Population Ecology

Yahyea
Baktiar
Laskar
ZOOHCC-102: Principles of Ecology (U2)
POPULATION ECOLOGY
Yahyea Baktiar Laskar, PhD
Assistant Professor
Department of Zoology
yahyeabaktiar.laskar@aus.ac.in
2022

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Baktiar
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Population Ecology
Unique and group attributes of population:
What is a population?
In ecology, a population consists of all the organisms of a particular species living in a given area. For instance, we could say that a
population of humans lives in Silchar town.
 Population Size and Density: The number of individuals in the population, or population size—N. Population density is the
number of individuals per area or volume of habitat.
 Size and density are both important in describing the current status of the population and, potentially, for making predictions about
how it could change in the future.
 Larger populations may be more stable than smaller populations because they’re likely to have greater genetic variability and thus
more potential to adapt to changes in the environment through natural selection.
 A member of a low-density population—where organisms are sparsely spread out—might have more trouble finding a mate to
reproduce with than an individual in a high-density population.
 The statistical study of populations and how they change over time is called demography.

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Population Ecology
Demography:
 The statistical study of any population, human or otherwise, is known as demography.
Importance of Demography:
 Populations can change in their numbers and structure—for example age and sex distribution—for various reasons. These changes
can affect how the population interacts with its physical environment and with other species.
 By tracking populations over time, ecologists can see how these populations have changed and may be able to predict how they're
likely to change in the future.
 Monitoring the size and structure of populations can also help ecologists manage populations—for example, by showing whether
conservation efforts are helping an endangered species increase in numbers.

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Population Ecology
Measuring population size:
 Counting all the organisms in a population may be too expensive in terms of time and money, or it may simply not be possible.
 For these reasons, scientists often estimate a population's size by taking one or more samples from the population and using these
samples to make inferences about the population as a whole. A variety of methods can be used to sample populations to
determine their size and density.
 Two of the most important methods: the quadrat and mark-recapture methods.

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Population Ecology
Measuring population size: Quadrat method
 For immobile organisms such as plants—or for very small and slow-moving
organisms—plots called quadrats may be used to determine population size and
density.
 Each quadrat marks off an area of the same size—typically, a square area—within the
habitat. A quadrat can be made by staking out an area with sticks and string or by using
a wood, plastic, or metal square placed on the ground, as shown in the picture below.
 After setting up quadrats, researchers count the number of individuals within the
boundaries of each one. Multiple quadrat samples are performed throughout the habitat
at several random locations, which ensures that the numbers recorded are representative
for the habitat overall. In the end, the data can be used to estimate the population size
and population density within the entire habitat.

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Population Ecology
Measuring population size: Mark-recapture method
 For organisms that move around, such as mammals, birds, or fish, a technique
called the mark-recapture method is often used to determine population size. This
method involves capturing a sample of animals and marking them in some way—
for instance, using tags, bands, paint, or other body markings, as shown below.
Then, the marked animals are released back into the environment and allowed to
mix with the rest of the population.
 Later, a new sample is collected. This new sample will include some individuals
that are marked—recaptures—and some individuals that are unmarked. Using the
ratio of marked to unmarked individuals, scientists can estimate how many
individuals are in the total population.

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Population Ecology
Measuring population size: Example of Mark-recapture method (Lincoln Index)
 Let’s say we want to find the size of a deer population.
 Suppose that we capture 80 deer, tag them, and release them back into the forest. After some time has passed—allowing the
marked deer to thoroughly mix with the rest of the population—we come back and capture another 100 deer. Out of these deer, we
find that 20 are already marked.
 Using this information, we can formulate the following relationship:

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Population Ecology
Measuring population size/density: Shannon-Weiner diversity index for community
 The Shannon Diversity Index (sometimes called the Shannon-Wiener Index) is a way to measure the diversity of species in a
community.
 Denoted as H, this index is calculated as:
H = -∑ pi × ln(pi)
Where, pi is the proportion of the entire community made up of species i.
 The higher the value of H, the higher the diversity of species in a particular community. The lower the value of H, the lower the
diversity. A value of H = 0 indicates a community that only has one species.

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Population Ecology
Measuring population size/density: Shannon-Weiner diversity index for community
A list different species of a community is given below with their relative abundance:
Species Frequency
Relative
abundance (pi)
ln (pi) pi × ln(pi)
A 40 0.38 -0.97 -0.37
B 20 0.19 -1.66 -0.32
C 15 0.14 -1.95 -0.28
D 8 0.08 -2.57 -0.20
E 22 0.21 -1.56 -0.33
H 1.49
The Shannon Diversity Index for this community is 1.49.

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Population Ecology
Species distribution:
 Often, in addition to knowing the number and density of individuals in an area, ecologists will also want to know their
distribution. Species dispersion patterns—or distribution patterns—refer to how the individuals in a population are distributed
in space at a given time.
 Uniform dispersion. In uniform dispersion, individuals of a population are spaced more or less evenly. One example of uniform
dispersion comes from plants that secrete toxins to inhibit growth of nearby individuals—a phenomenon called allelopathy. We
can also find uniform dispersion in animal species where individuals stake out and defend territories.
 Random dispersion. In random dispersion, individuals are distributed randomly, without a predictable pattern. An example of
random dispersion comes from dandelions and other plants that have wind-dispersed seeds. The seeds spread widely and sprout
where they happen to fall, as long as the environment is favorable—has enough soil, water, nutrients, and light.

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Population Ecology
Species distribution:
 Clumped dispersion. In a clumped dispersion, individuals are clustered in groups. A clumped dispersion may be seen in plants
that drop their seeds straight to the ground—such as oak trees—or animals that live in groups—schools of fish or herds of
elephants. Clumped dispersions also happen in habitats that are patchy, with only some patches suitable to live in.

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Population Ecology
Natality or Birth Rate
 The term natality is more commonly used in population biology when describing a study of the human population. Natality is
defined as the birth of an individual in a population, whereas the natality rate refers to the number of individuals produced per
female per unit of time.
 In population biology, natality can be divided into two groups: absolute natality or realized natality.
 Absolute natality, it is also known as the maximum or the physiological natality, it can be defined as the maximum number of
individuals that can be produced by the female in an ideal condition. Ideal condition refers to the condition where there is no
limitation of resources and no competition.
 Realized natality is also known as ecological or actual natality. It can be defined as the number of births per individual per unit
of time in their normal ecological habitat.

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Population Ecology
Natality or Birth Rate Determination
 Crude Natality Rate (B) =
𝑁𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑛𝑒𝑤 𝑖𝑛𝑑𝑖𝑣𝑖𝑑𝑢𝑎𝑙 𝑝𝑟𝑜𝑑𝑢𝑐𝑒𝑑
𝑆𝑝𝑒𝑐𝑖𝑓𝑖𝑐 𝑢𝑛𝑖𝑡 𝑜𝑓 𝑡𝑖𝑚𝑒
, or B=
𝑁𝑛
∆𝑡
 Specific Natality Rate (B) =
𝑁𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑛𝑒𝑤 𝑖𝑛𝑑𝑖𝑣𝑖𝑑𝑢𝑎𝑙𝑠 𝑝𝑒𝑟 𝑢𝑛𝑖𝑡 𝑡𝑖𝑚𝑒
𝑈𝑛𝑖𝑡 𝑜𝑓 𝑝𝑜𝑝𝑢𝑙𝑎𝑡𝑖𝑜𝑛
, or B=
𝑁𝑛
𝑁∆𝑡
**Specific Natality Rate is the average rate of change per unit population.
Suppose, a population of 50 protozoa in a pool increases to 150 in one hour. Then
 The crude natality rate is 100/hour.
 The specific natality rate will be =
100/1
50
= 2 per hour per individual

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Population Ecology
Mortality or Death Rate Determination
 The mortality rate can be defined as the number of individuals who died during a given period of time. The mortality rate is often
calculated as the number of deaths per 1000 individuals per year. It is important to note that the mortality rate is time specific.
 Mortality rate can be mathematically expressed as the following:
𝑇𝑜𝑡𝑎𝑙 𝑁𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑁𝑒𝑤 𝐷𝑒𝑎𝑡ℎ 𝑖𝑛 𝑎 𝐷𝑒𝑓𝑖𝑛𝑒𝑑 𝑃𝑒𝑟𝑖𝑜𝑑 𝑜𝑓 𝑇𝑖𝑚𝑒
𝑇𝑜𝑡𝑎𝑙 𝑁𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑡ℎ𝑒 𝐼𝑛𝑑𝑖𝑣𝑖𝑑𝑢𝑎𝑙 𝑎𝑡 𝑡ℎ𝑒 𝐺𝑖𝑣𝑒𝑛 𝑃𝑜𝑝𝑢𝑙𝑎𝑡𝑖𝑜𝑛 𝑖𝑛 𝑆𝑡𝑎𝑟𝑡
• Ecological or realized mortality: The loss of individuals under a given environmental condition- which is not constant and varies
with population and environmental conditions.
• Minimum Mortality: Theoretical concept representing the minimum loss under ideal or non-limiting conditions.

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Population Ecology
Life Tables
 A complete picture of mortality in a population is illustrated systematically by the life
tables, first introduced by Raymond Pearl into general biology.
 A life table records matters of life and death for a population. It summarizes the likelihood
that organisms in a population will live, die, and/or reproduce at different stages of their
lives.
 Let's start simply by taking the example of the Dall mountain sheep, a wild sheep of
northwestern North America.
An ecologist named Olaus Murie hiked around Mount McKinley National Park in Alaska for several years in the 1930s and 1940s.
Every time he came across the skull of a dead Dall mountain sheep, he used the size of its horns to estimate how old it must have been
when it died. From the ages of the 608 skulls he discovered, he estimated survival and death rates for the sheep across their lifespans.
On the next slide, we have a table based on Murie's skull collection data. To make it easier to read, the table is standardized to a
population of 1000 sheep.

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Population Ecology
Life Table of Dall mountain sheep
Age interval in years
Number surviving
individuals
Number dying in age
interval out of 1000
born
Age-specific mortality
rate
0-0.5 1000 54 0.054
0.5-1 946 145 0.1533
1-2 801 12 0.015
2-3 789 13 0.0165
3–4 776 12 0.0155
4–5 764 30 0.0393
5–6 734 46 0.0627
6–7 688 48 0.0698
7–8 640 69 0.1078
8–9 571 132 0.2312
9–10 439 187 0.426

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Population Ecology
Survivorship Curve
 When the data from the number of surviving
individuals are plotted with the time interval on
the horizontal axis and the number of survivors
on the vertical axis, the resulting curve is called
the Survivorship Curve.
 A survivorship curve shows what fraction of a
starting group is still alive at each successive age.
For example, the survivorship curve for Dall
mountain sheep is shown below:

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Population Ecology
Types of Survivorship Curve
 Different species have differently shaped survivorship curves. In general, we
can divide survivorship curves into three types based on their shapes:
 Type I Survivorship Curve: Humans and most primates have a Type I
survivorship curve. In a Type I curve, organisms tend not to die when they are
young or middle-aged but, instead, die when they become elderly. Species with
Type I curves usually have small numbers of offspring and provide lots of
parental care to make sure those offspring survive.
 Type II Survivorship Curve: Many bird species have a Type II survivorship
curve. In a Type II curve, organisms die more or less equally at each age
interval. Organisms with this type of survivorship curve may also have
relatively few offspring and provide significant parental care.

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Population Ecology
Types of Survivorship Curve
 Type III Survivorship Curve: Trees, marine invertebrates, and most fish have
a Type III survivorship curve. In a Type III curve, very few organisms survive
their younger years. However, the lucky ones that make it through youth are
likely to have pretty long lives after that. Species with this type of curve
usually have lots of offspring at once—such as a tree releasing thousands of
seeds—but don't provide much care for the offspring.

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Population Ecology
Fecundity Vs Fertility
 Fecundity can be described as the maximum
reproductive output potential of an individual
(specifically females) under ideal environmental
conditions
 Fertility can be described as the actual reproductive
performance of an individual (specifically females)
under prevailing environmental conditions.

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Population Ecology
Concept of Growth and Carrying Capacity
Carrying Capacity
 In biology and environmental science, the carrying capacity of a biological species in a particular habitat refers to the maximum
number of individuals (of that species) that the environment can carry and sustain, considering its geography or physical features.
 In ecology, carrying capacity is measured as the maximum load of an environment.
 Carrying capacity may also be defined as the population size at which the population growth rate equals zero.
 The physical features present in the environment act as limiting factors (e.g. food, water, competition, etc.). Thus, the population
limit can be expected to depend on these factors. In essence, food availability is an important variable as it affects the population
size of the species. It does so in such a way that if food demand is not met over a given period of time the population size will
eventually decrease until the resources become adequate. By contrast, when food supply exceeds demand then the population size
will soon increase and will stop increasing when the source is consequently depleted.

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Population Ecology
Carrying Capacity
 Carrying capacity (K) is calculated as:
K=
𝒓𝑵 (𝟏−𝑵)
𝒅𝑵
𝒅𝒕
where r is the intrinsic rate of increase, N is the population size, and dN/dt is the change in population size.
Carrying Capacity Examples: Turtle population
For example, a pond inhabited initially by 10 turtles will be sustainable for the species’ population. Because water, food, and space
abound, the turtles can thrive and reproduce at an exponential rate. However, as the population grows, competition is intensified as
well. Turtles compete for food, water, and space.
Male turtles compete with other males for mates. These factors will limit the biotic potential of the turtles. When the population seems
stable, e.g. at a population of 100 turtles, then, it can be said that the carrying capacity for that area is 100 turtles.

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Population Ecology
Population Growth
 Populations show characteristic pattern of increase, termed as population growth curve or forms. Based on the shapes of arithmetic
plots of growth curves can be designated as:
• J-shaped or Exponential growth curve
• S-shaped or sigmoid or logistic growth curve
In exponential growth, a population's per capita (per individual) growth rate stays the same regardless of population size, making the
population grow faster and faster as it gets larger.
In logistic growth, a population's per capita growth rate gets smaller and smaller as population size approaches a maximum imposed
by limited resources in the environment, known as the carrying capacity (K).

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Exponential Growth Curve
 Bacteria grown in the lab provide an excellent example of exponential
growth. In exponential growth, the population’s growth rate increases over time,
in proportion to the size of the population.
 The key concept of exponential growth is that the population growth rate —the
number of organisms added in each generation—increases as the population gets
larger.
 In an ideal condition where there is an unlimited supply of food and resources,
the population growth will follow an exponential order.

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Population Ecology
Exponential Growth Curve
 When population size (N) is plotted over time, a J-shaped growth curve is made.
 The exponential growth represented with an r or rmax.
 rmax is the maximum per capita rate of increase for a particular species under ideal
conditions, and it varies from species to species. For instance, bacteria can
reproduce much faster than humans, and would have a higher maximum per capita
rate of increase.
 Consider a population of size N and birth rate be represented as b, death rate as d,
the rate of change of N can be given by the equation
𝒅𝑵/𝒅𝒕= (b-d) × N
 The maximum population growth rate for a species, sometimes called its biotic
potential, is expressed in the following equation:
𝒅𝑵
𝒅𝒕
= 𝒓𝒎𝒂𝒙N

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Population Ecology
Logistic Growth Curve
 This model defines the concept of ‘survival of the fittest’. Thus, it considers the
fact that resources in nature are exhaustible. The term ‘Carrying capacity’
defines the limit of the resources beyond which they cannot support any number
of organisms. Let this carrying capacity be represented as K.
 The availability of limited resources cannot show exponential growth. As a result,
the graph will have a lag phase, followed by an exponential phase, then a
declining phase and ultimately an asymptote. This is known as Verhulst-Pearl
Logistic Growth and is represented using the equation:
𝒅𝑵
𝒅𝒕
= 𝒓𝒎𝒂𝒙 N (
𝑲−𝑵
𝑵
)

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Logistic Growth Curve
Examples of logistic growth
 Yeast, a microscopic fungus used to make bread and alcoholic
beverages, can produce a classic S-shaped curve when grown in
a test tube. In the graph shown, yeast growth levels off as the
population hits the limit of the available nutrients. (If we
followed the population for longer, it would likely crash, since
the test tube is a closed system – meaning that fuel sources
would eventually run out and wastes might reach toxic levels).

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Population Ecology
Population regulation
Growth Regulation
 All populations on Earth have limits to their growth.
 Even populations of bacteria don't grow infinitely large.
 All organism (population) will ultimately reach limits on population size imposed by the environment.
What exactly are these environmental limiting factors?
 The factors that regulate population growth can be divided into two main groups: density-dependent and density-independent.
 Density-dependent factors, in which the density of the population affects growth rate and mortality, and density-independent
factors, which cause mortality in a population regardless of population density.

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Density-dependent factors
 Density-dependent limiting factors as factors that affect the per capita growth rate of a population differently depending on how
dense the population already is.
 Most density-dependent factors make the per capita growth rate go down as the population increases. This is an example of
negative feedback that limits population growth.
 Density-dependent limiting factors can lead to a logistic pattern of growth, in which a population's size levels off at an
environmentally determined maximum called the carrying capacity.
 Density-dependent limiting factors tend to be biotic—living organism-related—as opposed to physical features of the
environment.
 Most density-dependent factors are biological in nature and include predation, inter- and intraspecific competition, and parasites.
Usually, the denser a population is, the greater its mortality rate.

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Population Ecology
Density-dependent factors
Some common examples of density-dependent limiting factors include:
 Competition within the population: When a population reaches a high density, there are more individuals trying to use the same
quantity of resources. This can lead to competition for food, water, shelter, mates, light, and other resources needed for survival
and reproduction.
 Predation: Higher-density populations may attract predators who wouldn’t bother with a sparser population. When these
predators eat individuals from the population, they decrease its numbers but may increase their own. This can produce interesting,
cyclical patterns.
 Disease and parasites: Disease is more likely to break out and result in deaths when more individuals are living together in the
same place. Parasites are also more likely to spread under these conditions.
 Waste accumulation: High population densities can lead to the accumulation of harmful waste products that kill individuals or
impair reproduction, reducing the population’s growth.

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Density-independent factors
 The second group of limiting factors consists of density-independent limiting factors that affect per capita growth rate independent
of how dense the population is.
 Many factors that are typically physical in nature cause mortality of a population regardless of its density. These factors include
weather, natural disasters, and pollution.
 An individual deer will be killed in a forest fire regardless of how many deer happen to be in that area. Its chances of survival are
the same whether the population density is high or low. The same holds true for cold winter weather.
 Unlike density-dependent limiting factors, density-independent limiting factors alone can’t keep a population at constant levels.
That’s because their strength doesn’t depend on the size of the population, so they don’t make a "correction" when the population
size gets too large. Instead, they may lead to erratic, abrupt shifts in population size. Small populations may be at risk of getting
wiped out by sporadic, density-independent events

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Population Ecology
Population Interactions
Population interaction is the interaction between different populations. It refers to the effects that the organisms in a
community have on one another.
 Organisms living together in a community influence each other directly or indirectly under natural conditions.
 All the vital process of living such as growth, nutrition and reproduction requires such interactions between individuals in the
same species (intraspecific) or between species (interspecific)
 These inter or intra relationships of individuals in a population or community of an ecosystem is called biological interactions or
population interactions.
 The interaction between organisms may not be always beneficial to all the interacting counter parts. Based on whether, the
interaction is beneficial to both interacting species or harmful to at least one interaction species, the ecological of biological
interactions are classified into two categories: Positive and Negative Interactions.

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Or
Biological Interactions
Positive Interaction Negative Interaction
• Mutualism
• Commensalism
• Proto-cooperation
• Ammensalism
• Parasitism
• Predation
• Cannibalism
• Competition

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Positive Population Interactions
 In positive interactions, the interacting populations help one another. The positive interaction may be in one way or reciprocal.
 The benefit may be in respect of food, shelter, substratum or transportation.
 The positive association may be continuous, transitory, obligate or facultative. The two interacting partners may be in close
contact in such a way that the tissues intermixed with each other; or they may live within a specific area of the other; or attached to
its surface.

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Mutualism
• Mutualism, also called as symbiosis, is also a positive type of ecological interaction.
• Mutualism is a symbiotic association between two organisms in which both the interacting partners are mutually benefitted.
• Mutualism is different from proto-cooperation in the sense that mutualism is obligatory and none of the partners of mutualism can
survive individually.
• In mutualism, the organisms enter into some sort of physical and physiological exchange.
Example:
Lichens are the symbiotic association between algae and fungi. The body of lichen composed
of fungal matrix in which the algal cells are embedded. The fungi provide protection to algal
components and also provide moisture and nutrients to them. The algal components in turn will
supply carbohydrates for fungus.

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Commensalism:
• Commensalism is a positive type of ecological interaction between two species in an ecosystem.
• In commensalism, the association occurs between members of two different species where one species is benefited the other is
neither benefited nor harmed.
• Here the two populations live together without entering into any kind of physical exchange, and one is benefited without any effect
on the other.
Examples:
Sucker fish is found attached to sharks with their suckers. Sucker fish gets protection
from predators whereas it has no effect on sharks.

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Proto-cooperation:
• These interactions are beneficial, but are not obligatory for survival.
• The interaction between species in protocooperation is simply for the gain they receive from the interaction. The interaction is
often considered a type of mutualism (facultative mutualism).
• Even though the interaction increases the chances of survival and supports growth, it is not necessary for the general growth and
survival of the species.
Examples:
The cattle egret is a type of bird that feeds on a wide range of insects ranging from maggots, moths,
spiders, and even earthworms. These birds also capture prey present on the surface of cattle, and it
has been found the birds are about 3.6 times more successful in capturing prey when present
around cattle. The interaction helps the animals as it gets rid of any external parasites of insects that
might be present on their bodies.

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Negative Population Interactions
Ammensalism:
• In Ammensalism one species is harmed or inhibited other is neither benefitted nor harmed.
• Some authors prefer to use the term antibiosis for commensalism.
• Antibiosis is the partial or complete inhibition or death of one organism by another through the production of some substances or
environmental conditions as a result of its metabolic pathway.
• In antibiosis none of them derives any benefit. The process of antibiosis is common in microbial populations and the chemical
substances produced by microbes for antibiosis are generally called as antibiotics.
Examples:
Chlorella vulgaris produces a toxin (chlorellin, an antibiotic) which is harmful to
other algae.

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Parasitism:
• Parasitism belongs to the ‘exploitation’ category of negative population interactions.
• In exploitation, one species harms the other by making its direct or indirect use for shelter or food.
• A parasite is the organism living on or in the body of another organisms and deriving food form its tissues. The harmed one is
called host, the benefitted one is called parasite. A parasite usually takes a host which is usually larger than its body size.
• Usually a specialized parasite does not kill the host at least until it has completed its reproductive cycle.
Examples:
Cuscuta is a total stem parasite which lives on the surface of other large plants. They are
devoid of chloroplasts and hence they cannot prepare their own food. Thy have
specialized absorptive structures called haustoria. In the case of complete parasite, the
haustoria will be inserted into the phloem tissue of host plants and they absorb the
prepared food materials from the host phloem.

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Population Ecology
Predation:
• Predation is a negative type of population interaction and it belongs to the ‘exploitation’
category of negative population interactions.
• In predation, one species kill and feeds on another species. The killer species is called
predator and the one who dead are called prey.
• The predators are usually larger and power-full than prey. Predation is very important in
community dynamics and it helps to maintain the constancy of number of different
trophic levels in the ecosystem and thereby maintain the stability of ecosystem.
Examples:
Lion, tiger and Beer are predators of forest ecosystem. They predate herbivores

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Population Ecology
Competition:
• Competition is the association of two or more species; each species is adversely affected by the presence of other species in
respect of food, shelter, space, light etc.
• Competition occurs when individuals attempt to obtain a resource that is inadequate to support all the individuals seeking it or
even if the resources are adequate individuals harm one another in trying to obtain it.
• The resources in the environment for which the individuals compete include raw materials for life such as water, light and
nutrients, space for occupying and selection of mates for sexual reproduction. The competition in the ecosystem may be of two
types: Intraspecific (between the individuals of the same population) and Interspecific (between populations of different species)
competition.

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Population Ecology

Population Ecology

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