ECOLOGY
ECOSYSTEMS
Dr. A. Arunachalam
Department of Forestry
North Eastern Regional Institute of Science & Technology
Nirjuli 791109
Arunachal Pradesh
Email: arunachalam_in@yahoo.com
Contents:
Introduction
Structure of Ecosystem
Functional Aspects of Ecosystem
Ecosystem Types
Ecosystem Degradation
DEFINITION AND CONCEPT
A complex system, in which interactions between the different components of environment occur, is referred to as
an ecosystem. Precisely, ecosystem is a structural and functional unit comprising of biotic and abiotic components
which interact with each other leading to exchange of matter between the living and non-living components and a
unidirectional flow of energy through different trophic levels (Fig. 1).
Environmental variability defines the distribution and abundance of species as well as structure of a biotic
community. However, it is equally true that the organisms themselves, in part, define the physical environment
(Fig 2). It is this inseparable link between the biological environment (the community) and the physical
environment that led Sir Authur G. Tansely, a British Ecologist (1935) to coin the term “ecosystem”. The
ecosystem is a spatial concept. For example, a pond ecosystem is clearly distinguished from the surrounding
terrestrial area when it is full of water. But there are sometimes problems in demarcating boundaries of
ecosystems. As for example, the contrast between the pond and surrounding terrestrial area is poor when the pond
is dry.
STRUCTURE OF AN ECOSYSTEM
Different components of an ecosystem have been depicted in Chart 1.
1. Abiotic components
The abiotic or non- living component of an ecosystem consists of soil, sediments, nutrients, temperature, light,
water and air.
2. Biotic components
A large number of individuals belonging to different species which adjust, adopt, interact with each other and share
the same general environment and resources form biotic component in an ecosystem based on the function and the
general manner in which organisms obtain their food material, biotic components can be grouped into:
Primary producers
These include green plants also called primary producers or photo-autoptophs. The photo-autotrophs use light
energy of the sun to transform inorganic components into organic compounds. Typically producers are chlorophyll
bearing plants, the algae of a pond, the grass of a field, the trees of a forest. They also include the purple bacteria
found in most ecosystems which also assimilate CO2 from inorganic compounds with sunlight. In certain stagnant
lakes rich in hydrogen sulfide these photosynthetic bacteria may account for 25% of the total photosynthesis.
Finally, they also cover chemosynthetic bacteria, all of which obtain energy by oxidizing simple inorganic
compounds. They play a substantial role in the movement of mineral nutrients in ecosystems. Under anaerobic
conditions these chemosynthesizers “rescue” energy in the form of organic compounds, energy that would
otherwise be “lost” through storage in sediments. Chart 2 shows the different categories of autotrophs.
Consumers or Heterotrophs
Heterotrophs are those organisms whose nutritional needs are met by feeding on other organisms. They are also
referred to as the secondary producers. A primary consumer, or, more commonly, a herbivore, is a heterotroph
which derives its nutrition directly from the plants. A carnivore or secondary consumer is a heterotroph deriving its
energy indirectly from the producer by way of feeding out the herbivores. Some ecosystems contain tertiary
consumers that feed on the secondary consumers. Omnivores are consumers that derive their energy from both
producers and herbivores.
Decomposers
Excreta of animals/plants and their dead bodies/parts are decomposed by the activity of bacteria, fungi and other
small organisms. These organisms, which bring the constituent elements of the plants and animals back to the
surrounding medium, are called as decomposers.
CO2
O2
SOLAR
ENERGY
PC
PP
SC
TC
DEAD ORGANIC MATER
DECOMPOSITION
IMPORT
FROM
OTHER
SYSTEMS
MINERAL NUTRIENT
PP = PRIMARTY PRODUCERS
PC = PRIMARY CONSUMERRS
EXPORT
FROM
OTHER
SYSTEMS
SC = SECONDARY CONSUMERS
TC = TERTIARY CONSUMERS
Fig.1. The structure of Natural Ecosystem. (Source: Asthana and Asthana, 1998).
Component of
Ecosystem
Climate
Inorganic
substances
Organic
compounds
Macroconsumers
or
Phagotrophs
Primary
producers
Abiotic components
Biotic components
Transition between abiotic
and biotic components
Microconsumers
or
Saprotrophs
Chart 1. Flowchart showing different components of ecosystem
Autotrophs
Photoautotrophs
Chlorophyll bearing
plants e.g. blue green
algae and higher
plants
Photosynthetic bacteria
Bacteria using H2S as a reductant
e.g.
green
sulphur
bacteria
(Chlorobacteriaceae)
Purple
sulphur bacteria (Thiorhodaceae)
Chemoautotrophs
e.g. nitrifiers (Nitrosomonas
and Nitrobacter) Beggiatoa
found in sulphur springs
Bacteria using organic compounds as
reductant e.g. purple and brown bacteria
(Anthiorhodaceae)
Chart 2. Flowchart showing the different categories of autotrophs
Respiration
SUN
Radiant
energy
CO2
CO2
Consumption
Litterfall
O2
H2 O
O2
Producers
H2 O
Translocation
Deposition
Nutrients
Abiotic elements
Nutrients
Consumers
Decomposition
Fig. 2. Schematic diagram of an ecosystem. The dashed lines represent the boundary of the system. The arrows
indicate interactions within the system and with the environment. (Source: Smith and Smith, 2000)
Trophic structure
The word ‘Trophic’ means ‘feeding’. The trophic structure in an ecosystem is based on the food chain.
Food chain
The sequence of organisms in which one organism feeds on the one preceding it is called food chain. Food chain
can be traced in an ecosystem such as grasses are eaten by grasshoppers which are eaten by frogs. Frogs are eaten
by snakes and snakes are eaten by vulture. A food chain can be represented as:
(i) Grass
Grasshopper
(ii) Grass
(iii) Phytoplankton
Deer
Frog
Snake
Vulture
Tiger
Zooplankton
Fish
Snake
Thus, there are four to five steps or links in a food chain where food energy is transferred to the next trophic level.
At each step a large portion of energy is lost as heat and only a small fraction goes to the eater. Therefore, the
quantity of energy goes on decreasing at a rapid rate successively from producers to top carnivores. The ratio of
energy input and that of output is called Ecological efficiency. There are two types of food chains:
a)
Grazing food chain: Such food chains are distinguished as the source of energy for the first trophic level.
Examples of grazing food chain are cattle grazing in a pasture land or deer browsing in the forests. Grazing food
chains dominate in grasslands, unpolluted freshwater bodies and detritus food chain dominate in a climax forest
and deep oceans.
b)
Detritus food chain: Such food chains are distinguished by the dead plant or animal material as the
source of energy to the first trophic level. For example, some bacteria using latter as the source of energy and these
bacteria are eaten by some nematodes. In detritus food chain, the source of energy is dead organic matter or
detritus. Detritus food chain dominates in a climax forest and deep oceans.
The quantity of energy flowing into a trophic level decreases with each increasing trophic level. This pattern
occurs because not all energy is used for production. An ecological rule of thumb is that only 10% of energy
transferred from a trophic level can be accumulated in the next trophic level. If herbivores eat 1000 Kcal of plant
energy about 100 Kcal will convert into herbivore tissue, 10 Kcal into first level carnivore production, and 1 Kcal
into second level carnivore production.
Food web
The food relations are rather complicated and many organisms behave differently in their food habits. The food
chain is nor linear. Resources are shared especially at the beginning of the chain. The plant species may be food for
a variety of herbivores, and a herbivore species may be food for several predators. Thus food chains link to form a
complex network, called food web (Fig.3). No matter how productive an ecosystem, each food chain rarely
exceeds four links, because the length is limited by the inefficiency of energy transfer. Highly productive
ecosystems do not support more species and therefore more complex food webs.
Fig. 3. Food web in a grassland ecosystem (Source: Smith and Smith, 2000)
Ecological pyramids
There is some sort of relationship between the numbers, biomass and energy content of the primary producers,
consumers of the first and second orders and top carnivores in any ecosystem. These relationships can be
represented in diagrammatic ways and are referred to as ecological pyramids. Thus if we sum all of the biomass or
energy contained in each trophic level, we can construct ecological pyramids. The ecological pyramids are of three
categories: a) of numbers, b) of biomass and c) of energy.
a)
Pyramids of numbers: Pyramids of numbers represents the relationships between the numbers of
primary producers and consumers of different order in an ecosystem. At the base of such a figure is always the
number of primary producers and the subsequent structures on this base are represented by the numbers of
consumers at successive trophic levels. The top of the pyramid represents the top carnivores in an ecosystem. The
pyramids of numbers may be upright or inverted. A forest ecosystem is an example of inverted pyramid where the
number of primary producers, the trees, is less than that of herbivore birds feeding upon the tree fruits. The number
of parasites like bugs and lice living and feeding upon birds is still higher. Thus, depending upon the size and
biomass the pyramid of numbers may not always pyramidal, it may even be completely inverted in shape.
b)
Pyramid of biomass: The pyramid of biomass depicts/shows the total bulk of organisms or fixed
energy in different trophic levels in an ecosystem present. Because some energy or material is lost at each
successive trophic level, the total mass supported at each level is limited by the next lower level. In general, the
biomass of primary producers must be greater than that of the herbivores they support, and the biomass of
herbivores must be greater than that of carnivores. Such a state is quite common in terrestrial ecosystems and is
shown in upright pyramids. In ecosystems such as lakes and open seas, primary production is largely a feature of
the microscopic algae which have a short life cycle. They are heavily grazed by herbivorous zooplankton that are
larger in size and have longer life span. As a result, despite a high primary production, the biomass of primary
producers is low compared to that of zooplankton herbivores. Such a situation is shown as an inverted pyramid.
c)
Pyramid of energy: The energy pyramid indicates energy flow at different trophic levels in an
ecosystem. Some organisms have a small biomass, but the total energy they assimilate and pass on may be
considerably greater than that of organisms with a much larger biomass. On a pyramid of biomass these organisms
would appear much less important in the ecosystem than they really are.
Energy pyramids are upright because energy transferred from a trophic is lower than the energy received by it in
accordance with the second law of thermodynamics. In instances in which the producers have less bulk than
consumers, as in open water, the energy they store and pass on must be greater than that of the next higher level.
This high energy flow is maintained by a rapid turnover of individuals rather than an increase in total mass.
A Grassland
A Forest
A forest
PYRAMID OF NUMBERS
A pond
PYRAMID OF BIOMASS
A Lake
PYRAMID OF ENERGY
Fig.4. Ecological Pyramids showing upright and vertical nature of different categories
FUNCTIONAL ASPECTS OF AN ECOSYSTEM
1. Production Process in an Ecosystem
Production in ecosystems involves the fixation and transfer of energy. Green plants fix solar energy by the process
of photosynthesis and fixation and transfer of energy in an ecosystem is governed by the laws of thermodynamics.
2. Ecosystems and the Laws of Thermodynamics
The fixation, loss and storage of energy are governed by the two laws of thermodynamics. The first law of
thermodynamics states that energy can neither be created nor destroyed; but it can be converted from one form to
another. The first law is also called the law of conservation of energy.
The second law of thermodynamics states that energy transformation from one form to the other involves loss of
some energy. The lost energy is dispersed into the universe in the from of unavailable heat energy. Energy to fuel
herbivore populations is limited to that flowing in the plant trophic level through photosynthesis. However,
utilization of plant biomass by herbivores involves transformation of energy in plant to the energy in the form of
animal biomass. Because this energy transformation can not be 100% efficient, the animals must have less energy,
and must therefore be rather than the plants they feed on. This trend continues through the successive energy
transformations of each trophic level. When energy is transferred from one trophic level to another trophic level, a
large part of that energy is lost as heat which is no longer transferable.
3. Energy Flow in an Ecosystem
Solar Energy input: The starting point of energy flow is sunlight. Of the solar radiation reaching the earth’s
atmosphere, about 30% is reflected into the space, about 20% is absorbed by the atmosphere and about 50% is
absorbed as heat by ground, water, or vegetation. Only less than 1% of the sunlight energy reaching the earth
surface is fixed through photosynthesis.
Energy flow through trophic structure: The flow of energy through most of the ecosystem starts with the
harvesting of sunlight by green plants, a process that in itself demands the expenditure of energy. A plant gets its
start by living on the food energy stored in the seed/vegetative propagule until it produces green leaves. Energy
accumulated by plants is called production, more specifically, Primary production, because it is the first and
basic form of energy storage. The rate at which energy accumulates in primary producers is primary productivity.
It is the organic matter produced by green plants or the energy trapped by the autotrophs which is passed through a
chain of consumers right up to the top of trophic structure in an ecosystem (Fig 5).The total energy storage by
autotrophs in an ecosystem is referred to as Gross Primary production (GPP). For most ecosystems, gross primary
production is equal to total photosynthesis. The plants themselves use a considerable amount of this energy in their
own respiration. The stored energy left after plant respiration is termed as net primary production (NPP). Thus,
Net Primary Production (NPP) = Gross Primary Production (GPP) – Respiration by Autotrophs(R)
Productivity is the term used to refer to production rate i.e., production per unit area and time. Energy storage at
consumer levels is referred to as secondary production and the rate of production is termed secondary productivity.
Usually, the terms “gross” and “net” are not applied to secondary production. Rather secondary production refers
to new biomass at a consumer trophic level i.e., to energy storage after the consumer’s respiration has already been
subtracted. “Assimilation” or “assimilated energy” is the term usually applied to the consumer analog of gross
primary production, that is,
Assimilation = Gross Energy intakes – Egested and Excretory energy.
Production is usually expressed in units of energy per unit area: Kilocalories/m2 (Kcal /m2). However, production
may also be expressed as the mass of dry organic matter grams per square meter (g/ m2). Productivity is usually
expressed as Kcal/ m2/year or as g/m2/year. Net primary production accumulates over time as plant biomass. The
amount of accumulated organic matter found in an area at a given time is the standing crop biomass. Like
production, biomass is usually expressed as g/ m2 or as Cal/m2. Biomass differs from productivity; biomass is the
amount present at any given time. Productivity is the rate at which organic matter is produced.
Certainly, a wide range in the efficiency of conversion (assimilation and production efficiencies) exists among
different feeding groups. Production efficiency in photosynthetic organisms (Net production/solar radiation) is
low, ranging from 0.34% in some phytoplankton to 0.8 to 0.9% in grassland vegetation.
Assimilation efficiencies vary widely among poikilotherms and homeotherms. Homeotherms are much more
efficient than poikilotherms. However, carnivorous animals, even poikilotherms, have high assimilation
efficiencies. Because of high maintenance and respiratory costs, homeotherms have low production efficiency
compared to poikilotherms. Only about 2 to 10% of energy consumed by herbivore homeotherms goes into
biomass production. However, poikilotherms convert about 17% of their consumption to herbivore biomass.
The ratio of phytoplankton to zooplankton production in open freshwater ecosystems is about 7.1:1 and the ratio of
herbivore zooplankton production to carnivore zooplankton production is 2.1:1. Efficiencies are lower in the
benthic (bottom dwelling organisms in an aquatic system) community - 2.2 for herbivores and 0.3 for carnivores.
Dead Bodies,
Excretions
Decomposers
Dead Bodies,
Excretions
Dead Bodies,
Excretions
Sun
Light
Dead Bodies,
Excretions
Dead Bodies,
Excretions
Heat
Producers
Food
Primary
Consumers
Food
Secondary
Consumers
Food
Tertiary
Consumers
Respiration
Respiration
Respiration
Respiration
Heat
Heat
Fig.5. Energy flow in an ecosystem (Source: Brewer, 1994)
Heat
Heat
Biogeochemical cycles
Energy and the materials flow through the ecosystem together as organic matter; one cannot be separated
from the other. The continuous cycle of materials together with the one way flow of energy keeps the
ecosystem functioning.
All nutrients flow from the non-living to the living and back to the nonliving components of the ecosystem
in a more or less cyclic path and this cycling is known as biogeochemical cycling.
Types of Biogeochemical cycles: There are two basic types of biogeochemical cycles:
1.
Gaseous cycles: In this type of biogeochemical cycles the reservoir of elements is the
atmosphere and the ocean. For example, Carbon, nitrogen and water cycling.
2.
Sedentary cycles: In this type, the reservoir pool is the earth’s crust. Mineral salts come directly
from earth’s crust through weathering. The soluble salts then enter the water cycle. With water they reach
the seas, where they remain indefinitely. Other salts return to earth’s crust through sedimentation. They
become incorporated into salt beds, silts and limestone. After weathering, they the cycle again. Phosphorous
cycling is a typical example of sedentary cycles.
Cycles such as the sulfur cycles are a hybrid between the gaseous and the sedimentary, because they have
reservoirs not only in earth’s crust but also in the atmosphere.
1.
The hydrologic cycle or water cycle: The major pathway of the water cycle is an
interchange between the earth’s surface and the atmosphere via precipitation, evaporation and transpiration,
the energy for which is derived from the sun (Fig.6). The cycle is a steady state one because the total
precipitation is balanced by total evaporation. Although there is a misbalance in favor of the precipitation
that occurs over land, there is greater evaporation from the ocean, the compensating factor being the runoff
from the land. Significant amount of water is incorporated by ecosystems in protoplasmic synthesis and a
substantial return to the atmosphere occurs by way of transpiration from living plants.
Of the total estimated water on the earth and in its atmosphere, only about 5 percent is actually or potentially
free and in circulation, and nearly 99 percent of that is in the ocean. In contrast, 95 percent of the earth’s
water is bound in the lithosphere and in sedimentary rocks. Freshwater amounts to only about 0.1 percent of
the total supply of water and three quarters of that is bound up in polar ice caps and glaciers. Water in ice
caps and glaciers is estimated to be equal to a 50 m deep water column over the entire surface of the earth.
1
Although total precipitation is an impressive quantity, it is even more significant to note that its source - the
atmospheric water vapor, constitutes an infinitesimal amount. It has been estimated that the amount of water
vapor in the atmosphere is equivalent of 2.5 cm of rain covering the entire surface of the earth.
Fig.6. Hydrological cycle (Source: Smith and Smith, 2000)
2.
Carbon Cycle: Carbon is a basic constituent of all organic compounds. The source of all carbon
in both living organisms and fossil deposits is carbon dioxide (CO2) in the atmosphere and the waters of
Earth. Photosynthesis draws CO2 from the air and water into the living component of the ecosystem. Just as
energy flows through the grazing food chain, carbon passes to herbivores and then to carnivores. Primary
producers and consumers release carbon back to the atmosphere in the form of CO2 by respiration, excreta
and litter (Fig.7). Decomposers release CO2 to the atmosphere through respiration and decomposition of
dead organic matter.
The rate at which carbon cycles through the ecosystem is determined by a number of processes, particularly
the rates of primary productivity and decomposition. Both processes are strongly influenced by
environmental conditions such as temperature and precipitation. In warm wet ecosystems such as tropical
rainforest, rates of productivity and decomposition are high and carbon cycles through the ecosystem
quickly. In cool dry ecosystems, the process is slower. In ecosystems where temperatures are very low,
2
decomposition is very slow and
dead organic matter accumulates. In swamps and marshes where dead
materials fall into the water, organic material does not completely decompose. Stored as raw humus or peat,
carbon circulates very slowly. Over geologic time, this build up of partially decomposed organic matter in
swamps and
marshes has formed fossil fuels.
Similarly in freshwater and marine ecosystems phytoplankton uses CO2 that diffuses into the upper layers of
water or is present as carbonates and converts it into plant tissue. The carbon then
passes from the
primary producers through aquatic food chain. The CO2 produced through respiration is either reutilized or
reintroduced to the atmosphere by diffusion from the water surface to the surrounding air. Significant
portion of carbon is bound as carbonates in bodies of mollusks and foraminifers. Some of these carbonates
dissolve back into solution, while some become buried in the bottom mud at varying depths when the
organisms die. Isolated from biotic activity this carbon is removed from cycling. Incorporated into bottom
sediments, over geologic time, it may appear in coral reefs and limestone rocks.
Fig.7. Carbon Cycle a) Pools and fluxes b) C-cycle in terrestrial ecosystem (excluding consumers) c) Ccycle in aquatic ecosystems (Source: Smith and Smith, 2000)
3
3.
Nitrogen cycle: Nitrogen is an essential constituent of proteins. Nitrogen, although present as a
major constituent in the atmosphere (79%), can be utilized by only a few organisms. To be used by
organisms, free molecular nitrogen is to be fixed. This fixation comes about two ways (Box 1). Another
source of nitrogen is organic matter broken down by decomposition and mineralization processes including
ammonification, nitrification and denitrification (Box 2).
Box 1. Ways of N2 fixation.
High energy fixation - Cosmic radiation, meteorite trails, and lightning provide the high energy needed to
combine nitrogen with oxygen and hydrogen of water. The resulting ammonia and nitrates are carried to
earth’s surface in rain water. Estimates suggests that less than 8 - 9 kg N/ha comes to earth annually in this
manner. About two thirds of this amount comes as ammonia and one third as nitric acid.
Biological fixation - This method produces 100 - 200 kg N/ha, or roughly 90% of the fixed nitrogen
contributed to earth each year. This fixation is accomplished by symbiotic bacteria living in association with
leguminous and non-leguminous plants, by free living aerobic bacteria, and by cyanobacteria (blue green
algae). Fixation splits molecular N2 into two free N-atoms. The free N-atoms then combine with hydrogen to
form two molecules of ammonia. This process requires considerable energy. To fix 1 kg of N2, N2-fixing
bacteria associated with the root system of a leguminous plant must expend about 10 gm of glucose.
In agricultural ecosystems about 200 species are the prominent N2 fixers. In non-agricultural ecosystems
some 12,000 species from cyanobacteria to nodule bearing plants are responsible for N2-fixation. Also
contributing to the fixation of N2 are free living soil bacteria. The prominent of the 15 known genera are the
aerobic Azotobacter and the anaerobic Clostridium. Cyanobacteria (blue green algae) are another important
group of largely non-symbiotic N2-fixers. Of some 40 known species, the most common are in genera
Nostoc and Ulothrix, which are found both in soil and in aquatic habitats. Certain lichens (Collema
tunaeforme and Peltigera rufescens) are also implicated in N2-fixation. Lichens with N2-fixing ability
possess N2-fixing cyanobacteria as their algal component.
Box 2. Crucial processes of N2 cycle
•
Ammonification: Decomposers break down the amino acids in dead organic material to release energy.
It is a one-way reaction. Ammonia is then directly absorbed by plant roots and incorporated into amino
acids, which pass through the food chain.
•
Nitrification: A biological process in which ammonia is oxidized to nitrite and nitrate, yielding energy.
Two groups of microorganisms are involved in the process. Nitrosomonas bacteria utilize the ammonia
in the soil as their sole source of energy. They promote its transformation into nitrite and water. Nitrite
is then further transformed into nitrate by another group of bacteria, Nitrobacter.
•
Denitrification: Nitrogen in the form of nitrate is transformed through this process into gaseous
nitrogen by denitrifiers, represented by fungi and Pseudomonas bacteria. Like nitrification,
denitrification takes place under certain conditions- a sufficient supply of oxygen, a pH range from 6-7
and optimum temperature of 600C.
4
Fig.8 shows the cycling of nitrogen in the biosphere. The sources of nitrogen under natural conditions are the
biological fixation of atmospheric nitrogen, additions of inorganic nitrogen in rainfall from such sources as
lightning and volcanic activity; ammonia absorption from atmosphere by plant and soils and nitrogen
accretion from windblown aerosols, which contain both organic and inorganic forms of nitrogen.
In terrestrial ecosystems, nitrogen is largely taken up by the plants in the form of ammonia or nitrates, which
convert it to amino acids. Eventually, animal wastes and dead plant and animal tissue are broken down by
bacteria and fungi into ammonia. Ammonia may be lost as gas to the atmosphere, acted upon by nitrifying
bacteria, or taken up directly by plants. Nitrates may be utilized by plants, immobilized by microbes, stored
in decomposing organic matter, or leached away. Leached material runs off to streams, lakes and eventually
to the sea, where it is available for use in aquatic ecosystems.
In aquatic ecosystems, nitrogen cycles in a similar manner, except that the large reservoir in the soil is
lacking. Life in the water contributes organic matter and dead organisms that undergo decomposition,
releasing ammonia and nitrate.
Under natural conditions, nitrogen lost from ecosystems by denitrification, volatilization, leaching, erosion
and windblown aerosols is balanced by biological fixation and other sources.
4.
Phosphorous cycle: Phosphorous is practically absent in the atmosphere. It can follow the
hydrological cycle but only over part of the way, from land to sea (Fig.9). The main reservoirs of
phosphorous are rocks and natural phosphate deposits, from which it is released by weathering, leaching and
erosion. Some phosphorous passes through the grazing food chain and is cycled back to soil and water by
excretion, death and decomposition. In terrestrial ecosystems organic phosphates unavailable to plants are
transformed by bacteria to inorganic phosphates. Some inorganic phosphates is absorbed by plants, some
gets transformed by various chemical processes into unavailable compounds and some get immobilized
within the bodies of microorganisms. Some of the phosphorous of terrestrial ecosystems escapes to lakes and
seas.
5
Fig.8. Nitrogen Cycle (Source: Smith and Smith, 2000)
In marine and freshwater ecosystems, phosphorous moves through three states: particulate organic
phosphorous, dissolved organic phosphates and inorganic phosphates. Organic phosphates are taken up
quickly by all forms of phytoplankton and eaten in turn by zooplanktons and detritus feeding organisms.
Zooplankton may excrete as much phosphorous daily as it stores in its biomass. More than half of the
phosphorous zooplankton excretes is inorganic phosphates, which is taken up
by
phytoplankton.
The
remainder of phosphorous in aquatic ecosystems is in the form of organic compounds that may be utilized by
6
bacteria which fail to regenerate much dissolved inorganic phosphate. Part of the phosphate is deposited in
shallow sediments and part in deep waters. In the ocean upwelling (the process of the movement of deep
waters to the surface) brings some phosphates from the deep to surface waters where light is available to
drive the process of photosynthesis. These phosphates are taken up by phytoplankton. Part of the
phosphorous contained in the bodies of plants and animals sinks to the bottom and is deposited as sediments.
As a result, surface waters may become depleted of phosphorous and the deep waters saturated. Humans
have altered the phosphorous cycle through the waste disposal and the application of phosphate fertilizers to
croplands. Some fertilizer phosphates react with Ca, Fe and Al in the soil and become immobilized. Another
part goes along with the harvested crop. Concentrations of P in the wastes of food processing plants and
feedlots add excessive phosphates to natural waters. Greater quantities are released in urban areas; elsewhere
the phosphate concentrates in sewage systems and is added to the waterways. In aquatic ecosystems
vegetation takes up phosphorous rapidly, causing a sudden increase in algal biomass. Eventually, all the
phosphorous mobilized by humans becomes immobilized in the soil or in the bottom sediments of aquatic
bodies leading to a process called ‘Eutrophication’. In many lakes worldwide, the input of large quantities of
phosphorous and nitrogen from agricultural runoff and sewage produces ideal conditions for high
phytoplankton activity. In such cases of Eutrophication (enrichment), the lake water becomes turbid
because of dense populations of phytoplankton and large water plants are out competed and disappear along
with their associated invertebrate populations. In addition, decomposition of the large phytoplankton
biomass may lead to low oxygen concentrations which kill fish and invertebrates. The outcome is a
productive community, but with low biotic diversity.
7
Fig.9. Phosphorus cycle (Source: Smith and Smith, 2000)
5.
Sulfur cycle: Sulfur is an essential component of protoplasm, occurring in various amino acids,
enzymes and other compounds such as those producing odors of garlic and skunks. Uptake by plants is
mostly as sulfate from soil (or water fro aquatic plants Fig.10.). The decay of plant and animal tissue by
various bacteria and fungi eventually yields sulfates or hydrogen sulfide. Sulfate is regenerated from H2S,
mostly in water, by chemosynthesis and photosynthetic sulfur bacteria. Oxidation of H2S to sulfate also
occurs abiotically in water and in the atmosphere. Sulfate is returned to land from the sea as windblown sea
salts. Globally more than one-fourth of the sulfur going into the atmosphere is SO2 released from the burning
of fossil fuels. Chemical reactions in the atmosphere yields sulfuric acid, producing acid rain. Acid rain is a
result of fossil fuel burning, which produces sulfur oxides, and nitric oxide which may combine with
atmospheric water to form sulfuric acid (H2SO4) and nitric acid (HNO3), respectively. The term ‘acid
deposition’ is more accurate as acid may also be deposited from the air in the form of snow, sleet and fog.
Acid rain reduces pH of soil and lakes, while acidification can also cause the death of trees and allow toxic
metals (e.g. aluminum and mercury) to be leached from soils and sediments. Water bodies in northern
temperate regions of Europe and North America have suffered from acidification due to ‘acid rain’.
8
Fig.10. Sulfur Cycle (Source: Brewer, 1994)
9
Table 1: Net Primary Production and Plant Biomass of World Ecosystems
Ecosystems (in order
of productivity)
World
Area
2
2
(10 km )
Net
Mean Biomass
Mean Net Primary
Primary
per Unit Area
Production per
Production
(kg/m2)
Unit Area (g/ m2/yr)
(109mtn/yr)
Continental
Tropical rain forest
17.0
2000.0
34.00
44.00
Tropical seasonal forest
7.5
1500.0
11.30
36.00
Temperate evergreen forest
5.0
1300.0
6.40
36.00
Temperate deciduous forest
7.0
1200.0
8.40
30.00
Boreal forest
12.0
800.0
9.50
20.00
Savannah
15.0
700.0
10.40
4.00
Cultivated land
14.0
644.0
9.10
1.10
Woodland and shrubland
8.0
600.0
4.90
6.80
Temperate grassland
9.0
500.0
4.40
1.60
Tundra and alpine meadow
8.0
144.0
1.10
0.67
Desert shrub
18.0
71.0
1.30
0.67
Rock, ice, sand
24.0
3.3
0.09
0.02
Swamp and marsh
2.0
2500.0
4.90
15.00
Lake and stream
2.5
500.0
1.30
0.02
149.0
720.0
107.09
12.30
Algal beds and reefs
0.6
2000.0
1.10
2.00
Estuaries
1.4
1800.0
2.40
1.00
Upwelling zones
0.4
500.0
0.22
0.02
Continental shelf
26.6
360.0
9.60
0.01
Open ocean
332.0
127.0
42.00
0.003
Total marine
361.0
153.0
55.32
0.01
Global total
510.0
320.0
162.41
3.62
Total continental
Marine
Source: Whittaker and Likens (1973)
5. State of Dynamic Equilibrium and System Homeostasis:
At each level in the trophic structure any fluctuation in the rate of input is countered by an equal and
opposite attraction of the output rates. Self regulation ability of ecosystem is evident in terms of a state of
10
dynamic equilibrium is referred to as system homeostasis. However, all ecosystems have the ability to
perform homeostatic adjustments only within certain limits. For example, when herbivores become
numerous, predator population rises to regulate the population of herbivores. Too much strain could cause
irreparable damage to an ecosystem.
Ecotones: The zone of vegetation separating two different types of communities is called ecotone or
tension zone. These are the marginal zones sometimes easily recognizable. The development of a
community is intimately related to the prevailing environment and as such difference in community structure
and physiognomy reflects a difference in their environments such as the moisture condition, biotic influence
or any other factors. Therefore, an ecotone is a region where the influence of two different patterns of
environment work together and hence the vegetation of ecotones is somewhat specialized. The width of an
ecotone may be narrow or wide. Usually, in ecotones the variety of species is greater than in any of the
adjacent communities. The phenomenon of increased variety and of plants at the community junctions is
called the edge effect and is essentially due to a wide range of suitable environmental conditions.
Ecotypes: The fact that two individuals belong to the same species does not guarantee that they will
respond identically to some ecological factors such as temperature or light. Individuals differ, of course, and
so may local populations. Populations having genetically based differences of ecological importance have
been termed ecotypes., though the term now seems to be used mainly to refer to plants showing striking
morphological adaptations to habitat. Simply, these are the products of genetic response of populations with
the environmental conditions. As against such heritable variants, there are a number of cases where
populations show different morphological appearance in different habitats i.e., the variations is not
genetically fixed and these are called as ecophene or ecads. For example, Euphorbia sp. is a common
herbaceous species showing morphological variations in disturbed and undisturbed habitats. Agrostis tenuis
is a common example for heavy-metal-tolerant ecotypes.
ECOSYSTEM TYPES
The first climatic map and standard ‘Koppen classification’ of climate were based on vegetation maps.
Koppen worked on the hypothesis that the distribution of vegetation reflected the underlying pattern of
climate. These great plant formations are what we call now as Biomes. Biomes represent the divisions of the
major community types of the world, and are characterized by specific climate conditions. According to
Whittaker, R. H. (1970), the major types of community, conceived in terms of physiognomy on a given
continent is a biome or formation. “Biome” is used when the concern is with both plants and animals and
11
formation is used when the concern is with plant communities only. These are the products of interaction of
regional climate with biota and substrate. A biome is a grouping of terrestrial ecosystems on a given
continent that are similar in vegetation structure (physiognomy), in the major features of environment to
which this structure is a response and in some characteristics of their animal communities. The biome
concept which is most widely applied to land ecosystems can also be applied in aquatic environment to such
zones as the worldwide structural type defined by major kind of organisms.
Ecosystems can be classified a number of ways such as based on the difference in growth medium of plants,
disturbances, management, vegetation, climate and maturity:
1. On the basis of human influence ecosystems can be grouped into:
a) Natural Ecosystems
b) Man made or Modified Ecosystems
2. On the basis of major habitat type:
a) Terrestrial Ecosystems – which operates on the land.
b) Aquatic Ecosystem – that operates in the aquatic habitats.
Both terrestrial and Aquatic Ecosystems may be natural as well as man – made ecosystems.
Terrestrial Ecosystems
Terrestrial ecosystems occurring on land or characterized by plant growth on typical soil can further be
divided into the following types:
1.
2.
3.
4.
5.
6.
Forest ecosystem
Grassland and Savannah ecosystem
Desert and shrubland ecosystem
Tundra ecosystem
Man made ecosystems such as agroecosystems, Gardens, etc…
Other special habitats such as Caves, Phytotelmata and Aeolian biome.
Aquatic Ecosystems
Aquatic ecosystems where plants grows in water can be divided into:
1.
2.
3.
4.
5.
Fresh water ecosystem
Pond and ecosystem
River and stream ecosystem
Wetland ecosystem such as Marsh, Bog, Fen, Shrub – Carr and Swamps.
Estuarine ecosystem – mud flats and mangrove forests.
1. FOREST ECOSYSTEMS
A forest is a plant community dominated by trees. The most diverse of these ecosystems are confined to the
tropics. Amongst forest ecosystems, the tropical rain forests are the most diverse. They have characteristic
12
features like drips tip of leaves, thin bark, superficial and buttressed roots. The forest type of a particular
region is particularly dependent on rainfall, temperature, latitude or altitude. Other forest types like the
Coniferous boreal forests are characteristic of cold climates and high elevation. Temperate forests occur at
lower latitudes, where there is sufficient rainfall, giving way to tropical rainforest toward the equator with
higher temperature and rainfalls. The boreal forests have long, cold winters with light precipitation in the
winter and more rain in the summer. The soils are podzols, acidic and humus rich. The climate of temperate
forests is also seasonal, with temperatures falling below freezing in winter and warm humid summer.
Rainfall is 70m to 190m per year. Soils are well developed and rich. In the tropics, the forest climate is nonseasonal and warm with frequent heavy rainfall; soils are acidified and poor nutrient.
Major Vegetation and animals
Forests tend to have a high net primary productivity and also a high biomass (Table 1). Boreal forests are
dominated by coniferous tree species which are adapted to minimize evapotranspiration and tissue damage
from freezing. Temperate forests are consist mainly of broad leaved, deciduous species and the complex
layered structure of the vegetation leads to high primary production. The high biodiversity of tropical
rainforests results from their great age and complex physical environment.
Temperate and boreal forests support herbivores mammals (e.g., Deer), and predatory species such as
wolves. Forests are important habitats for birds and in temperate forests, small mammals which inhabit the
dense understorey. Forest ecosystems also contain a wide range of specialist and generalist insect herbivores.
Tropical rain forests support tremendous animal diversity, particularly insects, amphibians, reptiles, birds
and small mammals. Many species are tree dwelling, feeding on fruit and seeds.
2. GRASSLAND AND SAVANNAH ECOSYSTEMS
Grassland occur where rainfall in intermediate between that of deserts and forests. There are two major types
of grasslands depending on the temperature:
a)
Savannah or tropical grassland – often with scattered trees is most extensive in Africa and also found in
Australia, South America and Southern Asia.
b)
Temperature grasslands – occur across large areas of Eastern Europe and Asia (steppe), central North
America (Prairie), and South America (Pampas).
Tropical grasslands may receive up to 1200mm of rain in the wet season, but none during the prolonged dry
season. Low soil moisture impedes nutrient cycling and reduces nutrient availability. Temperate grasslands
have between 250 and 600mm of rainfall per annum. The climate is continental with hot summers and cold
13
winters. Grassland soils receive a large amount of organic matter and are very rich, making them well suited
to the growing of arable crops such as Corn and Wheat.
Major Vegetation and animals
Grasslands have high primary productivity and relatively low biomass. Managed grasslands are used for
crops and for rangeland. Mainly dominated by grass species, but frequently included trees, such as the
Acacia, which are characteristics of African savannah. Temperate grasslands include broad – leaved
perennials which either flower early in the season or after the grasses have died down. The African savannah
supports large populations of grazing and browsing animals. Those herbivores in turn support large number
of mammalian carnivores. The uniformity of the vegetation structure, the absence of trees and the short
growing season limit the diversity of birds and amphibians.
3. DESERTS, SEMI-DESERTS AND SHRUBLAND ECOSYSTEM
Hot deserts are found around latitudes 30ºN and 30ºS. Semi-desert ecosystems occur in less arid regions, but
where water remains limiting. Temperate shrubland is formed around the shrubs of the Mediterranean Sea,
where it is known as Maquis and in Southern California where it is called Chaparral. Cool semi deserts occur
in parts of North America, Central Asia and mountainous region where the climate is too dry for grassland.
Climate and Soil
Deserts have less than 50mm of annual rainfall, hot days and cold nights. Soils are nutrient poor, thin and
freely drained. When rainfall does occur it usually penetrates the soil very quickly or runs over the surface in
temporary streams.
Chaparral and Maquis are seasonal and low rainfall with a prolonged dry season. Decomposition and soil
development is impeded by lack of moisture and frequent fires.
Vegetation and Animals
Hot desert vegetation includes thorny shrubs, ephemeral animals, underground corms and bulbs and
succulents such as cacti. All have adaptations allowing them to survive long periods of drought. Cool deserts
have denser shrub vegetation and abundant microflora. Chaparral contains species with small, thick, drought
– resistant leaves and one community is maintained by regular fire.
Reptiles and insects are most able to survive in desert conditions. However some mammals, including
rodents and camels, have evolved means of coping with arid conditions. Semi deserts and shrublands are
important habitats for reptiles, small mammals and birds.
14
4. TUNDRA ECOSYSTEMS
The arctic tundra forms a circum polar band between the Arctic Ocean and the polar ice caps to the north and
the coniferous forests to the South. Smaller, but ecologically similar regions found above the tree line on
high mountains are called alpine tundra. These are superficially barren, treeless regions where extreme
environmental conditions severely limit plant growth. Tundra occurs where the low temperature and short
growing season prevent the development of forest. The falls below that required for plant growth for most of
the year. Precipitation is low (usually<250mm) and occurs mainly as snow. Below a certain depth the
ground remains permanently frozen forming permafrost. The low productivity and limited microbial activity
result in thin soils.
Vegetation and Animals
Tundra has a low productivity but high species richness. The vegetation consists of low growing mat – and
hummock forming plants such as sedges (Carex spp.), lichens, mosses, grasses and dwarf willows (Salix
spp.) etc. The long day length during the summer, combined with higher temperatures, allow primary
productivity at this time of the year to be an order of magnitude higher than in the winter.
The extreme seasonality of tundra means that some animals are only present as summer migrants. Permanent
residents such as reindeer are migratory, ranging over vast areas in order to find enough food. Migratory
birds such as geese, sandpipers, ducks and other water fowl breed on tundra in summer feeding on the
vegetation and on the emergent insects.
5. MAN – MADE ECOSYSTEMS
Agricultural lands, gardens, parks, horticultural gardens even cities, towns, villages, industrial and mining
areas are some examples of man–made ecosystems. The space capsule which lasts for limited period is also
an artificial ecosystem. Unlike natural ecosystems, these man–made ecosystems have weaker self regulatory
mechanisms and depend on regular external input for their very existence.
Agroecosystem
Among man – made ecosystems, agroecosystems are extensively prevalent in all parts of the world in
various forms. The biotic components of such ecosystems vary with agroclimatic conditions and the societal
set up of the people practicing it.
Some of the agroecosystems are highly developed; exist by modern scientific inputs as intensive cultivation
practices, while some are still on traditional forms, practiced usually for subsistence needs.
Intensive cultivation of cereals, pulses, oil crops, vegetables, fruit crops, plantation crops etc. are some
examples of modern agroecosystems. Shifting agriculture practices in hilly areas of Asia and grasslands of
15
Africa are traditional agroecosystems. They also include small holders low input agriculture practiced
mainly for subsistence needs.
Agroecosystems also exist as monocultures (one major crop), mixed cropping, multiple cropping of annuals
and as agroforestry systems. Agroforestry systems, with woody perennials as predominant component mixed
with agriculture and/ or animal husbandry in various spatial and temporal arrangements is usually rich in
component diversity and somewhat self regulatory depicting a forest ecosystem. Presence of woody
perennials and the ecological interaction among the components (tree, crop, animal and abiotic) makes
agroforestry systems usually more efficient in terms of nutrient recycling and energy budgeting compared to
other agroecosystems.
Ecosystem Degradation
The activity of modern man has slowly been degrading the quality of global environment. The demand for
energy to cater the industrial need and to the rapid growing concentrations of populations, or urbanization
has brought environmental problems of unprecedented magnitude. Of all the anthropogenic impacts on
ecosystems, perhaps few are as awesome and on as grand a scale as the major and systematic destruction of
large sized ecosystems, such as forests-boreal, temperate, and tropical-and grassland. The major aspects of
human activity responsible for ecosystem degradation are overexploitation of natural resources and pollution
of the environment.
Major causes of ecosystem degradation: Ecosystem degradation is a consequence of over
exploitation of our natural ecosystems for space, energy and materials. The basic reasons for such
degradation are:
1.
Expansion of agriculture: As demand for agricultural products rise more and more land is brought
under cultivation for which forests are cleared, grasslands ploughed, uneven grounds leveled, marshes
drained and land under water reclaimed. However, this expansion is usually marked with more
ecological destruction than rationality. Agricultural practices have also taken their toll on tropical
forests. Poor landless peasants tend to move into the edges of the forests seeking farming areas. While
large areas are cleared for plantations (e.g., rubber and palm oil groves have replaced the rain forest on
the lower slopes of Malayasia; Manu African countries have destroyed forest areas for coffee and tea
cultivation. During the process of clearing land precious forest and vegetation is simply burned. In
tropical regions of the world as much of the mineral materials is lodged in the plant biomass, its
removal takes away large part of the nutrients. The soil being poorer is unable to support farming for
16
long durations. Since agriculture fails, the cleared land is put to use as cattle ranches. The bared soil is
subjected to massive erosion and degradation.
2.
Extension of cultivation on hill slopes: Outside humid tropical zone, in most of the third world
countries, major forests often occur on hill tops and slopes. Though agriculture has nearly always been
concentrated on plains and floors of valleys, farming on narrow terraces is an age old practice. It has
never been extensive because of the grueling labour and low productivity. However the ever rising
human population and their growing necessities have forced many to go to the mountain slopes for
cultivation. In the process, more and more slopes are cleared of plants and terraces prepared resulting
top soil to be washed away to the streams and rivers.
3.
Shifting cultivation: Shifting cultivation is often blamed for destruction of forests. In fact it is the
poor fertility of soil which has given rise to such a pattern of farming. A small patch of tropical forest is
cleared, vegetation slashed and burned. Crops are grown as long as the soil is productive, after which
the cultivation is abandoned and the cultivators move onto fresh patch of land. The abandoned land is
allowed to lay fallow for long periods during which regrowth of vegetation takes place and the natural
ecosystem is restored. When the fallow period was longer the shifting cultivators worked in harmony
with the nature. However, the demands of growing population have shortened the fallow period
drastically. The soil is unable to regain its fertility before it is put to use again. As the crops fail more
and more land is cleared of forests to be put similar overexploitation. The practice is prominent in many
tropical countries of Asia and Africa and in India it is extensively practiced by the tribal people of North
Eastern region in hill slopes (Ramakrishnan, 1992).
4.
Cattle ranching: Large areas of tropical forests in Central and South America have been cleared for
use as grazing land to raise cattle and cash in lucrative beef export to USA. But the soil has degenerated
within a short span pf time due to overgrazing and massive soil erosion occurred. Cattle ranching have
done much damage to the tropical forest cover in South and Central America.
5.
Firewood collection: To majority of rural population and a large number of people living in small
towns and cities of developing countries the only fuel is wood used for cooking and to heat in chilly
winters. Firewood collection contributes much to the depletion of forest cover. In case of lightly
wooded forests the pressure is higher compared to the dense forests. Outright felling of live trees to
meet firewood and charcoal requirement is common in light wooded areas in many countries. Every
form of fuel wood biomass, including wood, twigs, crop residues and grass, is especially important in
17
many third world and poor countries. Even developed countries are increasingly turning to obtain
energy from plants to bypass the use of fossil fuels.
6.
Timber harvesting: Logging and felling of forest trees for obtaining timber is an important cause of
degradation of forest ecosystems in third world countries. Commercial logging in tropical countries
usually involves felling of trees of only selected species which fetch better price. This process of
creaming or removing a few selected trees amidst dense vegetation on rather a delicate soil causes much
more destruction than the actual number of trees or the volume of timber taken out. In a study in
Indonesia, it was found that logging operations destroyed about 40% of the trees left behind. In many
third world countries, logging operation have been observed to lead to a permanent loss of forest cover.
Loggers after removing a select group of trees move onto other areas. They are usually followed up by
others who move into the cut over area hoping to start farming and put down roots. The remaining
vegetation is slashed and burnt and agriculture is attempted. When cultivation fails it is replaced by
cattle ranching or by useless tenacious grasses.
7.
Environmental Pollution: Pollution involves introduction of undesirable and harmful material in
the form of a gas, liquid or solid as such or in dissolved state in an ecosystem. Most of these pollutants,
even seemingly harmless materials also, adversely affect the biotic community in an ecosystem. The
major type of pollution which may have serious effect on the ecosystem include:
a)
Air pollution: Air pollution is the transfer of harmful amounts of natural and synthetic materials into
the atmosphere as a consequence of human activity. Pollutants can be added to the air directly (primary
pollutants), or they can be created in the air (secondary pollutants) from other pollutants under the
influence of electromagnetic radiation from the sun. The major air pollutants include - sulfur dioxide,
particulates viz., fumes, dust aerosols etc, Nitrogen oxide, hydrocarbons, carbon monoxide, ozone,
hydrogen sulfide, fluorides, nitric oxides, lead, mercury, etc. While all these pollutants have effects,
sources and control strategies is common. Mechanical processes that involve grinding or pulverizing of
materials are source of particulate air pollution. Combustion of fossil fuels releases sulfur dioxide,
carbon monoxide and nitrogen dioxide. Incomplete combustion of petrol from automobiles and aircrafts
accounts for hydrocarbon pollutants in air. Lead, a toxic metal is added to the atmosphere from
automobile exhausts and discharges from some industrial facilities.
b) Water pollution: Basically water pollution can be defined as any human action that impairs the use of
water as a resource. All water pollutants fall into one of four categories- biological agents, dissolved
18
chemicals, no dissolved chemicals and heat. Effluents that are discharged into water bodies are as varied
as man’s activities. Various effluents contain various pollutants.
AQUATIC ECOSYSTEMS
Water containing little or no chlorine is called freshwater. According to the Venice system, which classifies
brackish waters by their percentage of chlorine content, freshwater contains 0.03 percent or less of chlorine.
The following are the freshwater systems:
1. Freshwater ecosystems:
Freshwater ecosystems includes
a) Lake and Ponds
b) Rivers and Streams
c) Wetlands such as Bogs, Marshes, Fens, Shrub – Carr and Swamps.
These systems are fed by water and nutrients leaching from surrounding catchments area.
a) Lakes and Ponds
Lakes and ponds have very little or no current, allowing the water body to separate out into layers depending
upon the temperature and chemical composition. The illuminated, warm water is called the epilimnion. The
cooler water below, the metalimnion, becomes colder with depth. For every 1m depth, the temperature
declines by 1°C. When the temperature of the water reaches 4°C and its greatest density, it lives as a layer at
the bottom called the hypolimnion. In shallower lakes the stratification persists during the summer when
surface waters are warm.
On the basis of water depth and types of vegetation and animals there may be three zones in a lake or pond.
1.
Littoral
2.
Limnetic and
3.
Profundal
i)
The littoral zone is the shallow water region which is usually occupied by rooted plants.
ii)
The limnetic zone ranges from the shallow to the depth of effective light penetration and
associated organisms are small crustaceans, rotifers, insects and their larvae and algae.
iii)
The profundal zone is the deep water parts where there is no effective light penetration. The
associated organisms are snails, mussels, crabs and wounds.
Depending upon the substrate and the geology of the surrounding catchments, lakes can be nutrient rich
(eutrophic) or nutrient poor (oligotrophic). Eutrophication can also occur through organic and inorganic
pollution.
19
b) Rivers and Streams
Rivers and streams differ greatly, depending on their size. They also vary along their length from their
length, from their source in upland the sea. In general, as the mouth of a river is approached:
i)
The speed of water flow decreases, the water becomes less turbulent and oxygen level falls.
ii) The volume of water increases having accumulated as the river passes through its catchment.
iii) The energy of the river decreases, suspended material is deposited and river bed becomes composed of
finer particles and eventually silt.
iv) The river bed becomes less steep because the larger value of water erodes a broader channel.
v)
Human influences increase.
Streams high in the catchment that are unpolluted will support caddis fly (Trichoptera) and blackfly
(Simulium spp.) larvae feeding fine organic particles. The water will be too turbulent and nutrient poor for all
but aquatic mosses, liverworts and algae. Plankton communities consisting of algae, photosynthetic bacteria,
crustaceans, and rotifers, can develop further downstream where the volume of moving water is increased
and the current is reduced. Fish, reptiles, birds and mammals may be present.
WETLAND ECOSYSTEMS
Wetlands range along a gradient from permanently flooded to periodically saturated soil and support
hydrophytic vegetation at sometime during the growing season. Hydrophytic plants are adapted grow in
water or on soil that is periodically anaerobic because of excess water. Fresh water wetlands can be grouped
into:
a)
Bogs
b) Marshes
c)
Fen
d) Shrub – land
e)
Swamp
a) Bogs
Bogs are areas with waterlogged soil with spongy covering of mosses. These are the landforms characterized
by the accumulation of peat filled depressions in northern latitude especially glaciated regions.
b) Marshes
Marsh is a wetland dominated by graminoids i.e., grass like plants. Marsh may occur as a zone around a lake
or pond or alongside a river, but may also occur away from any water bodies in areas where the water table
20
is high. Marsh vegetation comprises cattails, reeds, bulrushes, spike rushes and wild rice. Deep marshes are
bordered by emergent vegetation such as pondweed, naiads, wild celery, and water lily.
c) Fen
A fen is an “alkaline bog”, a mineral rich peatland. Usually, fens occur at the base of slopes in the path of
mineral charged ground water that results in a near neutral to slightly alkaline pH. Swamp milkweed, marsh
bellflower, kalm’s lobelia, grass – of – Parnassus, dwarf birch etc. are common plants.
d) Shrub – Carr
Shrub – Carr is the wetland variety of thicket, shrubby vegetation may invade marshes, fens or bogs.
Dogwoods, willows, buttonbush and birches are the most common kinds of shrub in marsh.
e) Swamp
A swamp is a wooded wetland. Swamps may be:
Shrub Swamps – with waterlogged soil, often covered with 15cm or more of water. Alder, willow,
buttonbush, dogwoods are common vegetation. These are also nesting and feeding areas for ducks to limited
extent.
Wood Swamp – with soil waterlogged, often covered with 0.3m of water found along sluggish streams, flat
uplands and shallow lake basins. Tamarack, arborvitae, spruce, red maple, silver maple, water oak, over cup
oak, tupelo swamp, black gum, cypress etc. are usually found plants.
MARINE or OCEANIC ECOSYSTEMS
Oceans cover 70% of the world surface and include:
a)
Open oceans
b) Continental shelves
c)
Inter-tidal zone and coral reef
d) Salt marsh
a) Open Ocean
Ocean water can be upto 10km deep and much of this water receives little light, is nutrient poor and is
unproductive. Light penetration typically occurs to about 150m producing a surface zone known as photic
zone where photosynthesis is possible. Marine phytoplanktons include microscopic algae, flagellates,
diatoms and bacteria. These phytoplankton support large numbers of the zooplankton consisting largely of
the larval stages of marine and intertidal invertebrates. Planktonic organisms cannot control their movement
but those that are able to swim are part of the nektons.
21
Below the photic zone carnivorous and detrivorous animals occur, feeding on material from the communities
above. Light levels and productivity decline with depth. Bottom or benthic fauna is sparse except in regions
of hydrothermal vents. Three hundred thermal vent species have been described, from sulfur bacteria to
limpets, tubeworms, and fish are unique to this habitat.
b) Continental shelves
Continental shelves support some of the most productive marine ecosystems, particularly in areas of up
welling where currents bring nutrients from the deep to the surface water. Here kelp forests are found
formed from brown algae, such as Laminaria spp. This anchors itself to the substrate, growing to a length of
50m or more. The shelf benthos supports a diverse fauna including Polychaete worms, molluscs, sea squizts,
bryozoans, sponges, sea spiders, crustaceans and echinodermata and large fish population.
c) Intertidal zone and Coral reefs
Intertidal rocky shores are dominated by algae. Sand beaches provide an unstable, abrasine and nutrient poor
substrate inhabited by filter feeding burrowing animals which are themselves food for wading birds.
Diverse coral reef communities occur in warm and very shallow water. Corals are colonials’ animals which
produce structurally complex calcareous skeletons on which live algae, invertebrates and carnivorous fishes.
d) Salt Marsh
Saltmarsh occurs in shettered areas protected from wave action and provides a stable substrate for
colonization by salt tolerant higher plant.
ESTUARIANE ECOSYSTEM
Water of most streams and rivers eventually drain into the sea and the place where this fresh water joins salt
water is called an estuary. Estuaries usually develop two important types of ecosystems:
i) Mud flats
ii) Mangrove forests
i) Mud flats
Mud flats tend to retain organic matter deposited by the tide because of their small particle size. Estuarine
silts consist of river sediments which are very rich in organic matter. The absence of oxygen greatly restricts
the organisms’ ability to survive in these muds and slits. But the organisms that reaches very high densities,
provides a rich food source for other organisms including birds. In estuaries, where water is brackish,
phytoplankton, benthic microflora and invertebrates are very abundant making estuaries important nursery
grounds for shellfish and fish.
22
ii) Mangrove Forest
Mangrove forests replace salt marsh in warmer climates and develop on anoxic mud. They cover 60 – 70%
of the coastline of tropical regions. The dominant plants have shallow, widely spreading roots which emerge
from the trunk above ground and act as props. Many species have root extensions that take in oxygen for the
roots which are known as pneumatophores. Genus like Rhizophora dominates in the mangrove ecosystems.
These forests are faunally rich with a unique mixture of terrestrial and marine life. Fiddler crabs and tropical
land crabs burrow into the mud during low tide. In the well developed Indo – Malaysian mangrove forests
mud skippers (Periophthalmus) are found. These are fish that live in burrows in the mud and are able to
crawl about on top of it like amphibians.
23
REFERENCES AND FURTHER READINGS
1.
Smith, Robert Leo and Smith, Thomas M. 2000. Elements of Ecology. Addison Wesley Longman
Inc., San Francisco, USA, Fourth edition, 567p.
2.
Brewer, Richard.1994. The Science of Ecology. Sounders College Publishing, Orlando, Florida,
USA, Second Edition, 773p.
3.
Mackeenzie, Aulay, Ball, Andy S. and Virdee, Sonia R. 2001. Instant Notes in Ecology.Viva Books
Private Ltd., New Delhi, 321p.
4.
Kormondy, Edward J. 2000. Concepts of Ecology. Prentice Hall of India, New Delhi, Fourth
Edition, 559p.
5.
Ramakrishnan, P.S. 1992. Shifting agriculture and sustainable development. UNESCO, Paris, Man
and Biosphere Series, Vol. 10, 424p
6.
Ambasht, R.S and Ambasht, N.K. 1996. A text book of Plant Ecology. Students’ Friends and Co.,
Varanasi, India, Twelfth Edition, 408p.
7.
Rao, B.N. 1987. Watershed management and social forestry. In: Social forestry for rural
development, P.K. Khosla and R. K. Joshi (eds.), ISTS, Solan, H.P., 52 – 64pp.
8.
Vinod Kumar. 1999. Nursery and plantation practices in forestry. Scientific Publishers (India),
Jodhpur, 531p.
9.
Likens, Gene E., Bormann, F. Herbert, Pierce, Robert S., Eaton, John S. and Johnson, Noye M.
1977. Biogeochemistry of a forested ecosystem. Springer-Verlag New York Inc., New York, USA,
146p.
10. Lal, J.B. 1987. Environmental Conservation. International Book Distributors, Dehra Dun, India,
111p.
11. Maikhuri, R.K. and Ramakrishnan, P.S. 1991. Comparative analysis of village ecosystem function
of different living in the same area in Arunachal Pradesh in North-Eastern India. Agricultural
Systems, 35: 377 – 399.
12. Whittaker, R.H. and Likens, G.E. Carbon in biota. 1973. In: Carbon and the Biosphere Conf..
72501, G.M. Woodwell and E. V. Pecan (eds.). Springfield, VA: National Technical Information
Service, pp. 281 – 300.
13. Maikhuri, R.K. and Ramakrishnan, P.S. 1990. Ecological analysis of a cluster of villages
emphasizing land use of different tribes in Meghalaya in North-East India. Agriculture, ecosystem
and environment, 31: 17 – 37.
14. Mishra, B.K. and Ramakrishnan, P.S. (1982). Energy flow through a village ecosystem with slash
and burn agriculture in North Eastern India. Agricultural Systems, 9(1): 57 -72.
15. Asthana, D.K. and Asthana, Meera. 1998. Environment: problems and solutions. S. Chand &
Company Ltd., New Delhi, 323p.
16. Odum, H. T. and Odum, E. P. 1959. Fundamentals of Ecology. W. B. Saunders Co.,
Philadelphia, PA.
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