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Summary Ecology, Epidemiology and Control of Infectious Diseases (15/20)
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This is a summary of the course that is given in the Master Infectious and Tropical Diseases.
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Summarize: Ecology: Deel I1. Chapter 1: Population ecology
1.1. What is a population
Zoonoses is the term we use for infections that originated in animals and then jumps to humans
where it can causes a infection. For understanding infections and their spreading then we have to
understand mathematical models, these models predict how an infection will spread around in a
population. If we talk about a population then we don’t always talk about a human population. A
population can also be animals or micro-organisms. Furthermore we going to look at what the impact
is of a population on the environment and vice versa and what their relation will have as impact on
the infection.
A population is a group of individuals of one species, that are living together at a certain place in
space and time. This definition isn’t as practical as it could be, that’s why there is another definition
that is more practical in its use. This definition is as follow: a group of individuals of one species that
is under investigation. A group of individuals from different species that are living together is called a
community. Defining the boundaries or limits of a population isn’t always easy. Only when the
population is in a closed system then the boundaries are clear otherwise an individual will move
around and may even leave the system, an example of a closed environment is an isolated island.
The limits will depends on the question that we want an answer to.
If we talk about the spreading of an infection then the connectivity between the individuals in a
population is very important. This is because without connectivity between the individuals an
infection will not spread.
Like already been told a population isn’t always a human population. Sometimes it can be a parasitic
population, the problem with this is how are we going to tell what is a population of parasites. Are
we only talk about the parasites in one individual, are we talking about all the individuals with
parasites or are we talking about all parasites no matter if they are in a host or in the environment.
We can call these different population by different names namely:
- Infrapopulation: this describes all parasites in one host.
- Component population: this describes all parasites that are not in a host, so all parasites that are
in the environment.
- Suprapopulation: this describes all parasites sitting both in environment as in hosts
A parasite can have different species as his host in different stages of his life. A parasite has different
stages in his life and sometimes only a specific stage can infect a specific host.
1.2. Population size
If we talk about the population size then we have to keep track of two thing, namely:
- Abundance: This is the number of organisms in a population, combining intensity (density within
inhabited areas) and prevalence (number and size of inhabited areas)
- Density: This is the number of individuals per area or more specific per unit of resources. The
density of the population plays an important role in how easy it is for an infection to spread in a
population.
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,We can know the population size by counting the individuals in the population, this is only possible
with small populations. Furthermore there can be a discussion in which individuals belongs to a
population if that species goes through a metamorphosis in its life time. So again depending on the
question of your research you are going to count differently (remember the photo with the two frogs
and many frog eggs). Again with parasites it is even going to be more difficult. This is because we can
count them in many different ways. For parasites sometimes it is better to know how many hosts are
infected instead of knowing how much parasites there are in one host. But then again if the question
is: “Are more parasites leading to a more pathogenic image?” then we have to research the
individualistic load. So getting to know the abundance for a parasite is more difficult than to know
the abundance of for example humans.
1.3. Population dynamics
The population isn’t a set number it is going to fluctuate because there will be
mortality and natality in the population. So the abundance of a population is
constantly changing. So if we want to know the present population
then we have to take into account natality, mortality, immigration and
emigration. So the population (N) can be calculated as follow:
The different parameters all need to be described before we know something about
N. this model isn’t practical in real life because all parameters have a maximum.
1.3.1.Natality
The mother is the depending factor in species where we have sexual reproduction. This is because
one man can fertilize many women but if a women gets pregnant she cannot fertilize for some time.
So if the gestation is short the women can be fertilize quicker and thus give more offspring on the
same time as an organism with a long gestation. For this reason in many population models the
males aren’t counted. The natality is mainly dependent on females (in sexually reproductive species).
The number, age, reproductive capacity and realised reproductive capacity of women influence the
natality.
1.3.2.Mortality
There are many reasons why an organism dies. So in a
study we have to look at the organisms that dies and
look at the impact or effect they have on the future
and the population. For example by humans if old
people dies the impact is less then when young
people dies. So depending on the mortality we can
make a survivorship curve. We can see three different
types of survivorship curves.
- Type I: organism will have a long live and almost
everyone will live up till an old age. At an old we will see everybody die. Example for this one is
domestic dogs.
- Type II: organism will die at a constant rate
- Type III: a lot of death will occur after birth and only a few will grow old. Example of this are frogs
and their many frog eggs.
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,For humans the type of survivorship curve will depend on their living conditions. It is not enough to
know the amount of individuals in the population. We also have to look at the individuals that makes
up the population. So we have to look to their age and sex structure. If we look at the age structure
of a population then we can get some information about how a population is going to look like in the
future.
1.3.3.Immigration and emigration
These are very difficult to keep track of because there is problem to measure them. They usually
assume immigration = emigration. We know what the effect is of immigration and emigration is on a
population but not what triggers it. The effect of immigration on a population is the same as natality,
so there is another individual in the population that can helps the population to grow. The effect of
emigration is the same as death, the individual is no longer in the population and cannot help the
population. The effect of immigration can be very disruptive even more then natality. In many
populations we are going to say that the amount of immigration is equal to the amount of
emigration. This is only true if the quality of life is the same everywhere.
For a parasite migration is verry important. A parasite has to migrate. This means it invade a lot of
new areas. This means one infection event of a parasite can infect a whole population.
1.4. Population growth
We can measure the population growth by watching the change in population
over time. We can calculate this by following formula. B = births. D = deaths.
1.4.1.Net reproductive rate (R) and basic reproductive rate (R0)
The net reproductive rate tells something about how the population is changing. Reproductive rate
has a critical value, this is most of the times 1. If R > 1 then we have an increase in the population if R
< 1 then we have a decrease of the population. R can be changed to R0 which represents the basic
reproductive rate. R0 is the factor that tells us how much individuals will replace the parent individual
over the course of a generation. If R0 > 1 then we have an increase in the population if R0 < 1 then we
have a decrease in the population. R: R0
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, 1.4.2. Carrying capacity (K)
The increase of a population is mostly in an exponential way. So if
we have R0 = 2 then we going to see 2 becomes 4, 4 becomes 8, 8
becomes 16, … . This exponential growth cannot go on indefinitely,
this is because of resources are not limitless so an environment has a
carrying capacity for a population. So after an exponential grow we
are going to see that the curve is flattening or the curve will level
off when carrying capacity is reached. Then we will have a logistic
growth (sigmoid). When this happens we are going to have an equal
amount of birth and death. So our R = 1 on that point. The carrying
capacity will make it that each environment has its own equilibrium
in density for a population.
dN K−N
= rN
dt K
1.4.3.Density dependent population regulation
Depending on the density of the population we can have different birth and death rates. With a
stable birth rate we are going to have more deaths then births the moment we reach carrying
capacity. If we have a stable death rate then we are going to have less births when the population
reaches carrying capacity. But mostly it is a mix of those two. So at carrying capacity the deaths will
rise and the births will decrease. In the three models we see that we are going to have a decrease in
the population so the environment can carry the population again.
Also parasites depend on the density of the
population. If there are more parasites in the host
then each parasite will give less eggs because there
are not enough resources to produce the eggs. But if
we are going to see the total eggs of all the parasites
in the host then we will see an increase in the eggs
this is because a more parasites with fewer eggs give
more eggs then a few parasites with a lot of eggs.
This we can see in the next curve. So a population
will reach an equilibrium with its environment and
will stay at it as long as the environment doesn’t
change.
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