Population ecology for epidemiologists
Published on: Mar 4, 2016
Transcripts - Population ecology for epidemiologists
Population ecology, study of the processes that affect
the distribution and abundance of animal and plant
Dr. Bhoj R singh, Principal Scientist (VM)
I/C Epidemiology; Centre for Animal Disease Research and Diagnosis
Indian Veterinary Research Institute, Izatnagar-243122, Bareilly, UP, India.
• A population is a subset of individuals of one species that
occupies a particular geographic area and, in sexually
reproducing species, interbreeds.
– Closed: Defined territories, geographically isolated from, and
lack exchange with, other populations of the same species. The
number of individuals is governed by the rates of birth
(natality), growth, reproduction, and death (mortality).
– Open: show varying degrees of connectedness with other
populations of the same species. Besides natality and
mortality, immigration or emigration affects size and density.
– Metapopulation: Regional groups of interconnected populations
are called metapopulations. These metapopulations are, in
turn, connected to one another over broa As local populations
within a metapopulation fluctuate in size, they become
vulnerable to extinction during periods when their numbers are
low. der geographic ranges.
Types of populations
• r-selected: Populations are considered opportunistic
because their reproductive behaviour involves a high
intrinsic rate of growth (r)—individuals give birth once at an
early age to many offspring. Populations that exhibit this
strategy often have been shaped by an extremely variable
and uncertain environment. Because mortality occurs
randomly in this setting, quantity of progeny rather than
quality of care serves the species better.
• K-selected, populations tend to remain near the carrying
capacity (K), the maximum number of individuals that the
environment can sustain. Individuals in a K-selected
population give birth at a later age to fewer offspring. This
equilibrial life history is exhibited in more stable
environments where reproductive success depends more
on the fitness of the offspring than on their numbers.
Determination of Population size
• Direct method: Counting each individual
– Can be used where study area/ population size is small and
there is no movement of individuals or individuals are
– Drawback: labour intensive, not applicable when very large
area/ large population, lot of emigration/
immigration, organism size is small.
• Indirect methods: using sampling and mathematical
calculations. It is also applicable when it is difficult to
observe the organism. Common methods used are
observations of tracks, droppings, nests or other signs
• Mark and recapture methods.
Sampling for population size
• Sample size is based on population
density, demography of the area and size of
population and area to be covered.
• Useful when population is very large, area to
be mapped is large.
• When it is difficult to remember what and
which has been count and which is not.
• Useful for wildlife count.
Factors affecting populations
• Biotic factors: The biotic factors are the population density and its
relation with the food and pathogens.. It is ability to achieve the
maximum birth rate at ideal conditions, potential natality and
realized natality (actual number of births that take place in a given
• Vital index= Natality/ mortality
• Immigration/ emigration.
• Carrying capacity: The maximum number of the resources which
can support the maximum number of individuals in given area is
known as a carrying capacity. The population size at which growth
stops is generally called the carrying capacity (K).
• The natural calamities destabilize the ecosystem and population.
The most common calamities include earthquake, land, flood and
• Abiotic factors:
Sunlight, , temperature, humidity, rainfall, minerals, substratum
availability. The hibernation, aestivation are controlled by the
changes in weather. The hibernation, aestivation are controlled by
the changes in weather.
Biotic factors affecting the population size
• Factors limiting population may vary from one population
to other population leading to ‘Population change over’.
• Common biotic factors:
– Food- both the quantity and the quality of food are important.
Snails cannot reproduce successfully in an environment low in
– Predators- as a prey population becomes larger, predators may
also. If the number of predators suddenly falls, there may be
erruptive increase in prey population.
– Competitors- other organisms may require the same resources
from the environment, and so reduce growth of a population.
– Parasites- These may cause disease, and slow down the growth
and reproductive rate of organisms within a population.
Human intervention and Population size
• Humans can greatly influence the size of animal populations they
directly interact with. Some common methods are:
• Spaying - removing the ovaries and uterus of a female animal -
medical term = ovariohysterectomy.
• Neutering - removing the testes of a male animal - medical term
• Various humans activities (e.g.
hunting, farming, fishing, industrialization and urbanization) all
impact various animal populations.
• Culling, translocation, Animal population control is the practice of
intentionally altering the size of any animal population besides
humans. It may involve manipulation of the reproductive capability.
• Animal euthanasia is often used as a final resort to control animal
populations. It may be for control of diseases (depopulation at
some farm or in a territory.
• Irruptive: There is a quick increase in population which
occurs in a short time and increases the population density.
It is also followed by the reduction in the size of
• Cyclic: There is a slow increase in the population which
occurs in a long time and increases the population density.
It occurs due to the seasonal changes.
• Geometric (pulsed/ seasonal reproduction) and
Exponential rates (no seasonality, like in humans).
• Exponential growth: Low density, favourable environment.
Exponential growth may apply to populations establishing
new environments, during transient, favourable
conditions, and by populations with low initial population
• Logistic growth : As resources are depleted, population
growth rate slows and eventually stops.
Limitation of population
• Density dependent: Often biotic. Density-dependent factors include
disease, competition, and predation. Density-dependant factors can have
either a positive or a negative correlation to population size. Density-
dependant factors may influence the size of the population by changes in
reproduction or survival. Density dependant factors may also affect
population mortality and migration.
– Exploitation competition: a density dependent, often intraspecies or with related
interspecies, for common needs with limited resources.
– Interference competition: Often interspecies, certain individuals obtain an
adequate supply of the limited resource at the expense of other individuals in
• Density independent: Often abiotic. Many sources of environmental stress
affect population growth, irrespective of the density of the population.
Density-independent factors, such as environmental stressors and
catastrophe, are not influenced by population density change. Viz., food or
nutrient limitation, pollutants in the environment, and climate
extremes, including seasonal cycles such as monsoons. In
addition, catastrophic factors can also impact population growth, such as
earthquakes, volcanoes, floods, heavy snow, blizzards, fires and hurricanes.
– Inbreeding: It an important density independent biotic factor which affect
survival due to limited genetic diversity required for adaptation under stress of
change. This inbreeding depression may make inbred individuals more
susceptible to disease, less able to find food, or less likely to breed successfully.
Natural selection/ Stress factors
• Genetic conservation: The individuals in the environment
that are best adapted to survive will reproduce and pass
the genes for adaptation along to their off-springs.
• The resilience is defined as the ability of a population to
bear the changes caused by the changes in
temperature, biotic factors and the changes in humidity.
• Evolution: Over the time natural selection may lead to
change in bodies of the organisms and new strains/
varieties/ species may arise.
• Genetic variation is more easily sustained in large and open
populations than in small and closed populations. Through
the effects of random genetic drift, a genetic trait can be
lost from a small population relatively quickly.
Geometric and exponential growth rates
• Exponential growth: Populations, in which individuals live and reproduce
for many years and in which reproduction is distributed throughout the
year, grow exponentially.
– Exponential population growth can be determined by dividing the change in
population size (ΔN) by the time interval (Δt) for a certain population size (N).
– The growth curve of these populations is smooth and becomes increasingly
steeper over time.
• Geometric growth: Insects and plants that live for a single year/ short
duration and reproduce once before dying are examples of organisms
whose growth is geometric. In these species a population grows as a series
of increasingly steep steps rather than as a smooth curve.
• Logistic population growth: If growth is limited by resources such as
food, the exponential growth of the population begins to slow
as competition for those resources increases. The growth of the population
eventually slows nearly to zero as the population reaches the carrying
capacity (K) for the environment. The result is an S-shaped curve of
population growth known as the logistic curve. It is determined by the
Species interactions and population growth
• Community-level interactions are made up of the combined
interactions between species within the biological community where
the species coexist. The effects of one species upon another that
derive from these interactions may take one of three forms: positive
(+), negative (–), and neutral (0). Hence, interactions between any
two species in any given biological community can take any of six
• Mutualism (+, +), in which both species benefit from the interaction.
• Exploitation (+, –), in which one species benefits at the expense of
• Commensalism (+, 0), in which one species benefits from the
interaction while the other species neither benefits nor suffers.
• Interspecific competition (–, –), in which both species incur a cost of
the interaction between them.
• Amensalism (–, 0), in which one species suffers while the other incurs
no measurable cost of the interaction.
• Neutrality (0, 0), in which both species neither benefit nor suffer
from the interaction.
• The effects of species interactions on the population dynamics of the
species involved can be predicted by a pair of linked equations that
were developed independently during the 1920s by American
mathematician and physical scientist Alfred J. Lotka and Italian
physicist Vito Volterra.
• Lotka-Volterra equations are often used to assess the potential
benefits or demise of one species involved in competition with
dN1/dt = r1N1(1 – N1/K1 – α1,2N2/K2)dN2/dt = r2N2(1 – N2/K2 – α2,1N1/K1)
Here r = rate of increase, N = population size, and K = carrying
capacity of any given species. In the first equation, the change in
population size of species 1 over a specific period of time (dN1/dt) is
determined by its own population dynamics in the absence of species
2 (r1N1[1 – N1/K1]) as well as by its interaction with species 2
(α1,2N2/K2). As the formula implies, the effect of species 2 on species 1
(α1,2) in turn is determined by the population size and carrying
capacity of species 2 (N2 and K2).
Calculation of population growth rates
• Survivor curves
• Life tables
• Reproductive rates
Survivor curves and life history
• Type I survivorship curve : In K-selected species which have fewer
numbers of offspring but invest much time and energy in caring for
their young. This relatively flat curve reflects low juvenile
mortality, with most individuals living to old age.
• Type II survivorship curve: In r-selected species. A constant
probability of dying at any age, is evident as a straight line with a
constant slope that decreases over time toward zero. The species
produces many offspring but provide little care for them, mortality
is greatest among the youngest individuals.
• Type III survivorship curve: Life history is initially very
steep, reflectiing very high mortality among the young, but flattens
out as those individuals who reach maturity survive for a relatively
longer time; it is exhibited by animals such as
many insects or shellfish.
• Complex Type survivorship curve: Species commonly suffer high
mortality during the first year of life and a lower, more constant
rate of death in subsequent years.
• Life tables were originally developed by insurance
companies to provide a means of determining how long a
person of a particular age could be expected to live.
• They are used by plant, animal, and microbial ecologists to
make projections about the life expectancies of
populations, as well as the effects of variation
on demography and population growth.
• Life tables are designed to evaluate how rates of birth and
death influence the overall growth rate of a population.
• Life tables also are used to study population growth. The
average number of offspring left by a female at each age
together with the proportion of individuals surviving to
each age can be used to evaluate the rate at which the size
of the population changes over time.
• The average number of offspring that a female produces during her lifetime is called
the net reproductive rate (R0).
• If all females survived to the oldest possible age for that population, the net
reproductive rate would simply be the sum of the average number of offspring
produced by females at each age.
• The net reproductive rate for a set cohort is obtained by multiplying the proportion
of females surviving to each age (lx) by the average number of offspring produced at
each age (mx) and then adding the products from all the age groups: R0 = Σlxmx.
• A net reproductive rate of 1.0 indicates that a population is neither increasing nor
decreasing but replacing its numbers exactly. This rate indicates population stability.
Any number below 1.0 indicates a decrease in population, while any number above
indicates an increase.
• Mean generation time = T = (Σxlxmx)/(R0) = 6.08 years, Generation time is the
average interval between the birth of an individual and the birth of its offspring. To
determine the mean generation time of a population, the age of the individuals (x) is
multiplied by the proportion of females surviving to that age (lx) and the average
number of offspring left by females at that age (mx).
• Intrinsic rate of natural increase of the population = r = approximately 1nR0 / T =
2.101/6.08 = 0.346, n=number of females. Intrinsic rate of natural increase (r), or the
Malthusian parameter. It is the number of births minus the number of deaths per
generation time—in other words, the reproduction rate less the death rate. To derive
this value using a life table, the natural logarithm of the net reproductive rate is
divided by the mean generation time. Values above zero indicate that the population
is increasing; the higher the value, the faster the growth rate.