Biology Recapture (Lecture 20-24)
Lecture 20 – Population Ecology
Virulence and Transmission
Remember, a parasite does not want to fully kill the host. The host individual needs to be kept alive so
that they can further infect others (new hosts). This is known as the trade-off hypothesis.
There is a particular version of this trade-off hypothesis and is known as the Transmission mode
hypothesis. It states that there is an optimal virulence of a parasite and is dependent on the how the
parasite actually moves between the hosts.
Some virulence requires direct contact, whereas some other individuals do not require direct contact.
Some parasites can be spread through indirect means. An example of this is the transmission of
contaminated water, and other hosts such as bugs. Pathogens can be released from inanimate objects
as well. This is seen in public places such as washrooms or hospitals.
Parasites that can be spread through indirect forms tend to have a more virulent effect. Many direct
transmission methods are less effective or virulent. There is a lot of evidence supporting the
Transmission mode hypothesis which is the reasoning for its great acceptance biologically.
There are practical implications of this method of transmission then just defining the principles. By
understanding these concepts, we can determine the ways in which virulent diseases will evolve over
time. From a physical perspective, if we reduce chances for indirect transmission (i.e. Water supplies),
parasites will rely more on the direct transmission methods, which eventually promote evolution to
reduce the virulence. Overall, this will reduce the fitness of the individual parasite, but will be more
beneficial to humans.
A population is a group of individuals of the same species that live in a defined geographical area.
Although this is an arbitrary definition, there is a common acceptance of what the location may be. Any
location is a viable explanation of location. We will be looking at a variety of scales.
Characteristics of Populations
Populations have many attributes that are not associated with the individuals. They consist of:
- Allele Frequencies, genotype frequencies
- Population size(N)
- Geographical Range
- Sex Ratio – Males to Female
- Age Structure – Sexual Maturity or Not?
- Rate of Growth Estimation of Population Size
In the human population, we have a constant total of individuals that will be extrapolated to help us
determine the population. If you are dealing with organisms that do not move, you can simply count all
the individuals in a said location to get the population and rate of dispersion. If the organisms move
however, there is the need for additions counting systems.
Methods of Estimation
There are multiple ways to estimate population size when we do not have access to all individuals
conclusively. The Mark Recapture method is the most common method of tracking individuals. You go
out into the field, catch a bunch of individuals, and place a marking on them. At some point in the
future, you come back and capture a bunch of individuals. This new set of individuals are inspected for
the number of individuals that were recaptured (had already been marked). We can then calculate the
number of recaptures, which will allow us to measure the total population size.
Mark Recapture Method
R/C = M/N, Where R is the Recaptures, C is the number captured in the second visit, M is the number of
Marked individuals. We then isolate for N, which is the number of individuals in the whole population.
We clip the toes of 100 frogs in a lake. You then return and Capture 75 Frogs, that of which have 10
clipped toes. How many individuals are there in the lake?
There are 750 individuals.
Mark recapture method works well only if the marked and unmarked animals are equally likely to be
captured. If an animal is marked, it should not be more likely to be found the second visit. Also, the
marks on the population must not affect the survival of the individuals and the marks cannot wear off.
This means that although the marks may come off in a few years’ time. As long as the marks are still
present during the second capture, the method will work well. The time between the first and second
visit should be fairly close together to avoid Births, Deaths, Immigration and Emigration.
In a mark-recapture study of blackbird, many individuals became trap happy, where after they are
caught a first time, they are very easily caught again. How will this affect the estimation of population
This will cause us to under estimate the true value for population size.
If you were to use washable mark to mark the wings of butterflies in a mark recapture study. Some
marks wash off before the second sample is captured. How will this affect your estimate of N?
This will cause an underestimation of recapture and an over estimation of the total population size.
Sometimes we are not as interested in the number of individuals in a population, but rather the
dispersion with respect to area. There is typically an inversely proportional relation of body size to
density of individuals. If there are very small individuals, there will be greater density.
Why does density matter?
Density influences the overall access to resources. If the density is too high, the per capita growth rate
may decrease. The denser a population is, the smaller the rate of growth is. Remember, there are both
Density Dependant, and Density Independent factors that will play a role. If the density is too low, per
the per capita growth rate may decrease. This is known as the Allee effect. If individuals struggle to find a mate because there are so few individuals, they will have a decrease in growth. If individual hunts in a
pack, their performance and success might also drop.
With a specific density, there are three types of dispersions that actually occur. Most populations have
the clumped method of organization. The three types are:
We see that if there is a food source in a specific location, the population will be clumped towards that
location. Uniform dispersion is found mostly in the plant population, or animals that have very similar
competition over the land mass. There is no need to have competition if there are many resources
Dispersions can change overtime. In the breeding season of birds, there will be the uniform dispersion
patter, but in the winter time, there will be the formation of flocks, which represents the clumped
What are some advantages of living in a group or clumped dispersion?
- Raising Offspring
- Cooperative Hunting
- Ease of Finding Mates
What are some disadvantages of living in a group or clumped dispersion?
- Competition for Resources
- Disease Control
Characteristics of Populations That Affect Population Growth Rates
Which population would grow fastest? A population with more females than males will be most
successful. Feamles are a limiting resource in a population. In all different situations, there may be the
need to have a variety of combinations with respect to male to female ratio. In most populations,
females are the limiting factor, but this is not always the case.
The faster the generation time, the faster the population will be able to grow. Imagine that there are
two populations, both with an average of offspring produced per female per lifetime, and only differ in
their generation time.
Population 1 reproduces at age 16, and Population 2 reproduces at age 32. The population that
reproduces at age 16 will continuously have many more offspring produced over the time. Increasing
the generation time very quickly decreases the overall population number.
Education of females is the best way to increase generation time and decreasing population growth. Lecture 21 – Population Ecology
Read Section 46.1, 46.4
There are final Exam notes posted in the Test Resources Folder.
1. Age structure and population growth
2. Life tables and survivorship curves
3. Life history strategies
4. Models of population growth
Population Growth Rate
Age structure encompasses the ages of individuals in a population that are able to reproduce.
Many age structure pyramids or histograms can explain if a population will grow fast, slow, or not at all.
When the young individuals are in abundance, the population will experience rapid growth. This is the
most desirable conditions that could occur.
Negative Growth occurs when there are very few pre-reproductive individuals, while there is an
abundance of post-reproductive individuals
Life Table Analysis
A life table analysis summarizes survival and reproduction for a cohort of individuals. A cohort is a set of
individuals that are very similar in age. (I.e. a school year or so). We should be able to fully follow a
cohort through the development. This data can be collected and placed into a life table. It classifies the
cohorts based on their age, and what ranges they fall in. The measurement is based around how long
each individual species lives. Does the species live to the next generation of species? We can use this
data to calculate age specific mortality rate (death). We can also calculate the survivorship of each age
Survivorship curves are no really based around number of individuals or when they are born. A
survivorship curve is based around the period of time when many individuals go through mortality.
Large animals tend to go through the type 1 survivorship curve; there is very little chance or mortality
until the species becomes very old. There is very little juvenile mortality. Smaller individuals tend to
follow the type 2 survivorship curve. In this type, most individuals are not dying because of old age, but
rather environmental factors. All of these factors are unrelated to the age of the individual. There are
some organisms that will actually follow the type 3 survivorship curve. In this case, most mortality
occurs in the early years of individuals. Very few individuals make it past the first bit of life. Once they
make it through the system, the majority of the lifespan is constant.
Life History Characteristics
Life history is the study of how organisms allocate energy and resources given the fact that they have
many competing demands in their live. All creatures must partition energy and resources all the time.
Demands consist of:
- Self-Maintenance(repair) Each of these is examples of components that must be balanced as a category, and also in a sub-
Each individual will have a different life history strategy, which is a denotation of how resources will be
allocated over the lifespan to themselves and offspring.
Different environments place various stresses on the delegation of resources due to the specific needs.
Quantity and Quality(R strategist and K Strategists)
All individuals fall somewhere between r selected species and K selected species. When individuals
reproduce very early in their lives, they are most likely an ‘r’ strategy type system. These species tend to
invest very little in the offspring. K species are very different compared to R species. They tend to devote
lots of energy to growth, self-maintenance and repair. The time required to reproduce in K species takes
long, but the average life if much longer. There is a lot more parental investment, which leads to lower
juvenile mortality. R species are producing high quantity of offspring, while K species produce high
Population Growth Predictions
To calculate growth of a system, we must integrate all of the factors (Births, Immigration, Deaths, and
Emigration). The equation includes the summation of Mortality components, and then the subtraction
of the natality components. (B+I) – (D+E) = Growth rate.
This equation is modelled over time, and the Growth rate is model in terms of change in population
before (currently) and after (future).
Birth and Death Rates per capita are extremely useful. There is a system where there are no immigrants
or emigrants. We will to calculate the birth rate per capita. This is modelled as B/N. This is the same for
the death rate, and is modeled by d=D/N.
Rate of Growth
Once we know the per capita death and growth rate, we can calculate the intrinsic growth rate = r.
In the exponential model of growth, r is independent of the total population size. The rate of change in
the growth of a population is the intrinsic growth rate(r) multiplied by the total population, yielding (rN).
In exponential growth, we see that the overall population will increase in size if R is greater than 0. If the
R value is less than zero, then the total population will decrease. The doubling time of the population
can be found by taking 70 and dividing it by the r value as a percentage.
Remember, this all works in theory, but it is often unrealistic to assume that growth is independent of
the N value.
Logistic Population Growth
For any population, in any given environment, there may be a maximum number of individuals that the
environment can sustain. This is known as the carrying capacity value and = K. As N increases, the rate of
growth will decrease. (K-N)/K is the growing room that the system has. This is an extremely important value, as it measures
how much room for growth in this system is possible.
The rate at which a population will growth in a logistic model is represented by:
The only difference between an exponential growth curve and a logistical growth curve is the factor of
resources. If resources are limited, there is the development of a carrying capacity, which leads to the
logistical growth pattern. If the resources are completely abundant, then the exponential growth
pattern will happen.
If there are 900 individuals in a population, and the carrying capacity is 1000, along with a r value of 0.1
,then after one year, the population will grow to 0.1 x 900 x (1000-900)/1000 = 9 individuals per year. If
there is no carrying capacity, the growth would simply be 0.1 x 900, which would be 900+90 after 1 year,
leading to a net value of 990.
When N= K, the carrying capacity has been hit theoretically.
In an exponential growing population, the r value will maintain the same value over time, but in the case
of a logistical curve, the r value will decrease as the system reaches the carrying capacity.
When N is very small, the R value is extremely high.
If there is a group of salmon that could have a maximum population size of 500 individuals, what
population size at the time would have the most aggressive (greatest) growth rate?
Logistical Model Misleading
When N is 0.5K, the growth rate is maximized, as this is the derivative of the curve. In theory however,
not everything that grows logistically will follow the pattern and rules 100%. The Allee effect will cause
the population to die out if they fall below some minimal density in a location. There is also the potential
of a time lag where the population may overshoot K and then dip below K.
Density Dependant Factors
- Accumulation of wastes
Density Independent Factors
- Act Regardless of the population size
- Server Weather
There is the possibility for both density dependant and density independent factors can interact with
each other at times which may drive the said population right into the ground. In Haiti, we saw the blending of both of these factors. The DIF was the earthquake that wiped out part of the population,
while the DDF was the access to food and other sicknesses that occurred later.
Human Population Growth
Most K-selected species show logistic growth, but what about humans? Humans have been exhibiting
exponential growth over the last thousands of years. Why is this the case though?
Lecture 22 – Community Ecology 1
Why is it that humans do not follow the rules of logistic specific growth? Humans are still under their
carrying capacity significantly, which allows for lots of expansion. We also have technology present,
unlike most species, which allows for us to continually adapt to the situation. We also have access to
resources that we can manipulate in any way we wish, unlike other animals. We also have no other
predators at the time which reduces the competition for most consumer based goods.
Why is the human population still growing?
There is in fact, no specific reasoning, but we have utilized the equated based representation of the
population to a point where we have been able to increase the carrying capacity overall. We have
improved all aspects of the N value in our population. We have increased carrying capacity, reduced
death rates, and increased birth rates. We have also made adjustments so that we can increase our
Variation in R.
We have projected population sizes for future years, and we see very little trend. However, we see that
Europe will most likely decrease while Asia increased. From the standpoint of North America, it appears
as though we will make no significant change.
R reflects the overall age structure of a geographical region. In the case of the USA, we see that there
are many individuals that are pre-reproductive and reproductive. In the case of Mexico however, we see
that there are just many pre-reproductive individuals.
Increasing N, but Decreasing r
Globally, we are increase at about 1.2% annually, per person per year. Our r value is overall decreasing,
but our N value is still increase with great demand. In the last few years, we have been significantly
increasing our value of N, which is shown by the 7 billionth person in existence.
Remember that r represents the slope of the population. So although we are increasing overtime, the
rate at which we are increasing is become lesser.
Over time, human societies have become more advanced, and wealthier overall. Birthrates have been
decreasing overall, while death rates are decreasing. In the case of some third world countries, the birth
and death rates are extremely high, which leads to a balance, and an extremely low value of R. When
these populations increase in success, they move towards cities, which allows for improvement in
concepts that prevent as many death rates. As medical conditions improve worldwide, the numbers of death rates decrease greatly. At this point in time, there is a large change in r value positively. The post-
industrial stage is when there is zero population group (when birth and death rates are equivalent,
disregarding immigration and emigration).
Most of these models apply strictly to first world countries that are extremely bias and advanced. This
may suggest that the models will not accurately predict systems in third world countries.
Population Size isn’t Everything
These models explain that most populations will decrease in size, as they will mostly be entering the
post-industrialized situations. In terms of population, size may not be everything. The resource use per
person may be a much more important factor than that of the actual growth. North American resource
use is approximately 50 times larger than that of thirds world countries. This lead sus to have a much
great ecological and carbon based footprint.
A community is a set of populations that interact with each other. Population ecology looks at one
species at a time, and the said interactions, whereas the community involvement looks at multiple