1 per cent (15 Kcal) of it is converted
at first trophic level. Thus net primary production comes to be at 15 Kcal. Secondary
productivity (P2 and P3 in the diagram) is about 10 percent at successive consumer trophic
levels in other words at the levels of herbivores and the carnivores. However, efficiency may
be sometimes higher as 20 per cent, at the carnivore level as shown (or P3=0.3 Kcal) in the
diagram.
It may be concluded from the above studies as under:
(1) There is a successive reduction in energy flow at successive trophic levels. Thus
shorter the food chain, greater would be the available food energy. The reason is
with an increase in the length of food chain, there is a corresponding more loss of
energy.
(2) With a reduction in energy flow (shown as ‘pipes’ in the diagram) at each successive
trophic level, there is also a corresponding decrease in standing crop or biomass
(shown as ‘boxes’ in the diagram). However, it does not mean that there exists any
correlation between the biomass and energy. Indeed energy as taken here represents
rate functions or production rates. The relationships between biomass and content
are not fixed. They may differ according to the situations. For example, one gram
of an algae (lesser biomass) may be equal to many grams (more biomass) of a forest
tree leaves as the rate of production of the algae is higher than that of tree leaves.
Y-shaped energy flow model-Two channel energy flow model
Following the example of Lindeman, several authors described energy flow modes for
different kinds of ecosystems. Two illustrations are here:
(1) Teal (1957) prepared an energy flow diagram of Root Spring in U.S.A.
(2) H.T. Odum (1957) prepared energy flow model for Silver Springs, Florida, U.S.A.
(3) 30, 810 Kcal/m2 y remained for net production.
In model given by Teal (1957) for Root Springs, most of the energy rich material eaten
by heterotrophs entered the systems as plant debris. On the other hand in the model given
by H.T. Odum (1957) for Silver Spring, most of the heterotroph’s food in food chain was
produced by green with in some systems heterotrophs consume living plants while in others
they feel on dead plant parts (detritus).
(1) In Root Springs, the chain began with dead plant parts.
(2) In Silver Springs the chain began with live plant parts.
On the basis of the studies E.P. Odum pointed out that in nature there are present two
basic food chains in any system:
(1) The grazing food chain beginning with green plant base going to herbivores and
then to carnivores, and
(2) The detritus food chain beginning with dead organic matter acted by microbes,
then passing to detritivores and their consumers (predators).
The figure given below present one of the first published energy flow models as pioneered
by H.T. Odum in 1956.
P
1
P
P3 1
P
P N
Fig. 3.3 First Energy Flow Model (1956)
The above figure illustrates energy flow in a community with a large import and
smaller export of organic matter.
P indicates gross primary production; PN indicates net primary production. P2.
P
2...............P5 indicate secondary production at the shown levels.
Gross Primary production GPP = Total photosynthetic C fixation Autotrophic Respiration,
RA = GPP-NPP
Net primary Production, NPP-RA
Heterotrophic Respiration, RH = respiration of consumers and decomposers.
Ecosystem Production, NEP = GPP-RE
The three major steps in energy flow correspond to:—
(a) Exploitation efficiency,
(b) Assimilation efficiency,
(c) Net production efficiency.
The product of the assimilation net production efficiencies gives gross production
efficiency i.e. by the fraction of the eaten material eventually transformed into consumer
biomass. The whole food web may be taken to be the product of the gross production
efficiency and the exploitation efficiency. The various kinds of energetic efficiencies can be
defined as under:
Exploitation efficiency = Ingestion of food/prey production.
Assimilation efficiency = Assimilation/ingestion;
Net production efficiency = Production/assimilation.
Ecological efficiency = Exploitation efficiency × Assimilation efficiency × Net
production efficiency;
= Consumer production/prey production;
= Production/ingestion.
Gross production = Production/ingestion.
In animals, rate of production appears to depend on body mass. Per unit body mass,
small animals are found more productive than big animals. Again invertebrates are less
productive than mammals. Molluscs, annelids, isopods, and insects are invertebrates of
intermediates size between copepods and echinoids.
Some conclusions regarding energy flow in the ecosystem are as under:
There is no quantitative relationship between the production of a certain trophic level
and the production of the next lower trophic level (both in calorific terms) except for the very
high or very low values of the former. This applies to the “phytoplankton-filter feeders” and
as well as “filtrators-invertebrate predators” trophic links in the plankton food chain.
The utilization of primary production in pelagic zone often depends on the nature of
dominant species of producers and consumers. In a system containing phyto-planktonic
algae-macroconsumers effective utilization occurs mostly via grazing. In the case of larger
algae and smaller consumers, primary production is mainly utilized via bacterial detritus
medium.
The energy transfer efficiency from the filtrator’s trophic level to their invertebrate
predators is often higher than from phytoplankton to filtrators.
ECOLOGICAL SUCCESSION-MEANING AND TYPES
Meaning of Succession
Biotic communities are not static. Instead they change through time. This change can
be understood on several levels. The simplest level is the growth, interaction and death of
individual organisms as they pass through their life cycles, affected by the cycles of seasons
and other natural phenomena. Some other levels of community change act over longer time
spans and that account for much larger changes in community composition and structure.
These include ecological succession and community evolution.
It is evident from the above said that the term succession denotes a sequence of changes
in the species composition of a community, which is generally associated with a sequence
of changes in its structural and functional properties. The term is generally used for temporal
sequence (in terms of years, decades or centuries) of vegetation on a site; although only
short term changes can be observed directly and the long term ones are inferred from
spatial sequences.
The changes associated with succession are usually progressive or directional. This fact
enables one to predict which species are likely to replace other in the course of a succession.
Sucession tends to continue until the species combinations best suited to the regional climate
and the particular site are established.
Historical Background
The basic idea of succession was in the beginning forwarded by Anon Kerner (1863) in
his book “Plant Life of the Danube Basin” during the description of the regeneration of a
swamp forest. The term ecological succession was first of all used by Hult (1885) in the study
of communities of Southern Sweden. H.C. Cowles held that communities are not static but
dynamic. This changed understanding be visualized as an orderly, directional and predictable
phenomenon. It was added that succession is autogenic i.e. regulated by biotic interactions
within the community. The central foundation of the classical theory was that early
communities alter the environment to their detriment and favour later successional
communities. It was revealed by the later studies that allogenesis was perhaps more common
and dominant than autogenesis; allogenesis means the control of community dynamics by
factors originating outside the community boundaries.
The succession of animals on these dunes was studied by ‘Shelford (1913). Later on,
Olson (1958) restudies the ecosystem development on these dunes and has given us an
updated information about it. Federick Clements (1907-1936) elaborates the principles and
theory of succession. He proposed the monoclimax hypotesis of succession. During the later
years certain other hypotheses were proposed by various ecologists to explain the nature of
climax communities: for example, polyclimx hypothesis by Braun-Blanquet (1932) and Tansley
(1939): climax pattern hypothesis by Whittaker (1953), Mac intosh (1958) and Sellack (1960):
and stored energy theory of information theory by Fosberg (1965, 1967) and Odum (1969).
Odum (1969) defined succession in terms of 3 parameters, viz.:
(1) Succession is an orderly process of reasonable directional and fairly predictable
community development;
(2) Succession results from modification of the physical environment by a community,
i.e. succession is largely community controlled.
(3) Succession culminates in a stabilized ecosystem in which maximum biomass and
symbiotic function between organisms are maintained per unit of available energy
flow. Whittaker (1975), held that through the course of succession community
production, height and mass, species-diversity, relative stability, and soil depth and
differentiation generally all tend to increase. The culminating point of succession
is a climax community of relatively stable species composition and steady-state
function, It is adapted to its habitat. It is permanent in its habitat if it is not
disturbed.
Illustrations
Ecological succession can be explained with the help of illustrations as under: -
1. Lake
When a lake fills with silt it changes gradually from a deep to a shallow lake of pond,
then to a marsh, and beyond this, in some cases, to a dry-land forest.
2. Crop field
When a crop field is deserted or a forest is severely burned over, it is just like a plot
of bare ground and a series of plant communities grow up there and replace on another -
firest annual weeds, then perennial weeds and grasses, then shrubs, and trees-until a forest
ends the development.
In this way, ecological succession is an orderly and progressive replacement of one
community by another until a relatively stable community, called the climax community,
occupies the area.
(1) In the first example the principal cause of the change in the community was
physical process-the filling in of the lake with silt.
(2) In the second example, a principal cause was the growth of plants on an existing
soil.
Development
Ecological succession develops as under:
1. Pioneers
The first organisms to become established in an ecosystem undergoing succession are
called pioneers; the stable community that ends the succession is termed the climax
community.
2. Sere
The whole series of communities which are involved in the ecological succession at a
given area. For example, from grass to shrub to forest, and which terminates in a final
stable climax community, is called as sere.
3. Seral stage
Each of the changes that take place is a seral stage.
4. Community
Each seral stage is a community, although temporary, with its own characteristics. It
may remain for a very short time or for many years.
Classification of Seres
Seres are sometimes classified according to the predominant force that is bringing them
about. These forces are biotic, climatic, physiographic, and geologic. Their resultant seres
are commonly called bioseres, cliseres, eoseres and geoseres.
Types of Succession
The succession may be of the following two types:
1. Primary Succession
Primary Succession is the process of species colonization and replacement in which the
environment is initially virtually free of life. In the other words the process starts with base
rose or sand dune or river delta or glacial debris and it ends when climax is reached. The
sere involved in primary succession is called presere.
2. Secondary Succession
Secondary succession is the process of change that occurs after an ecosystem is disrupted
but not totally obliterated. In this situation, organic matter and some organisms from the
original community will remain; thus the successional process does not start from scratch.
As a result, secondary succession is more rapid than primary. It is seen in areas burned by
fire or cut by farmers for cultivation. The sere involved in secondary succession is called
subsere.
Types of Succession
The primary and secondary successions may be of three types. The classification is on
the basis of the moisture contents:
(a) Hydrach of Hydrosere
The succession when starts in the aquatic environment such as ponds, lakes, streams,
swamps, bogs, etc. is called hydrach or hydrosere.
(b) Mesarch
The succession when begins in and area, where adequate moisture is present, is called
mesarch.
(c) Xerach or Exerosere
The succession when starts in xeric or dry habitat having minimum amounts of moisture,
such as dry deserts, rocks, etc. is called xerach. A temporary community in an ecological
succession on dry as sterile habitat is called xerosere. It may be of three types as under:-
(1) Iithosere-succession initiating on sand;
(2) Psammosere-succession initiating on sand;
(3) Halosere-succession starting on saline water or soil.
Autogenic Community
Autogenic community is the succession progressing entirely as a result of interactions
of the organisms and their environment (i.e. “driving force” is internal to the community)
for example succession on sand dunes.
Allegonic Community
Allegonic community is the succession moving under the influence of external factors,
as input of nutrients, succession in a small pond or bog.
Autotrophic And Heterotrophic Succession
Sometimes, succession is classified as autotrophic and heterotrophic on the basis of
community metabolism:
(1) Autotrophic succession is characterized by early and continued dominance of
autotrophic organisms like green plants. It begins in a predominantly inorganic
environment. In it the energy flow is maintained indefinitely.
(2) Heterotrophic succession is characterized by early dominance of heterotrophs, such
as bacteria, actinomycetes, fungi and animals. This sort of succession begins in a
predominantly organic environment and there is a progressive decline in the energy
content.
Serule
The miniature succession of micro-organic environment and different types of fungi on
the fallen logs of the decaying wood, tree bark, etc. is called serule.
Drury and Nisbet (1973) classified succession into three main types:
(a) Category I includes many classical types of secondary succession and some primary
successions. It involves temporal sequences on one site with climate and
physiography mostly remaining stable.
(b) Category II includes many primary successions (especially those in ponds and
lakes) and a few secondary successions. In this, temporal sequences on site with
the local environment changes under the influence of such external factors as
climate, erosion, drainage, nutrient inputs, etc.
(c) Category III includes those changes, which take place over long (geological) time
scale, and cover spatial sequences on adjacent sites.
Common attributes of Ecological Succession
Some Common attributes of ecological succession are as under:
Table 3.3 Ecosystem Attributes
Ecosystem attributes Development Stage Mature Stages
Community Energetics More or less than 1 About 1 (or
Gross production/community respiration approaches 1)
Gross production/standing crop biomass High Low
Biomass supported/unit energy flow Low High
Net community production (yield) High Low
Food Chains Mainly grazing; detrital Predominantly linear
web like
B. Community Structure
Total organic matter Less More
Inorganic nutrients Extrabiotic Intrabiotic
Species diversity Low High
Biochemical diversity Low High
Spatial heterogeneity and stratification Poor Well-organized
(Contd.
C. Life History
Niche specialization Broad Narrow
Organism size Small Big
Life Cycle Simple & short Complex, Long
D. Nutrient Cycling
Mineral cycles Open Closed
Nutrients exchange rate Rapid Slow
Role of detritus in nutrient regeneration
E. Selection
Growth For rapid growth “I” Mainly for feedback
selection control (k-selection
Quantity Quality
F. Overall Homeostasis
Internal symbiosis Undeveloped Developed
Nutrient conservation Poor Good
Stability (resistance to external perturbations Poor Good
Entropy High Low
SUCCESSION: GENERAL PROCESS, CLIMAX
General Process
The process of succession being with a bare area or nudation formed by several reasons,
such as volcanic eruption, landslide, following sequential steps.
1. Nudation
The process of succession begins with a bare area or nudation formed by several reasons,
such as volcanic eruption, Landslide, flooding, erosion, deposite, fire, disease, or other
catastrophic agency. Man also may be reason of formation of new lifeless bare areas for
example, walls, stone quarrying, burning, digging, flooding large land areas under reservoirs,
etc.
2. Invasion
The invasion means the arrival of the reproductive bodies or propagules of various
organisms and their settlement in the new or bare area. Plants are the first invaders
(pioneers) in any area the animals depend on them for food. The invasion includes the
following three steps:
(a) Dispersal or migration: The seeds, spores or other propagules of the species
reach the bare area through air, water or animals.
(b) Ecesis: Ecesis is the successful establishment of migrated plant species into the
new area. It includes germination of seeds or propagules, growth of seedlings and
starting of reproduction by adult plants.
(c) Aggregation: In this stage, the successful immigrant individuals of a species
increase their number by reproduction and aggregate in large population in the
area. As a result individuals of the species come close to one another.
QUESTIONS1. Explain the concept of an ecosystem with their structure and function.
2. Write the procedures of ecosystem in your own words.
3. Draw an energy flow in the ecosystem.
4. What is Ecological Succession, and food chain ? Explain in your own words.
5. Write short notes on:
(a) Ecological pyramids,
(b) Types and characteristics,
(c) Structure and functions of ecosystems.
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