1. Population Ecology
Population – n. a group of individuals of a single species that occupies the same general area.

2. How are Populations Measured and Distributed?
Is it possible to count EVERY individual in a population?
Scientists perform “mark and recapture” experiments to estimate population sizes
Ex. “Something’s Fishy”
Randomly
No pattern at all
Clumped
Groups of pop. concentrations
Uniform
Evenly spread out or spaced in the environment

3. Population Dispersion

4. Why Different Types?

5. How Do Populations Grow? There are several mathematical models…
Exponential growth model – the rate of expansion of a population under “ideal” conditions
Population-limiting factors – hunting, amount of space suitable for breeding, restricted population growth, food availability, etc.
Logistic growth model – idealized population; growth slowed by limiting factors as the population size increases
Carrying capacity – the maximum population size that an environment can support at a particular time, without degradation of the habitat

6. Exponential growth of bacteria

7. r = 1.0
r = 0.5
Growth Without Limits
r = population growth rate

8. What if it stops growing
What if it stops growing? Logistic growth, compared to exponential growth
K = carrying capacity

9. dN/dt is maximized when N*r is maximized
Maximizing Yield
dN/dt is maximized when N*r is maximized

10. New or Changing Environment (no competition / limits)
Imposition of limits
dN/dt = r  N  (K-N)/K
Impact of Limits
New or Changing Environment (no competition / limits)

11. Growth of a population of fur seals (logistic growth model)

12. What does the logistic growth model suggest about real populations in nature?
A population’s growth rate will be small when the population size is either small or large and highest when the population is at an intermediate level relative to the carrying capacity.
Limiting factors make the birth rate decrease, the death rate increase, or both
Eventually the population will stabilize at the carrying capacity, when the birth rate equals the death rate
(These are mathematical models and no wild population fits either model perfectly!)

13. Some factors that limit population growth
As density of song sparrows increase, the number of eggs laid decreases because of food shortages
Plants grown under crowded conditions tend to be smaller and less likely to survive
Disease transmission or accumulation of toxic waste products can increase mortality

14. Continued……
A predator may capture more of a particular kind of prey as the prey becomes abundant
White-footed mice stop reproducing at a colony size of even when food and shelter are provided. Stress?
The graph shows aphids which feed on the phloem sap of plants; increase in population in the summer and then die-off in the fall and winter

15. Continued….
Some populations remain fairly stable in size close to carrying capacity
Most populations fluctuate as seen at the left
This graph shows song sparrow populations, with periodic catastrophic reductions due to severe winter weather

16. Boom and bust cycles
Hare cycles may be caused by increasing food shortages during winter caused by overgrazing
They may be due to predator-prey interactions
Cycles could be affected by a combination of food resource limitation and excessive predation
Predators reproduce more slowly than their prey so they always lag behind prey in population growth.

17. Why Does Population Size Change?
Density Independent Forces
Forces that are at work irrespective of the population density
Doesn’t matter how many individuals there are in an area: ALL are affected the same way
Density Dependent Forces
Forces that vacillate depending on the population density
May be caused by either low or high densities

18. Density Independent Forces
Examples
Climate
Topography
Latitude
Altitude
Rainfall
Sunlight
In Sum: Abiotic factors
Exceptions do exist!

19. Density-Independent Factors (e.g., weather)
Good Times!
(in Australia)

20. Density Dependent Forces
Examples
Within species
Breeding spaces
Food
Mates
Foraging spots
Between species
Predation
Parasitism
Pollinators
Competition
In Sum: Biotic factors
Exceptions do exist!

21. Density-Dependent Limits (to max = K)
Competition increases

22. Indeterminate Factors
Most influences are pretty constant and Deterministic
Opposite of deterministic factors is Stochastic forces
Examples
Environmental: Droughts, floods, asteroids, volcanoes, fires, etc.
Demographic: Crash in effective population size (ex. passenger Pigeon), series of single sex born (ex. Alligators).

23. Growth Matters! r-selected species Why most weeds are “weedy”
Edge species are typically r-selected
Invasive species are often r-selected

24. Growth Matters! K-selected species
Why don’t we get many species of oaks in most young forests?
Climax communities
Susceptible to habitat fragmentation

25. Boom and then Bust r-like
Water flee (Daphnia magna) is adapted to exploit new environment: high growth rate, resistant eggs produced before crash.

26. Boom and then really Bust
Reindeer introduced to Pribilov island. Initial exponential growth, crash, then complete extinction.
r-like

27. Boom and sort of Bust K-like? r-like?
Predators were removed from Kaibab plateau. Mule deer population size increased from 4,000 to hundred thousand, then dropped and stabilzed at 10,000.
K-like?
r-like?

28. Boom but not much Bust r & K-like
Sheep introduced to Tasmania: rapid initial growth, overshoot, drop, fluctuation around carrying capacity.
r & K-like

29. Boom & Bust & Boom & Bust & Boom & Bust
Hare r tendencies kept under control
by predation or by
their food supply?
The familiar year hare-lynx cycle might not be true. Biased data. (

30. Exponential Population Growth Equation Derivation
Which measured population growth components can change?
They are:
Birth
Death
Immigration
Emigration
Relationship between these?
No + B + I - D – E

31. Exponential Population Growth Equation Derivation
The equation for population change over a unit t (time)
N / t = No + B + I - D – E
Simplify the equation
Assume a closed population
Eliminate migration (I, E)
N / t = No + B - D
Create a growth rate (r) = (B-D)/t
N / t = (r)(No)
This is the basic exponential growth equation

32. Exponential Population Growth Equation - Implications
N / t = (r)(No)
What can be experimentally changed here and how does our close-to-home example apply?
Only r can change
r in humans has been continually increasing with technology
When r = 0, the population growth has stopped
What is this time-point called?

33. Logistic Population Growth Equation Derivation
Add the Carrying Capacity (K) – how?
N / t = (r)(No)
Base Expon. Equation
N / t = (r)(No)(1-(N/K))
Base Logistic equation
(1-(N/K)) is the unoccupied portion of the carrying capacity

34. Logistic Population Growth Equation - Implications
N / t = (r)(No)(1-(N/K))
Base Logistic equation
Implications:
As N becomes approx. equal to K, population stops increasing
Logistic is a special case of Exponential, when K = infinity

35. Large Variation in Pop. Size

36. Life tables – compiled by life insurance agents

37. Survivorship curves
Type I curve – parents produce few offspring and give them good care
Type III curve – high death rates for infants then a period when death rates are lower for those who survive to a certain age

38. Exponential growth of the human population
Throughout human history parents had many children but only two on average survived to adulthood
Estimates that by 2025 the world will have to double food production, 2/3 of the available fresh water on earth will be in use, 60,000 plant species will be lost to support the population
Issues: overgrazing, rivers running dry, decrease in groundwater, energy?

39. Human Population Growth

42. Population Density

43. Human carrying capacity estimates
Ecological footprint (food, fuel & water consumption, housing size, and waste production)
Calculates current demand on world resources by each country, in hectares of land per person
World ecological capacity is approx. 1.7 ha per person alive in 1997

44. How to achieve population stability?
Zero population growth – when birth rates equal death rates
Two ways to reach ZPG. High birth and death rates or low birth and death rates.
Demographic transition is moving from the first to the second. Most developed countries have made the transition
See the demographic transition in Mexico at the left.

46. Question: Why do locusts destroy crops?
nymph
aggregating nymphs
adults feeding
swarming
destructamundo

47. Limits: Locust Freedom Without Responsibility
I’ve got my rights!
It’s a
free country!
Who’s going to stop me?
= Destroyed Crops
(destruction of environment)

48. Question: Why do some microbes make us sick?
©Phage et al.
Question: Why do some microbes make us sick?

49. Limits: Freedom Without Responsibility
It’s a free country
Who’s going
to stop me?
I’ve got my rights!
= Disease!
(destruction of the body environment)
Bacterial
pathogens

50. Human Freedom Without Responsibility
urban sprawl
global warming
deforestation
desertification
overpopulation
air polution
water polution
loss of
habitat
overconsumption
conspicuous consumption
loss of farmland
overfishing
greenhouse effect
ozone hole
mass extinction
greed
loss of wetlands
NIMBY
= “not in my
backyard”
lack of cooperation
out-of-control
materialism
special interests
destruction
TEOTWAWKI = “the end of the world as we know it”
radical anti-environmentalism
might makes right
short-term thinking
fish kills
toxic algal
blooms
erosion
loss of topsoil
bigger is better
monoculture
pesticides
the bottom line
Who’s going to stop me?
It’s a free country
I’ve got my rights!
= Destructamundo!
(destruction of environment)