1. Unit 2.6 Changes in Populations
DP Biology 12
Unit 2.6
Changes in Populations

2. 2.6.1 Explain the concepts of limiting
factors and carrying capacity in the
context of population growth.

3. Populations Factors affecting population size Natality Mortality
Birth rate
Mortality
Death rate
Immigration
Rate at which individuals join the population
Emigration
Rate at which individuals leave the population

4. Populations
Factors affecting population size

5. Populations Carrying capacity
In nature population sizes can not grow infinitely due to limiting factors
Disease
Parasites
Accidents
Disasters
Hunting and predation
Competition for resources
Limit to available resources

6. Populations Carrying capacity
The maximum number of organisms of a given species that a habitat can support without environmental degradation
Represented as “K”

7. 2.6.2
Describe and explain S and J
population curves.

8. Populations
J-curves
Many populations are capable of exponential growth
Doubling at a predictable rate
Graphed on a J-curve

9. Populations
Sigmoid growth curve

10. Populations Sigmoid growth curve
When populations move into a new environment, they follow a very predictable pattern of development:
Exponential phase
Transitional phase
Plateau phase

11. Populations
Sigmoid growth curve 3 2
Population Size 1
Time

12. Sigmoid growth curve
Population is expanding into a habitat offering initial low limiting factors
Exponential Phase of rapid population growth rate
Slowing yet still high growth rate against time or generations
Plateau stage when the population remains stable with time
There maybe slight oscillations

13. Populations Sigmoid growth curve Exponential phase
Population doubles per unit time producing exponential/geometric growth
No limiting factors
Nutrients, oxygen, space in ample supply
Does NOT mean no death or emigration!
Examples:
Germinating annual plants in a new season
Bacteria during the initial phases of an infection

14. Populations Sigmoid growth curve Transitional phase
Population growth continues, but at an ever-decreasing rate
Limiting factors slow growth rate
Nutrients, oxygen, space in ever-shorter supply
More selection (survival and reproduction) of those individuals within the population best suited to accessing the resources

15. Populations Sigmoid growth curve Plateau phase
Population growth slows to zero (stable)
Limiting factors inhibit growth
Nutrients, oxygen, space in short supply
Natality + Immigration = Mortality + Emigration
Population size is determined by the carrying capacity of the habitat
How large a population it can support

16. Populations
Comparison

17. Populations Comparison
In nature, J-curves often become S- curves over time
Environmental resistance = the effect of factors that limit population growth
Some species experience exponential growth when conditions are good, then die-back below the K

18. Populations

20. 2.6.3 Describe the role of density‑dependent and
density‑independent factors, and
internal and external factors, in the
regulation of populations.

21. Populations Factors limiting population growth
With unlimited resources, populations would increase exponentially
In reality, environmental resistance limits the population growth
Determines the carrying capacity of habitat

22. Populations Factors limiting population growth
Density-dependent factors:
Depend on the size of the population
Become greater as population grows
Act as negative feedback

23. Populations Factors limiting population growth
Density-dependent factors:
INTERNAL – within a single species
Competition
Reduced fertility
Shortage of food or prey
Accumulation of waste
Shortage of space or territory
Mortality due to disease
Predation or parasitism

24. Populations Factors limiting population growth
Density-dependent factors:
EXTERNAL – between species
Mortality due to disease
Predation or parasitism

25. Populations Factors limiting population growth
Density-independent factors:
Do not depend on population size
Tend to be abiotic

26. Populations Factors limiting population growth
Density-independent factors:
Mortality due to weather
Mortality due to natural disasters

27. 2.6.4 Describe the principles associated
with survivorship curves including,
K‑ and r‑strategists.

28. Population ecology Reproduction strategies
Natality has a strong affect on population size
Different animals have different reproductive patterns to be fit
Two main reproductive strategies:
r-strategies
K-strategies

29. Population ecology Reproduction strategies r-Strategies
Reproduce very rapidly
Produce a large number of offspring
Low parental care
Short generation time
Short lifespan

30. Population ecology Reproduction strategies r-Strategies
Pioneer organisms
Move into new habitats
Migration and dispersal are important
Achieve rapid colonization
Often small organisms
Breed quickly

31. Population ecology Reproduction strategies r-Strategies
Unstable habitats
Environment changes quickly
Larger population may have more variation
Easier to adapt
More likely that some survive
Easier to restore large population
If many die during a disturbance
J-curves!

32. Population ecology Reproduction strategies r-Strategies Examples:
Many insects
Many rodents
Weeds
Some fish

33. Population ecology Reproduction strategies K-Strategies
Reproduce slowly
Produce few offspring
High parental care
Have a long life span
Constant population size
Near carrying capacity
Compete successfully for resources
Often near carrying capacity (K)

34. Population ecology Reproduction strategies K-Strategies
Stable habitats
Adapted to competition for resources
Specific adaptations and behaviors
Invest into young to help them be fit
Smaller population
Near carrying capacity – S-curves!
Often larger organisms

35. Population ecology Reproduction strategies K-Strategies Examples:
Large mammals
Some birds

36. Population ecology

37. Population ecology Reproduction strategies r/K-Strategies
Most animals somewhere in-between
Some organisms are at extremes
Display some r-traits and some K-traits

38. Do humans follow r- or K- strategies?
TOK QUESTION:
Do humans follow r- or K- strategies?
There are many stereotypes and ideas about how socioeconomic class and international cultures affect how people raise their children.
How can this be explained using the concept of reproductive strategies in population ecology?

39. 2.6.5 Describe the concept and processes of succession in a named
habitat.

40. Ecosystems and biomes Succession Ecological succession
Process of colonization when new land is made available
A sequence of different communities (sere)
Transition from first colonizers to most “fit”
Goes through several stages (seres) until reaching a climax community

41. Ecosystems and biomes Succession Ecological succession Xerosere
Succession on land
Hydrosere
Succesion in water

42. Ecosystems and biomes Succession Two types: Primary succession
Succession beginning on completely new land without established soil (prisere)
Secondary succession
Succession beginning on existing soil
Site has sustained life before

43. Ecosystems and biomes Succession Primary succession
New land is formed:
Dried river deltas
Overturned sand dunes
Cooled lava
New exposed rock (erosion)
New water is collected:
New pond formed (flooding or rain)

44. Ecosystems and biomes Succession Primary succession
Site has never sustained life before
First step is always to create soil

46. Ecosystems and biomes Succession Primary succession Example:
In 1980, Mount St. Helens in America erupted
57 people and thousands of animals were killed
Hundreds of square kilometers were destroyed

47. Ecosystems and biomes Succession Primary succession Example:
In 1980, Mount St. Helens in the USA erupted
25 years later, plants and animals are coming back

48. Ecosystems and biomes
Succession
Primary succession
Example:

49. Ecosystems and biomes Succession Primary succession
Mosses and lichens break down rocks
Using acid
Creates small amounts of soil
First colonizers die
Detrivores break them down
Adding nutrients and minerals to the soil
Can take many years

50. Ecosystems and biomes Succession Primary succession
More plants will be able to grow
Hold the soil with their roots
Can not blow away
Minimizing erosion and water-loss
Breaking wind and creating shadow
Attracts consumers
Come to eat the plants

51. Ecosystems and biomes
Succession
Primary succession

52. Ecosystems and biomes Succession Primary succession Over time:
Diversity increases
Productivity increases

53. Ecosystems and biomes Succession Primary succession
Bare, inorganic habitat
Colonization
Establishment
Competition
Stabilization
Seral climax

54. Ecosystems and biomes Succession Secondary succession
Site has sustained life before
Soil is present
Usually happens very quickly
Land is destroyed:
Forest fires
Hurricanes/typhoons/cyclones
Deforestation

55. Ecosystems and biomes Succession Secondary succession Example
Fires burn down forests
ie. Near Athens, Greece in 2007
Burned vegetation adds nutrients to the soil

56. Ecosystems and biomes Succession Secondary succession Example
Once fire goes out, seeds are blown back in
Many resources and little competition

57. Ecosystems and biomes
Succession
Secondary succession
Example

58. Ecosystems and biomes
Succession

59. 2.6.6 Explain the changes in energy flow, gross and net productivity,
diversity and mineral cycling in
different stages of succession.

60. Ecosystems and biomes Succession Gross productivity
The rate of production of new biomass
Increases through succession

61. Ecosystems and biomes Succession Gross productivity
Low during early stages
Few produces
No soil for new growth
High in later stages
More producers

62. Ecosystems and biomes Succession Net productivity
GP – R (respiration = energy used)

63. Ecosystems and biomes Succession Net productivity
High during early stages
System is adding biomass
Little energy lost as numbers are low and little competition
Falls in later stages
More respiration as more energy needed to compete without adding new biomass
Ratio of GP:R approaches 1:1

64. Ecosystems and biomes
Succession
Biodiversity
Species richness!

65. Ecosystems and biomes Succession Biodiversity Low during early stages
Few pioneer species can live on bare rock
Few producers = little food for consumers
High in later stages
Food webs become more complex
Pioneers still survive, but new species immigrate
Falls slightly at climax
Specialized niches with high competition
Pioneers are out-competed (competitive exclusion)

66. Ecosystems and biomes Succession Mineral cycling
Low during early stages
Few producers to break down rock
Limited time for erosion
Limited soil for decomposers
Increases throughout succession
Remains high
Especially in hot tropical zones

67. 2.6.7
Describe factors affecting the
nature of climax communities.

68. Ecosystems and biomes Subclimax community
Often a factor prevents succession from reaching climax
Climate or lack of nutrients
Later species can not survive as niches lacking or too few nutrients or soil
Succession is “stuck” until limiting factor is removed

69. Ecosystems and biomes Zonation
Set of habitats in different seral zones due to gradient of some limiting factor
Pioneer species in a forest when an old tree falls
Distance from the beach as sand dunes are blown by wind
Distance from ocean as salinity decreases
Altitude as temperature drops

70. Ecosystems and biomes Plagioclimax
Humans can maintain highest productivity of early stages
Maximize NP by limiting R

71. Ecosystems and biomes Plagioclimax Preventing waste of energy
Cut crops as soon as they mature to limit time of competition
Prevent other species from competing
Prevent natural predators, diseases and parasites
Maintain optimal conditions for growth

72. Ecosystems and biomes Plagioclimax
Much of Europe is kept in a permanent “plagiosere”