Chapter 9: Population Dynamics, Carrying Capacity, and
9-1: Population Dynamics and Carrying Capacity
Populations are dynamic, which means that they actively change.
Characteristics of a population that can be measured include:
1. Size (number of individuals)
2. Density (number of individuals in a given area)
3. Dispersion (the way in which the population is arranged)
A. Clumped – populations are found in groups with otherwise empty spaces
(ex. – a herd of cattle)
B. Uniform – populations are evenly distributed (ex. – territorial animals,
such as nesting penguins)
C. Random – population has no discernable pattern (ex. – flowers with
4. Age distribution (percentage of individuals in different age groups)
Four factors affect the size of a population. Births and immigrants (individuals moving
to an area) cause the population to rise. Deaths and emigrants (individuals moving
away from an area) cause the population to fall. The change in a population can be
expressed as a simple equation.
Population change = (births + immigrants) – (deaths + emigrants)
Populations also vary in the speed in which they can grow, sometimes called their
biotic potential. The rate at which a population would grow if there were no limiting
factors is signified by the symbol r.
Some species have very high rates of potential growth because they
Become sexually mature at young age
Have short generation times
Have many offspring each time they reproduce
Most insects are examples of species with a high r. On the other hand, elephants have
a relatively low r, because their reproductive characteristics are opposite of those
Species with a low r are more susceptible to extinction. If their population drops
below a certain number, called the minimum viable population (MVP), it is unlikely
to recover back to previous levels. This may be happening with certain species of
baleen whales, such as the blue whale.
A real world environment will almost always have limiting factors which will prevent a
population from reaching its growth potential for very long.
These limiting factors provide a carrying capacity (K) for the population for a
particular place and time. A carrying capacity is the number of individuals that can be
sustained indefinitely in a given space.
If a population initially has few limiting factors, the population will grow
exponentially, creating a J-shaped curve on a graph.
Typically, the population will eventually reach its carrying capacity as determined by
the limiting factors of the environment. The exponential growth will slow and the
population will level off. This type of growth is called logistic and produces an S-
shaped curve on a graph.
Often times a population will temporarily exceed its true carrying capacity. This is
called an overshoot. Overshoots are unsustainable because the population is higher
than the carrying capacity of the environment.
The consequence of an overshoot is usually a population crash, which is a short period
of time where the death rate increases to compensate for the population being over
the carrying capacity.
The carrying capacity for a population in an area is not necessarily a fixed, stable
number. As factors in the environment change, the carrying capacity can change
along with it.
For example, the Irish potato blight of 1845 destroyed much of the food crop for that
year and created a new, severe limiting factor of food. The carrying capacity of the
island was now much lower than it had been. One million people died and three
million emigrated from Ireland to other countries as the population crashed.
The carrying capacity of the world as a whole for humans has increased dramatically
over the past few thousand years, mostly through a tremendous jump in the ability to
produce food. As the human population continues to grow exponentially, at what
point will we reach carrying capacity? Or are we already past it and don’t know it yet?
Limiting factors that act on a population regardless of its density are called density-
independent population controls.
Natural disasters (flood, hurricane, tornado, earthquake)
Odd weather conditions (early or late frost, excessive heat)
Pollution (pesticides, lead, mercury, etc.)
Other limiting factors have a greater effect on a population only when the density is
sufficiently high. These are called density-dependent population controls.
Competition for resources
Predation (easier to catch prey)
Parasitism (easier to spread)
Disease (easier to spread)
There are four general types of fluctuations found in natural populations.
1. Stable – the population is relatively constant, hovering around the carrying
2. Irruptive – the population is usually constant but occasionally there is a rapid
overshoot, followed by a corresponding crash. This may occur because a new,
but temporary resource becomes available and then runs out (algae bloom). It
may also be a natural part of the life cycle of some animals, such as the 17
3. Cyclic – the population rises and falls noticeably in a predictable pattern,
usually in response to a corresponding predator or prey.
4. Irregular – the population changes noticeably, but not in any type of pattern
that anyone can figure out.
9-2: The Role of Predation in Controlling Population Size
Predator-prey populations often change in synchronized cycles. As the predator
population increases, the prey population decreases. The decreasing prey population
causes the predator population to also drop. With a drop in predators, the prey
population increases. Logically, the predator population will also then increase,
leading back to the beginning of the cycle.
The question is ‘Are the prey controlling the predators, or are the predators
controlling the prey?’
If the number of predators determines the number of prey, it is called the top-down
hypothesis. In this case the pressure of the predator is the limiting factor and
otherwise the prey population would be higher.
If the number of prey determines the number of predators, it is called the bottom-up
hypothesis. This can occur if the number of prey is being determined by a different
limiting factor, such as the availability of food. The amount of food determines the
number of prey, which then in turn determines the carrying capacity of that particular
9-3: Reproductive Patterns and Survival
Asexual reproduction is when one parent is able to create clones of itself. Each
offspring is genetically identical to its one parent. This type of reproduction is most
common among simple organisms such as bacteria, sponges, and other small
Sexual reproduction is when two different parents (male and female) create a new
offspring by combining their genetic material. Normally this process creates unique
offspring that are similar in many ways to their parents, but are not clones.
Sexual reproduction has many difficulties that asexual reproduction does not have.
1. Each female must produce twice as many surviving offspring to compensate for
the fact that males don’t produce any.
2. The process of combing the DNA from two different individuals is complicated
and often leads to mistakes.
3. The mating process is time-consuming (courtship), energy-consuming (egg
production), and dangerous (disease, danger from predation).
Yet 97% of all species reproduce sexually. There must be a huge advantage that
offsets all of these potential disadvantages.
It is the greater genetic diversity that sexual reproduction provides. Each individual in
a population turns out a little different than every other because of the different
combinations of genetic material provided by the parents.
When the environment changes, the chances are greater that at least some of the
individuals in a sexually-reproduced population can survive compared to an asexually
For example, suppose a new disease infects the population. If every member is the
same, then either they will all survive, or they will all die. If every member is
different, then likely some will die, but some will survive and the population as a
whole will carry on without being completely wiped out.
So the main advantage of sex is that it allows the population as a whole to be better
prepared to handle any significant changes that occur in the environment.
Species can also be categorized into two different groups based on their reproductive
Species with a very high r are called r-selected species. They have the ability to
reproduce very quickly and in high numbers. Characteristics of r-selected species
Ability to reproduce early in life
Production of many, small offspring
Little or no parental care of offspring
Very high infant mortality rate
Tendency to be generalists
Typically short life span
The idea here is to produce so many offspring that by sheer numbers at least a couple
will survive. Because parental care is kept at a minimum, the vast majority of
offspring die young.
The advantage of this reproductive strategy is the ability to take advantage of a
resource quickly and so they are excellent at building up a population from scratch in
a short time. Being generalists also helps because they don’t need a specific set of
environmental conditions to succeed.
This strategy becomes much less effective if the carrying capacity has been reached.
There is no room for all of the offspring being produced and so most will die right off
Species with a much lower r are called K-selected species. They do not have the
ability to reproduce quickly or increase their population rapidly. Characteristics of K-
selected species include:
Fewer offspring, which are born large
High level of parental care
Long period of sexual immaturity after birth
Typically long life span
Tendency to be specialists
Much lower infant mortality rate
K-selected species are most successful when the population is near carrying capacity
and conditions are relatively stable. Their style of producing offspring allows them to
keep a constant population without the massive amount of death that occurs with r-
strategists. Because they are typically specialists, they are very good at what they do
and can usually outcompete generalists under normal conditions.
The big disadvantage of K strategy is the inability to recover quickly from a population
drop. Whereas r-selected species can rapidly rebuild former numbers, it may take K-
selected species years or decades to recapture past population numbers.
Common examples of r-selected species include bacteria, most insects, algae, and
Common examples of K-selected species include humans, whales, eagles, and large
Some species are intermediate between r and K and have characteristics of both. For
example sea turtles show characteristics of an r-selected species by laying 70-100 eggs
at a time and not providing any parental care, but also show characteristics of a K-
selected species by having a long period of sexual immaturity after birth and a very
long life span.
A survivorship curve is a way to represent the rate at which individuals of a species
die at different stages in their lives. There are three general types of survivorship
1. Late loss curve – a large percentage of offspring survive infancy and middle-
age; not until the end of the life span does the mortality rate become severe;
typical of K-selected species. Ex. – elephants, modern humans.
2. Early loss curve – the vast majority of offspring die very young; those
individuals that do survive infancy tend to live for an extended time; typical of
r-selected species. Ex. – most fish (Nemo), most plants, insects.
3. Constant loss curve – the species has a fairly consistent mortality rate at all
points in its life, trending neither towards death at a very young or very old
age. Ex. – birds, lizards, small mammals.
9-4: Conservation Biology: Sustaining Wildlife Populations
Conservation biology is the study of the best way to preserve species and entire
We will cover the material in this section in more depth in other units.
9-5: Human Impacts on Ecosystems: Learning from Nature
This section is repetitive and the information in it is covered elsewhere.