1. Population Ecology Topic 3
This trio of sea otters is part of the population that lives near Monterey, California. Sea otters often rest by wrapping themselves in kelp to keep from drifting away.

2. Kelp Forest Ecosystem Kelp forest Sea Otters
America’s Pacific Northwest is home to an underwater “forest” that is the basis for a very diverse ecosystem.
What does kelp provide?
food & habitat
Sea Otters are an important member of the kelp forest. Otters eat a lot of food (sea urchins) to stay warm in the cold water
Sea urchins, in turn, feed on kelp
The relationships along this food chain set the stage for a classic tale of population growth and decline.
Kelp forest
Sea Otters
Sea Otters are Keystone Species
A keystone is the stone at the top of an arch that supports the other stones and keeps the whole arch from falling.
**A Keystone species is a species on which the persistence of a large number of other species in the ecosystem depends.

3. Kelp Forest Continued
A century ago, otters were nearly eliminated by hunting.
Sea urchin populations increased greatly, and kelp forests nearly disappeared. Why?
Then, sea otters were declared an endangered species and were protected from hunting
Otter populations recovered, sea urchin numbers dropped, and kelp grew back
But now, some otter populations are shrinking again…
Click to Watch

4. How sea otters help save the planet
The sea otter, a voracious urchin-eater and possessor of the densest fur in the animal kingdom. Photograph: Alamy
Sea otters floating in kelp. They help kelp forests to flourish by keeping sea urchin numbers under control. Photograph: Frans Lanting/Getty Images

5. Sea otters use rocks to crack open the spiny shells of sea urchins
Sea otters use rocks to crack open the spiny shells of sea urchins. Photograph: Noel Hendrickson/Getty Images

6. Jaguar (keystone predator)

7. Prairie Dog (keystone modifier)

8. Hummingbird (keystone mutualist)

9. Beaver (keystone modifier)

10. Saguaro Cactus (keystone host)

11. Snowshoe Hare (keystone prey)

12. …back to our topic, Populations
A group of organisms of the same species that live in the same place
There are Four Important Characteristics of Populations:
1. Geographic Distribution
(Range)
2. Density
3. Growth Rate
4. Age Structure

13. 1. Geographic Distribution (Range)
Describes the area inhabited by a population
The range can vary in size from a few cm3 to millions of km2
Which has a larger range?

15. 2. Population Density The number of individuals per unit area
This number can vary tremendously depending on the species and its ecosystem
The tall saguaro cactuses in this Arizona desert have a low population density compared to the smaller desert plants

16. Density of Whitetail Deer in the United States
Which states have the highest density?

17. 3. Growth Rate
How the number of individuals in a population changes over time
Three factors can affect population size:
Number of births
Number of deaths
Number of individuals that enter and leave the population
Immigration: the movement of individuals into an area
Emigration: the movement of individuals out of an area
What happens when the birth rate is greater than the death rate?
What happens when life expectancy improves?

18. Feedback Loop in Populations
A feedback loop is a process in which the system responds to a disturbance either:
In the same direction (positive feedback)
In the opposite direction (negative feedback)

19. Example
Positive Feedback Loop = increase in birth rate (reinforcing)

20. Negative Feedback Loop = increase in death rate (diminishing)
Click to Watch
Negative Feedback Loop = increase in death rate (diminishing)

21. 4. Age Structure
Population growth depends, in part, on how many individuals of different ages make up a population
Age-structure diagrams—graph of the number of individuals in different age groups

22. There are two models or types of growth curves:
GROWTH CURVES REPRESENT THE GROWTH OR DECLINE OF A POPULATION GRAPHICALLY
There are two models or types of growth curves:
1. Exponential (J-shaped)—occurs when the individuals in a population reproduce at a constant rate
2. Logistic (S-shaped)—occurs when a population’s growth slows or stops following a period of exponential growth

23. EXPONENTIAL GROWTH
In the presence of unlimited resources and in the absence of predation and disease (ideal conditions), a population will grow exponentially
If this growth pattern continued for several days without slowing down bacteria would cover the planet!

24. LOGISTIC GROWTH
Since bacteria doesn’t cover the planet, this means that exponential growth does not continue in natural populations for very long
As resources become less available, the growth of a population slows or stops

25. Carrying Capacity
In logistic growth, why does the growth slow or stop?
The ecosystem reaches the carrying capacity, or the largest number of individuals of a population that a given environment can support
Note that even when the population growth is said to stop, it still rises and falls, but averages out to a certain population size.
Limiting Factors

26. Let’s Review… Concept Map Population Growth Logistic Exponential
can be
Exponential
growth
Logistic
characterized by
represented by
characterized by
represented by
Falling
growth rate
S-shaped
curve
Limits on
growth
No limits on
J-shaped
Constant
Unlimited
resources
which cause a

27. Limits to growth
Many factors may cause a decline in a population over time.
Remember…predators such as killer whales are now feeding on populations of sea otters 
Another Example: primary productivity of an ecosystem can be reduced when there is an insufficient supply of a particular nutrient – limiting nutrient
This is an example of a larger concept – a limiting factor.

28. Limiting Factors Cause population growth to decrease
Ex. Competition, Predation, Parasitism, Disease, Drought, Human disturbances
Think of it like a dam holding water back.

29. Density-Dependent Limiting Factors
Depends on population size—only becomes limiting when the density (number of organisms per unit area) reaches a certain level
This includes factors such as competition, predation, parasitism, & disease
Ex. Where is your chance of getting the flu the highest?

30. Predator-prey relationships
The regulation of a population by predation is one of the best-known mechanisms of population control

32. How many moose does a wolf need to eat to survive a year?
In order to survive a 50 kg wolf needs to eat about 2700 kg of moose/yr
The average mass of a moose is 385 kg
How many “average” moose does a wolf need to eat each year? ~7
If there are 8 wolves in a pack?
2700 x 8 =21,600/385 = ~56

33. Density-Independent Limiting Factors
Affect all populations in similar ways, regardless of the population size
This includes factors such as weather, natural disasters, seasonal cycles, & human activity

34. In response to such factors, many species show a characteristic crash in population size
What could be the potential cause for this crash?

35. Human Population Growth
Like other organisms, the size of the human pop. tends to increase over time.
Industrial
Revolution
begins
Agriculture
begins
Bubonic
plague
Plowing
and
irrigation
To begin: Life was often difficult…many limiting factors
Until fairly recently only ½ of children survived to adulthood
When the food supply became more reliable, technology, sanitation, medicine, & healthcare reduced death rate.

36. Demography – the scientific study of human populations
Thomas Malthus predicted that human populations would not grow indefinitely
due to war, famine, disease providing limits
How large is the human population?
NPR population video
Birthrates, death rates, and the age structure of a population help predict why some countries have high growth rates while other countries grow more slowly.

37. The Demographic Transition
All countries begin here
As countries modernize, they have better nutrition, sanitation, and medicine which lowers the death rate
As modernization continues, families have fewer children which lowers the birth rate
D. The transition is complete when population growth stops D. B.
Know how changes to birth & death rates affect the population.

38. Age-Structure Diagrams: used for predicting future growth
Section 5-3
U.S. Population
Rwandan Population
Males
Females
Males
Females
Click to Watch
Population growth depends on how many people of
different ages make up a population.

39. Animal Societies
Some of the most extreme examples of relatedness, and the most complex animal societies (apart from humans) are found among social insects!
An animal society is a group of animals of the same species that interact closely and often cooperate
Members of a society are often related to one another
An Ant Society
Click to Watch
Figure 1: Social insects have well protected or defended nests, including termites (a), wasps (b), and bees (c).
Click to Watch

40. Animal Societies
Figure 1: Social groups which have formed to improve the probability of survival and reproduction of individual members: pack of wolves (a) and school of fish (b)

41. Group Behavior
Figure 1: Group-living is widespread in the animal kingdom.
Although many species form temporary associations, such as (a) flamingo (Phoenicopterus minor) colonies and (b) zebra (Equus quagga) herds, some species such as (c) African elephants (Loxodonta africana), (d) snapping shrimps (Synalpheus brooksi), and (e) superb starlings (Lamprotornis superbus) form more permanent social groups and live together year-round.

42. Great Migrations Click to Watch
Tens of millions of bison once rumbled across the Great Plains on a quest for grazing. By the late 1800s nearly all had been slaughtered. Today most of the half million remaining bison are in captivity, like these on the Triple U ranch in South Dakota.

43. The Genius of Swarms Click to Watch
A single ant or bee isn't smart, but their colonies are. The study of swarm intelligence is providing insights that can help humans manage complex systems, from truck routing to military robots.