Diversity is not only a characteristic of living organisms but
also of content in biology textbooks. Biology is presented either
as botany, zoology and microbiology or as classical and
modern. The latter is a euphemism for molecular aspects of
biology. Luckily we have many threads which weave the
different areas of biological information into a unifying
principle. Ecology is one such thread which gives us a holistic
perspective to biology. The essence of biological understanding
is to know how organisms, while remaining an individual,
interact with other organisms and physical habitats as a group
and hence behave like organised wholes, i.e., population,
community, ecosystem or even as the whole biosphere.
Ecology explains to us all this. A particular aspect of this is the
study of anthropogenic environmental degradation and the
socio-political issues it has raised. This unit describes as well as
takes a critical view of the above aspects.
Chapter 13
Organisms and Populations
Chapter 14
Ecosystem
Chapter 15
Biodiversity and Conservation
Chapter 16
Environmental Issues
2020-21
Ramdeo Misra is revered as the Father of Ecology in India. Born on 26 August
1908, Ramdeo Misra obtained Ph.D in Ecology (1937) under Prof. W. H. Pearsall,
FRS, from Leeds University in UK. He established teaching and research in
ecology at the Department of Botany of the Banaras Hindu University,
Varanasi. His research laid the foundations for understanding of tropical
communities and their succession, environmental responses of plant
populations and productivity and nutrient cycling in tropical forest and
grassland ecosystems. Misra formulated the first postgraduate course in
ecology in India. Over 50 scholars obtained Ph. D degree under his supervision
and moved on to other universities and research institutes to initiate ecology
teaching and research across the country.
He was honoured with the Fellowships of the Indian National Science
Academy and World Academy of Arts and Science, and the prestigious Sanjay
Gandhi Award in Environment and Ecology. Due to his efforts, the
Government of India established the National Committee for Environmental
Planning and Coordination (1972) which, in later years, paved the way
for the establishment of the Ministry of Environment and Forests (1984).
RAMDEO MISRA
(1908-1998)
2020-21
Our living world is fascinatingly diverse and amazingly
complex. We can try to understand its complexity by
investigating processes at various levels of biological
organisation–macromolecules, cells, tissues, organs,
individual organisms, population, communities,
ecosystems and biomes. At any level of biological
organisation we can ask two types of questions – for
example, when we hear the bulbul singing early morning
in the garden, we may ask – ‘How does the bird sing?’
Or, ‘Why does the bird sing ?’ The ‘how-type’ questions
seek the mechanism behind the process while the ‘why-
type’ questions seek the significance of the process. For
the first question in our example, the answer might be in
terms of the operation of the voice box and the vibrating
bone in the bird, whereas for the second question the
answer may lie in the bird’s need to communicate with its
mate during breeding season. When you observe nature
around you with a scientific frame of mind you will
certainly come up with many interesting questions of both
types - Why are night-blooming flowers generally white?
How does the bee know which flower has nectar? Why
does cactus have so many thorns? How does the chick
spures recognise her own mother?, and so on.
CHAPTER 13
ORGANISMS AND POPULATIONS
13.1 Organism and Its
Environment
13.2 Populations
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You have already learnt in previous classes that Ecology is a subject
which studies the interactions among organisms and between the
organism and its physical (abiotic) environment.
Ecology is basically concerned with four levels of biological
organisation – organisms, populations, communities and biomes. In this
chapter we explore ecology at organismic and population levels.
13.1 ORGANISM AND ITS ENVIRONMENT
Ecology at the organismic level is essentially physiological ecology which
tries to understand how different organisms are adapted to their
environments in terms of not only survival but also r
eproduction. You
may have learnt in earlier classes how the rotation of our planet around
the Sun and the tilt of its axis cause annual variations in the intensity
and duration of temperature, resulting in distinct seasons. These
variations together with annual variation in precipitation (remember
precipitation includes both rain and snow) account for the formation of
major biomes such as desert, rain forest and tundra (Figure 13.1).
Figure 13.1 Biome distribution with respect to annual temperature and precipitation
Regional and local variations within each biome lead to the formation of a
wide variety of habitats. Major biomes of India are shown in Figure 13.2.
On planet Earth, life exists not just in a few favourable habitats but even
in extreme and harsh habitats – scorching Rajasthan desert, rain-soaked
Meghalaya forests, deep ocean trenches, torrential streams, permafrost
(snow laden) polar regions, high mountain tops, thermal springs, and
stinking compost pits, to name a few. Even our intestine is a unique
habitat for hundreds of species of microbes.
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What are the key elements that lead to so much variation in the
physical and chemical conditions of different habitats? The most
important ones are temperature, water, light and soil. We must remember
that the physico-chemical (abiotic) components alone do not characterise
the habitat of an organism completely; the habitat includes biotic
components also – pathogens, parasites, predators and competitors – of
the organism with which they interact constantly. We assume that over a
period of time, the organism had through natural selection, evolved
adaptations to optimise its survival and reproduction in its habitat.
Each organism has an invariably defined range of conditions that it
can tolerate, diversity in the resources it utilises and a distinct functional
role in the ecological system, all these together comprise its niche.
13.1.1 Major Abiotic Factors
Temperature: Temperature is the most important ecologically relevant
environmental factor. You are aware that the average temperature on
land varies seasonally, decreases progressively from the equator towards
the poles and from plains to the mountain tops. It ranges from subzero
levels in polar areas and high altitudes to >50
0
C in tropical deserts in
summer. There are, however, unique habitats such as thermal springs
and deep-sea hydrothermal vents where average temperatures exceed
100
0
C. It is general knowledge that mango trees do not and cannot grow
Figure 13.2 Major biomes of India : (a) Tropical rain forest; (b) Deciduous forest;
(c) Desert; (d) Sea coast
(a)
(b)
(c)
(d)
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in temperate countries like Canada and Germany, snow leopards are not
found in Kerala forests and tuna fish are rarely caught beyond tropical
latitudes in the ocean. You can appreciate the significance of temperature
to living organisms when you realise that it affects the kinetics of enzymes
and through it the metabolic activity and other physiological functions of
the organism. A few organisms can tolerate and thrive in a wide range of
temperatures (they are called eurythermal), but, a vast majority of them
are restricted to a narrow range of temperatures (such organisms are called
stenothermal). The levels of thermal tolerance of different species determine
to a large extent their geographical distribution. Can you think of a few
eurythermal and stenothermal animals and plants?
In recent years, there has been a growing concern about the gradually
increasing average global temperatures (Chapter 16). If this trend continues,
would you expect the distributional range of some species to be affected?
Water: Water is another the most important factor influencing the life of
organisms. In fact, life on earth originated in water and is unsustainable
without water. Its availability is so limited in deserts that only special
adaptations make it possible for organisms to live there. The productivity
and distribution of plants is also heavily dependent on water. You might
think that organisms living in oceans, lakes and rivers should not face
any water-related problems, but it is not true. For aquatic organisms the
quality (chemical composition, pH) of water becomes important. The salt
concentration (measured as salinity in parts per thousand), is less than
5 in inland waters, 30-35 in the sea and > 100 in some hypersaline
lagoons. Some organisms are tolerant of a wide range of salinities
(euryhaline) but others are restricted to a narrow range (stenohaline).
Many freshwater animals cannot live for long in sea water and vice versa
because of the osmotic problems, they would face.
Light: Since plants produce food through photosynthesis, a process which
is only possible when sunlight is available as a source of energy, we can
quickly understand the importance of light for living organisms,
particularly autotrophs. Many species of small plants (herbs and shrubs)
growing in forests are adapted to photosynthesise optimally under very
low light conditions because they are constantly overshadowed by tall,
canopied trees. Many plants are also dependent on sunlight to meet their
photoperiodic requirement for flowering. For many animals too, light is
important in that they use the diurnal and seasonal variations in light
intensity and duration (photoperiod) as cues for timing their foraging,
reproductive and migratory activities. The availability of light on land is
closely linked with that of temperature since the sun is the source for both.
But, deep (>500m) in the oceans, the environment is dark and its inhabitants
are not aware of the existence of a celestial source of energy called Sun.
What, then is their source of energy?. The spectral quality of solar radiation
is also important for life. The UV component of the spectrum is harmful to
many organisms while not all the colour components of the visible spectrum
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are available for marine plants living at different depths of the ocean. Among
the red, green and brown algae that inhabit the sea, which is likely to
be found in the deepest waters? Why?
Soil: The nature and properties of soil in different places vary; it is
dependent on the climate, the weathering process, whether soil is
transported or sedimentary and how soil development occurred. Various
characteristics of the soil such as soil composition, grain size and
aggregation determine the percolation and water holding capacity of the
soils. These characteristics along with parameters such as pH, mineral
composition and topography determine to a large extent the vegetation in
any area. This in turn dictates the type of animals that can be supported.
Similarly, in the aquatic environment, the sediment-characteristics often
determine the type of benthic animals that can thrive there.
13.1.2 Responses to Abiotic Factors
Having realised that the abiotic conditions of many habitats may vary
drastically in time, we now ask–how do the organisms living in such
habitats cope or manage with stressful conditions? But before attempting
to answer this question, we should perhaps ask first why a highly variable
external environment should bother organisms after all. One would expect
that during the course of millions of years of their existence, many species
would have evolved a relatively constant internal (within the body)
environment that permits all biochemical reactions and physiological
functions to proceed with maximal efficiency
and thus, enhance the overall ‘fitness’ of the
species. This constancy, for example, could
be in terms of optimal temperature and
osmotic concentration of body fluids. Ideally
then, the organism should try to maintain
the constancy of its internal environment (a
process called homeostasis) despite varying
external environmental conditions that tend
to upset its homeostasis. Let us take an
analogy to clarify this important concept.
Suppose a person is able to perform his/her
best when the temperature is 25
0
C and
wishes to maintain it so, even when it is
scorchingly hot or freezingly cold outside. It
could be achieved at home, in the car while
travelling, and at workplace by using an air conditioner in summer and
heater in winter. Then his/her performance would be always maximal
regardless of the weather around him/her. Here the person’s homeostasis
is accomplished, not through physiological, but artificial means. How do
other living organisms cope with the situation? Let us look at various
possibilities (Figure 13.3).
Figure 13.3 Diagrammatic representation of
organismic response
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(i) Regulate: Some organisms are able to maintain homeostasis by
physiological (sometimes behavioural also) means which ensures
constant body temperature, constant osmotic concentration, etc.
All birds and mammals, and a very few lower vertebrate and
invertebrate species are indeed capable of such regulation
(thermoregulation and osmoregulation). Evolutionary biologists
believe that the ‘success’ of mammals is largely due to their ability
to maintain a constant body temperature and thrive whether they
live in Antarctica or in the Sahara desert.
The mechanisms used by most mammals to regulate their body
temperature are similar to the ones that we humans use. We maintain
a constant body temperature of 37
0
C. In summer, when outside
temperature is more than our body temperature, we sweat profusely.
The resulting evaporative cooling, similar to what happens with a
desert cooler in operation, brings down the body temperature. In
winter when the temperature is much lower than 37
0
C, we start to
shiver, a kind of exercise which produces heat and raises the body
temperature. Plants, on the other hand, do not have such
mechanisms to maintain internal temperatures.
(ii) Conform: An overwhelming majority (99 per cent) of animals and
nearly all plants cannot maintain a constant internal environment.
Their body temperature changes with the ambient temperature. In
aquatic animals, the osmotic concentration of the body fluids
change with that of the ambient air, water osmotic concentration.
These animals and plants are simply conformers. Considering the
benefits of a constant internal environment to the organism, we must
ask why these conformers had not evolved to become regulators.
Recall the human analogy we used above; much as they like, how
many people can really afford an air conditioner? Many simply
‘sweat it out’ and resign themselves to suboptimal performance in
hot summer months. Thermoregulation is energetically expensive
for many organisms. This is particularly true for small animals like
shrews and humming birds. Heat loss or heat gain is a function of
surface area. Since small animals have a larger surface area relative
to their volume, they tend to lose body heat very fast when it is cold
outside; then they have to expend much energy to generate body
heat through metabolism. This is the main reason why very small
animals are rarely found in polar regions. During the course of
evolution, the costs and benefits of maintaining a constant internal
environment are taken into consideration. Some species have evolved
the ability to regulate, but only over a limited range of environmental
conditions, beyond which they simply conform.
If the stressful external conditions are localised or remain only
for a short duration, the organism has two other alternatives for
survival.
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(iii) Migrate: The organism can move away temporarily from the
stressful habitat to a more hospitable area and return when stressful
period is over. In human analogy, this strategy is like a person
moving from Delhi to Shimla for the duration of summer. Many
animals, particularly birds, during winter undertake long-distance
migrations to more hospitable areas. Every winter the famous
Keolado National Park (Bharatpur) in Rajasthan host thousands of
migratory birds coming from Siberia and other extremely cold
northern regions.
(iv) Suspend: In bacteria, fungi and lower plants, various kinds of thick-
walled spores are formed which help them to survive unfavourable
conditions – these germinate on availability of suitable environment.
In higher plants, seeds and some other vegetative reproductive
structures serve as means to tide over periods of stress besides helping
in dispersal – they germinate to form new plants under favourable
moisture and temperature conditions. They do so by reducing their
metabolic activity and going into a state of ‘dormancy’.
In animals, the organism, if unable to migrate, might avoid the
stress by escaping in time. The familiar case of bears going into
hibernation during winter is an example of escape in time. Some
snails and fish go into aestivation to avoid summer–related
problems-heat and dessication. Under unfavourable conditions
many zooplankton species in lakes and ponds are known to enter
diapause, a stage of suspended development.
13.1.3 Adaptations
While considering the various alternatives available to organisms for
coping with extremes in their environment, we have seen that some are
able to respond through certain physiological adjustments while others
do so behaviourally (migrating temporarily to a less stressful habitat).
These responses are also actually, their adaptations. So, we can say that
adaptation is any attribute of the organism (morphological, physiological,
behavioural) that enables the organism to survive and reproduce in its
habitat. Many adaptations have evolved over a long evolutionary time
and are genetically fixed. In the absence of an external source of water,
the kangaroo rat in North American deserts is capable of meeting all its
water requirements through its internal fat oxidation (in which water is
a by product). It also has the ability to concentrate its urine so that
minimal volume of water is used to remove excretory products.
Many desert plants have a thick cuticle on their leaf surfaces and
have their stomata arranged in deep pits (sunken) to minimise water loss
through transpiration. They also have a special photosynthetic pathway
(CAM) that enables their stomata to remain closed during day time. Some
desert plants like Opuntia, have no leaves – they are reduced to spines–
and the photosynthetic function is taken over by the flattened stems.
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Mammals from colder climates generally have shorter ears and limbs
to minimise heat loss. (This is called the Allen’s Rule.) In the polar seas
aquatic mammals like seals have a thick layer of fat (blubber) below their
skin that acts as an insulator and reduces loss of body heat.
Some organisms possess adaptations that are physiological which
allow them to respond quickly to a stressful situation. If you had ever
been to any high altitude place (>3,500m Rohtang Pass near Manali and
Leh you must have experienced what is called altitude sickness. Its
symptoms include nausea, fatigue and heart palpitations. This is because
in the low atmospheric pressure of high altitudes, the body does not get
enough oxygen. But, gradually you get acclimatised and stop experiencing
altitude sickness. How did your body solve this problem? The body
compensates low oxygen availability by increasing red blood cell
production, decreasing the binding affinity of hemoglobin and by
increasing breathing rate. Many tribes live in the high altitude of
Himalayas. Find out if they normally have a higher red blood cell count
(or total hemoglobin) than people living in the plains.
In most animals, the metabolic reactions and hence all the
physiological functions proceed optimally in a narrow temperature range
(in humans, it is 37
0
C). But there are microbes (archaebacteria) that
flourish in hot springs and deep sea hydrothermal vents where
temperatures far exceed 100
0
C. How is this possible?
Many fish thrive in Antarctic waters where the temperature is always
below zero. How do they manage to prevent their body fluids from freezing?
A large variety of marine invertebrates and fish live at great depths in
the ocean where the pressure could be >100 times the normal atmospheric
pressure that we experience. How do they live under such high pressures
and do they have any special enzymes? Organisms living in such extreme
environments show a fascinating array of biochemical adaptations.
Some organisms show behavioural responses to cope up with
variations in their environment. Desert lizards lack the physiological ability
that mammals have to deal with the high temperatures of their habitat,
but manage to keep their body temperature fairly constant by behavioural
means. They bask in the sun and absorb heat when their body
temperature drops below the comfort zone, but move into shade when
the ambient temperature starts increasing. Some species are capable of
burrowing into the soil to hide and escape from the above-ground heat.
13.2 POPULATIONS
13.2.1 Population Attributes
In nature, we rarely find isolated, single individuals of any species; majority
of them live in groups in a well defined geographical area, share or compete
for similar resources, potentially interbreed and thus constitute a
population. Although the term interbreeding implies sexual reproduction,
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a group of individuals resulting from even asexual reproduction is also
generally considered a population for the purpose of ecological studies.
All the cormorants in a wetland, rats in an abandoned dwelling, teakwood
trees in a forest tract, bacteria in a culture plate and lotus plants in a
pond, are some examples of a population. In earlier chapters you have
learnt that although an individual organism is the one that has to cope
with a changed environment, it is at the population level that natural
selection operates to evolve the desired traits. Population ecology is,
therefore, an important area because it links ecology to population genetics
and evolution.
A population has certain attributes whereas, an individual organism
does not. An individual may have births and deaths, but a population has
birth rates and death rates. In a population these rates refer to per capita
births and deaths. The rates, hence, expressed are change in numbers
(increase or decrease) with respect to members of the population. Here is an
example. If in a pond there were 20 lotus plants last year and through
reproduction 8 new plants are added, taking the current population to 28,
we calculate the birth rate as 8/20 = 0.4 offspring per lotus per year. If 4
individuals in a laboratory population of 40 fruitflies died during a specified
time interval, say a week, the death rate in the population during that period
is 4/40 = 0.1 individuals per fruitfly per week.
Another attribute characteristic of a population is sex ratio. An
individual is either a male or a female but a population has a sex ratio
(e.g., 60 per cent of the population are females and 40 per cent males).
A population at any given time is composed of individuals of
different ages. If the age distribution (per cent individuals of a given
age or age group) is plotted for the population, the resulting structure
is called an age pyramid (Figure 13.4). For human population, the
age pyramids generally show age distribution of males and females in
a diagram. The shape of the pyramids reflects the growth status of
the population - (a) whether it is growing, (b) stable or (c) declining.
The size of the population tells us a lot about its status in the habitat.
Whatever ecological processes we wish to investigate in a population, be
it the outcome of competition with another species, the impact of a
Figure 13.4 Representation of age pyramids for human population
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predator or the effect of a pesticide application, we always evaluate them
in terms of any change in the population size. The size, in nature, could
be as low as <10 (Siberian cranes at Bharatpur wetlands in any year) or
go into millions (Chlamydomonas in a pond). Population size, technically
called population density (designated as N), need not necessarily be
measured in numbers only. Although total number is generally the most
appropriate measure of population density, it is in some cases either
meaningless or difficult to determine. In an area, if there are 200 carrot
grass (Parthenium hysterophorus) plants but only a single huge banyan
tree with a large canopy, stating that the population density of banyan is
low relative to that of carrot grass amounts to underestimating the
enormous role of the Banyan in that community. In such cases, the per
cent cover or biomass is a more meaningful measure of the population
size. Total number is again not an easily adoptable measure if the
population is huge and counting is impossible or very time-consuming.
If you have a dense laboratory culture of bacteria in a petri dish what is
the best measure to report its density? Sometimes, for certain ecological
investigations, there is no need to know the absolute population densities;
relative densities serve the purpose equally well. For instance, the number
of fish caught per trap is good enough measure of its total population
density in the lake. We are mostly obliged to estimate population sizes
indirectly, without actually counting them or seeing them. The tiger census
in our national parks and tiger reserves is often based on pug marks and
fecal pellets.
13.2.2 Population Growth
The size of a population for any species is not a static parameter. It keeps
changing with time, depending on various factors including food
availability, predation pressure and adverse weather. In fact, it is these
changes in population density that give us some idea of what is happening
to the population – whether it is flourishing or declining. Whatever might
be the ultimate reasons, the density of a population in a given habitat
during a given period, fluctuates due to changes in four basic processes,
two of which (natality and immigration) contribute to an increase in
population density and two (mortality and emigration) to a decrease.
(i) Natality refers to the number of births during a given period in the
population that are added to the initial density.
(ii) Mortality is the number of deaths in the population during a given
period.
(iii) Immigration is the number of individuals of the same species that
have come into the habitat from elsewhere during the time period
under consideration.
(iv) Emigration is the number of individuals of the population who
left the habitat and gone elsewhere during the time period under
consideration.
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So, if N is the population density at time t, then its density at time t +1 is
N
t+1
= N
t
+ [(B + I) – (D + E)]
You can see from the above equation (Fig. 13.5) that population
density will increase if the number of births plus the number of
immigrants (B + I) is more than the number of deaths plus the number
of emigrants (D + E). Under normal conditions, births and deaths are
the most important factors influencing population density, the other
two factors assuming importance only under special conditions. For
instance, if a new habitat is just being colonised, immigration may
contribute more significantly to population growth than birth rates.
Growth Models : Does the growth of a population with time show any
specific and predictable pattern? We have been concerned about
unbridled human population growth and problems created by it in our
country and it is therefore natural for us to be curious if different animal
populations in nature behave the same way or show some restraints on
growth. Perhaps we can learn a lesson or two from nature on how to
control population growth.
(i) Exponential growth: Resource (food and space) availability is
obviously essential for the unimpeded growth of a population.
Ideally, when resources in the habitat are unlimited, each species
has the ability to realise fully its innate potential to grow in number,
as Darwin observed while developing his theory of natural
selection. Then the population grows in an exponential or
Figure 13.5
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geometric fashion. If in a population of size N, the birth rates (not
total number but per capita births) are represented as b and death
rates (again, per capita death rates) as d, then the increase or
decrease in N during a unit time period t (dN/dt) will be
dN/dt = (b – d) × N
Let (b–d) = r, then
dN/dt = rN
The r in this equation is called the ‘intrinsic rate of natural increase’
and is a very important parameter chosen for assessing impacts of
any biotic or abiotic factor on population growth.
To give you some idea about the magnitude of r values, for the
Norway rat the r is 0.015, and for the flour beetle it is 0.12. In
1981, the r value for human population in India was 0.0205. Find
out what the current r value is. For calculating it, you need to
know the birth rates and death rates.
The above equation describes the exponential or geometric growth
pattern of a population (Figure 13.6) and results in a J-shaped curve
when we plot N in relation to time. If you are familiar with basic
calculus, you can derive the integral form of the
exponential growth equation as
N
t
= N
0
e
rt
where
N
t
= Population density after time t
N
0
= Population density at time zero
r = intrinsic rate of natural increase
e = the base of natural logarithms (2.71828)
Any species growing exponentially under unlimited
resource conditions can reach enormous population
densities in a short time. Darwin showed how even
a slow growing animal like elephant could reach
enormous numbers in the absence of checks. The
following is an anecdote popularly narrated to
demonstrate dramatically how fast a huge
population could build up when growing
exponentially.
The king and the minister sat for a chess game. The king, confident
of winning the game, was ready to accept any bet proposed by the
minister. The minister humbly said that if he won, he wanted only
some wheat grains, the quantity of which is to be calculated by placing
on the chess board one grain in Square 1, then two in Square 2,
then four in Square 3, and eight in Square 4, and so on, doubling each
time the previous quantity of wheat on the next square until all the 64
squares were filled. The king accepted the seemingly silly bet and started
Figure 13.6 Population growth curve
a when responses are not
limiting the growth, plot is
exponential,
b when responses are limiting
the growth, plot is logistic,
K is carrying capacity
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the game, but unluckily for him, the minister won. The king felt that fulfilling
the minister’s bet was so easy. He started with a single grain on
the first square and proceeded to fill the other squares following
minister’s suggested procedure, but by the time he covered half the
chess board, the king realised to his dismay that all the wheat
produced in his entire kingdom pooled together would still be
inadequate to cover all the 64 squares. Now think of a tiny
Paramecium starting with just one individual and through binary
fission, doubling in numbers every day, and imagine what a mind-
boggling population size it would reach in 64 days. (provided food
and space remain unlimited)
(ii) Logistic growth: No population of any species in nature has at its
disposal unlimited resources to permit exponential growth. This
leads to competition between individuals for limited resources.
Eventually, the ‘fittest’ individual will survive and reproduce. The
governments of many countries have also realised this fact and
introduced various restraints with a view to limit human population
growth. In nature, a given habitat has enough resources to support
a maximum possible number, beyond which no further growth is
possible. Let us call this limit as nature’s carrying capacity (K) for
that species in that habitat.
A population growing in a habitat with limited resources show
initially a lag phase, followed by phases of acceleration and
deceleration and finally an asymptote, when the population density
reaches the carrying capacity. A plot of N in relation to time (t)
results in a sigmoid curve. This type of population growth is called
Verhulst-Pearl Logistic Growth (Figure 13.6) and is described by
the following equation:
dN/dt =
K N
rN
K
−
 
 
ï£ ï£¸
Where N = Population density at time t
r = Intrinsic rate of natural increase
K = Carrying capacity
Since resources for growth for most animal populations are finite
and become limiting sooner or later, the logistic growth model is
considered a more realistic one.
Gather from Government Census data the population figures
for India for the last 100 years, plot them and check which growth
pattern is evident.
13.2.3 Life History Variation
Populations evolve to maximise their reproductive fitness, also called
Darwinian fitness (high r value), in the habitat in which they live. Under
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a particular set of selection pressures, organisms evolve towards the most
efficient reproductive strategy. Some organisms breed only once in their
lifetime (Pacific salmon fish, bamboo) while others breed many times
during their lifetime (most birds and mammals). Some produce a large
number of small-sized offspring (Oysters, pelagic fishes) while others
produce a small number of large-sized offspring (birds, mammals). So,
which is desirable for maximising fitness? Ecologists suggest that life
history traits of organisms have evolved in relation to the constraints
imposed by the abiotic and biotic components of the habitat in which
they live. Evolution of life history traits in different species is currently an
important area of research being conducted by ecologists.
13.2.4 Population Interactions
Can you think of any natural habitat on earth that is inhabited just by a
single species? There is no such habitat and such a situation is even
inconceivable. For any species, the minimal requirement is one more
species on which it can feed. Even a plant species, which makes its own
food, cannot survive alone; it needs soil microbes to break down the organic
matter in soil and return the inorganic nutrients for absorption. And then,
how will the plant manage pollination without an animal agent? It is
obvious that in nature, animals, plants and microbes do not and cannot
live in isolation but interact in various ways to form a biological
community. Even in minimal communities, many interactive linkages
exist, although all may not be readily apparent.
Interspecific interactions arise from the interaction of populations of
two different species. They could be beneficial, detrimental or neutral
(neither harm nor benefit) to one of the species or both. Assigning a ‘+’
sign for beneficial interaction, ‘-’ sign for detrimental and 0 for neutral
interaction, let us look at all the possible outcomes of interspecific
interactions (Table13.1).
Both the species benefit in mutualism and both lose in competition in
their interactions with each other. In both parasitism and predation only
one species benefits (parasite and predator, respectively) and the interaction
Species A Species B Name of Interaction
+ + Mutualism
– – Competition
+ – Predation
+ – Parasitism
+ 0 Commensalism
– 0 Amensalism
Table 13.1 : Population Interactions
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is detrimental to the other species (host and prey, respectively).
The interaction where one species is benefitted and the other is neither
benefitted nor harmed is called commensalism. In amensalism on
the other hand one species is harmed whereas the other is
unaffected. Predation, parasitism and commensalism share a common
characteristic– the interacting species live closely together.
(i) Predation: What would happen to all the energy fixed by
autotrophic organisms if the community has no animals to eat the
plants? You can think of predation as nature’s way of transferring
to higher trophic levels the energy fixed by plants. When we think
of predator and prey, most probably it is the tiger and the deer that
readily come to our mind, but a sparrow eating any seed is no less
a predator. Although animals eating plants are categorised
separately as herbivores, they are, in a broad ecological context,
not very different from predators.
Besides acting as ‘conduits’ for energy transfer across trophic
levels, predators play other important roles. They keep prey
populations under control. But for predators, prey species could
achieve very high population densities and cause ecosystem
instability. When certain exotic species are introduced into a
geographical area, they become invasive and start spreading fast
because the invaded land does not have its natural predators. The
prickly pear cactus introduced into Australia in the early 1920’s
caused havoc by spreading rapidly into millions of hectares of
rangeland. Finally, the invasive cactus was brought under control
only after a cactus-feeding predator (a moth) from its natural habitat
was introduced into the country. Biological control methods adopted
in agricultural pest control are based on the ability of the predator
to regulate prey population. Predators also help in maintaining
species diversity in a community, by reducing the intensity of
competition among competing prey species. In the rocky intertidal
communities of the American Pacific Coast the starfish Pisaster
is
an important predator. In a field experiment, when all the starfish
were removed from an enclosed intertidal area, more than 10 species
of invertebrates became extinct within a year, because of inter-
specific competition.
If a predator is too efficient and overexploits its prey, then the
prey might become extinct and following it, the predator will also
become extinct for lack of food. This is the reason why predators in
nature are ‘prudent’. Prey species have evolved various defenses to
lessen the impact of predation. Some species of insects and frogs
are cryptically-coloured (camouflaged) to avoid being detected easily
by the predator. Some are poisonous and therefore avoided by the
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predators. The Monarch butterfly is highly distasteful to its predator
(bird) because of a special chemical present in its body.
Interestingly, the butterfly acquires this chemical during its
caterpillar stage by feeding on a poisonous weed.
For plants, herbivores are the predators. Nearly 25 per cent of
all insects are known to be phytophagous (feeding on plant sap
and other parts of plants). The problem is particularly severe for
plants because, unlike animals, they cannot run away from their
predators. Plants therefore have evolved an astonishing variety of
morphological and chemical defences against herbivores. Thorns
(Acacia, Cactus) are the most common morphological means of
defence. Many plants produce and store chemicals that make the
herbivore sick when they are eaten, inhibit feeding or digestion,
disrupt its reproduction or even kill it. You must have seen the
weed Calotropis growing in abandoned fields. The plant produces
highly poisonous cardiac glycosides and that is why you never see
any cattle or goats browsing on this plant. A wide variety of chemical
substances that we extract from plants on a commercial scale
(nicotine, caffeine, quinine, strychnine, opium, etc.,) are produced
by them actually as defences against grazers and browsers.
(ii) Competition: When Darwin spoke of the struggle for existence and
survival of the fittest in nature, he was convinced that interspecific
competition is a potent force in organic evolution. It is generally
believed that competition occurs when closely related species
compete for the same resources that are limiting, but this is not
entirely true. Firstly, totally unrelated species could also compete
for the same resource. For instance, in some shallow South
American lakes, visiting flamingoes and resident fishes compete for
their common food, the zooplankton in the lake. Secondly,
resources need not be limiting for competition to occur; in
interference competition, the feeding efficiency of one species might
be reduced due to the interfering and inhibitory presence of the
other species, even if resources (food and space) are abundant.
Therefore, competition is best defined as a process in which the
fitness of one species (measured in terms of its ‘r’ the intrinsic rate
of increase) is significantly lower in the presence of another species.
It is relatively easy to demonstrate in laboratory experiments, as
Gause and other experimental ecologists did, when resources are
limited the competitively superior species will eventually eliminate
the other species, but evidence for such competitive exclusion
occurring in nature is not always conclusive. Strong and persuasive
circumstantial evidence does exist however in some cases. The
Abingdon tortoise in Galapagos Islands became extinct within a
decade after goats were introduced on the island, apparently due
to the greater browsing efficiency of the goats. Another evidence for
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the occurrence of competition in nature comes from what is called
‘competitive release’. A species whose distribution is restricted to a
small geographical area because of the presence of a competitively
superior species, is found to expand its distributional range
dramatically when the competing species is experimentally removed.
Connell’s elegant field experiments showed that on the rocky sea
coasts of Scotland, the larger and competitively superior barnacle
Balanus dominates the intertidal area, and excludes the smaller
barnacle Chathamalus from that zone. In general, herbivores and
plants appear to be more adversely affected by competition than
carnivores.
Gause’s ‘Competitive Exclusion Principle’ states that two
closely related species competing for the same resources cannot
co-exist indefinitely and the competitively inferior one will be
eliminated eventually. This may be true if resources are limiting,
but not otherwise. More recent studies do not support such gross
generalisations about competition. While they do not rule out the
occurrence of interspecific competition in nature, they point out
that species facing competition might evolve mechanisms that
promote co-existence rather than exclusion. One such mechanism
is ‘resource partitioning’. If two species compete for the same
resource, they could avoid competition by choosing, for instance,
different times for feeding or different foraging patterns. MacArthur
showed that five closely related species of warblers living on the
same tree were able to avoid competition and co-exist due to
behavioural differences in their foraging activities.
(iii) Parasitism: Considering that the parasitic mode of life ensures
free lodging and meals, it is not surprising that parasitism has
evolved in so many taxonomic groups from plants to higher
vertebrates. Many parasites have evolved to be host-specific (they
can parasitise only a single species of host) in such a way that both
host and the parasite tend to co-evolve; that is, if the host evolves
special mechanisms for rejecting or resisting the parasite, the
parasite has to evolve mechanisms to counteract and neutralise
them, in order to be successful with the same host species. In
accordance with their life styles, parasites evolved special
adaptations such as the loss of unnecessary sense organs, presence
of adhesive organs or suckers to cling on to the host, loss of digestive
system and high reproductive capacity. The life cycles of parasites
are often complex, involving one or two intermediate hosts or vectors
to facilitate parasitisation of its primary host. The human liver fluke
(a trematode parasite) depends on two intermediate hosts (a snail
and a fish) to complete its life cycle. The malarial parasite needs a
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vector (mosquito) to spread to other hosts. Majority of the parasites
harm the host; they may reduce the survival, growth and
reproduction of the host and reduce its population density. They
might render the host more vulnerable to predation by making it
physically weak. Do you believe that an ideal parasite should be
able to thrive within the host without harming it? Then why didn’t
natural selection lead to the evolution of such totally harmless
parasites?
Parasites that feed on the external surface of the host organism
are called ectoparasites. The most familiar examples of this group
are the lice on humans and ticks on dogs. Many marine fish are
infested with ectoparasitic copepods. Cuscuta, a parasitic plant that
is commonly found growing on hedge plants, has lost its chlorophyll
and leaves in the course of evolution. It derives its nutrition from
the host plant which it parasitises. The female mosquito is not
considered a parasite, although it needs our blood for reproduction.
Can you explain why?
In contrast, endoparasites are those that live inside the host
body at differ
ent sites (liver, kidney, lungs, red blood cells, etc.).
The life cycles of endoparasites are more complex because of their
extreme specialisation. Their morphological and anatomical features
are greatly simplified while emphasising their reproductive potential.
Brood parasitism in birds is a fascinating example of parasitism
in which the parasitic bird lays its eggs in the nest of its host and
lets the host incubate them. During the course of evolution, the
eggs of the parasitic bird have evolved to resemble the host’s egg in
size and colour to reduce the chances of the host bird detecting the
foreign eggs and ejecting them from the nest. Try to follow the
movements of the cuckoo (koel) and the crow in your neighborhood
park during the breeding season (spring to summer) and watch
brood parasitism in action.
(iv) Commensalism: This is the interaction in which one species benefits
and the other is neither harmed nor benefited. An orchid growing
as an epiphyte on a mango branch, and barnacles growing on the
back of a whale benefit while neither the mango tree nor the whale
derives any apparent benefit. The cattle egret and grazing cattle in
close association, a sight you are most likely to catch if you live in
farmed rural areas, is a classic example of commensalism. The
egrets always forage close to where the cattle are grazing because
the cattle, as they move, stir up and flush out insects from the
vegetation that otherwise might be difficult for the egrets to find
and catch. Another example of commensalism is the interaction
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between sea anemone that has stinging tentacles and the clown
fish that lives among them. The fish gets protection from predators
which stay away from the stinging tentacles. The anemone does
not appear to derive any benefit by hosting the clown fish.
(v) Mutualism: This interaction confers benefits on both the interacting
species. Lichens represent an intimate mutualistic relationship
between a fungus and photosynthesising algae or cyanobacteria.
Similarly, the mycorrhizae are associations between fungi and the
roots of higher plants. The fungi help the plant in the absorption of
essential nutrients from the soil while the plant in turn provides the
fungi with energy-yielding carbohydrates.
The most spectacular and evolutionarily fascinating examples
of mutualism are found in plant-animal relationships. Plants need
the help of animals for pollinating their flowers and dispersing their
seeds. Animals obviously have to be paid ‘fees’ for the services that
plants expect from them. Plants offer rewards or fees in the form of
pollen and nectar for pollinators and juicy and nutritious fruits for
seed dispersers. But the mutually beneficial system should also
be safeguarded against ‘cheaters’, for example, animals that try to
steal nectar without aiding in pollination. Now you can see why
plant-animal interactions often involve co-evolution of the
mutualists, that is, the evolutions of the flower and its pollinator
species are tightly linked with one another. In many species of fig
trees, there is a tight one-to-one relationship with the pollinator
species of wasp (Figure 13.7). It means that a given fig species can
be pollinated only by its ‘partner’ wasp species and no other species.
The female wasp uses the fruit not only as an oviposition (egg-laying)
site but uses the developing seeds within the fruit for nourishing
Figure 13.7 Mutual relationship between fig tree and wasp: (a) Fig flower is pollinated
by wasp; (b) Wasp laying eggs in a fig fruit
(a) (b)
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its larvae. The wasp pollinates the fig inflorescence while
searching for suitable egg-laying sites. In return for the
favour of pollination the fig offers the wasp some of its
developing seeds, as food for the developing wasp larvae.
Orchids show a bewildering diversity of floral
patterns many of which have evolved to attract the right
pollinator insect (bees and bumblebees) and ensure
guaranteed pollination by it (Figure 13.8). Not all
orchids offer rewards. The Mediterranean orchid
Ophrys employs ‘sexual deceit’ to get pollination done
by a species of bee. One petal of its flower bears an
uncanny resemblance to the female of the bee in size,
colour and markings. The male bee is attracted to what
it perceives as a female, ‘pseudocopulates’ with the
flower, and during that process is dusted with pollen
from the flower. When this same bee ‘pseudocopulates’
with another flower, it transfers pollen to it and thus,
pollinates the flower. Here you can see how co-evolution
operates. If the female bee’s colour patterns change even slightly for any
reason during evolution, pollination success will be reduced unless the
orchid flower co-evolves to maintain the resemblance of its petal to the
female bee.
Figure 13.8 Showing bee-a pollinator
on orchid flower
SUMMARY
As a branch of biology, Ecology is the study of the relationships of
living organisms with the abiotic (physico-chemical factors) and biotic
components (other species) of their environment. It is concerned
with four levels of biological organisation-organisms, populations,
communities and biomes.
Temperature, light, water and soil are the most important
physical factors of the environment to which the organisms are
adapted in various ways. Maintenance of a constant internal
environment (homeostasis) by the organisms contributes to optimal
performance, but only some organisms (regulators) are capable of
homeostasis in the face of changing external environment. Others
either partially regulate their internal environment or simply
conform. A few other species have evolved adaptations to avoid
unfavourable conditions in space (migration) or in time (aestivation,
hibernation, and diapause).
Evolutionary changes through natural selection take place at
the population level and hence, population ecology is an important
area of ecology. A population is a group of individuals of a given
species sharing or competing for similar resources in a defined
geographical area. Populations have attributes that individual
organisms do not- birth rates and death rates, sex ratio and age
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distribution. The proportion of different age groups of males and
females in a population is often presented graphically as age pyramid;
its shape indicates whether a population is stationary, growing or
declining.
Ecological effects of any factors on a population are generally
reflected in its size (population density), which may be expressed in
different ways (numbers, biomass, per cent cover, etc.,) depending
on the species.
Populations grow through births and immigration and decline
through deaths and emigration. When resources are unlimited, the
growth is usually exponential but when resources become
progressively limiting, the growth pattern turns logistic. In either
case, growth is ultimately limited by the carrying capacity of the
environment. The intrinsic rate of natural increase (r) is a measure
of the inherent potential of a population to grow.
In nature populations of different species in a habitat do not live
in isolation but interact in many ways. Depending on the outcome,
these interactions between two species are classified as competition
(both species suffer), predation and parasitism (one benefits and the
other suffers), commensalism (one benefits and the other is
unaffected), amensalism (one is harmed, other unaffected) and
mutualism (both species benefit). Predation is a very important
process through which trophic energy transfer is facilitated and some
predators help in controlling their prey populations. Plants have
evolved diverse morphological and chemical defenses against
herbivory. In competition, it is presumed that the superior competitor
eliminates the inferior one (the Competitive Exclusion Principle), but
many closely related species have evolved various mechanisms which
facilitate their co-existence. Some of the most fascinating cases of
mutualism in nature are seen in plant-pollinator interactions.
EXERCISES
1. How is diapause different from hibernation?
2. If a marine fish is placed in a fresh water aquarium, will the fish be
able to survive? Why or why not?
3. Most living organisms cannot survive at temperature above 45
0
C. How
are some microbes able to live in habitats with temperatures exceeding
100
0
C?
4. List the attributes that populations possess but not individuals.
5. If a population growing exponentially double in size in 3 years, what is
the intrinsic rate of increase (r) of the population?
6. Name important defence mechanisms in plants against herbivory.
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7. An orchid plant is growing on the branch of mango tree. How do you
describe this interaction between the orchid and the mango tree?
8. What is the ecological principle behind the biological control method of
managing with pest insects?
9. Distinguish between the following:
(a) Hibernation and Aestivation
(b) Ectotherms and Endotherms
10. Write a short note on
(a) Adaptations of desert plants and animals
(b) Adaptations of plants to water scarcity
(c) Behavioural adaptations in animals
(d) Importance of light to plants
(e) Effect of temperature or water scarcity and the adaptations of animals.
11. List the various abiotic environmental factors.
12. Give an example for:
(a) An endothermic animal
(b) An ectothermic animal
(c) An organism of benthic zone
13. Define population and community.
14. Define the following terms and give one example for each:
(a) Commensalism
(b) Parasitism
(c) Camouflage
(d) Mutualism
(e) Interspecific competition
15. With the help of suitable diagram describe the logistic population
growth curve.
16. Select the statement which explains best parasitism.
(a) One organism is benefited.
(b) Both the organisms are benefited.
(c) One organism is benefited, other is not affected.
(d) One organism is benefited, other is affected.
17. List any three important characteristics of a population and explain.
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