1. Introduction to Ecology
Midlands State University
Session 2 – Population Ecology

2. Population Ecology Goals for the day Why is this field important?
What is a population?
Why does a population change in size?
Unlimited, exponential population growth
Logistic population growth
Exponential vs. Logistic growth
Population interactions

3. Population Ecology Goals for the day Why is this field important?
What is a population?
Why does a population change in size?
Unlimited, exponential population growth
Logistic population growth
Exponential vs. Logistic growth

4. Important Applications of Population Ecology - Local
Growth rates of introduced species
Population Viability Analyses of endangered species
What is the minimum number of individuals needed to ensure a 90% chance of survival for 100 years
Population genetics of endangered species

5. Important Applications of Population Ecology - Regional
Metapopulation analyses
Tracing the survival of all component populations
More in a bit
Captive Breeding projects at zoos
Applied metapopulation analyses

6. Population Ecology Goals for the day Why is this field important?
What is a population?
Why does a population change in size?
Unlimited, exponential population growth
Logistic population growth
Exponential vs. Logistic growth

7. Only minimal genetic flow, at most
What is a Population?
Components?
Definition :
One species
One area
Isolated from other areas
Able to interbreed
Example:
Only minimal genetic flow, at most

8. What is a Metapopulation?
Components?
Definition :
One species
Multiple areas
Isolated from other areas, further away
Able to interbreed
Example:
Only minimal genetic flow, at most

9. Population Characteristics
Population ecology is the study of interactions within populations (i.e., intraspecific interactions)
Recall that populations are groups of interacting conspecifics (e.g., inter-mating)
We can characterize individual populations in terms of there…
Size (average vs. variation)
Density (& impacts on size; density dependence)
Patterns of Dispersion
Demographics (age structure, sex ratios)
Rates of growth (or decline)
Limits on population growth
Population Characteristics

10. Characteristics of a Population
What features can we measure of a population?
Features:
Size
Age structure
Sex ratios
Effective population size
Birth rate
Death rate
Immigration
Emigration

11. Population Ecology Goals for the day Why is this field important?
What is a population?
Why does a population change in size?
Unlimited, exponential population growth
Logistic population growth
Exponential vs. Logistic growth

12. Why Does Population Size Change?
Density Independent Forces
Forces that are at work irrespective of the population density
Density Dependent Forces
Forces that vacillate depending on the population density

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

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

15. Indeterminate Factors
Most influences are pretty constant and Deterministic
Opposite of deterministic factors is Stochastic forces e.g droughts, floods
volcanoes, asteroids, fires etc
Examples
Environmental: Droughts, floods, asteroids, volcanoes, fires, etc.
Demographic: Crash in effective population size, series of single sex born, etc.

16. Small Populations Usually at great risk Why?
Small population size
Small genetic diversity
Highly susceptible to stochastic forces
Poor competitors with resident biota
Severely limited adaptability

17. Population Dispersion

18. Why Different Types?

19. Age-Structure Pyramids
cohort

20. (Cohort) Survivorship Curves

21. Reproduction Age of first reproduction (~sexual maturity) Clutch size
Investment in individual progeny
Tradeoff between reproduction and survival
Number of reproductive episodes per lifetime
Upside of Semelparity
Upside of Iteroparity
Reproduction

22. Semelparity

23. Iteroparity

24. Population Ecology Goals for the day Why is this field important?
What is a population?
Why does a population change in size?
Unlimited, exponential population growth
Logistic population growth
Exponential vs. Logistic growth

25. Types of Population Growth
Exponential
Unlimited, rapid growth
Often called Malthusian
Growth without bounds
Logistic
Growth within natural limits
What sets that limit?
What is the limit?
More in a moment…

26. Exponential Population Growth
Examples of this?
Invasive alien species water hycinth
Often an unnatural occurrence
Conditions under which this occurs naturally
Introduced species
Nutritionally enriched environments
Cultural innovations?

27. 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

28. 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

29. 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 timepoint called?

30. Population Ecology Goals for the day Why is this field important?
What is a population?
Why does a population change in size?
Unlimited, exponential population growth
Logistic population growth
Exponential vs. Logistic growth

31. Carrying Capacity – Unique to Logistic Growth
Definition?
A summary of all factors regulating population sizes
Density dependent
Density independent
Determinate
Stochastic
Site and species specific value

32. Logistic Population Growth
What is added in this form of population growth?
The Carrying Capacity is added
What is it and what determines it?
Typically summarized as K
How would we modify the exponential population growth equation to reflect this?

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 ~ K, population increase stops
Logistic is a special case of Exponential, when K = infinity

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

37. 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)

38. Environmental ResistanceK N
N/K = “Environmental Resistance”

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

40. Fitting Curves to Real Pops.
Logistic growth model does not consider the effects of predators or interspecific competition, so fails to predict the complexities of the density of many natural populations as a function of time
Nevertheless, it serves as good null hypothesis

41. Oscillations about K
Time N K
overshoot
oscillations

42. r versus K Selection

43. Growth Matters! How many humans can we expect? May be unlimited?
What about implications of Ecological Footprint exercise?
Currently 6 billion people
Hotly contested

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

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

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

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

48. 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?

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

50. Boom & Bust & Boom & Bust & Boom & Bust
Hare r tendencies
kept under control
by 1predation or by
their food supply?
The familiar year hare-lynx cycle might not be true. Biased data. (

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

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

53. Density-Dependent vs. Independent Limits

54. Question: Why are humans destroying the earth?
destructamundo
Question: Why are humans destroying the earth?

55. Limits: Paul Ehrlich and the Population Bomb
Impact = Population * Affluence * Technology
Impact = Population * Affluence * Efficiency
Consumption per Baby = Resource * Efficiency
(or afluence * technology)
Consumption Damages Resources (a.k.a., the environment)
There are only so many resources to use up!

56. Human Population Growth

57. 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)

58. Proximate Ecological Fields - Revisited
Trends down pyramid:
Increase in geographic scale
From single species to multiple species
Increasing number of ecological factors that may be influential
Decreasing certainty in results
Population
Community
Ecosystem

59. Next Week: The Tour of Ecology Continues
Population ecology
Community ecology
Next week’s emphasis
Ecosystem ecology
Conservation Issues