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Summary of the courses of Ecology Part Leirs
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Academic year: 2022-2023ECOLOGY, EPIDEMIOLOGY AND
CONTROL OF INFECTIOUS
DISEASES
By Joke Stynen
EXAM:
• 1 modelling exercise: make a model for an infectious disease and make some
calculations (what happens if you vaccinate 80% of the children...) → bring your
own computer
o Are open book: can use notes, papers, but not internet (computer in flight
mode)
o Maybe show the results on the computerscreen
• Theory part: don’t know the equations by heart
Joke Stynen
1-12-2022
,PART1-PARTIM LEIRS
H1: POPULATION ECOLOGY
1. WHAT IS A POPULATION
• Not only human populations but also populations of animals…
• Population =
o A group of individuals of one species, living at a certain place in space and time.
▪ Difficulties: a certain place is not easy to define and time is a moving window.
▪ Tricky definition → so often use another definition.
o A group of individuals of one species under investigation.
▪ Same species can have different characteristics → differences in population.
• Populations vary: old people, people that aren’t born yet, people that have died…
2. DETEREMINING POPU LA TION SIZE
• To do this, we need to know the limits of a population
o Geographical?
o Social?
o Connectivity?
o Time?
• Identifying the individuals that belong to your population can be difficult.
o The borders are not always clear cut.
▪ E.g. on an island it is clear but for a forest it is not.
o Example: population of Belgium:
▪ Borders are not always clear.
▪ Some people live in Belgium but work in France
▪ The residents are only a part of the population, not the whole population.
▪ So: how do you define living in Belgium? What are the criteria?
→depending on the question you can include or exclude people
• PARASITE POPULATIONS:
o Hosts form a population (and the parasites form
also populations)
o Parasites can live in different hosts
▪ There are interactions between hosts:
e.g. eggs that are taken up by new hosts
or parasites that are not host specific.
o Infrapopulation: population of parasites in 1
hosts.
o Suprapopulation: population of parasites over
all the hosts.
o Component population: 1 life stage of the parasite population.
1
,3. EXPRESSING POPULATIO N
ABUNDANCE
• The number of organisms in a population, combining ‘intensity’ (density within inhabited
areas) and ‘prevalence’ (number and size of inhabited areas).
o Prevalence for parasitic infections: proportion of hosts that are infected with a
certain parasite.
• Absolute number!
• This doesn’t tell you anything about the contact with other individuals or food per individual.
• Effects of abundance on transmission of infections among humans and from
reservoirs/vectors to humans:
o Direct:
▪ Higher abundance → more sources of infection.
▪ Higher abundance → higher prevalence (often).
o Indirect:
▪ Effects on other hosts/vectors.
▪ Effects on infrastructure, health system, food, …
DENSITY
• Number of individuals per area (or more general: per unit of resource).
• Density is an abstraction!
o (But maybe more relevant for the individual that experiences it).
USE OF ABUNDANCE OR DENSITY?
• This is a matter of perspective/question/type of infection!
o Ebola → prevalence is most important.
▪ If the question is about the risk of coming into contact with the infection:
prevalence.
o Worms: takes a few months between ingestion of egg and development of new eggs.
▪ If you want to know the possibility of coming into contact with eggs of worm
→ intensity.
▪ If you want to know how many people you’ll have to treat → abundance.
POPULATION SIZE
• Counting which individuals?
o This depends on the research question.
o Example: frogs:
▪ How many frogs are there? → 2 adults.
▪ Will the population grow → eggs are important, don’t only look at adults.
o Eggs are also individuals, that will develop in the future.
o For parasites:
▪ At a certain moment: only adults can spread the infection.
▪ Eggs will in the (near) future be able to spread the infection.
2
, • Counting parasites
o Prevalence: proportion of hosts that are
infected with the parasite.
o 10 host individuals + 2 species of parasites +
6 red infected hosts + 4 yellow infected
hosts
o Intensity of the infection: how much of the
health of the individuals will be affected?
o Abundance: some hosts are not infected.
4. POPULATION DYNA MICS
*
• Immigration and birth add individuals to the population.
• Emigration and mortality remove individuals from the population.
• Population size now = population size then + number of births – number of deaths number
of immigrations – number of emigrations
NATALITY OR BIRTH
• Mainly dependent on females (in most sexually reproducing species):
o Number: how many females are there?
o Age: how old are thes females?
o Reproductive capacity:
▪ Not easy to determine.
▪ This is not the max reproductive capacity because in reality they produce far
less offspring.
▪ Realised reproductive capacity is far more important.
➔ SO the sex-structure is important.
MORTALITY
• Many factors can effect mortality.
o Knowing how old individuals can become is not useful for populations → realized
lifespan is more relevant and often shorter than the maximum lifespan .
o Whether individuals die or not is also important: young children die → period where
fewer individuals are able to reproduce because they died pre-reproductive.
3
, • Effects depending on which part of the population dies
o SURVIVORSHIP CURVES:
▪ Type I: young individuals have a high
propbabilty of survival until they are
old. E.g. humans in western Europe
▪ Type III: at a young age, many
inidviduals die. After that there is a
gradual mortality. E.g. humans in poor
countries, frogs
▪ Type II: organisms will die at a
constant rate
o POPULATION COMPOSITION:depending on the situation, populations will have a
different survivorship → always necessary to understand the agestructure of a
population.
▪ Ratio females/males is important for reproductive capacity!
▪ Population of children VS old people: in the population with children there
will possibly be natality in the future VS the older population where possibly
mortality will occur.
▪ Age distributions differ at different levels: within and between populations
e.g. human age pyramids.
IMMIGRATION AND EMIGRATION
• Very difficult to study (in animal populations).
• Usually assumed: immigration= emigration → obviously a FAULTY ASSUMPTION .
• These are of utmost importance for parasite ecology!
o TRANSMISSION BETWEEN HOSTS
▪ Emigration: leaving of one host.
▪ Immigration: in to another host.
▪ Every new infection is an act of immigration for one host and emigration
from another host!
o INVASION OF INFECTION IN NEW AREA
▪ When immigrants arrive, does the infection spread?
4
, POPULATION GROWTH
𝜟𝑵
=𝑩−𝑫
𝜟𝒕
• Difference in population size over the time = number of births – number of deaths (+
immigrants – emigrants)
• R = population growth rate = net reproductive rate
o = population at a certain time +1 / population at a certain time
o = 1 if a population remains stable → R=1 is the critical value
• Basic reproductive rate = R0 = Rnought
o So R0 is the factor, on average, with which an individual
replaces itself over the course of a generation.
o At which rate are individuals replacing themselves (with
another generation)
▪ If every generation the population is doubled: R0= 2
• Expressed with birth and death rate
• Intrinsic rate of natural increase r
o How is the population changing over time?
o Critical value of r = 0; positive = growth; negative = decrease.
• Exponential growth: population continues growing
o This does not happen a lot in IRL.
o Example: bacteria has exponential growth until human
dies or immune system suppresses the infection.
• Logistic growth (sigmoid growth)
o Resources are limited (”carrying capacity”).
o The more animals, the more difficult for individual animals.
→ Density dependent mortality.
o Population growth will reach an equilibrium.
▪ K= carrying capacity.
▪ If population is very small → N is very
small → K/K will be around 1.
▪ In the beginning N is big, by the end N is as big as K → rN = 0 → equal
amounts of births and deaths, no growth of population.
o There can be a change in natality or mortality which leads to the equilibrium.
5
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