5.1 Population Potential
1. Identify factors that contribute to population growth and explain how they contribute to population size.
2. Compare real population growth curves to models
3. Explain what factors determine carrying capacity.
Readings: Chapter 13: 343-355; selections from Plan B 3.) by Lester Brown (follow link in Wk 5 Learning Module)
Reminders: Reading Quiz 5 due before the start of today’s class
Assignment #2 on Food Webs due before start of today’s class
Midterm Friday Oct 9th from 12:00 to 12:50 in your normal lecture room
What is a population?:
Individuals of the same species living in the same geographic area
at the same time
The rate at which populations grow is determined by 4 key factors:
Factor Effect on population size
Birth Adds new individuals increase
Death Removes individuals decrease
Immigration New individuals enter population from elsewhere increase
Emigration Individuals leave population to go somewhere else decrease
Generation time or The younger the age of the parents when they reproduce… Faster increase
The older the parents when they reproduce… Slower increase
Amount of Healthier individuals living longer….lowers death rate… Increase
available resources AND/OR
(don’t think about Individuals with more resources for growth (and reproduction)….can
Homo sapiens) delay reproduction and spend more time growing eg long-lived trees Slower increase
Individuals with few resources are less healthy….increases death Decrease or slower
rate….and they have less energy for birthing healthy offspring increase
If resources are limiting….organisms may put available resources
into reproduction and then die shortly afterward
(big-bang reproduction eg weedy plants) Faster increase
How can we reconcile the seemingly contradictory effects that factors like the amount of available resources can have
on population growth? How can the existence of plentiful resources potentially both increase or decrease population
We should consider the typical pattern of growth and reproduction for an individual species first (its life history) and
then consider the available resources second.
Life history: how an individual within a species allocates resources to growth and reproduction as well as activities and
adaptations it has related to its survival
Life history traits form a continuum:
Some species put most of their energy into
reproduction and little energy into actively insuring
the survival of those offspring eg. A tree frog lays a
large mass of eggs but does not protect the eggs and
offspring nor does it nurture the offspring. A
fireweed plant is shown at the far right releasing
seeds for wind dispersal.
Some species give birth to few offspring but put relatively more energy into
insuring the survival of those few offspring.
eg: An elephant usually only gives birth to one offspring at at time after
gestating 22 months, the mother feeds her young offspring milk and keeps it
with her protecting and teaching the young for ~4 years.
eg2 A coconut tree only produces a relatively small number of seeds
(coconuts) each season but each seed has a large energy store for
germination and early growth and the seed itself is well protected against
For their life history all species fall somewhere in between these two extremes of high reproduction vs. high
survivorship. The life history strategy employed by a species will affect how a population responds to environmental
changes such as resource limitation.
What order would you place these species on a continuum of reproduction (aka fecundity above left shown by crabs) vs.
survivorship far right shown by coconuts)?
QUESTION 1. Number the species from 1 to 5 with 1 being the species most focused on reproducing high numbers of
organisms and 5 putting the most emphasis on survivorship of offspring.
American coot puffball lizards Japlura swinhonts Paramecium caudatum chimpanzees
Fulica americana Lycoperdon (amoeboid protist) Pan troglodytes
#4 #1 #5
(Highest (Chimpanzees take longer to
Fecundity) teach offspring stuffs)
QUESTION: 2. If you had any trouble deciding the order of some of these species what questions would help you better
decide on the order?
- How often do they reproduce?
- What is the generation time?
- How many offspring in total does an individual have?
- How much energy is invested per offspring?
How do populations grow?
Whooping cranes (Grus americana) were
near extinction mainly due to habitat loss.
Intensive recovery efforts have been ongoing
since the 1970s including protecting
remaining habitat, introducing whooping
crane eggs into sandhill crane nests, creating
1 non-migratory population in Florida, and
creating a second more easterly migratory
QUESTIONS: This migratory crane population appears to still be increasing (the last data point is from 2007). 3. What is
permitting the population to continue to grow?
- Few competitors (less competition)
- Available food (because they move around)
- Few predators
- Migration may favour a larger population
- Available nesting sites
4. What would limit its growth? (Hint: remember your lectures on Biotic factors)
- lack of habitat
- lack of food
- ( - unfavourable abiotic conditions )
This graph shows increases in the number of wolves in
the Rocky Mts of the United States after recovery
efforts occurred. NWMT..= wolves in Montana, GYA.. =
wolves in the greater Yellowstone area and CID.. =
Idaho. Approximately the last 5 years on the graph
indicate population growth from the wolves is now only
coming from wolves in Idaho, the other two areas have
reached a stable or maximal size.
QUESTION: 5.Why is the Idaho population likely still
growing while the other two are not?
- Bigger capacity
- Available food
- More available habitat
- Fewer predators
- Less contact w/ humans (who feel threatened
and kill them :X )
QUESTION 6: For this population of yeast cells growing in a culture in a test tube draw a simple plot below of this data
and predict the population size at 120 and 135 minutes.
Time in Number of cells
QUESTION: 7.What has begun to happen beyond 60 minutes?
- Resources (eg: food, gases) have become limiting (this happens at the beginning of stationary phase)
Carrying capacity: The maximal population size a habitat can support without any habitat degradation.
The population growth curves for whooping cranes and wolves were shown for populations in recovery because most
natural populations we observe are not starting from scratch moving into a new habitat and beginning to grow. They
have already reached some particular level and are either stable or showing some type of fluctuations depending on
abiotic and biotic conditions.
Estimating what population level a habitat can support is a key question for population biologists. For example here is a
graph generated by wildlife biologists in Wisconsin concerned with their wolf population:
The growth shown by many populations in initially exponential as shown by the first part of the above S-shaped curve.
Once a population gets close to carrying capacity (where the graph flattens out) population growth slows. This overall
trend is called logistic growth as shown by Fig 13.6 in your text.
When a population is dense and close to its carrying capacity a number of things can occur:
-wastes build up and can harm the health of the species itself as well as other species in the same habitat
-diseases may increase
QUESTION 8. What else?
- You begin to see the effect of competition between organisms within the population
- Food supplies can become limiting
These factors which affect a population that is close or at carrying capacity are called density-dependent factors.
***QUESTION: 9. Considering that a population at carrying capacity is at the maximum number of individuals that can
be supported without habitat degradation, has the human population surpassed its carrying capacity?
Populations can also be affected by density-independent factors that can affect a population no matter what size it is.
These factors tend to be abiotic.
QUESTION: 10. What could be influencing the population size of the song sparrow
population from Mandarte Island (BC) shown left?
- Extreme weather events